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Submitted URL: https://tools.ietf.org/html/rfc9110#section-15.5.1
Effective URL: https://datatracker.ietf.org/doc/html/rfc9110
Submission: On November 25 via manual from US — Scanned from US
Effective URL: https://datatracker.ietf.org/doc/html/rfc9110
Submission: On November 25 via manual from US — Scanned from US
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RFC 9110
Internet Standard
Title HTTP Semantics Document Document type RFC - Internet Standard
June 2022
View errata Report errata IPR
Obsoletes RFC 7538, RFC 7233, RFC 2818, RFC 7694, RFC 7232, RFC 7615, RFC 7230,
RFC 7235, RFC 7231
Updates RFC 3864
Was draft-ietf-httpbis-semantics (httpbis WG)
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RFC 9110 draft-ietf-httpbis-semantics-19 draft-ietf-httpbis-semantics-18
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Authors Roy T. Fielding , Mark Nottingham , Julian Reschke
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RFC 9110 HTTP Semantics June 2022 Fielding, et al. Standards Track [Page]
Stream: Internet Engineering Task Force (IETF) RFC: 9110 STD: 97 Obsoletes:
2818, 7230, 7231, 7232, 7233, 7235, 7538, 7615, 7694 Updates: 3864 Category:
Standards Track Published: June 2022 ISSN: 2070-1721 Authors:
R. Fielding, Ed.
Adobe
M. Nottingham, Ed.
Fastly
J. Reschke, Ed.
greenbytes
RFC 9110
HTTP SEMANTICS
ABSTRACT
The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol
for distributed, collaborative, hypertext information systems. This document
describes the overall architecture of HTTP, establishes common terminology, and
defines aspects of the protocol that are shared by all versions. In this
definition are core protocol elements, extensibility mechanisms, and the "http"
and "https" Uniform Resource Identifier (URI) schemes.¶
This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235,
7538, 7615, 7694, and portions of 7230.¶
STATUS OF THIS MEMO
This is an Internet Standards Track document.¶
This document is a product of the Internet Engineering Task Force (IETF). It
represents the consensus of the IETF community. It has received public review
and has been approved for publication by the Internet Engineering Steering Group
(IESG). Further information on Internet Standards is available in Section 2 of
RFC 7841.¶
Information about the current status of this document, any errata, and how to
provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9110.¶
COPYRIGHT NOTICE
Copyright (c) 2022 IETF Trust and the persons identified as the document
authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions
Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on
the date of publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect to this
document. Code Components extracted from this document must include Revised BSD
License text as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.¶
This document may contain material from IETF Documents or IETF Contributions
published or made publicly available before November 10, 2008. The person(s)
controlling the copyright in some of this material may not have granted the IETF
Trust the right to allow modifications of such material outside the IETF
Standards Process. Without obtaining an adequate license from the person(s)
controlling the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may not be
created outside the IETF Standards Process, except to format it for publication
as an RFC or to translate it into languages other than English.¶
▲
TABLE OF CONTENTS
* 1. Introduction
* 1.1. Purpose
* 1.2. History and Evolution
* 1.3. Core Semantics
* 1.4. Specifications Obsoleted by This Document
* 2. Conformance
* 2.1. Syntax Notation
* 2.2. Requirements Notation
* 2.3. Length Requirements
* 2.4. Error Handling
* 2.5. Protocol Version
* 3. Terminology and Core Concepts
* 3.1. Resources
* 3.2. Representations
* 3.3. Connections, Clients, and Servers
* 3.4. Messages
* 3.5. User Agents
* 3.6. Origin Server
* 3.7. Intermediaries
* 3.8. Caches
* 3.9. Example Message Exchange
* 4. Identifiers in HTTP
* 4.1. URI References
* 4.2. HTTP-Related URI Schemes
* 4.2.1. http URI Scheme
* 4.2.2. https URI Scheme
* 4.2.3. http(s) Normalization and Comparison
* 4.2.4. Deprecation of userinfo in http(s) URIs
* 4.2.5. http(s) References with Fragment Identifiers
* 4.3. Authoritative Access
* 4.3.1. URI Origin
* 4.3.2. http Origins
* 4.3.3. https Origins
* 4.3.4. https Certificate Verification
* 4.3.5. IP-ID Reference Identity
* 5. Fields
* 5.1. Field Names
* 5.2. Field Lines and Combined Field Value
* 5.3. Field Order
* 5.4. Field Limits
* 5.5. Field Values
* 5.6. Common Rules for Defining Field Values
* 5.6.1. Lists (#rule ABNF Extension)
* 5.6.1.1. Sender Requirements
* 5.6.1.2. Recipient Requirements
* 5.6.2. Tokens
* 5.6.3. Whitespace
* 5.6.4. Quoted Strings
* 5.6.5. Comments
* 5.6.6. Parameters
* 5.6.7. Date/Time Formats
* 6. Message Abstraction
* 6.1. Framing and Completeness
* 6.2. Control Data
* 6.3. Header Fields
* 6.4. Content
* 6.4.1. Content Semantics
* 6.4.2. Identifying Content
* 6.5. Trailer Fields
* 6.5.1. Limitations on Use of Trailers
* 6.5.2. Processing Trailer Fields
* 6.6. Message Metadata
* 6.6.1. Date
* 6.6.2. Trailer
* 7. Routing HTTP Messages
* 7.1. Determining the Target Resource
* 7.2. Host and :authority
* 7.3. Routing Inbound Requests
* 7.3.1. To a Cache
* 7.3.2. To a Proxy
* 7.3.3. To the Origin
* 7.4. Rejecting Misdirected Requests
* 7.5. Response Correlation
* 7.6. Message Forwarding
* 7.6.1. Connection
* 7.6.2. Max-Forwards
* 7.6.3. Via
* 7.7. Message Transformations
* 7.8. Upgrade
* 8. Representation Data and Metadata
* 8.1. Representation Data
* 8.2. Representation Metadata
* 8.3. Content-Type
* 8.3.1. Media Type
* 8.3.2. Charset
* 8.3.3. Multipart Types
* 8.4. Content-Encoding
* 8.4.1. Content Codings
* 8.4.1.1. Compress Coding
* 8.4.1.2. Deflate Coding
* 8.4.1.3. Gzip Coding
* 8.5. Content-Language
* 8.5.1. Language Tags
* 8.6. Content-Length
* 8.7. Content-Location
* 8.8. Validator Fields
* 8.8.1. Weak versus Strong
* 8.8.2. Last-Modified
* 8.8.2.1. Generation
* 8.8.2.2. Comparison
* 8.8.3. ETag
* 8.8.3.1. Generation
* 8.8.3.2. Comparison
* 8.8.3.3. Example: Entity Tags Varying on Content-Negotiated Resources
* 9. Methods
* 9.1. Overview
* 9.2. Common Method Properties
* 9.2.1. Safe Methods
* 9.2.2. Idempotent Methods
* 9.2.3. Methods and Caching
* 9.3. Method Definitions
* 9.3.1. GET
* 9.3.2. HEAD
* 9.3.3. POST
* 9.3.4. PUT
* 9.3.5. DELETE
* 9.3.6. CONNECT
* 9.3.7. OPTIONS
* 9.3.8. TRACE
* 10. Message Context
* 10.1. Request Context Fields
* 10.1.1. Expect
* 10.1.2. From
* 10.1.3. Referer
* 10.1.4. TE
* 10.1.5. User-Agent
* 10.2. Response Context Fields
* 10.2.1. Allow
* 10.2.2. Location
* 10.2.3. Retry-After
* 10.2.4. Server
* 11. HTTP Authentication
* 11.1. Authentication Scheme
* 11.2. Authentication Parameters
* 11.3. Challenge and Response
* 11.4. Credentials
* 11.5. Establishing a Protection Space (Realm)
* 11.6. Authenticating Users to Origin Servers
* 11.6.1. WWW-Authenticate
* 11.6.2. Authorization
* 11.6.3. Authentication-Info
* 11.7. Authenticating Clients to Proxies
* 11.7.1. Proxy-Authenticate
* 11.7.2. Proxy-Authorization
* 11.7.3. Proxy-Authentication-Info
* 12. Content Negotiation
* 12.1. Proactive Negotiation
* 12.2. Reactive Negotiation
* 12.3. Request Content Negotiation
* 12.4. Content Negotiation Field Features
* 12.4.1. Absence
* 12.4.2. Quality Values
* 12.4.3. Wildcard Values
* 12.5. Content Negotiation Fields
* 12.5.1. Accept
* 12.5.2. Accept-Charset
* 12.5.3. Accept-Encoding
* 12.5.4. Accept-Language
* 12.5.5. Vary
* 13. Conditional Requests
* 13.1. Preconditions
* 13.1.1. If-Match
* 13.1.2. If-None-Match
* 13.1.3. If-Modified-Since
* 13.1.4. If-Unmodified-Since
* 13.1.5. If-Range
* 13.2. Evaluation of Preconditions
* 13.2.1. When to Evaluate
* 13.2.2. Precedence of Preconditions
* 14. Range Requests
* 14.1. Range Units
* 14.1.1. Range Specifiers
* 14.1.2. Byte Ranges
* 14.2. Range
* 14.3. Accept-Ranges
* 14.4. Content-Range
* 14.5. Partial PUT
* 14.6. Media Type multipart/byteranges
* 15. Status Codes
* 15.1. Overview of Status Codes
* 15.2. Informational 1xx
* 15.2.1. 100 Continue
* 15.2.2. 101 Switching Protocols
* 15.3. Successful 2xx
* 15.3.1. 200 OK
* 15.3.2. 201 Created
* 15.3.3. 202 Accepted
* 15.3.4. 203 Non-Authoritative Information
* 15.3.5. 204 No Content
* 15.3.6. 205 Reset Content
* 15.3.7. 206 Partial Content
* 15.3.7.1. Single Part
* 15.3.7.2. Multiple Parts
* 15.3.7.3. Combining Parts
* 15.4. Redirection 3xx
* 15.4.1. 300 Multiple Choices
* 15.4.2. 301 Moved Permanently
* 15.4.3. 302 Found
* 15.4.4. 303 See Other
* 15.4.5. 304 Not Modified
* 15.4.6. 305 Use Proxy
* 15.4.7. 306 (Unused)
* 15.4.8. 307 Temporary Redirect
* 15.4.9. 308 Permanent Redirect
* 15.5. Client Error 4xx
* 15.5.1. 400 Bad Request
* 15.5.2. 401 Unauthorized
* 15.5.3. 402 Payment Required
* 15.5.4. 403 Forbidden
* 15.5.5. 404 Not Found
* 15.5.6. 405 Method Not Allowed
* 15.5.7. 406 Not Acceptable
* 15.5.8. 407 Proxy Authentication Required
* 15.5.9. 408 Request Timeout
* 15.5.10. 409 Conflict
* 15.5.11. 410 Gone
* 15.5.12. 411 Length Required
* 15.5.13. 412 Precondition Failed
* 15.5.14. 413 Content Too Large
* 15.5.15. 414 URI Too Long
* 15.5.16. 415 Unsupported Media Type
* 15.5.17. 416 Range Not Satisfiable
* 15.5.18. 417 Expectation Failed
* 15.5.19. 418 (Unused)
* 15.5.20. 421 Misdirected Request
* 15.5.21. 422 Unprocessable Content
* 15.5.22. 426 Upgrade Required
* 15.6. Server Error 5xx
* 15.6.1. 500 Internal Server Error
* 15.6.2. 501 Not Implemented
* 15.6.3. 502 Bad Gateway
* 15.6.4. 503 Service Unavailable
* 15.6.5. 504 Gateway Timeout
* 15.6.6. 505 HTTP Version Not Supported
* 16. Extending HTTP
* 16.1. Method Extensibility
* 16.1.1. Method Registry
* 16.1.2. Considerations for New Methods
* 16.2. Status Code Extensibility
* 16.2.1. Status Code Registry
* 16.2.2. Considerations for New Status Codes
* 16.3. Field Extensibility
* 16.3.1. Field Name Registry
* 16.3.2. Considerations for New Fields
* 16.3.2.1. Considerations for New Field Names
* 16.3.2.2. Considerations for New Field Values
* 16.4. Authentication Scheme Extensibility
* 16.4.1. Authentication Scheme Registry
* 16.4.2. Considerations for New Authentication Schemes
* 16.5. Range Unit Extensibility
* 16.5.1. Range Unit Registry
* 16.5.2. Considerations for New Range Units
* 16.6. Content Coding Extensibility
* 16.6.1. Content Coding Registry
* 16.6.2. Considerations for New Content Codings
* 16.7. Upgrade Token Registry
* 17. Security Considerations
* 17.1. Establishing Authority
* 17.2. Risks of Intermediaries
* 17.3. Attacks Based on File and Path Names
* 17.4. Attacks Based on Command, Code, or Query Injection
* 17.5. Attacks via Protocol Element Length
* 17.6. Attacks Using Shared-Dictionary Compression
* 17.7. Disclosure of Personal Information
* 17.8. Privacy of Server Log Information
* 17.9. Disclosure of Sensitive Information in URIs
* 17.10. Application Handling of Field Names
* 17.11. Disclosure of Fragment after Redirects
* 17.12. Disclosure of Product Information
* 17.13. Browser Fingerprinting
* 17.14. Validator Retention
* 17.15. Denial-of-Service Attacks Using Range
* 17.16. Authentication Considerations
* 17.16.1. Confidentiality of Credentials
* 17.16.2. Credentials and Idle Clients
* 17.16.3. Protection Spaces
* 17.16.4. Additional Response Fields
* 18. IANA Considerations
* 18.1. URI Scheme Registration
* 18.2. Method Registration
* 18.3. Status Code Registration
* 18.4. Field Name Registration
* 18.5. Authentication Scheme Registration
* 18.6. Content Coding Registration
* 18.7. Range Unit Registration
* 18.8. Media Type Registration
* 18.9. Port Registration
* 18.10. Upgrade Token Registration
* 19. References
* 19.1. Normative References
* 19.2. Informative References
* Appendix A. Collected ABNF
* Appendix B. Changes from Previous RFCs
* B.1. Changes from RFC 2818
* B.2. Changes from RFC 7230
* B.3. Changes from RFC 7231
* B.4. Changes from RFC 7232
* B.5. Changes from RFC 7233
* B.6. Changes from RFC 7235
* B.7. Changes from RFC 7538
* B.8. Changes from RFC 7615
* B.9. Changes from RFC 7694
* Acknowledgements
* Index
* Authors' Addresses
1. INTRODUCTION
1.1. PURPOSE
The Hypertext Transfer Protocol (HTTP) is a family of stateless,
application-level, request/response protocols that share a generic interface,
extensible semantics, and self-descriptive messages to enable flexible
interaction with network-based hypertext information systems.¶
HTTP hides the details of how a service is implemented by presenting a uniform
interface to clients that is independent of the types of resources provided.
Likewise, servers do not need to be aware of each client's purpose: a request
can be considered in isolation rather than being associated with a specific type
of client or a predetermined sequence of application steps. This allows
general-purpose implementations to be used effectively in many different
contexts, reduces interaction complexity, and enables independent evolution over
time.¶
HTTP is also designed for use as an intermediation protocol, wherein proxies and
gateways can translate non-HTTP information systems into a more generic
interface.¶
One consequence of this flexibility is that the protocol cannot be defined in
terms of what occurs behind the interface. Instead, we are limited to defining
the syntax of communication, the intent of received communication, and the
expected behavior of recipients. If the communication is considered in
isolation, then successful actions ought to be reflected in corresponding
changes to the observable interface provided by servers. However, since multiple
clients might act in parallel and perhaps at cross-purposes, we cannot require
that such changes be observable beyond the scope of a single response.¶
1.2. HISTORY AND EVOLUTION
HTTP has been the primary information transfer protocol for the World Wide Web
since its introduction in 1990. It began as a trivial mechanism for low-latency
requests, with a single method (GET) to request transfer of a presumed hypertext
document identified by a given pathname. As the Web grew, HTTP was extended to
enclose requests and responses within messages, transfer arbitrary data formats
using MIME-like media types, and route requests through intermediaries. These
protocols were eventually defined as HTTP/0.9 and HTTP/1.0 (see [HTTP/1.0]).¶
HTTP/1.1 was designed to refine the protocol's features while retaining
compatibility with the existing text-based messaging syntax, improving its
interoperability, scalability, and robustness across the Internet. This included
length-based data delimiters for both fixed and dynamic (chunked) content, a
consistent framework for content negotiation, opaque validators for conditional
requests, cache controls for better cache consistency, range requests for
partial updates, and default persistent connections. HTTP/1.1 was introduced in
1995 and published on the Standards Track in 1997 [RFC2068], revised in 1999
[RFC2616], and revised again in 2014 ([RFC7230] through [RFC7235]).¶
HTTP/2 ([HTTP/2]) introduced a multiplexed session layer on top of the existing
TLS and TCP protocols for exchanging concurrent HTTP messages with efficient
field compression and server push. HTTP/3 ([HTTP/3]) provides greater
independence for concurrent messages by using QUIC as a secure multiplexed
transport over UDP instead of TCP.¶
All three major versions of HTTP rely on the semantics defined by this document.
They have not obsoleted each other because each one has specific benefits and
limitations depending on the context of use. Implementations are expected to
choose the most appropriate transport and messaging syntax for their particular
context.¶
This revision of HTTP separates the definition of semantics (this document) and
caching ([CACHING]) from the current HTTP/1.1 messaging syntax ([HTTP/1.1]) to
allow each major protocol version to progress independently while referring to
the same core semantics.¶
1.3. CORE SEMANTICS
HTTP provides a uniform interface for interacting with a resource (Section 3.1)
-- regardless of its type, nature, or implementation -- by sending messages that
manipulate or transfer representations (Section 3.2).¶
Each message is either a request or a response. A client constructs request
messages that communicate its intentions and routes those messages toward an
identified origin server. A server listens for requests, parses each message
received, interprets the message semantics in relation to the identified target
resource, and responds to that request with one or more response messages. The
client examines received responses to see if its intentions were carried out,
determining what to do next based on the status codes and content received.¶
HTTP semantics include the intentions defined by each request method (Section
9), extensions to those semantics that might be described in request header
fields, status codes that describe the response (Section 15), and other control
data and resource metadata that might be given in response fields.¶
Semantics also include representation metadata that describe how content is
intended to be interpreted by a recipient, request header fields that might
influence content selection, and the various selection algorithms that are
collectively referred to as "content negotiation" (Section 12).¶
1.4. SPECIFICATIONS OBSOLETED BY THIS DOCUMENT
Table 1 Title Reference See HTTP Over TLS [RFC2818] B.1 HTTP/1.1 Message Syntax
and Routing [*] [RFC7230] B.2 HTTP/1.1 Semantics and Content [RFC7231] B.3
HTTP/1.1 Conditional Requests [RFC7232] B.4 HTTP/1.1 Range Requests [RFC7233]
B.5 HTTP/1.1 Authentication [RFC7235] B.6 HTTP Status Code 308 (Permanent
Redirect) [RFC7538] B.7 HTTP Authentication-Info and Proxy-Authentication-Info
Response Header Fields [RFC7615] B.8 HTTP Client-Initiated Content-Encoding
[RFC7694] B.9
[*] This document only obsoletes the portions of RFC 7230 that are independent
of the HTTP/1.1 messaging syntax and connection management; the remaining bits
of RFC 7230 are obsoleted by "HTTP/1.1" [HTTP/1.1].¶
2. CONFORMANCE
2.1. SYNTAX NOTATION
This specification uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC5234], extended with the notation for case-sensitivity in strings defined in
[RFC7405].¶
It also uses a list extension, defined in Section 5.6.1, that allows for compact
definition of comma-separated lists using a "#" operator (similar to how the "*"
operator indicates repetition). Appendix A shows the collected grammar with all
list operators expanded to standard ABNF notation.¶
As a convention, ABNF rule names prefixed with "obs-" denote obsolete grammar
rules that appear for historical reasons.¶
The following core rules are included by reference, as defined in Appendix B.1
of [RFC5234]: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL
(controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG (hexadecimal
0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed), OCTET (any 8-bit sequence
of data), SP (space), and VCHAR (any visible US-ASCII character).¶
Section 5.6 defines some generic syntactic components for field values.¶
This specification uses the terms "character", "character encoding scheme",
"charset", and "protocol element" as they are defined in [RFC6365].¶
2.2. REQUIREMENTS NOTATION
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when,
and only when, they appear in all capitals, as shown here.¶
This specification targets conformance criteria according to the role of a
participant in HTTP communication. Hence, requirements are placed on senders,
recipients, clients, servers, user agents, intermediaries, origin servers,
proxies, gateways, or caches, depending on what behavior is being constrained by
the requirement. Additional requirements are placed on implementations, resource
owners, and protocol element registrations when they apply beyond the scope of a
single communication.¶
The verb "generate" is used instead of "send" where a requirement applies only
to implementations that create the protocol element, rather than an
implementation that forwards a received element downstream.¶
An implementation is considered conformant if it complies with all of the
requirements associated with the roles it partakes in HTTP.¶
A sender MUST NOT generate protocol elements that do not match the grammar
defined by the corresponding ABNF rules. Within a given message, a sender MUST
NOT generate protocol elements or syntax alternatives that are only allowed to
be generated by participants in other roles (i.e., a role that the sender does
not have for that message).¶
Conformance to HTTP includes both conformance to the particular messaging syntax
of the protocol version in use and conformance to the semantics of protocol
elements sent. For example, a client that claims conformance to HTTP/1.1 but
fails to recognize the features required of HTTP/1.1 recipients will fail to
interoperate with servers that adjust their responses in accordance with those
claims. Features that reflect user choices, such as content negotiation and
user-selected extensions, can impact application behavior beyond the protocol
stream; sending protocol elements that inaccurately reflect a user's choices
will confuse the user and inhibit choice.¶
When an implementation fails semantic conformance, recipients of that
implementation's messages will eventually develop workarounds to adjust their
behavior accordingly. A recipient MAY employ such workarounds while remaining
conformant to this protocol if the workarounds are limited to the
implementations at fault. For example, servers often scan portions of the
User-Agent field value, and user agents often scan the Server field value, to
adjust their own behavior with respect to known bugs or poorly chosen defaults.¶
2.3. LENGTH REQUIREMENTS
A recipient SHOULD parse a received protocol element defensively, with only
marginal expectations that the element will conform to its ABNF grammar and fit
within a reasonable buffer size.¶
HTTP does not have specific length limitations for many of its protocol elements
because the lengths that might be appropriate will vary widely, depending on the
deployment context and purpose of the implementation. Hence, interoperability
between senders and recipients depends on shared expectations regarding what is
a reasonable length for each protocol element. Furthermore, what is commonly
understood to be a reasonable length for some protocol elements has changed over
the course of the past three decades of HTTP use and is expected to continue
changing in the future.¶
At a minimum, a recipient MUST be able to parse and process protocol element
lengths that are at least as long as the values that it generates for those same
protocol elements in other messages. For example, an origin server that
publishes very long URI references to its own resources needs to be able to
parse and process those same references when received as a target URI.¶
Many received protocol elements are only parsed to the extent necessary to
identify and forward that element downstream. For example, an intermediary might
parse a received field into its field name and field value components, but then
forward the field without further parsing inside the field value.¶
2.4. ERROR HANDLING
A recipient MUST interpret a received protocol element according to the
semantics defined for it by this specification, including extensions to this
specification, unless the recipient has determined (through experience or
configuration) that the sender incorrectly implements what is implied by those
semantics. For example, an origin server might disregard the contents of a
received Accept-Encoding header field if inspection of the User-Agent header
field indicates a specific implementation version that is known to fail on
receipt of certain content codings.¶
Unless noted otherwise, a recipient MAY attempt to recover a usable protocol
element from an invalid construct. HTTP does not define specific error handling
mechanisms except when they have a direct impact on security, since different
applications of the protocol require different error handling strategies. For
example, a Web browser might wish to transparently recover from a response where
the Location header field doesn't parse according to the ABNF, whereas a systems
control client might consider any form of error recovery to be dangerous.¶
Some requests can be automatically retried by a client in the event of an
underlying connection failure, as described in Section 9.2.2.¶
2.5. PROTOCOL VERSION
HTTP's version number consists of two decimal digits separated by a "." (period
or decimal point). The first digit (major version) indicates the messaging
syntax, whereas the second digit (minor version) indicates the highest minor
version within that major version to which the sender is conformant (able to
understand for future communication).¶
While HTTP's core semantics don't change between protocol versions, their
expression "on the wire" can change, and so the HTTP version number changes when
incompatible changes are made to the wire format. Additionally, HTTP allows
incremental, backwards-compatible changes to be made to the protocol without
changing its version through the use of defined extension points (Section 16).¶
The protocol version as a whole indicates the sender's conformance with the set
of requirements laid out in that version's corresponding specification(s). For
example, the version "HTTP/1.1" is defined by the combined specifications of
this document, "HTTP Caching" [CACHING], and "HTTP/1.1" [HTTP/1.1].¶
HTTP's major version number is incremented when an incompatible message syntax
is introduced. The minor number is incremented when changes made to the protocol
have the effect of adding to the message semantics or implying additional
capabilities of the sender.¶
The minor version advertises the sender's communication capabilities even when
the sender is only using a backwards-compatible subset of the protocol, thereby
letting the recipient know that more advanced features can be used in response
(by servers) or in future requests (by clients).¶
When a major version of HTTP does not define any minor versions, the minor
version "0" is implied. The "0" is used when referring to that protocol within
elements that require a minor version identifier.¶
3. TERMINOLOGY AND CORE CONCEPTS
HTTP was created for the World Wide Web (WWW) architecture and has evolved over
time to support the scalability needs of a worldwide hypertext system. Much of
that architecture is reflected in the terminology used to define HTTP.¶
3.1. RESOURCES
The target of an HTTP request is called a "resource". HTTP does not limit the
nature of a resource; it merely defines an interface that might be used to
interact with resources. Most resources are identified by a Uniform Resource
Identifier (URI), as described in Section 4.¶
One design goal of HTTP is to separate resource identification from request
semantics, which is made possible by vesting the request semantics in the
request method (Section 9) and a few request-modifying header fields. A resource
cannot treat a request in a manner inconsistent with the semantics of the method
of the request. For example, though the URI of a resource might imply semantics
that are not safe, a client can expect the resource to avoid actions that are
unsafe when processing a request with a safe method (see Section 9.2.1).¶
HTTP relies upon the Uniform Resource Identifier (URI) standard [URI] to
indicate the target resource (Section 7.1) and relationships between resources.¶
3.2. REPRESENTATIONS
A "representation" is information that is intended to reflect a past, current,
or desired state of a given resource, in a format that can be readily
communicated via the protocol. A representation consists of a set of
representation metadata and a potentially unbounded stream of representation
data (Section 8).¶
HTTP allows "information hiding" behind its uniform interface by defining
communication with respect to a transferable representation of the resource
state, rather than transferring the resource itself. This allows the resource
identified by a URI to be anything, including temporal functions like "the
current weather in Laguna Beach", while potentially providing information that
represents that resource at the time a message is generated [REST].¶
The uniform interface is similar to a window through which one can observe and
act upon a thing only through the communication of messages to an independent
actor on the other side. A shared abstraction is needed to represent ("take the
place of") the current or desired state of that thing in our communications.
When a representation is hypertext, it can provide both a representation of the
resource state and processing instructions that help guide the recipient's
future interactions.¶
A target resource might be provided with, or be capable of generating, multiple
representations that are each intended to reflect the resource's current state.
An algorithm, usually based on content negotiation (Section 12), would be used
to select one of those representations as being most applicable to a given
request. This "selected representation" provides the data and metadata for
evaluating conditional requests (Section 13) and constructing the content for
200 (OK), 206 (Partial Content), and 304 (Not Modified) responses to GET
(Section 9.3.1).¶
3.3. CONNECTIONS, CLIENTS, AND SERVERS
HTTP is a client/server protocol that operates over a reliable transport- or
session-layer "connection".¶
An HTTP "client" is a program that establishes a connection to a server for the
purpose of sending one or more HTTP requests. An HTTP "server" is a program that
accepts connections in order to service HTTP requests by sending HTTP
responses.¶
The terms client and server refer only to the roles that these programs perform
for a particular connection. The same program might act as a client on some
connections and a server on others.¶
HTTP is defined as a stateless protocol, meaning that each request message's
semantics can be understood in isolation, and that the relationship between
connections and messages on them has no impact on the interpretation of those
messages. For example, a CONNECT request (Section 9.3.6) or a request with the
Upgrade header field (Section 7.8) can occur at any time, not just in the first
message on a connection. Many implementations depend on HTTP's stateless design
in order to reuse proxied connections or dynamically load balance requests
across multiple servers.¶
As a result, a server MUST NOT assume that two requests on the same connection
are from the same user agent unless the connection is secured and specific to
that agent. Some non-standard HTTP extensions (e.g., [RFC4559]) have been known
to violate this requirement, resulting in security and interoperability
problems.¶
3.4. MESSAGES
HTTP is a stateless request/response protocol for exchanging "messages" across a
connection. The terms "sender" and "recipient" refer to any implementation that
sends or receives a given message, respectively.¶
A client sends requests to a server in the form of a "request" message with a
method (Section 9) and request target (Section 7.1). The request might also
contain header fields (Section 6.3) for request modifiers, client information,
and representation metadata, content (Section 6.4) intended for processing in
accordance with the method, and trailer fields (Section 6.5) to communicate
information collected while sending the content.¶
A server responds to a client's request by sending one or more "response"
messages, each including a status code (Section 15). The response might also
contain header fields for server information, resource metadata, and
representation metadata, content to be interpreted in accordance with the status
code, and trailer fields to communicate information collected while sending the
content.¶
3.5. USER AGENTS
The term "user agent" refers to any of the various client programs that initiate
a request.¶
The most familiar form of user agent is the general-purpose Web browser, but
that's only a small percentage of implementations. Other common user agents
include spiders (web-traversing robots), command-line tools, billboard screens,
household appliances, scales, light bulbs, firmware update scripts, mobile apps,
and communication devices in a multitude of shapes and sizes.¶
Being a user agent does not imply that there is a human user directly
interacting with the software agent at the time of a request. In many cases, a
user agent is installed or configured to run in the background and save its
results for later inspection (or save only a subset of those results that might
be interesting or erroneous). Spiders, for example, are typically given a start
URI and configured to follow certain behavior while crawling the Web as a
hypertext graph.¶
Many user agents cannot, or choose not to, make interactive suggestions to their
user or provide adequate warning for security or privacy concerns. In the few
cases where this specification requires reporting of errors to the user, it is
acceptable for such reporting to only be observable in an error console or log
file. Likewise, requirements that an automated action be confirmed by the user
before proceeding might be met via advance configuration choices, run-time
options, or simple avoidance of the unsafe action; confirmation does not imply
any specific user interface or interruption of normal processing if the user has
already made that choice.¶
3.6. ORIGIN SERVER
The term "origin server" refers to a program that can originate authoritative
responses for a given target resource.¶
The most familiar form of origin server are large public websites. However, like
user agents being equated with browsers, it is easy to be misled into thinking
that all origin servers are alike. Common origin servers also include home
automation units, configurable networking components, office machines,
autonomous robots, news feeds, traffic cameras, real-time ad selectors, and
video-on-demand platforms.¶
Most HTTP communication consists of a retrieval request (GET) for a
representation of some resource identified by a URI. In the simplest case, this
might be accomplished via a single bidirectional connection (===) between the
user agent (UA) and the origin server (O).¶
request >
UA ======================================= O
< response
Figure 1
3.7. INTERMEDIARIES
HTTP enables the use of intermediaries to satisfy requests through a chain of
connections. There are three common forms of HTTP "intermediary": proxy,
gateway, and tunnel. In some cases, a single intermediary might act as an origin
server, proxy, gateway, or tunnel, switching behavior based on the nature of
each request.¶
> > > >
UA =========== A =========== B =========== C =========== O
< < < <
Figure 2
The figure above shows three intermediaries (A, B, and C) between the user agent
and origin server. A request or response message that travels the whole chain
will pass through four separate connections. Some HTTP communication options
might apply only to the connection with the nearest, non-tunnel neighbor, only
to the endpoints of the chain, or to all connections along the chain. Although
the diagram is linear, each participant might be engaged in multiple,
simultaneous communications. For example, B might be receiving requests from
many clients other than A, and/or forwarding requests to servers other than C,
at the same time that it is handling A's request. Likewise, later requests might
be sent through a different path of connections, often based on dynamic
configuration for load balancing.¶
The terms "upstream" and "downstream" are used to describe directional
requirements in relation to the message flow: all messages flow from upstream to
downstream. The terms "inbound" and "outbound" are used to describe directional
requirements in relation to the request route: inbound means "toward the origin
server", whereas outbound means "toward the user agent".¶
A "proxy" is a message-forwarding agent that is chosen by the client, usually
via local configuration rules, to receive requests for some type(s) of absolute
URI and attempt to satisfy those requests via translation through the HTTP
interface. Some translations are minimal, such as for proxy requests for "http"
URIs, whereas other requests might require translation to and from entirely
different application-level protocols. Proxies are often used to group an
organization's HTTP requests through a common intermediary for the sake of
security services, annotation services, or shared caching. Some proxies are
designed to apply transformations to selected messages or content while they are
being forwarded, as described in Section 7.7.¶
A "gateway" (a.k.a. "reverse proxy") is an intermediary that acts as an origin
server for the outbound connection but translates received requests and forwards
them inbound to another server or servers. Gateways are often used to
encapsulate legacy or untrusted information services, to improve server
performance through "accelerator" caching, and to enable partitioning or load
balancing of HTTP services across multiple machines.¶
All HTTP requirements applicable to an origin server also apply to the outbound
communication of a gateway. A gateway communicates with inbound servers using
any protocol that it desires, including private extensions to HTTP that are
outside the scope of this specification. However, an HTTP-to-HTTP gateway that
wishes to interoperate with third-party HTTP servers needs to conform to user
agent requirements on the gateway's inbound connection.¶
A "tunnel" acts as a blind relay between two connections without changing the
messages. Once active, a tunnel is not considered a party to the HTTP
communication, though the tunnel might have been initiated by an HTTP request. A
tunnel ceases to exist when both ends of the relayed connection are closed.
Tunnels are used to extend a virtual connection through an intermediary, such as
when Transport Layer Security (TLS, [TLS13]) is used to establish confidential
communication through a shared firewall proxy.¶
The above categories for intermediary only consider those acting as participants
in the HTTP communication. There are also intermediaries that can act on lower
layers of the network protocol stack, filtering or redirecting HTTP traffic
without the knowledge or permission of message senders. Network intermediaries
are indistinguishable (at a protocol level) from an on-path attacker, often
introducing security flaws or interoperability problems due to mistakenly
violating HTTP semantics.¶
For example, an "interception proxy" [RFC3040] (also commonly known as a
"transparent proxy" [RFC1919]) differs from an HTTP proxy because it is not
chosen by the client. Instead, an interception proxy filters or redirects
outgoing TCP port 80 packets (and occasionally other common port traffic).
Interception proxies are commonly found on public network access points, as a
means of enforcing account subscription prior to allowing use of non-local
Internet services, and within corporate firewalls to enforce network usage
policies.¶
3.8. CACHES
A "cache" is a local store of previous response messages and the subsystem that
controls its message storage, retrieval, and deletion. A cache stores cacheable
responses in order to reduce the response time and network bandwidth consumption
on future, equivalent requests. Any client or server MAY employ a cache, though
a cache cannot be used while acting as a tunnel.¶
The effect of a cache is that the request/response chain is shortened if one of
the participants along the chain has a cached response applicable to that
request. The following illustrates the resulting chain if B has a cached copy of
an earlier response from O (via C) for a request that has not been cached by UA
or A.¶
> >
UA =========== A =========== B - - - - - - C - - - - - - O
< <
Figure 3
A response is "cacheable" if a cache is allowed to store a copy of the response
message for use in answering subsequent requests. Even when a response is
cacheable, there might be additional constraints placed by the client or by the
origin server on when that cached response can be used for a particular request.
HTTP requirements for cache behavior and cacheable responses are defined in
[CACHING].¶
There is a wide variety of architectures and configurations of caches deployed
across the World Wide Web and inside large organizations. These include national
hierarchies of proxy caches to save bandwidth and reduce latency, content
delivery networks that use gateway caching to optimize regional and global
distribution of popular sites, collaborative systems that broadcast or multicast
cache entries, archives of pre-fetched cache entries for use in off-line or
high-latency environments, and so on.¶
3.9. EXAMPLE MESSAGE EXCHANGE
The following example illustrates a typical HTTP/1.1 message exchange for a GET
request (Section 9.3.1) on the URI "http://www.example.com/hello.txt":¶
Client request:¶
GET /hello.txt HTTP/1.1
User-Agent: curl/7.64.1
Host: www.example.com
Accept-Language: en, mi
¶
Server response:¶
HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
Server: Apache
Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
ETag: "34aa387-d-1568eb00"
Accept-Ranges: bytes
Content-Length: 51
Vary: Accept-Encoding
Content-Type: text/plain
Hello World! My content includes a trailing CRLF.
¶
4. IDENTIFIERS IN HTTP
Uniform Resource Identifiers (URIs) [URI] are used throughout HTTP as the means
for identifying resources (Section 3.1).¶
4.1. URI REFERENCES
URI references are used to target requests, indicate redirects, and define
relationships.¶
The definitions of "URI-reference", "absolute-URI", "relative-part",
"authority", "port", "host", "path-abempty", "segment", and "query" are adopted
from the URI generic syntax. An "absolute-path" rule is defined for protocol
elements that can contain a non-empty path component. (This rule differs
slightly from the path-abempty rule of RFC 3986, which allows for an empty path,
and path-absolute rule, which does not allow paths that begin with "//".) A
"partial-URI" rule is defined for protocol elements that can contain a relative
URI but not a fragment component.¶
URI-reference = <URI-reference, see [URI], Section 4.1>
absolute-URI = <absolute-URI, see [URI], Section 4.3>
relative-part = <relative-part, see [URI], Section 4.2>
authority = <authority, see [URI], Section 3.2>
uri-host = <host, see [URI], Section 3.2.2>
port = <port, see [URI], Section 3.2.3>
path-abempty = <path-abempty, see [URI], Section 3.3>
segment = <segment, see [URI], Section 3.3>
query = <query, see [URI], Section 3.4>
absolute-path = 1*( "/" segment )
partial-URI = relative-part [ "?" query ]
¶
Each protocol element in HTTP that allows a URI reference will indicate in its
ABNF production whether the element allows any form of reference
(URI-reference), only a URI in absolute form (absolute-URI), only the path and
optional query components (partial-URI), or some combination of the above.
Unless otherwise indicated, URI references are parsed relative to the target URI
(Section 7.1).¶
It is RECOMMENDED that all senders and recipients support, at a minimum, URIs
with lengths of 8000 octets in protocol elements. Note that this implies some
structures and on-wire representations (for example, the request line in
HTTP/1.1) will necessarily be larger in some cases.¶
4.2. HTTP-RELATED URI SCHEMES
IANA maintains the registry of URI Schemes [BCP35] at
<https://www.iana.org/assignments/uri-schemes/>. Although requests might target
any URI scheme, the following schemes are inherent to HTTP servers:¶
Table 2 URI Scheme Description Section http Hypertext Transfer Protocol 4.2.1
https Hypertext Transfer Protocol Secure 4.2.2
Note that the presence of an "http" or "https" URI does not imply that there is
always an HTTP server at the identified origin listening for connections. Anyone
can mint a URI, whether or not a server exists and whether or not that server
currently maps that identifier to a resource. The delegated nature of registered
names and IP addresses creates a federated namespace whether or not an HTTP
server is present.¶
4.2.1. HTTP URI SCHEME
The "http" URI scheme is hereby defined for minting identifiers within the
hierarchical namespace governed by a potential HTTP origin server listening for
TCP ([TCP]) connections on a given port.¶
http-URI = "http" "://" authority path-abempty [ "?" query ]
¶
The origin server for an "http" URI is identified by the authority component,
which includes a host identifier ([URI], Section 3.2.2) and optional port number
([URI], Section 3.2.3). If the port subcomponent is empty or not given, TCP port
80 (the reserved port for WWW services) is the default. The origin determines
who has the right to respond authoritatively to requests that target the
identified resource, as defined in Section 4.3.2.¶
A sender MUST NOT generate an "http" URI with an empty host identifier. A
recipient that processes such a URI reference MUST reject it as invalid.¶
The hierarchical path component and optional query component identify the target
resource within that origin server's namespace.¶
4.2.2. HTTPS URI SCHEME
The "https" URI scheme is hereby defined for minting identifiers within the
hierarchical namespace governed by a potential origin server listening for TCP
connections on a given port and capable of establishing a TLS ([TLS13])
connection that has been secured for HTTP communication. In this context,
"secured" specifically means that the server has been authenticated as acting on
behalf of the identified authority and all HTTP communication with that server
has confidentiality and integrity protection that is acceptable to both client
and server.¶
https-URI = "https" "://" authority path-abempty [ "?" query ]
¶
The origin server for an "https" URI is identified by the authority component,
which includes a host identifier ([URI], Section 3.2.2) and optional port number
([URI], Section 3.2.3). If the port subcomponent is empty or not given, TCP port
443 (the reserved port for HTTP over TLS) is the default. The origin determines
who has the right to respond authoritatively to requests that target the
identified resource, as defined in Section 4.3.3.¶
A sender MUST NOT generate an "https" URI with an empty host identifier. A
recipient that processes such a URI reference MUST reject it as invalid.¶
The hierarchical path component and optional query component identify the target
resource within that origin server's namespace.¶
A client MUST ensure that its HTTP requests for an "https" resource are secured,
prior to being communicated, and that it only accepts secured responses to those
requests. Note that the definition of what cryptographic mechanisms are
acceptable to client and server are usually negotiated and can change over
time.¶
Resources made available via the "https" scheme have no shared identity with the
"http" scheme. They are distinct origins with separate namespaces. However,
extensions to HTTP that are defined as applying to all origins with the same
host, such as the Cookie protocol [COOKIE], allow information set by one service
to impact communication with other services within a matching group of host
domains. Such extensions ought to be designed with great care to prevent
information obtained from a secured connection being inadvertently exchanged
within an unsecured context.¶
4.2.3. HTTP(S) NORMALIZATION AND COMPARISON
URIs with an "http" or "https" scheme are normalized and compared according to
the methods defined in Section 6 of [URI], using the defaults described above
for each scheme.¶
HTTP does not require the use of a specific method for determining equivalence.
For example, a cache key might be compared as a simple string, after
syntax-based normalization, or after scheme-based normalization.¶
Scheme-based normalization (Section 6.2.3 of [URI]) of "http" and "https" URIs
involves the following additional rules:¶
* If the port is equal to the default port for a scheme, the normal form is to
omit the port subcomponent.¶
* When not being used as the target of an OPTIONS request, an empty path
component is equivalent to an absolute path of "/", so the normal form is to
provide a path of "/" instead.¶
* The scheme and host are case-insensitive and normally provided in lowercase;
all other components are compared in a case-sensitive manner.¶
* Characters other than those in the "reserved" set are equivalent to their
percent-encoded octets: the normal form is to not encode them (see Sections
2.1 and 2.2 of [URI]).¶
For example, the following three URIs are equivalent:¶
http://example.com:80/~smith/home.html
http://EXAMPLE.com/%7Esmith/home.html
http://EXAMPLE.com:/%7esmith/home.html
¶
Two HTTP URIs that are equivalent after normalization (using any method) can be
assumed to identify the same resource, and any HTTP component MAY perform
normalization. As a result, distinct resources SHOULD NOT be identified by HTTP
URIs that are equivalent after normalization (using any method defined in
Section 6.2 of [URI]).¶
4.2.4. DEPRECATION OF USERINFO IN HTTP(S) URIS
The URI generic syntax for authority also includes a userinfo subcomponent
([URI], Section 3.2.1) for including user authentication information in the URI.
In that subcomponent, the use of the format "user:password" is deprecated.¶
Some implementations make use of the userinfo component for internal
configuration of authentication information, such as within command invocation
options, configuration files, or bookmark lists, even though such usage might
expose a user identifier or password.¶
A sender MUST NOT generate the userinfo subcomponent (and its "@" delimiter)
when an "http" or "https" URI reference is generated within a message as a
target URI or field value.¶
Before making use of an "http" or "https" URI reference received from an
untrusted source, a recipient SHOULD parse for userinfo and treat its presence
as an error; it is likely being used to obscure the authority for the sake of
phishing attacks.¶
4.2.5. HTTP(S) REFERENCES WITH FRAGMENT IDENTIFIERS
Fragment identifiers allow for indirect identification of a secondary resource,
independent of the URI scheme, as defined in Section 3.5 of [URI]. Some protocol
elements that refer to a URI allow inclusion of a fragment, while others do not.
They are distinguished by use of the ABNF rule for elements where fragment is
allowed; otherwise, a specific rule that excludes fragments is used.¶
Note: The fragment identifier component is not part of the scheme definition for
a URI scheme (see Section 4.3 of [URI]), thus does not appear in the ABNF
definitions for the "http" and "https" URI schemes above.¶
4.3. AUTHORITATIVE ACCESS
Authoritative access refers to dereferencing a given identifier, for the sake of
access to the identified resource, in a way that the client believes is
authoritative (controlled by the resource owner). The process for determining
whether access is granted is defined by the URI scheme and often uses data
within the URI components, such as the authority component when the generic
syntax is used. However, authoritative access is not limited to the identified
mechanism.¶
Section 4.3.1 defines the concept of an origin as an aid to such uses, and the
subsequent subsections explain how to establish that a peer has the authority to
represent an origin.¶
See Section 17.1 for security considerations related to establishing authority.¶
4.3.1. URI ORIGIN
The "origin" for a given URI is the triple of scheme, host, and port after
normalizing the scheme and host to lowercase and normalizing the port to remove
any leading zeros. If port is elided from the URI, the default port for that
scheme is used. For example, the URI¶
https://Example.Com/happy.js
¶
would have the origin¶
{ "https", "example.com", "443" }
¶
which can also be described as the normalized URI prefix with port always
present:¶
https://example.com:443
¶
Each origin defines its own namespace and controls how identifiers within that
namespace are mapped to resources. In turn, how the origin responds to valid
requests, consistently over time, determines the semantics that users will
associate with a URI, and the usefulness of those semantics is what ultimately
transforms these mechanisms into a resource for users to reference and access in
the future.¶
Two origins are distinct if they differ in scheme, host, or port. Even when it
can be verified that the same entity controls two distinct origins, the two
namespaces under those origins are distinct unless explicitly aliased by a
server authoritative for that origin.¶
Origin is also used within HTML and related Web protocols, beyond the scope of
this document, as described in [RFC6454].¶
4.3.2. HTTP ORIGINS
Although HTTP is independent of the transport protocol, the "http" scheme
(Section 4.2.1) is specific to associating authority with whomever controls the
origin server listening for TCP connections on the indicated port of whatever
host is identified within the authority component. This is a very weak sense of
authority because it depends on both client-specific name resolution mechanisms
and communication that might not be secured from an on-path attacker.
Nevertheless, it is a sufficient minimum for binding "http" identifiers to an
origin server for consistent resolution within a trusted environment.¶
If the host identifier is provided as an IP address, the origin server is the
listener (if any) on the indicated TCP port at that IP address. If host is a
registered name, the registered name is an indirect identifier for use with a
name resolution service, such as DNS, to find an address for an appropriate
origin server.¶
When an "http" URI is used within a context that calls for access to the
indicated resource, a client MAY attempt access by resolving the host identifier
to an IP address, establishing a TCP connection to that address on the indicated
port, and sending over that connection an HTTP request message containing a
request target that matches the client's target URI (Section 7.1).¶
If the server responds to such a request with a non-interim HTTP response
message, as described in Section 15, then that response is considered an
authoritative answer to the client's request.¶
Note, however, that the above is not the only means for obtaining an
authoritative response, nor does it imply that an authoritative response is
always necessary (see [CACHING]). For example, the Alt-Svc header field [ALTSVC]
allows an origin server to identify other services that are also authoritative
for that origin. Access to "http" identified resources might also be provided by
protocols outside the scope of this document.¶
4.3.3. HTTPS ORIGINS
The "https" scheme (Section 4.2.2) associates authority based on the ability of
a server to use the private key corresponding to a certificate that the client
considers to be trustworthy for the identified origin server. The client usually
relies upon a chain of trust, conveyed from some prearranged or configured trust
anchor, to deem a certificate trustworthy (Section 4.3.4).¶
In HTTP/1.1 and earlier, a client will only attribute authority to a server when
they are communicating over a successfully established and secured connection
specifically to that URI origin's host. The connection establishment and
certificate verification are used as proof of authority.¶
In HTTP/2 and HTTP/3, a client will attribute authority to a server when they
are communicating over a successfully established and secured connection if the
URI origin's host matches any of the hosts present in the server's certificate
and the client believes that it could open a connection to that host for that
URI. In practice, a client will make a DNS query to check that the origin's host
contains the same server IP address as the established connection. This
restriction can be removed by the origin server sending an equivalent ORIGIN
frame [RFC8336].¶
The request target's host and port value are passed within each HTTP request,
identifying the origin and distinguishing it from other namespaces that might be
controlled by the same server (Section 7.2). It is the origin's responsibility
to ensure that any services provided with control over its certificate's private
key are equally responsible for managing the corresponding "https" namespaces or
at least prepared to reject requests that appear to have been misdirected
(Section 7.4).¶
An origin server might be unwilling to process requests for certain target URIs
even when they have the authority to do so. For example, when a host operates
distinct services on different ports (e.g., 443 and 8000), checking the target
URI at the origin server is necessary (even after the connection has been
secured) because a network attacker might cause connections for one port to be
received at some other port. Failing to check the target URI might allow such an
attacker to replace a response to one target URI (e.g.,
"https://example.com/foo") with a seemingly authoritative response from the
other port (e.g., "https://example.com:8000/foo").¶
Note that the "https" scheme does not rely on TCP and the connected port number
for associating authority, since both are outside the secured communication and
thus cannot be trusted as definitive. Hence, the HTTP communication might take
place over any channel that has been secured, as defined in Section 4.2.2,
including protocols that don't use TCP.¶
When an "https" URI is used within a context that calls for access to the
indicated resource, a client MAY attempt access by resolving the host identifier
to an IP address, establishing a TCP connection to that address on the indicated
port, securing the connection end-to-end by successfully initiating TLS over TCP
with confidentiality and integrity protection, and sending over that connection
an HTTP request message containing a request target that matches the client's
target URI (Section 7.1).¶
If the server responds to such a request with a non-interim HTTP response
message, as described in Section 15, then that response is considered an
authoritative answer to the client's request.¶
Note, however, that the above is not the only means for obtaining an
authoritative response, nor does it imply that an authoritative response is
always necessary (see [CACHING]).¶
4.3.4. HTTPS CERTIFICATE VERIFICATION
To establish a secured connection to dereference a URI, a client MUST verify
that the service's identity is an acceptable match for the URI's origin server.
Certificate verification is used to prevent server impersonation by an on-path
attacker or by an attacker that controls name resolution. This process requires
that a client be configured with a set of trust anchors.¶
In general, a client MUST verify the service identity using the verification
process defined in Section 6 of [RFC6125]. The client MUST construct a reference
identity from the service's host: if the host is a literal IP address (Section
4.3.5), the reference identity is an IP-ID, otherwise the host is a name and the
reference identity is a DNS-ID.¶
A reference identity of type CN-ID MUST NOT be used by clients. As noted in
Section 6.2.1 of [RFC6125], a reference identity of type CN-ID might be used by
older clients.¶
A client might be specially configured to accept an alternative form of server
identity verification. For example, a client might be connecting to a server
whose address and hostname are dynamic, with an expectation that the service
will present a specific certificate (or a certificate matching some externally
defined reference identity) rather than one matching the target URI's origin.¶
In special cases, it might be appropriate for a client to simply ignore the
server's identity, but it must be understood that this leaves a connection open
to active attack.¶
If the certificate is not valid for the target URI's origin, a user agent MUST
either obtain confirmation from the user before proceeding (see Section 3.5) or
terminate the connection with a bad certificate error. Automated clients MUST
log the error to an appropriate audit log (if available) and SHOULD terminate
the connection (with a bad certificate error). Automated clients MAY provide a
configuration setting that disables this check, but MUST provide a setting which
enables it.¶
4.3.5. IP-ID REFERENCE IDENTITY
A server that is identified using an IP address literal in the "host" field of
an "https" URI has a reference identity of type IP-ID. An IP version 4 address
uses the "IPv4address" ABNF rule, and an IP version 6 address uses the
"IP-literal" production with the "IPv6address" option; see Section 3.2.2 of
[URI]. A reference identity of IP-ID contains the decoded bytes of the IP
address.¶
An IP version 4 address is 4 octets, and an IP version 6 address is 16 octets.
Use of IP-ID is not defined for any other IP version. The iPAddress choice in
the certificate subjectAltName extension does not explicitly include the IP
version and so relies on the length of the address to distinguish versions; see
Section 4.2.1.6 of [RFC5280].¶
A reference identity of type IP-ID matches if the address is identical to an
iPAddress value of the subjectAltName extension of the certificate.¶
5. FIELDS
HTTP uses "fields" to provide data in the form of extensible name/value pairs
with a registered key namespace. Fields are sent and received within the header
and trailer sections of messages (Section 6).¶
5.1. FIELD NAMES
A field name labels the corresponding field value as having the semantics
defined by that name. For example, the Date header field is defined in Section
6.6.1 as containing the origination timestamp for the message in which it
appears.¶
field-name = token
¶
Field names are case-insensitive and ought to be registered within the
"Hypertext Transfer Protocol (HTTP) Field Name Registry"; see Section 16.3.1.¶
The interpretation of a field does not change between minor versions of the same
major HTTP version, though the default behavior of a recipient in the absence of
such a field can change. Unless specified otherwise, fields are defined for all
versions of HTTP. In particular, the Host and Connection fields ought to be
recognized by all HTTP implementations whether or not they advertise conformance
with HTTP/1.1.¶
New fields can be introduced without changing the protocol version if their
defined semantics allow them to be safely ignored by recipients that do not
recognize them; see Section 16.3.¶
A proxy MUST forward unrecognized header fields unless the field name is listed
in the Connection header field (Section 7.6.1) or the proxy is specifically
configured to block, or otherwise transform, such fields. Other recipients
SHOULD ignore unrecognized header and trailer fields. Adhering to these
requirements allows HTTP's functionality to be extended without updating or
removing deployed intermediaries.¶
5.2. FIELD LINES AND COMBINED FIELD VALUE
Field sections are composed of any number of "field lines", each with a "field
name" (see Section 5.1) identifying the field, and a "field line value" that
conveys data for that instance of the field.¶
When a field name is only present once in a section, the combined "field value"
for that field consists of the corresponding field line value. When a field name
is repeated within a section, its combined field value consists of the list of
corresponding field line values within that section, concatenated in order, with
each field line value separated by a comma.¶
For example, this section:¶
Example-Field: Foo, Bar
Example-Field: Baz
¶
contains two field lines, both with the field name "Example-Field". The first
field line has a field line value of "Foo, Bar", while the second field line
value is "Baz". The field value for "Example-Field" is the list "Foo, Bar,
Baz".¶
5.3. FIELD ORDER
A recipient MAY combine multiple field lines within a field section that have
the same field name into one field line, without changing the semantics of the
message, by appending each subsequent field line value to the initial field line
value in order, separated by a comma (",") and optional whitespace (OWS, defined
in Section 5.6.3). For consistency, use comma SP.¶
The order in which field lines with the same name are received is therefore
significant to the interpretation of the field value; a proxy MUST NOT change
the order of these field line values when forwarding a message.¶
This means that, aside from the well-known exception noted below, a sender MUST
NOT generate multiple field lines with the same name in a message (whether in
the headers or trailers) or append a field line when a field line of the same
name already exists in the message, unless that field's definition allows
multiple field line values to be recombined as a comma-separated list (i.e., at
least one alternative of the field's definition allows a comma-separated list,
such as an ABNF rule of #(values) defined in Section 5.6.1).¶
Note: In practice, the "Set-Cookie" header field ([COOKIE]) often appears in a
response message across multiple field lines and does not use the list syntax,
violating the above requirements on multiple field lines with the same field
name. Since it cannot be combined into a single field value, recipients ought to
handle "Set-Cookie" as a special case while processing fields. (See Appendix
A.2.3 of [Kri2001] for details.)¶
The order in which field lines with differing field names are received in a
section is not significant. However, it is good practice to send header fields
that contain additional control data first, such as Host on requests and Date on
responses, so that implementations can decide when not to handle a message as
early as possible.¶
A server MUST NOT apply a request to the target resource until it receives the
entire request header section, since later header field lines might include
conditionals, authentication credentials, or deliberately misleading duplicate
header fields that could impact request processing.¶
5.4. FIELD LIMITS
HTTP does not place a predefined limit on the length of each field line, field
value, or on the length of a header or trailer section as a whole, as described
in Section 2. Various ad hoc limitations on individual lengths are found in
practice, often depending on the specific field's semantics.¶
A server that receives a request header field line, field value, or set of
fields larger than it wishes to process MUST respond with an appropriate 4xx
(Client Error) status code. Ignoring such header fields would increase the
server's vulnerability to request smuggling attacks (Section 11.2 of
[HTTP/1.1]).¶
A client MAY discard or truncate received field lines that are larger than the
client wishes to process if the field semantics are such that the dropped
value(s) can be safely ignored without changing the message framing or response
semantics.¶
5.5. FIELD VALUES
HTTP field values consist of a sequence of characters in a format defined by the
field's grammar. Each field's grammar is usually defined using ABNF
([RFC5234]).¶
field-value = *field-content
field-content = field-vchar
[ 1*( SP / HTAB / field-vchar ) field-vchar ]
field-vchar = VCHAR / obs-text
obs-text = %x80-FF
¶
A field value does not include leading or trailing whitespace. When a specific
version of HTTP allows such whitespace to appear in a message, a field parsing
implementation MUST exclude such whitespace prior to evaluating the field
value.¶
Field values are usually constrained to the range of US-ASCII characters
[USASCII]. Fields needing a greater range of characters can use an encoding,
such as the one defined in [RFC8187]. Historically, HTTP allowed field content
with text in the ISO-8859-1 charset [ISO-8859-1], supporting other charsets only
through use of [RFC2047] encoding. Specifications for newly defined fields
SHOULD limit their values to visible US-ASCII octets (VCHAR), SP, and HTAB. A
recipient SHOULD treat other allowed octets in field content (i.e., obs-text) as
opaque data.¶
Field values containing CR, LF, or NUL characters are invalid and dangerous, due
to the varying ways that implementations might parse and interpret those
characters; a recipient of CR, LF, or NUL within a field value MUST either
reject the message or replace each of those characters with SP before further
processing or forwarding of that message. Field values containing other CTL
characters are also invalid; however, recipients MAY retain such characters for
the sake of robustness when they appear within a safe context (e.g., an
application-specific quoted string that will not be processed by any downstream
HTTP parser).¶
Fields that only anticipate a single member as the field value are referred to
as "singleton fields".¶
Fields that allow multiple members as the field value are referred to as
"list-based fields". The list operator extension of Section 5.6.1 is used as a
common notation for defining field values that can contain multiple members.¶
Because commas (",") are used as the delimiter between members, they need to be
treated with care if they are allowed as data within a member. This is true for
both list-based and singleton fields, since a singleton field might be
erroneously sent with multiple members and detecting such errors improves
interoperability. Fields that expect to contain a comma within a member, such as
within an HTTP-date or URI-reference element, ought to be defined with
delimiters around that element to distinguish any comma within that data from
potential list separators.¶
For example, a textual date and a URI (either of which might contain a comma)
could be safely carried in list-based field values like these:¶
Example-URIs: "http://example.com/a.html,foo",
"http://without-a-comma.example.com/"
Example-Dates: "Sat, 04 May 1996", "Wed, 14 Sep 2005"
¶
Note that double-quote delimiters are almost always used with the quoted-string
production (Section 5.6.4); using a different syntax inside double-quotes will
likely cause unnecessary confusion.¶
Many fields (such as Content-Type, defined in Section 8.3) use a common syntax
for parameters that allows both unquoted (token) and quoted (quoted-string)
syntax for a parameter value (Section 5.6.6). Use of common syntax allows
recipients to reuse existing parser components. When allowing both forms, the
meaning of a parameter value ought to be the same whether it was received as a
token or a quoted string.¶
Note: For defining field value syntax, this specification uses an ABNF rule
named after the field name to define the allowed grammar for that field's value
(after said value has been extracted from the underlying messaging syntax and
multiple instances combined into a list).¶
5.6. COMMON RULES FOR DEFINING FIELD VALUES
5.6.1. LISTS (#RULE ABNF EXTENSION)
A #rule extension to the ABNF rules of [RFC5234] is used to improve readability
in the definitions of some list-based field values.¶
A construct "#" is defined, similar to "*", for defining comma-delimited lists
of elements. The full form is "<n>#<m>element" indicating at least <n> and at
most <m> elements, each separated by a single comma (",") and optional
whitespace (OWS, defined in Section 5.6.3).¶
5.6.1.1. SENDER REQUIREMENTS
In any production that uses the list construct, a sender MUST NOT generate empty
list elements. In other words, a sender has to generate lists that satisfy the
following syntax:¶
1#element => element *( OWS "," OWS element )
¶
and:¶
#element => [ 1#element ]
¶
and for n >= 1 and m > 1:¶
<n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element )
¶
Appendix A shows the collected ABNF for senders after the list constructs have
been expanded.¶
5.6.1.2. RECIPIENT REQUIREMENTS
Empty elements do not contribute to the count of elements present. A recipient
MUST parse and ignore a reasonable number of empty list elements: enough to
handle common mistakes by senders that merge values, but not so much that they
could be used as a denial-of-service mechanism. In other words, a recipient MUST
accept lists that satisfy the following syntax:¶
#element => [ element ] *( OWS "," OWS [ element ] )
¶
Note that because of the potential presence of empty list elements, the RFC 5234
ABNF cannot enforce the cardinality of list elements, and consequently all cases
are mapped as if there was no cardinality specified.¶
For example, given these ABNF productions:¶
example-list = 1#example-list-elmt
example-list-elmt = token ; see Section 5.6.2
¶
Then the following are valid values for example-list (not including the double
quotes, which are present for delimitation only):¶
"foo,bar"
"foo ,bar,"
"foo , ,bar,charlie"
¶
In contrast, the following values would be invalid, since at least one non-empty
element is required by the example-list production:¶
""
","
", ,"
¶
5.6.2. TOKENS
Tokens are short textual identifiers that do not include whitespace or
delimiters.¶
token = 1*tchar
tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*"
/ "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
/ DIGIT / ALPHA
; any VCHAR, except delimiters
¶
Many HTTP field values are defined using common syntax components, separated by
whitespace or specific delimiting characters. Delimiters are chosen from the set
of US-ASCII visual characters not allowed in a token (DQUOTE and
"(),/:;<=>?@[\]{}").¶
5.6.3. WHITESPACE
This specification uses three rules to denote the use of linear whitespace: OWS
(optional whitespace), RWS (required whitespace), and BWS ("bad" whitespace).¶
The OWS rule is used where zero or more linear whitespace octets might appear.
For protocol elements where optional whitespace is preferred to improve
readability, a sender SHOULD generate the optional whitespace as a single SP;
otherwise, a sender SHOULD NOT generate optional whitespace except as needed to
overwrite invalid or unwanted protocol elements during in-place message
filtering.¶
The RWS rule is used when at least one linear whitespace octet is required to
separate field tokens. A sender SHOULD generate RWS as a single SP.¶
OWS and RWS have the same semantics as a single SP. Any content known to be
defined as OWS or RWS MAY be replaced with a single SP before interpreting it or
forwarding the message downstream.¶
The BWS rule is used where the grammar allows optional whitespace only for
historical reasons. A sender MUST NOT generate BWS in messages. A recipient MUST
parse for such bad whitespace and remove it before interpreting the protocol
element.¶
BWS has no semantics. Any content known to be defined as BWS MAY be removed
before interpreting it or forwarding the message downstream.¶
OWS = *( SP / HTAB )
; optional whitespace
RWS = 1*( SP / HTAB )
; required whitespace
BWS = OWS
; "bad" whitespace
¶
5.6.4. QUOTED STRINGS
A string of text is parsed as a single value if it is quoted using double-quote
marks.¶
quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE
qdtext = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text
¶
The backslash octet ("\") can be used as a single-octet quoting mechanism within
quoted-string and comment constructs. Recipients that process the value of a
quoted-string MUST handle a quoted-pair as if it were replaced by the octet
following the backslash.¶
quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )
¶
A sender SHOULD NOT generate a quoted-pair in a quoted-string except where
necessary to quote DQUOTE and backslash octets occurring within that string. A
sender SHOULD NOT generate a quoted-pair in a comment except where necessary to
quote parentheses ["(" and ")"] and backslash octets occurring within that
comment.¶
5.6.5. COMMENTS
Comments can be included in some HTTP fields by surrounding the comment text
with parentheses. Comments are only allowed in fields containing "comment" as
part of their field value definition.¶
comment = "(" *( ctext / quoted-pair / comment ) ")"
ctext = HTAB / SP / %x21-27 / %x2A-5B / %x5D-7E / obs-text
¶
5.6.6. PARAMETERS
Parameters are instances of name/value pairs; they are often used in field
values as a common syntax for appending auxiliary information to an item. Each
parameter is usually delimited by an immediately preceding semicolon.¶
parameters = *( OWS ";" OWS [ parameter ] )
parameter = parameter-name "=" parameter-value
parameter-name = token
parameter-value = ( token / quoted-string )
¶
Parameter names are case-insensitive. Parameter values might or might not be
case-sensitive, depending on the semantics of the parameter name. Examples of
parameters and some equivalent forms can be seen in media types (Section 8.3.1)
and the Accept header field (Section 12.5.1).¶
A parameter value that matches the token production can be transmitted either as
a token or within a quoted-string. The quoted and unquoted values are
equivalent.¶
Note: Parameters do not allow whitespace (not even "bad" whitespace) around the
"=" character.¶
5.6.7. DATE/TIME FORMATS
Prior to 1995, there were three different formats commonly used by servers to
communicate timestamps. For compatibility with old implementations, all three
are defined here. The preferred format is a fixed-length and single-zone subset
of the date and time specification used by the Internet Message Format
[RFC5322].¶
HTTP-date = IMF-fixdate / obs-date
¶
An example of the preferred format is¶
Sun, 06 Nov 1994 08:49:37 GMT ; IMF-fixdate
¶
Examples of the two obsolete formats are¶
Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
¶
A recipient that parses a timestamp value in an HTTP field MUST accept all three
HTTP-date formats. When a sender generates a field that contains one or more
timestamps defined as HTTP-date, the sender MUST generate those timestamps in
the IMF-fixdate format.¶
An HTTP-date value represents time as an instance of Coordinated Universal Time
(UTC). The first two formats indicate UTC by the three-letter abbreviation for
Greenwich Mean Time, "GMT", a predecessor of the UTC name; values in the asctime
format are assumed to be in UTC.¶
A "clock" is an implementation capable of providing a reasonable approximation
of the current instant in UTC. A clock implementation ought to use NTP
([RFC5905]), or some similar protocol, to synchronize with UTC.¶
Preferred format:¶
IMF-fixdate = day-name "," SP date1 SP time-of-day SP GMT
; fixed length/zone/capitalization subset of the format
; see Section 3.3 of [RFC5322]
day-name = %s"Mon" / %s"Tue" / %s"Wed"
/ %s"Thu" / %s"Fri" / %s"Sat" / %s"Sun"
date1 = day SP month SP year
; e.g., 02 Jun 1982
day = 2DIGIT
month = %s"Jan" / %s"Feb" / %s"Mar" / %s"Apr"
/ %s"May" / %s"Jun" / %s"Jul" / %s"Aug"
/ %s"Sep" / %s"Oct" / %s"Nov" / %s"Dec"
year = 4DIGIT
GMT = %s"GMT"
time-of-day = hour ":" minute ":" second
; 00:00:00 - 23:59:60 (leap second)
hour = 2DIGIT
minute = 2DIGIT
second = 2DIGIT
¶
Obsolete formats:¶
obs-date = rfc850-date / asctime-date
¶
rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT
date2 = day "-" month "-" 2DIGIT
; e.g., 02-Jun-82
day-name-l = %s"Monday" / %s"Tuesday" / %s"Wednesday"
/ %s"Thursday" / %s"Friday" / %s"Saturday"
/ %s"Sunday"
¶
asctime-date = day-name SP date3 SP time-of-day SP year
date3 = month SP ( 2DIGIT / ( SP 1DIGIT ))
; e.g., Jun 2
¶
HTTP-date is case sensitive. Note that Section 4.2 of [CACHING] relaxes this for
cache recipients.¶
A sender MUST NOT generate additional whitespace in an HTTP-date beyond that
specifically included as SP in the grammar. The semantics of day-name, day,
month, year, and time-of-day are the same as those defined for the Internet
Message Format constructs with the corresponding name ([RFC5322], Section 3.3).¶
Recipients of a timestamp value in rfc850-date format, which uses a two-digit
year, MUST interpret a timestamp that appears to be more than 50 years in the
future as representing the most recent year in the past that had the same last
two digits.¶
Recipients of timestamp values are encouraged to be robust in parsing timestamps
unless otherwise restricted by the field definition. For example, messages are
occasionally forwarded over HTTP from a non-HTTP source that might generate any
of the date and time specifications defined by the Internet Message Format.¶
Note: HTTP requirements for timestamp formats apply only to their usage within
the protocol stream. Implementations are not required to use these formats for
user presentation, request logging, etc.¶
6. MESSAGE ABSTRACTION
Each major version of HTTP defines its own syntax for communicating messages.
This section defines an abstract data type for HTTP messages based on a
generalization of those message characteristics, common structure, and capacity
for conveying semantics. This abstraction is used to define requirements on
senders and recipients that are independent of the HTTP version, such that a
message in one version can be relayed through other versions without changing
its meaning.¶
A "message" consists of the following:¶
* control data to describe and route the message,¶
* a headers lookup table of name/value pairs for extending that control data
and conveying additional information about the sender, message, content, or
context,¶
* a potentially unbounded stream of content, and¶
* a trailers lookup table of name/value pairs for communicating information
obtained while sending the content.¶
Framing and control data is sent first, followed by a header section containing
fields for the headers table. When a message includes content, the content is
sent after the header section, potentially followed by a trailer section that
might contain fields for the trailers table.¶
Messages are expected to be processed as a stream, wherein the purpose of that
stream and its continued processing is revealed while being read. Hence, control
data describes what the recipient needs to know immediately, header fields
describe what needs to be known before receiving content, the content (when
present) presumably contains what the recipient wants or needs to fulfill the
message semantics, and trailer fields provide optional metadata that was unknown
prior to sending the content.¶
Messages are intended to be "self-descriptive": everything a recipient needs to
know about the message can be determined by looking at the message itself, after
decoding or reconstituting parts that have been compressed or elided in transit,
without requiring an understanding of the sender's current application state
(established via prior messages). However, a client MUST retain knowledge of the
request when parsing, interpreting, or caching a corresponding response. For
example, responses to the HEAD method look just like the beginning of a response
to GET but cannot be parsed in the same manner.¶
Note that this message abstraction is a generalization across many versions of
HTTP, including features that might not be found in some versions. For example,
trailers were introduced within the HTTP/1.1 chunked transfer coding as a
trailer section after the content. An equivalent feature is present in HTTP/2
and HTTP/3 within the header block that terminates each stream.¶
6.1. FRAMING AND COMPLETENESS
Message framing indicates how each message begins and ends, such that each
message can be distinguished from other messages or noise on the same
connection. Each major version of HTTP defines its own framing mechanism.¶
HTTP/0.9 and early deployments of HTTP/1.0 used closure of the underlying
connection to end a response. For backwards compatibility, this implicit framing
is also allowed in HTTP/1.1. However, implicit framing can fail to distinguish
an incomplete response if the connection closes early. For that reason, almost
all modern implementations use explicit framing in the form of length-delimited
sequences of message data.¶
A message is considered "complete" when all of the octets indicated by its
framing are available. Note that, when no explicit framing is used, a response
message that is ended by the underlying connection's close is considered
complete even though it might be indistinguishable from an incomplete response,
unless a transport-level error indicates that it is not complete.¶
6.2. CONTROL DATA
Messages start with control data that describe its primary purpose. Request
message control data includes a request method (Section 9), request target
(Section 7.1), and protocol version (Section 2.5). Response message control data
includes a status code (Section 15), optional reason phrase, and protocol
version.¶
In HTTP/1.1 ([HTTP/1.1]) and earlier, control data is sent as the first line of
a message. In HTTP/2 ([HTTP/2]) and HTTP/3 ([HTTP/3]), control data is sent as
pseudo-header fields with a reserved name prefix (e.g., ":authority").¶
Every HTTP message has a protocol version. Depending on the version in use, it
might be identified within the message explicitly or inferred by the connection
over which the message is received. Recipients use that version information to
determine limitations or potential for later communication with that sender.¶
When a message is forwarded by an intermediary, the protocol version is updated
to reflect the version used by that intermediary. The Via header field (Section
7.6.3) is used to communicate upstream protocol information within a forwarded
message.¶
A client SHOULD send a request version equal to the highest version to which the
client is conformant and whose major version is no higher than the highest
version supported by the server, if this is known. A client MUST NOT send a
version to which it is not conformant.¶
A client MAY send a lower request version if it is known that the server
incorrectly implements the HTTP specification, but only after the client has
attempted at least one normal request and determined from the response status
code or header fields (e.g., Server) that the server improperly handles higher
request versions.¶
A server SHOULD send a response version equal to the highest version to which
the server is conformant that has a major version less than or equal to the one
received in the request. A server MUST NOT send a version to which it is not
conformant. A server can send a 505 (HTTP Version Not Supported) response if it
wishes, for any reason, to refuse service of the client's major protocol
version.¶
A recipient that receives a message with a major version number that it
implements and a minor version number higher than what it implements SHOULD
process the message as if it were in the highest minor version within that major
version to which the recipient is conformant. A recipient can assume that a
message with a higher minor version, when sent to a recipient that has not yet
indicated support for that higher version, is sufficiently backwards-compatible
to be safely processed by any implementation of the same major version.¶
6.3. HEADER FIELDS
Fields (Section 5) that are sent or received before the content are referred to
as "header fields" (or just "headers", colloquially).¶
The "header section" of a message consists of a sequence of header field lines.
Each header field might modify or extend message semantics, describe the sender,
define the content, or provide additional context.¶
Note: We refer to named fields specifically as a "header field" when they are
only allowed to be sent in the header section.¶
6.4. CONTENT
HTTP messages often transfer a complete or partial representation as the message
"content": a stream of octets sent after the header section, as delineated by
the message framing.¶
This abstract definition of content reflects the data after it has been
extracted from the message framing. For example, an HTTP/1.1 message body
(Section 6 of [HTTP/1.1]) might consist of a stream of data encoded with the
chunked transfer coding -- a sequence of data chunks, one zero-length chunk, and
a trailer section -- whereas the content of that same message includes only the
data stream after the transfer coding has been decoded; it does not include the
chunk lengths, chunked framing syntax, nor the trailer fields (Section 6.5).¶
Note: Some field names have a "Content-" prefix. This is an informal convention;
while some of these fields refer to the content of the message, as defined
above, others are scoped to the selected representation (Section 3.2). See the
individual field's definition to disambiguate.¶
6.4.1. CONTENT SEMANTICS
The purpose of content in a request is defined by the method semantics (Section
9).¶
For example, a representation in the content of a PUT request (Section 9.3.4)
represents the desired state of the target resource after the request is
successfully applied, whereas a representation in the content of a POST request
(Section 9.3.3) represents information to be processed by the target resource.¶
In a response, the content's purpose is defined by the request method, response
status code (Section 15), and response fields describing that content. For
example, the content of a 200 (OK) response to GET (Section 9.3.1) represents
the current state of the target resource, as observed at the time of the message
origination date (Section 6.6.1), whereas the content of the same status code in
a response to POST might represent either the processing result or the new state
of the target resource after applying the processing.¶
The content of a 206 (Partial Content) response to GET contains either a single
part of the selected representation or a multipart message body containing
multiple parts of that representation, as described in Section 15.3.7.¶
Response messages with an error status code usually contain content that
represents the error condition, such that the content describes the error state
and what steps are suggested for resolving it.¶
Responses to the HEAD request method (Section 9.3.2) never include content; the
associated response header fields indicate only what their values would have
been if the request method had been GET (Section 9.3.1).¶
2xx (Successful) responses to a CONNECT request method (Section 9.3.6) switch
the connection to tunnel mode instead of having content.¶
All 1xx (Informational), 204 (No Content), and 304 (Not Modified) responses do
not include content.¶
All other responses do include content, although that content might be of zero
length.¶
6.4.2. IDENTIFYING CONTENT
When a complete or partial representation is transferred as message content, it
is often desirable for the sender to supply, or the recipient to determine, an
identifier for a resource corresponding to that specific representation. For
example, a client making a GET request on a resource for "the current weather
report" might want an identifier specific to the content returned (e.g.,
"weather report for Laguna Beach at 20210720T1711"). This can be useful for
sharing or bookmarking content from resources that are expected to have changing
representations over time.¶
For a request message:¶
* If the request has a Content-Location header field, then the sender asserts
that the content is a representation of the resource identified by the
Content-Location field value. However, such an assertion cannot be trusted
unless it can be verified by other means (not defined by this specification).
The information might still be useful for revision history links.¶
* Otherwise, the content is unidentified by HTTP, but a more specific
identifier might be supplied within the content itself.¶
For a response message, the following rules are applied in order until a match
is found:¶
1. If the request method is HEAD or the response status code is 204 (No
Content) or 304 (Not Modified), there is no content in the response.¶
2. If the request method is GET and the response status code is 200 (OK), the
content is a representation of the target resource (Section 7.1).¶
3. If the request method is GET and the response status code is 203
(Non-Authoritative Information), the content is a potentially modified or
enhanced representation of the target resource as provided by an
intermediary.¶
4. If the request method is GET and the response status code is 206 (Partial
Content), the content is one or more parts of a representation of the target
resource.¶
5. If the response has a Content-Location header field and its field value is a
reference to the same URI as the target URI, the content is a representation
of the target resource.¶
6. If the response has a Content-Location header field and its field value is a
reference to a URI different from the target URI, then the sender asserts
that the content is a representation of the resource identified by the
Content-Location field value. However, such an assertion cannot be trusted
unless it can be verified by other means (not defined by this
specification).¶
7. Otherwise, the content is unidentified by HTTP, but a more specific
identifier might be supplied within the content itself.¶
6.5. TRAILER FIELDS
Fields (Section 5) that are located within a "trailer section" are referred to
as "trailer fields" (or just "trailers", colloquially). Trailer fields can be
useful for supplying message integrity checks, digital signatures, delivery
metrics, or post-processing status information.¶
Trailer fields ought to be processed and stored separately from the fields in
the header section to avoid contradicting message semantics known at the time
the header section was complete. The presence or absence of certain header
fields might impact choices made for the routing or processing of the message as
a whole before the trailers are received; those choices cannot be unmade by the
later discovery of trailer fields.¶
6.5.1. LIMITATIONS ON USE OF TRAILERS
A trailer section is only possible when supported by the version of HTTP in use
and enabled by an explicit framing mechanism. For example, the chunked transfer
coding in HTTP/1.1 allows a trailer section to be sent after the content
(Section 7.1.2 of [HTTP/1.1]).¶
Many fields cannot be processed outside the header section because their
evaluation is necessary prior to receiving the content, such as those that
describe message framing, routing, authentication, request modifiers, response
controls, or content format. A sender MUST NOT generate a trailer field unless
the sender knows the corresponding header field name's definition permits the
field to be sent in trailers.¶
Trailer fields can be difficult to process by intermediaries that forward
messages from one protocol version to another. If the entire message can be
buffered in transit, some intermediaries could merge trailer fields into the
header section (as appropriate) before it is forwarded. However, in most cases,
the trailers are simply discarded. A recipient MUST NOT merge a trailer field
into a header section unless the recipient understands the corresponding header
field definition and that definition explicitly permits and defines how trailer
field values can be safely merged.¶
The presence of the keyword "trailers" in the TE header field (Section 10.1.4)
of a request indicates that the client is willing to accept trailer fields, on
behalf of itself and any downstream clients. For requests from an intermediary,
this implies that all downstream clients are willing to accept trailer fields in
the forwarded response. Note that the presence of "trailers" does not mean that
the client(s) will process any particular trailer field in the response; only
that the trailer section(s) will not be dropped by any of the clients.¶
Because of the potential for trailer fields to be discarded in transit, a server
SHOULD NOT generate trailer fields that it believes are necessary for the user
agent to receive.¶
6.5.2. PROCESSING TRAILER FIELDS
The "Trailer" header field (Section 6.6.2) can be sent to indicate fields likely
to be sent in the trailer section, which allows recipients to prepare for their
receipt before processing the content. For example, this could be useful if a
field name indicates that a dynamic checksum should be calculated as the content
is received and then immediately checked upon receipt of the trailer field
value.¶
Like header fields, trailer fields with the same name are processed in the order
received; multiple trailer field lines with the same name have the equivalent
semantics as appending the multiple values as a list of members. Trailer fields
that might be generated more than once during a message MUST be defined as a
list-based field even if each member value is only processed once per field line
received.¶
At the end of a message, a recipient MAY treat the set of received trailer
fields as a data structure of name/value pairs, similar to (but separate from)
the header fields. Additional processing expectations, if any, can be defined
within the field specification for a field intended for use in trailers.¶
6.6. MESSAGE METADATA
Fields that describe the message itself, such as when and how the message has
been generated, can appear in both requests and responses.¶
6.6.1. DATE
The "Date" header field represents the date and time at which the message was
originated, having the same semantics as the Origination Date Field (orig-date)
defined in Section 3.6.1 of [RFC5322]. The field value is an HTTP-date, as
defined in Section 5.6.7.¶
Date = HTTP-date
¶
An example is¶
Date: Tue, 15 Nov 1994 08:12:31 GMT
¶
A sender that generates a Date header field SHOULD generate its field value as
the best available approximation of the date and time of message generation. In
theory, the date ought to represent the moment just before generating the
message content. In practice, a sender can generate the date value at any time
during message origination.¶
An origin server with a clock (as defined in Section 5.6.7) MUST generate a Date
header field in all 2xx (Successful), 3xx (Redirection), and 4xx (Client Error)
responses, and MAY generate a Date header field in 1xx (Informational) and 5xx
(Server Error) responses.¶
An origin server without a clock MUST NOT generate a Date header field.¶
A recipient with a clock that receives a response message without a Date header
field MUST record the time it was received and append a corresponding Date
header field to the message's header section if it is cached or forwarded
downstream.¶
A recipient with a clock that receives a response with an invalid Date header
field value MAY replace that value with the time that response was received.¶
A user agent MAY send a Date header field in a request, though generally will
not do so unless it is believed to convey useful information to the server. For
example, custom applications of HTTP might convey a Date if the server is
expected to adjust its interpretation of the user's request based on differences
between the user agent and server clocks.¶
6.6.2. TRAILER
The "Trailer" header field provides a list of field names that the sender
anticipates sending as trailer fields within that message. This allows a
recipient to prepare for receipt of the indicated metadata before it starts
processing the content.¶
Trailer = #field-name
¶
For example, a sender might indicate that a signature will be computed as the
content is being streamed and provide the final signature as a trailer field.
This allows a recipient to perform the same check on the fly as it receives the
content.¶
A sender that intends to generate one or more trailer fields in a message SHOULD
generate a Trailer header field in the header section of that message to
indicate which fields might be present in the trailers.¶
If an intermediary discards the trailer section in transit, the Trailer field
could provide a hint of what metadata was lost, though there is no guarantee
that a sender of Trailer will always follow through by sending the named
fields.¶
7. ROUTING HTTP MESSAGES
HTTP request message routing is determined by each client based on the target
resource, the client's proxy configuration, and establishment or reuse of an
inbound connection. The corresponding response routing follows the same
connection chain back to the client.¶
7.1. DETERMINING THE TARGET RESOURCE
Although HTTP is used in a wide variety of applications, most clients rely on
the same resource identification mechanism and configuration techniques as
general-purpose Web browsers. Even when communication options are hard-coded in
a client's configuration, we can think of their combined effect as a URI
reference (Section 4.1).¶
A URI reference is resolved to its absolute form in order to obtain the "target
URI". The target URI excludes the reference's fragment component, if any, since
fragment identifiers are reserved for client-side processing ([URI], Section
3.5).¶
To perform an action on a "target resource", the client sends a request message
containing enough components of its parsed target URI to enable recipients to
identify that same resource. For historical reasons, the parsed target URI
components, collectively referred to as the "request target", are sent within
the message control data and the Host header field (Section 7.2).¶
There are two unusual cases for which the request target components are in a
method-specific form:¶
* For CONNECT (Section 9.3.6), the request target is the host name and port
number of the tunnel destination, separated by a colon.¶
* For OPTIONS (Section 9.3.7), the request target can be a single asterisk
("*").¶
See the respective method definitions for details. These forms MUST NOT be used
with other methods.¶
Upon receipt of a client's request, a server reconstructs the target URI from
the received components in accordance with their local configuration and
incoming connection context. This reconstruction is specific to each major
protocol version. For example, Section 3.3 of [HTTP/1.1] defines how a server
determines the target URI of an HTTP/1.1 request.¶
Note: Previous specifications defined the recomposed target URI as a distinct
concept, the "effective request URI".¶
7.2. HOST AND :AUTHORITY
The "Host" header field in a request provides the host and port information from
the target URI, enabling the origin server to distinguish among resources while
servicing requests for multiple host names.¶
In HTTP/2 [HTTP/2] and HTTP/3 [HTTP/3], the Host header field is, in some cases,
supplanted by the ":authority" pseudo-header field of a request's control data.¶
Host = uri-host [ ":" port ] ; Section 4
¶
The target URI's authority information is critical for handling a request. A
user agent MUST generate a Host header field in a request unless it sends that
information as an ":authority" pseudo-header field. A user agent that sends Host
SHOULD send it as the first field in the header section of a request.¶
For example, a GET request to the origin server for
<http://www.example.org/pub/WWW/> would begin with:¶
GET /pub/WWW/ HTTP/1.1
Host: www.example.org
¶
Since the host and port information acts as an application-level routing
mechanism, it is a frequent target for malware seeking to poison a shared cache
or redirect a request to an unintended server. An interception proxy is
particularly vulnerable if it relies on the host and port information for
redirecting requests to internal servers, or for use as a cache key in a shared
cache, without first verifying that the intercepted connection is targeting a
valid IP address for that host.¶
7.3. ROUTING INBOUND REQUESTS
Once the target URI and its origin are determined, a client decides whether a
network request is necessary to accomplish the desired semantics and, if so,
where that request is to be directed.¶
7.3.1. TO A CACHE
If the client has a cache [CACHING] and the request can be satisfied by it, then
the request is usually directed there first.¶
7.3.2. TO A PROXY
If the request is not satisfied by a cache, then a typical client will check its
configuration to determine whether a proxy is to be used to satisfy the request.
Proxy configuration is implementation-dependent, but is often based on URI
prefix matching, selective authority matching, or both, and the proxy itself is
usually identified by an "http" or "https" URI.¶
If an "http" or "https" proxy is applicable, the client connects inbound by
establishing (or reusing) a connection to that proxy and then sending it an HTTP
request message containing a request target that matches the client's target
URI.¶
7.3.3. TO THE ORIGIN
If no proxy is applicable, a typical client will invoke a handler routine
(specific to the target URI's scheme) to obtain access to the identified
resource. How that is accomplished is dependent on the target URI scheme and
defined by its associated specification.¶
Section 4.3.2 defines how to obtain access to an "http" resource by establishing
(or reusing) an inbound connection to the identified origin server and then
sending it an HTTP request message containing a request target that matches the
client's target URI.¶
Section 4.3.3 defines how to obtain access to an "https" resource by
establishing (or reusing) an inbound secured connection to an origin server that
is authoritative for the identified origin and then sending it an HTTP request
message containing a request target that matches the client's target URI.¶
7.4. REJECTING MISDIRECTED REQUESTS
Once a request is received by a server and parsed sufficiently to determine its
target URI, the server decides whether to process the request itself, forward
the request to another server, redirect the client to a different resource,
respond with an error, or drop the connection. This decision can be influenced
by anything about the request or connection context, but is specifically
directed at whether the server has been configured to process requests for that
target URI and whether the connection context is appropriate for that request.¶
For example, a request might have been misdirected, deliberately or
accidentally, such that the information within a received Host header field
differs from the connection's host or port. If the connection is from a trusted
gateway, such inconsistency might be expected; otherwise, it might indicate an
attempt to bypass security filters, trick the server into delivering non-public
content, or poison a cache. See Section 17 for security considerations regarding
message routing.¶
Unless the connection is from a trusted gateway, an origin server MUST reject a
request if any scheme-specific requirements for the target URI are not met. In
particular, a request for an "https" resource MUST be rejected unless it has
been received over a connection that has been secured via a certificate valid
for that target URI's origin, as defined by Section 4.2.2.¶
The 421 (Misdirected Request) status code in a response indicates that the
origin server has rejected the request because it appears to have been
misdirected (Section 15.5.20).¶
7.5. RESPONSE CORRELATION
A connection might be used for multiple request/response exchanges. The
mechanism used to correlate between request and response messages is version
dependent; some versions of HTTP use implicit ordering of messages, while others
use an explicit identifier.¶
All responses, regardless of the status code (including interim responses) can
be sent at any time after a request is received, even if the request is not yet
complete. A response can complete before its corresponding request is complete
(Section 6.1). Likewise, clients are not expected to wait any specific amount of
time for a response. Clients (including intermediaries) might abandon a request
if the response is not received within a reasonable period of time.¶
A client that receives a response while it is still sending the associated
request SHOULD continue sending that request unless it receives an explicit
indication to the contrary (see, e.g., Section 9.5 of [HTTP/1.1] and Section 6.4
of [HTTP/2]).¶
7.6. MESSAGE FORWARDING
As described in Section 3.7, intermediaries can serve a variety of roles in the
processing of HTTP requests and responses. Some intermediaries are used to
improve performance or availability. Others are used for access control or to
filter content. Since an HTTP stream has characteristics similar to a
pipe-and-filter architecture, there are no inherent limits to the extent an
intermediary can enhance (or interfere) with either direction of the stream.¶
Intermediaries are expected to forward messages even when protocol elements are
not recognized (e.g., new methods, status codes, or field names) since that
preserves extensibility for downstream recipients.¶
An intermediary not acting as a tunnel MUST implement the Connection header
field, as specified in Section 7.6.1, and exclude fields from being forwarded
that are only intended for the incoming connection.¶
An intermediary MUST NOT forward a message to itself unless it is protected from
an infinite request loop. In general, an intermediary ought to recognize its own
server names, including any aliases, local variations, or literal IP addresses,
and respond to such requests directly.¶
An HTTP message can be parsed as a stream for incremental processing or
forwarding downstream. However, senders and recipients cannot rely on
incremental delivery of partial messages, since some implementations will buffer
or delay message forwarding for the sake of network efficiency, security checks,
or content transformations.¶
7.6.1. CONNECTION
The "Connection" header field allows the sender to list desired control options
for the current connection.¶
Connection = #connection-option
connection-option = token
¶
Connection options are case-insensitive.¶
When a field aside from Connection is used to supply control information for or
about the current connection, the sender MUST list the corresponding field name
within the Connection header field. Note that some versions of HTTP prohibit the
use of fields for such information, and therefore do not allow the Connection
field.¶
Intermediaries MUST parse a received Connection header field before a message is
forwarded and, for each connection-option in this field, remove any header or
trailer field(s) from the message with the same name as the connection-option,
and then remove the Connection header field itself (or replace it with the
intermediary's own control options for the forwarded message).¶
Hence, the Connection header field provides a declarative way of distinguishing
fields that are only intended for the immediate recipient ("hop-by-hop") from
those fields that are intended for all recipients on the chain ("end-to-end"),
enabling the message to be self-descriptive and allowing future
connection-specific extensions to be deployed without fear that they will be
blindly forwarded by older intermediaries.¶
Furthermore, intermediaries SHOULD remove or replace fields that are known to
require removal before forwarding, whether or not they appear as a
connection-option, after applying those fields' semantics. This includes but is
not limited to:¶
* Proxy-Connection (Appendix C.2.2 of [HTTP/1.1])¶
* Keep-Alive (Section 19.7.1 of [RFC2068])¶
* TE (Section 10.1.4)¶
* Transfer-Encoding (Section 6.1 of [HTTP/1.1])¶
* Upgrade (Section 7.8)¶
A sender MUST NOT send a connection option corresponding to a field that is
intended for all recipients of the content. For example, Cache-Control is never
appropriate as a connection option (Section 5.2 of [CACHING]).¶
Connection options do not always correspond to a field present in the message,
since a connection-specific field might not be needed if there are no parameters
associated with a connection option. In contrast, a connection-specific field
received without a corresponding connection option usually indicates that the
field has been improperly forwarded by an intermediary and ought to be ignored
by the recipient.¶
When defining a new connection option that does not correspond to a field,
specification authors ought to reserve the corresponding field name anyway in
order to avoid later collisions. Such reserved field names are registered in the
"Hypertext Transfer Protocol (HTTP) Field Name Registry" (Section 16.3.1).¶
7.6.2. MAX-FORWARDS
The "Max-Forwards" header field provides a mechanism with the TRACE (Section
9.3.8) and OPTIONS (Section 9.3.7) request methods to limit the number of times
that the request is forwarded by proxies. This can be useful when the client is
attempting to trace a request that appears to be failing or looping mid-chain.¶
Max-Forwards = 1*DIGIT
¶
The Max-Forwards value is a decimal integer indicating the remaining number of
times this request message can be forwarded.¶
Each intermediary that receives a TRACE or OPTIONS request containing a
Max-Forwards header field MUST check and update its value prior to forwarding
the request. If the received value is zero (0), the intermediary MUST NOT
forward the request; instead, the intermediary MUST respond as the final
recipient. If the received Max-Forwards value is greater than zero, the
intermediary MUST generate an updated Max-Forwards field in the forwarded
message with a field value that is the lesser of a) the received value
decremented by one (1) or b) the recipient's maximum supported value for
Max-Forwards.¶
A recipient MAY ignore a Max-Forwards header field received with any other
request methods.¶
7.6.3. VIA
The "Via" header field indicates the presence of intermediate protocols and
recipients between the user agent and the server (on requests) or between the
origin server and the client (on responses), similar to the "Received" header
field in email (Section 3.6.7 of [RFC5322]). Via can be used for tracking
message forwards, avoiding request loops, and identifying the protocol
capabilities of senders along the request/response chain.¶
Via = #( received-protocol RWS received-by [ RWS comment ] )
received-protocol = [ protocol-name "/" ] protocol-version
; see Section 7.8
received-by = pseudonym [ ":" port ]
pseudonym = token
¶
Each member of the Via field value represents a proxy or gateway that has
forwarded the message. Each intermediary appends its own information about how
the message was received, such that the end result is ordered according to the
sequence of forwarding recipients.¶
A proxy MUST send an appropriate Via header field, as described below, in each
message that it forwards. An HTTP-to-HTTP gateway MUST send an appropriate Via
header field in each inbound request message and MAY send a Via header field in
forwarded response messages.¶
For each intermediary, the received-protocol indicates the protocol and protocol
version used by the upstream sender of the message. Hence, the Via field value
records the advertised protocol capabilities of the request/response chain such
that they remain visible to downstream recipients; this can be useful for
determining what backwards-incompatible features might be safe to use in
response, or within a later request, as described in Section 2.5. For brevity,
the protocol-name is omitted when the received protocol is HTTP.¶
The received-by portion is normally the host and optional port number of a
recipient server or client that subsequently forwarded the message. However, if
the real host is considered to be sensitive information, a sender MAY replace it
with a pseudonym. If a port is not provided, a recipient MAY interpret that as
meaning it was received on the default port, if any, for the received-protocol.¶
A sender MAY generate comments to identify the software of each recipient,
analogous to the User-Agent and Server header fields. However, comments in Via
are optional, and a recipient MAY remove them prior to forwarding the message.¶
For example, a request message could be sent from an HTTP/1.0 user agent to an
internal proxy code-named "fred", which uses HTTP/1.1 to forward the request to
a public proxy at p.example.net, which completes the request by forwarding it to
the origin server at www.example.com. The request received by www.example.com
would then have the following Via header field:¶
Via: 1.0 fred, 1.1 p.example.net
¶
An intermediary used as a portal through a network firewall SHOULD NOT forward
the names and ports of hosts within the firewall region unless it is explicitly
enabled to do so. If not enabled, such an intermediary SHOULD replace each
received-by host of any host behind the firewall by an appropriate pseudonym for
that host.¶
An intermediary MAY combine an ordered subsequence of Via header field list
members into a single member if the entries have identical received-protocol
values. For example,¶
Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
¶
could be collapsed to¶
Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
¶
A sender SHOULD NOT combine multiple list members unless they are all under the
same organizational control and the hosts have already been replaced by
pseudonyms. A sender MUST NOT combine members that have different
received-protocol values.¶
7.7. MESSAGE TRANSFORMATIONS
Some intermediaries include features for transforming messages and their
content. A proxy might, for example, convert between image formats in order to
save cache space or to reduce the amount of traffic on a slow link. However,
operational problems might occur when these transformations are applied to
content intended for critical applications, such as medical imaging or
scientific data analysis, particularly when integrity checks or digital
signatures are used to ensure that the content received is identical to the
original.¶
An HTTP-to-HTTP proxy is called a "transforming proxy" if it is designed or
configured to modify messages in a semantically meaningful way (i.e.,
modifications, beyond those required by normal HTTP processing, that change the
message in a way that would be significant to the original sender or potentially
significant to downstream recipients). For example, a transforming proxy might
be acting as a shared annotation server (modifying responses to include
references to a local annotation database), a malware filter, a format
transcoder, or a privacy filter. Such transformations are presumed to be desired
by whichever client (or client organization) chose the proxy.¶
If a proxy receives a target URI with a host name that is not a fully qualified
domain name, it MAY add its own domain to the host name it received when
forwarding the request. A proxy MUST NOT change the host name if the target URI
contains a fully qualified domain name.¶
A proxy MUST NOT modify the "absolute-path" and "query" parts of the received
target URI when forwarding it to the next inbound server except as required by
that forwarding protocol. For example, a proxy forwarding a request to an origin
server via HTTP/1.1 will replace an empty path with "/" (Section 3.2.1 of
[HTTP/1.1]) or "*" (Section 3.2.4 of [HTTP/1.1]), depending on the request
method.¶
A proxy MUST NOT transform the content (Section 6.4) of a response message that
contains a no-transform cache directive (Section 5.2.2.6 of [CACHING]). Note
that this does not apply to message transformations that do not change the
content, such as the addition or removal of transfer codings (Section 7 of
[HTTP/1.1]).¶
A proxy MAY transform the content of a message that does not contain a
no-transform cache directive. A proxy that transforms the content of a 200 (OK)
response can inform downstream recipients that a transformation has been applied
by changing the response status code to 203 (Non-Authoritative Information)
(Section 15.3.4).¶
A proxy SHOULD NOT modify header fields that provide information about the
endpoints of the communication chain, the resource state, or the selected
representation (other than the content) unless the field's definition
specifically allows such modification or the modification is deemed necessary
for privacy or security.¶
7.8. UPGRADE
The "Upgrade" header field is intended to provide a simple mechanism for
transitioning from HTTP/1.1 to some other protocol on the same connection.¶
A client MAY send a list of protocol names in the Upgrade header field of a
request to invite the server to switch to one or more of the named protocols, in
order of descending preference, before sending the final response. A server MAY
ignore a received Upgrade header field if it wishes to continue using the
current protocol on that connection. Upgrade cannot be used to insist on a
protocol change.¶
Upgrade = #protocol
protocol = protocol-name ["/" protocol-version]
protocol-name = token
protocol-version = token
¶
Although protocol names are registered with a preferred case, recipients SHOULD
use case-insensitive comparison when matching each protocol-name to supported
protocols.¶
A server that sends a 101 (Switching Protocols) response MUST send an Upgrade
header field to indicate the new protocol(s) to which the connection is being
switched; if multiple protocol layers are being switched, the sender MUST list
the protocols in layer-ascending order. A server MUST NOT switch to a protocol
that was not indicated by the client in the corresponding request's Upgrade
header field. A server MAY choose to ignore the order of preference indicated by
the client and select the new protocol(s) based on other factors, such as the
nature of the request or the current load on the server.¶
A server that sends a 426 (Upgrade Required) response MUST send an Upgrade
header field to indicate the acceptable protocols, in order of descending
preference.¶
A server MAY send an Upgrade header field in any other response to advertise
that it implements support for upgrading to the listed protocols, in order of
descending preference, when appropriate for a future request.¶
The following is a hypothetical example sent by a client:¶
GET /hello HTTP/1.1
Host: www.example.com
Connection: upgrade
Upgrade: websocket, IRC/6.9, RTA/x11
¶
The capabilities and nature of the application-level communication after the
protocol change is entirely dependent upon the new protocol(s) chosen. However,
immediately after sending the 101 (Switching Protocols) response, the server is
expected to continue responding to the original request as if it had received
its equivalent within the new protocol (i.e., the server still has an
outstanding request to satisfy after the protocol has been changed, and is
expected to do so without requiring the request to be repeated).¶
For example, if the Upgrade header field is received in a GET request and the
server decides to switch protocols, it first responds with a 101 (Switching
Protocols) message in HTTP/1.1 and then immediately follows that with the new
protocol's equivalent of a response to a GET on the target resource. This allows
a connection to be upgraded to protocols with the same semantics as HTTP without
the latency cost of an additional round trip. A server MUST NOT switch protocols
unless the received message semantics can be honored by the new protocol; an
OPTIONS request can be honored by any protocol.¶
The following is an example response to the above hypothetical request:¶
HTTP/1.1 101 Switching Protocols
Connection: upgrade
Upgrade: websocket
[... data stream switches to websocket with an appropriate response
(as defined by new protocol) to the "GET /hello" request ...]
¶
A sender of Upgrade MUST also send an "Upgrade" connection option in the
Connection header field (Section 7.6.1) to inform intermediaries not to forward
this field. A server that receives an Upgrade header field in an HTTP/1.0
request MUST ignore that Upgrade field.¶
A client cannot begin using an upgraded protocol on the connection until it has
completely sent the request message (i.e., the client can't change the protocol
it is sending in the middle of a message). If a server receives both an Upgrade
and an Expect header field with the "100-continue" expectation (Section 10.1.1),
the server MUST send a 100 (Continue) response before sending a 101 (Switching
Protocols) response.¶
The Upgrade header field only applies to switching protocols on top of the
existing connection; it cannot be used to switch the underlying connection
(transport) protocol, nor to switch the existing communication to a different
connection. For those purposes, it is more appropriate to use a 3xx
(Redirection) response (Section 15.4).¶
This specification only defines the protocol name "HTTP" for use by the family
of Hypertext Transfer Protocols, as defined by the HTTP version rules of Section
2.5 and future updates to this specification. Additional protocol names ought to
be registered using the registration procedure defined in Section 16.7.¶
8. REPRESENTATION DATA AND METADATA
8.1. REPRESENTATION DATA
The representation data associated with an HTTP message is either provided as
the content of the message or referred to by the message semantics and the
target URI. The representation data is in a format and encoding defined by the
representation metadata header fields.¶
The data type of the representation data is determined via the header fields
Content-Type and Content-Encoding. These define a two-layer, ordered encoding
model:¶
representation-data := Content-Encoding( Content-Type( data ) )
¶
8.2. REPRESENTATION METADATA
Representation header fields provide metadata about the representation. When a
message includes content, the representation header fields describe how to
interpret that data. In a response to a HEAD request, the representation header
fields describe the representation data that would have been enclosed in the
content if the same request had been a GET.¶
8.3. CONTENT-TYPE
The "Content-Type" header field indicates the media type of the associated
representation: either the representation enclosed in the message content or the
selected representation, as determined by the message semantics. The indicated
media type defines both the data format and how that data is intended to be
processed by a recipient, within the scope of the received message semantics,
after any content codings indicated by Content-Encoding are decoded.¶
Content-Type = media-type
¶
Media types are defined in Section 8.3.1. An example of the field is¶
Content-Type: text/html; charset=ISO-8859-4
¶
A sender that generates a message containing content SHOULD generate a
Content-Type header field in that message unless the intended media type of the
enclosed representation is unknown to the sender. If a Content-Type header field
is not present, the recipient MAY either assume a media type of
"application/octet-stream" ([RFC2046], Section 4.5.1) or examine the data to
determine its type.¶
In practice, resource owners do not always properly configure their origin
server to provide the correct Content-Type for a given representation. Some user
agents examine the content and, in certain cases, override the received type
(for example, see [Sniffing]). This "MIME sniffing" risks drawing incorrect
conclusions about the data, which might expose the user to additional security
risks (e.g., "privilege escalation"). Furthermore, distinct media types often
share a common data format, differing only in how the data is intended to be
processed, which is impossible to distinguish by inspecting the data alone. When
sniffing is implemented, implementers are encouraged to provide a means for the
user to disable it.¶
Although Content-Type is defined as a singleton field, it is sometimes
incorrectly generated multiple times, resulting in a combined field value that
appears to be a list. Recipients often attempt to handle this error by using the
last syntactically valid member of the list, leading to potential
interoperability and security issues if different implementations have different
error handling behaviors.¶
8.3.1. MEDIA TYPE
HTTP uses media types [RFC2046] in the Content-Type (Section 8.3) and Accept
(Section 12.5.1) header fields in order to provide open and extensible data
typing and type negotiation. Media types define both a data format and various
processing models: how to process that data in accordance with the message
context.¶
media-type = type "/" subtype parameters
type = token
subtype = token
¶
The type and subtype tokens are case-insensitive.¶
The type/subtype MAY be followed by semicolon-delimited parameters (Section
5.6.6) in the form of name/value pairs. The presence or absence of a parameter
might be significant to the processing of a media type, depending on its
definition within the media type registry. Parameter values might or might not
be case-sensitive, depending on the semantics of the parameter name.¶
For example, the following media types are equivalent in describing HTML text
data encoded in the UTF-8 character encoding scheme, but the first is preferred
for consistency (the "charset" parameter value is defined as being
case-insensitive in [RFC2046], Section 4.1.2):¶
text/html;charset=utf-8
Text/HTML;Charset="utf-8"
text/html; charset="utf-8"
text/html;charset=UTF-8
¶
Media types ought to be registered with IANA according to the procedures defined
in [BCP13].¶
8.3.2. CHARSET
HTTP uses "charset" names to indicate or negotiate the character encoding scheme
([RFC6365], Section 2) of a textual representation. In the fields defined by
this document, charset names appear either in parameters (Content-Type), or, for
Accept-Encoding, in the form of a plain token. In both cases, charset names are
matched case-insensitively.¶
Charset names ought to be registered in the IANA "Character Sets" registry
(<https://www.iana.org/assignments/character-sets>) according to the procedures
defined in Section 2 of [RFC2978].¶
Note: In theory, charset names are defined by the "mime-charset" ABNF rule
defined in Section 2.3 of [RFC2978] (as corrected in [Err1912]). That rule
allows two characters that are not included in "token" ("{" and "}"), but no
charset name registered at the time of this writing includes braces (see
[Err5433]).¶
8.3.3. MULTIPART TYPES
MIME provides for a number of "multipart" types -- encapsulations of one or more
representations within a single message body. All multipart types share a common
syntax, as defined in Section 5.1.1 of [RFC2046], and include a boundary
parameter as part of the media type value. The message body is itself a protocol
element; a sender MUST generate only CRLF to represent line breaks between body
parts.¶
HTTP message framing does not use the multipart boundary as an indicator of
message body length, though it might be used by implementations that generate or
process the content. For example, the "multipart/form-data" type is often used
for carrying form data in a request, as described in [RFC7578], and the
"multipart/byteranges" type is defined by this specification for use in some 206
(Partial Content) responses (see Section 15.3.7).¶
8.4. CONTENT-ENCODING
The "Content-Encoding" header field indicates what content codings have been
applied to the representation, beyond those inherent in the media type, and thus
what decoding mechanisms have to be applied in order to obtain data in the media
type referenced by the Content-Type header field. Content-Encoding is primarily
used to allow a representation's data to be compressed without losing the
identity of its underlying media type.¶
Content-Encoding = #content-coding
¶
An example of its use is¶
Content-Encoding: gzip
¶
If one or more encodings have been applied to a representation, the sender that
applied the encodings MUST generate a Content-Encoding header field that lists
the content codings in the order in which they were applied. Note that the
coding named "identity" is reserved for its special role in Accept-Encoding and
thus SHOULD NOT be included.¶
Additional information about the encoding parameters can be provided by other
header fields not defined by this specification.¶
Unlike Transfer-Encoding (Section 6.1 of [HTTP/1.1]), the codings listed in
Content-Encoding are a characteristic of the representation; the representation
is defined in terms of the coded form, and all other metadata about the
representation is about the coded form unless otherwise noted in the metadata
definition. Typically, the representation is only decoded just prior to
rendering or analogous usage.¶
If the media type includes an inherent encoding, such as a data format that is
always compressed, then that encoding would not be restated in Content-Encoding
even if it happens to be the same algorithm as one of the content codings. Such
a content coding would only be listed if, for some bizarre reason, it is applied
a second time to form the representation. Likewise, an origin server might
choose to publish the same data as multiple representations that differ only in
whether the coding is defined as part of Content-Type or Content-Encoding, since
some user agents will behave differently in their handling of each response
(e.g., open a "Save as ..." dialog instead of automatic decompression and
rendering of content).¶
An origin server MAY respond with a status code of 415 (Unsupported Media Type)
if a representation in the request message has a content coding that is not
acceptable.¶
8.4.1. CONTENT CODINGS
Content coding values indicate an encoding transformation that has been or can
be applied to a representation. Content codings are primarily used to allow a
representation to be compressed or otherwise usefully transformed without losing
the identity of its underlying media type and without loss of information.
Frequently, the representation is stored in coded form, transmitted directly,
and only decoded by the final recipient.¶
content-coding = token
¶
All content codings are case-insensitive and ought to be registered within the
"HTTP Content Coding Registry", as described in Section 16.6¶
Content-coding values are used in the Accept-Encoding (Section 12.5.3) and
Content-Encoding (Section 8.4) header fields.¶
8.4.1.1. COMPRESS CODING
The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding [Welch] that
is commonly produced by the UNIX file compression program "compress". A
recipient SHOULD consider "x-compress" to be equivalent to "compress".¶
8.4.1.2. DEFLATE CODING
The "deflate" coding is a "zlib" data format [RFC1950] containing a "deflate"
compressed data stream [RFC1951] that uses a combination of the Lempel-Ziv
(LZ77) compression algorithm and Huffman coding.¶
Note: Some non-conformant implementations send the "deflate" compressed data
without the zlib wrapper.¶
8.4.1.3. GZIP CODING
The "gzip" coding is an LZ77 coding with a 32-bit Cyclic Redundancy Check (CRC)
that is commonly produced by the gzip file compression program [RFC1952]. A
recipient SHOULD consider "x-gzip" to be equivalent to "gzip".¶
8.5. CONTENT-LANGUAGE
The "Content-Language" header field describes the natural language(s) of the
intended audience for the representation. Note that this might not be equivalent
to all the languages used within the representation.¶
Content-Language = #language-tag
¶
Language tags are defined in Section 8.5.1. The primary purpose of
Content-Language is to allow a user to identify and differentiate
representations according to the users' own preferred language. Thus, if the
content is intended only for a Danish-literate audience, the appropriate field
is¶
Content-Language: da
¶
If no Content-Language is specified, the default is that the content is intended
for all language audiences. This might mean that the sender does not consider it
to be specific to any natural language, or that the sender does not know for
which language it is intended.¶
Multiple languages MAY be listed for content that is intended for multiple
audiences. For example, a rendition of the "Treaty of Waitangi", presented
simultaneously in the original Maori and English versions, would call for¶
Content-Language: mi, en
¶
However, just because multiple languages are present within a representation
does not mean that it is intended for multiple linguistic audiences. An example
would be a beginner's language primer, such as "A First Lesson in Latin", which
is clearly intended to be used by an English-literate audience. In this case,
the Content-Language would properly only include "en".¶
Content-Language MAY be applied to any media type -- it is not limited to
textual documents.¶
8.5.1. LANGUAGE TAGS
A language tag, as defined in [RFC5646], identifies a natural language spoken,
written, or otherwise conveyed by human beings for communication of information
to other human beings. Computer languages are explicitly excluded.¶
HTTP uses language tags within the Accept-Language and Content-Language header
fields. Accept-Language uses the broader language-range production defined in
Section 12.5.4, whereas Content-Language uses the language-tag production
defined below.¶
language-tag = <Language-Tag, see [RFC5646], Section 2.1>
¶
A language tag is a sequence of one or more case-insensitive subtags, each
separated by a hyphen character ("-", %x2D). In most cases, a language tag
consists of a primary language subtag that identifies a broad family of related
languages (e.g., "en" = English), which is optionally followed by a series of
subtags that refine or narrow that language's range (e.g., "en-CA" = the variety
of English as communicated in Canada). Whitespace is not allowed within a
language tag. Example tags include:¶
fr, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
¶
See [RFC5646] for further information.¶
8.6. CONTENT-LENGTH
The "Content-Length" header field indicates the associated representation's data
length as a decimal non-negative integer number of octets. When transferring a
representation as content, Content-Length refers specifically to the amount of
data enclosed so that it can be used to delimit framing (e.g., Section 6.2 of
[HTTP/1.1]). In other cases, Content-Length indicates the selected
representation's current length, which can be used by recipients to estimate
transfer time or to compare with previously stored representations.¶
Content-Length = 1*DIGIT
¶
An example is¶
Content-Length: 3495
¶
A user agent SHOULD send Content-Length in a request when the method defines a
meaning for enclosed content and it is not sending Transfer-Encoding. For
example, a user agent normally sends Content-Length in a POST request even when
the value is 0 (indicating empty content). A user agent SHOULD NOT send a
Content-Length header field when the request message does not contain content
and the method semantics do not anticipate such data.¶
A server MAY send a Content-Length header field in a response to a HEAD request
(Section 9.3.2); a server MUST NOT send Content-Length in such a response unless
its field value equals the decimal number of octets that would have been sent in
the content of a response if the same request had used the GET method.¶
A server MAY send a Content-Length header field in a 304 (Not Modified) response
to a conditional GET request (Section 15.4.5); a server MUST NOT send
Content-Length in such a response unless its field value equals the decimal
number of octets that would have been sent in the content of a 200 (OK) response
to the same request.¶
A server MUST NOT send a Content-Length header field in any response with a
status code of 1xx (Informational) or 204 (No Content). A server MUST NOT send a
Content-Length header field in any 2xx (Successful) response to a CONNECT
request (Section 9.3.6).¶
Aside from the cases defined above, in the absence of Transfer-Encoding, an
origin server SHOULD send a Content-Length header field when the content size is
known prior to sending the complete header section. This will allow downstream
recipients to measure transfer progress, know when a received message is
complete, and potentially reuse the connection for additional requests.¶
Any Content-Length field value greater than or equal to zero is valid. Since
there is no predefined limit to the length of content, a recipient MUST
anticipate potentially large decimal numerals and prevent parsing errors due to
integer conversion overflows or precision loss due to integer conversion
(Section 17.5).¶
Because Content-Length is used for message delimitation in HTTP/1.1, its field
value can impact how the message is parsed by downstream recipients even when
the immediate connection is not using HTTP/1.1. If the message is forwarded by a
downstream intermediary, a Content-Length field value that is inconsistent with
the received message framing might cause a security failure due to request
smuggling or response splitting.¶
As a result, a sender MUST NOT forward a message with a Content-Length header
field value that is known to be incorrect.¶
Likewise, a sender MUST NOT forward a message with a Content-Length header field
value that does not match the ABNF above, with one exception: a recipient of a
Content-Length header field value consisting of the same decimal value repeated
as a comma-separated list (e.g, "Content-Length: 42, 42") MAY either reject the
message as invalid or replace that invalid field value with a single instance of
the decimal value, since this likely indicates that a duplicate was generated or
combined by an upstream message processor.¶
8.7. CONTENT-LOCATION
The "Content-Location" header field references a URI that can be used as an
identifier for a specific resource corresponding to the representation in this
message's content. In other words, if one were to perform a GET request on this
URI at the time of this message's generation, then a 200 (OK) response would
contain the same representation that is enclosed as content in this message.¶
Content-Location = absolute-URI / partial-URI
¶
The field value is either an absolute-URI or a partial-URI. In the latter case
(Section 4), the referenced URI is relative to the target URI ([URI], Section
5).¶
The Content-Location value is not a replacement for the target URI (Section
7.1). It is representation metadata. It has the same syntax and semantics as the
header field of the same name defined for MIME body parts in Section 4 of
[RFC2557]. However, its appearance in an HTTP message has some special
implications for HTTP recipients.¶
If Content-Location is included in a 2xx (Successful) response message and its
value refers (after conversion to absolute form) to a URI that is the same as
the target URI, then the recipient MAY consider the content to be a current
representation of that resource at the time indicated by the message origination
date. For a GET (Section 9.3.1) or HEAD (Section 9.3.2) request, this is the
same as the default semantics when no Content-Location is provided by the
server. For a state-changing request like PUT (Section 9.3.4) or POST (Section
9.3.3), it implies that the server's response contains the new representation of
that resource, thereby distinguishing it from representations that might only
report about the action (e.g., "It worked!"). This allows authoring applications
to update their local copies without the need for a subsequent GET request.¶
If Content-Location is included in a 2xx (Successful) response message and its
field value refers to a URI that differs from the target URI, then the origin
server claims that the URI is an identifier for a different resource
corresponding to the enclosed representation. Such a claim can only be trusted
if both identifiers share the same resource owner, which cannot be
programmatically determined via HTTP.¶
* For a response to a GET or HEAD request, this is an indication that the
target URI refers to a resource that is subject to content negotiation and
the Content-Location field value is a more specific identifier for the
selected representation.¶
* For a 201 (Created) response to a state-changing method, a Content-Location
field value that is identical to the Location field value indicates that this
content is a current representation of the newly created resource.¶
* Otherwise, such a Content-Location indicates that this content is a
representation reporting on the requested action's status and that the same
report is available (for future access with GET) at the given URI. For
example, a purchase transaction made via a POST request might include a
receipt document as the content of the 200 (OK) response; the
Content-Location field value provides an identifier for retrieving a copy of
that same receipt in the future.¶
A user agent that sends Content-Location in a request message is stating that
its value refers to where the user agent originally obtained the content of the
enclosed representation (prior to any modifications made by that user agent). In
other words, the user agent is providing a back link to the source of the
original representation.¶
An origin server that receives a Content-Location field in a request message
MUST treat the information as transitory request context rather than as metadata
to be saved verbatim as part of the representation. An origin server MAY use
that context to guide in processing the request or to save it for other uses,
such as within source links or versioning metadata. However, an origin server
MUST NOT use such context information to alter the request semantics.¶
For example, if a client makes a PUT request on a negotiated resource and the
origin server accepts that PUT (without redirection), then the new state of that
resource is expected to be consistent with the one representation supplied in
that PUT; the Content-Location cannot be used as a form of reverse content
selection identifier to update only one of the negotiated representations. If
the user agent had wanted the latter semantics, it would have applied the PUT
directly to the Content-Location URI.¶
8.8. VALIDATOR FIELDS
Resource metadata is referred to as a "validator" if it can be used within a
precondition (Section 13.1) to make a conditional request (Section 13).
Validator fields convey a current validator for the selected representation
(Section 3.2).¶
In responses to safe requests, validator fields describe the selected
representation chosen by the origin server while handling the response. Note
that, depending on the method and status code semantics, the selected
representation for a given response is not necessarily the same as the
representation enclosed as response content.¶
In a successful response to a state-changing request, validator fields describe
the new representation that has replaced the prior selected representation as a
result of processing the request.¶
For example, an ETag field in a 201 (Created) response communicates the entity
tag of the newly created resource's representation, so that the entity tag can
be used as a validator in later conditional requests to prevent the "lost
update" problem.¶
This specification defines two forms of metadata that are commonly used to
observe resource state and test for preconditions: modification dates (Section
8.8.2) and opaque entity tags (Section 8.8.3). Additional metadata that reflects
resource state has been defined by various extensions of HTTP, such as Web
Distributed Authoring and Versioning [WEBDAV], that are beyond the scope of this
specification.¶
8.8.1. WEAK VERSUS STRONG
Validators come in two flavors: strong or weak. Weak validators are easy to
generate but are far less useful for comparisons. Strong validators are ideal
for comparisons but can be very difficult (and occasionally impossible) to
generate efficiently. Rather than impose that all forms of resource adhere to
the same strength of validator, HTTP exposes the type of validator in use and
imposes restrictions on when weak validators can be used as preconditions.¶
A "strong validator" is representation metadata that changes value whenever a
change occurs to the representation data that would be observable in the content
of a 200 (OK) response to GET.¶
A strong validator might change for reasons other than a change to the
representation data, such as when a semantically significant part of the
representation metadata is changed (e.g., Content-Type), but it is in the best
interests of the origin server to only change the value when it is necessary to
invalidate the stored responses held by remote caches and authoring tools.¶
Cache entries might persist for arbitrarily long periods, regardless of
expiration times. Thus, a cache might attempt to validate an entry using a
validator that it obtained in the distant past. A strong validator is unique
across all versions of all representations associated with a particular resource
over time. However, there is no implication of uniqueness across representations
of different resources (i.e., the same strong validator might be in use for
representations of multiple resources at the same time and does not imply that
those representations are equivalent).¶
There are a variety of strong validators used in practice. The best are based on
strict revision control, wherein each change to a representation always results
in a unique node name and revision identifier being assigned before the
representation is made accessible to GET. A collision-resistant hash function
applied to the representation data is also sufficient if the data is available
prior to the response header fields being sent and the digest does not need to
be recalculated every time a validation request is received. However, if a
resource has distinct representations that differ only in their metadata, such
as might occur with content negotiation over media types that happen to share
the same data format, then the origin server needs to incorporate additional
information in the validator to distinguish those representations.¶
In contrast, a "weak validator" is representation metadata that might not change
for every change to the representation data. This weakness might be due to
limitations in how the value is calculated (e.g., clock resolution), an
inability to ensure uniqueness for all possible representations of the resource,
or a desire of the resource owner to group representations by some
self-determined set of equivalency rather than unique sequences of data.¶
An origin server SHOULD change a weak entity tag whenever it considers prior
representations to be unacceptable as a substitute for the current
representation. In other words, a weak entity tag ought to change whenever the
origin server wants caches to invalidate old responses.¶
For example, the representation of a weather report that changes in content
every second, based on dynamic measurements, might be grouped into sets of
equivalent representations (from the origin server's perspective) with the same
weak validator in order to allow cached representations to be valid for a
reasonable period of time (perhaps adjusted dynamically based on server load or
weather quality). Likewise, a representation's modification time, if defined
with only one-second resolution, might be a weak validator if it is possible for
the representation to be modified twice during a single second and retrieved
between those modifications.¶
Likewise, a validator is weak if it is shared by two or more representations of
a given resource at the same time, unless those representations have identical
representation data. For example, if the origin server sends the same validator
for a representation with a gzip content coding applied as it does for a
representation with no content coding, then that validator is weak. However, two
simultaneous representations might share the same strong validator if they
differ only in the representation metadata, such as when two different media
types are available for the same representation data.¶
Strong validators are usable for all conditional requests, including cache
validation, partial content ranges, and "lost update" avoidance. Weak validators
are only usable when the client does not require exact equality with previously
obtained representation data, such as when validating a cache entry or limiting
a web traversal to recent changes.¶
8.8.2. LAST-MODIFIED
The "Last-Modified" header field in a response provides a timestamp indicating
the date and time at which the origin server believes the selected
representation was last modified, as determined at the conclusion of handling
the request.¶
Last-Modified = HTTP-date
¶
An example of its use is¶
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
¶
8.8.2.1. GENERATION
An origin server SHOULD send Last-Modified for any selected representation for
which a last modification date can be reasonably and consistently determined,
since its use in conditional requests and evaluating cache freshness ([CACHING])
can substantially reduce unnecessary transfers and significantly improve service
availability and scalability.¶
A representation is typically the sum of many parts behind the resource
interface. The last-modified time would usually be the most recent time that any
of those parts were changed. How that value is determined for any given resource
is an implementation detail beyond the scope of this specification.¶
An origin server SHOULD obtain the Last-Modified value of the representation as
close as possible to the time that it generates the Date field value for its
response. This allows a recipient to make an accurate assessment of the
representation's modification time, especially if the representation changes
near the time that the response is generated.¶
An origin server with a clock (as defined in Section 5.6.7) MUST NOT generate a
Last-Modified date that is later than the server's time of message origination
(Date, Section 6.6.1). If the last modification time is derived from
implementation-specific metadata that evaluates to some time in the future,
according to the origin server's clock, then the origin server MUST replace that
value with the message origination date. This prevents a future modification
date from having an adverse impact on cache validation.¶
An origin server without a clock MUST NOT generate a Last-Modified date for a
response unless that date value was assigned to the resource by some other
system (presumably one with a clock).¶
8.8.2.2. COMPARISON
A Last-Modified time, when used as a validator in a request, is implicitly weak
unless it is possible to deduce that it is strong, using the following rules:¶
* The validator is being compared by an origin server to the actual current
validator for the representation and,¶
* That origin server reliably knows that the associated representation did not
change twice during the second covered by the presented validator;¶
or¶
* The validator is about to be used by a client in an If-Modified-Since,
If-Unmodified-Since, or If-Range header field, because the client has a cache
entry for the associated representation, and¶
* That cache entry includes a Date value which is at least one second after the
Last-Modified value and the client has reason to believe that they were
generated by the same clock or that there is enough difference between the
Last-Modified and Date values to make clock synchronization issues unlikely;¶
or¶
* The validator is being compared by an intermediate cache to the validator
stored in its cache entry for the representation, and¶
* That cache entry includes a Date value which is at least one second after the
Last-Modified value and the cache has reason to believe that they were
generated by the same clock or that there is enough difference between the
Last-Modified and Date values to make clock synchronization issues unlikely.¶
This method relies on the fact that if two different responses were sent by the
origin server during the same second, but both had the same Last-Modified time,
then at least one of those responses would have a Date value equal to its
Last-Modified time.¶
8.8.3. ETAG
The "ETag" field in a response provides the current entity tag for the selected
representation, as determined at the conclusion of handling the request. An
entity tag is an opaque validator for differentiating between multiple
representations of the same resource, regardless of whether those multiple
representations are due to resource state changes over time, content negotiation
resulting in multiple representations being valid at the same time, or both. An
entity tag consists of an opaque quoted string, possibly prefixed by a weakness
indicator.¶
ETag = entity-tag
entity-tag = [ weak ] opaque-tag
weak = %s"W/"
opaque-tag = DQUOTE *etagc DQUOTE
etagc = %x21 / %x23-7E / obs-text
; VCHAR except double quotes, plus obs-text
¶
Note: Previously, opaque-tag was defined to be a quoted-string ([RFC2616],
Section 3.11); thus, some recipients might perform backslash unescaping. Servers
therefore ought to avoid backslash characters in entity tags.¶
An entity tag can be more reliable for validation than a modification date in
situations where it is inconvenient to store modification dates, where the
one-second resolution of HTTP-date values is not sufficient, or where
modification dates are not consistently maintained.¶
Examples:¶
ETag: "xyzzy"
ETag: W/"xyzzy"
ETag: ""
¶
An entity tag can be either a weak or strong validator, with strong being the
default. If an origin server provides an entity tag for a representation and the
generation of that entity tag does not satisfy all of the characteristics of a
strong validator (Section 8.8.1), then the origin server MUST mark the entity
tag as weak by prefixing its opaque value with "W/" (case-sensitive).¶
A sender MAY send the ETag field in a trailer section (see Section 6.5).
However, since trailers are often ignored, it is preferable to send ETag as a
header field unless the entity tag is generated while sending the content.¶
8.8.3.1. GENERATION
The principle behind entity tags is that only the service author knows the
implementation of a resource well enough to select the most accurate and
efficient validation mechanism for that resource, and that any such mechanism
can be mapped to a simple sequence of octets for easy comparison. Since the
value is opaque, there is no need for the client to be aware of how each entity
tag is constructed.¶
For example, a resource that has implementation-specific versioning applied to
all changes might use an internal revision number, perhaps combined with a
variance identifier for content negotiation, to accurately differentiate between
representations. Other implementations might use a collision-resistant hash of
representation content, a combination of various file attributes, or a
modification timestamp that has sub-second resolution.¶
An origin server SHOULD send an ETag for any selected representation for which
detection of changes can be reasonably and consistently determined, since the
entity tag's use in conditional requests and evaluating cache freshness
([CACHING]) can substantially reduce unnecessary transfers and significantly
improve service availability, scalability, and reliability.¶
8.8.3.2. COMPARISON
There are two entity tag comparison functions, depending on whether or not the
comparison context allows the use of weak validators:¶
"Strong comparison": two entity tags are equivalent if both are not weak and
their opaque-tags match character-by-character.¶ "Weak comparison": two entity
tags are equivalent if their opaque-tags match character-by-character,
regardless of either or both being tagged as "weak".¶
The example below shows the results for a set of entity tag pairs and both the
weak and strong comparison function results:¶
Table 3 ETag 1 ETag 2 Strong Comparison Weak Comparison W/"1" W/"1" no match
match W/"1" W/"2" no match no match W/"1" "1" no match match "1" "1" match match
8.8.3.3. EXAMPLE: ENTITY TAGS VARYING ON CONTENT-NEGOTIATED RESOURCES
Consider a resource that is subject to content negotiation (Section 12), and
where the representations sent in response to a GET request vary based on the
Accept-Encoding request header field (Section 12.5.3):¶
>> Request:¶
GET /index HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip
¶
In this case, the response might or might not use the gzip content coding. If it
does not, the response might look like:¶
>> Response:¶
HTTP/1.1 200 OK
Date: Fri, 26 Mar 2010 00:05:00 GMT
ETag: "123-a"
Content-Length: 70
Vary: Accept-Encoding
Content-Type: text/plain
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
¶
An alternative representation that does use gzip content coding would be:¶
>> Response:¶
HTTP/1.1 200 OK
Date: Fri, 26 Mar 2010 00:05:00 GMT
ETag: "123-b"
Content-Length: 43
Vary: Accept-Encoding
Content-Type: text/plain
Content-Encoding: gzip
...binary data...
¶
Note: Content codings are a property of the representation data, so a strong
entity tag for a content-encoded representation has to be distinct from the
entity tag of an unencoded representation to prevent potential conflicts during
cache updates and range requests. In contrast, transfer codings (Section 7 of
[HTTP/1.1]) apply only during message transfer and do not result in distinct
entity tags.¶
9. METHODS
9.1. OVERVIEW
The request method token is the primary source of request semantics; it
indicates the purpose for which the client has made this request and what is
expected by the client as a successful result.¶
The request method's semantics might be further specialized by the semantics of
some header fields when present in a request if those additional semantics do
not conflict with the method. For example, a client can send conditional request
header fields (Section 13.1) to make the requested action conditional on the
current state of the target resource.¶
HTTP is designed to be usable as an interface to distributed object systems. The
request method invokes an action to be applied to a target resource in much the
same way that a remote method invocation can be sent to an identified object.¶
method = token
¶
The method token is case-sensitive because it might be used as a gateway to
object-based systems with case-sensitive method names. By convention,
standardized methods are defined in all-uppercase US-ASCII letters.¶
Unlike distributed objects, the standardized request methods in HTTP are not
resource-specific, since uniform interfaces provide for better visibility and
reuse in network-based systems [REST]. Once defined, a standardized method ought
to have the same semantics when applied to any resource, though each resource
determines for itself whether those semantics are implemented or allowed.¶
This specification defines a number of standardized methods that are commonly
used in HTTP, as outlined by the following table.¶
Table 4 Method Name Description Section GET Transfer a current representation of
the target resource. 9.3.1 HEAD Same as GET, but do not transfer the response
content. 9.3.2 POST Perform resource-specific processing on the request content.
9.3.3 PUT Replace all current representations of the target resource with the
request content. 9.3.4 DELETE Remove all current representations of the target
resource. 9.3.5 CONNECT Establish a tunnel to the server identified by the
target resource. 9.3.6 OPTIONS Describe the communication options for the target
resource. 9.3.7 TRACE Perform a message loop-back test along the path to the
target resource. 9.3.8
All general-purpose servers MUST support the methods GET and HEAD. All other
methods are OPTIONAL.¶
The set of methods allowed by a target resource can be listed in an Allow header
field (Section 10.2.1). However, the set of allowed methods can change
dynamically. An origin server that receives a request method that is
unrecognized or not implemented SHOULD respond with the 501 (Not Implemented)
status code. An origin server that receives a request method that is recognized
and implemented, but not allowed for the target resource, SHOULD respond with
the 405 (Method Not Allowed) status code.¶
Additional methods, outside the scope of this specification, have been specified
for use in HTTP. All such methods ought to be registered within the "Hypertext
Transfer Protocol (HTTP) Method Registry", as described in Section 16.1.¶
9.2. COMMON METHOD PROPERTIES
9.2.1. SAFE METHODS
Request methods are considered "safe" if their defined semantics are essentially
read-only; i.e., the client does not request, and does not expect, any state
change on the origin server as a result of applying a safe method to a target
resource. Likewise, reasonable use of a safe method is not expected to cause any
harm, loss of property, or unusual burden on the origin server.¶
This definition of safe methods does not prevent an implementation from
including behavior that is potentially harmful, that is not entirely read-only,
or that causes side effects while invoking a safe method. What is important,
however, is that the client did not request that additional behavior and cannot
be held accountable for it. For example, most servers append request information
to access log files at the completion of every response, regardless of the
method, and that is considered safe even though the log storage might become
full and cause the server to fail. Likewise, a safe request initiated by
selecting an advertisement on the Web will often have the side effect of
charging an advertising account.¶
Of the request methods defined by this specification, the GET, HEAD, OPTIONS,
and TRACE methods are defined to be safe.¶
The purpose of distinguishing between safe and unsafe methods is to allow
automated retrieval processes (spiders) and cache performance optimization
(pre-fetching) to work without fear of causing harm. In addition, it allows a
user agent to apply appropriate constraints on the automated use of unsafe
methods when processing potentially untrusted content.¶
A user agent SHOULD distinguish between safe and unsafe methods when presenting
potential actions to a user, such that the user can be made aware of an unsafe
action before it is requested.¶
When a resource is constructed such that parameters within the target URI have
the effect of selecting an action, it is the resource owner's responsibility to
ensure that the action is consistent with the request method semantics. For
example, it is common for Web-based content editing software to use actions
within query parameters, such as "page?do=delete". If the purpose of such a
resource is to perform an unsafe action, then the resource owner MUST disable or
disallow that action when it is accessed using a safe request method. Failure to
do so will result in unfortunate side effects when automated processes perform a
GET on every URI reference for the sake of link maintenance, pre-fetching,
building a search index, etc.¶
9.2.2. IDEMPOTENT METHODS
A request method is considered "idempotent" if the intended effect on the server
of multiple identical requests with that method is the same as the effect for a
single such request. Of the request methods defined by this specification, PUT,
DELETE, and safe request methods are idempotent.¶
Like the definition of safe, the idempotent property only applies to what has
been requested by the user; a server is free to log each request separately,
retain a revision control history, or implement other non-idempotent side
effects for each idempotent request.¶
Idempotent methods are distinguished because the request can be repeated
automatically if a communication failure occurs before the client is able to
read the server's response. For example, if a client sends a PUT request and the
underlying connection is closed before any response is received, then the client
can establish a new connection and retry the idempotent request. It knows that
repeating the request will have the same intended effect, even if the original
request succeeded, though the response might differ.¶
A client SHOULD NOT automatically retry a request with a non-idempotent method
unless it has some means to know that the request semantics are actually
idempotent, regardless of the method, or some means to detect that the original
request was never applied.¶
For example, a user agent can repeat a POST request automatically if it knows
(through design or configuration) that the request is safe for that resource.
Likewise, a user agent designed specifically to operate on a version control
repository might be able to recover from partial failure conditions by checking
the target resource revision(s) after a failed connection, reverting or fixing
any changes that were partially applied, and then automatically retrying the
requests that failed.¶
Some clients take a riskier approach and attempt to guess when an automatic
retry is possible. For example, a client might automatically retry a POST
request if the underlying transport connection closed before any part of a
response is received, particularly if an idle persistent connection was used.¶
A proxy MUST NOT automatically retry non-idempotent requests. A client SHOULD
NOT automatically retry a failed automatic retry.¶
9.2.3. METHODS AND CACHING
For a cache to store and use a response, the associated method needs to
explicitly allow caching and to detail under what conditions a response can be
used to satisfy subsequent requests; a method definition that does not do so
cannot be cached. For additional requirements see [CACHING].¶
This specification defines caching semantics for GET, HEAD, and POST, although
the overwhelming majority of cache implementations only support GET and HEAD.¶
9.3. METHOD DEFINITIONS
9.3.1. GET
The GET method requests transfer of a current selected representation for the
target resource. A successful response reflects the quality of "sameness"
identified by the target URI (Section 1.2.2 of [URI]). Hence, retrieving
identifiable information via HTTP is usually performed by making a GET request
on an identifier associated with the potential for providing that information in
a 200 (OK) response.¶
GET is the primary mechanism of information retrieval and the focus of almost
all performance optimizations. Applications that produce a URI for each
important resource can benefit from those optimizations while enabling their
reuse by other applications, creating a network effect that promotes further
expansion of the Web.¶
It is tempting to think of resource identifiers as remote file system pathnames
and of representations as being a copy of the contents of such files. In fact,
that is how many resources are implemented (see Section 17.3 for related
security considerations). However, there are no such limitations in practice.¶
The HTTP interface for a resource is just as likely to be implemented as a tree
of content objects, a programmatic view on various database records, or a
gateway to other information systems. Even when the URI mapping mechanism is
tied to a file system, an origin server might be configured to execute the files
with the request as input and send the output as the representation rather than
transfer the files directly. Regardless, only the origin server needs to know
how each resource identifier corresponds to an implementation and how that
implementation manages to select and send a current representation of the target
resource.¶
A client can alter the semantics of GET to be a "range request", requesting
transfer of only some part(s) of the selected representation, by sending a Range
header field in the request (Section 14.2).¶
Although request message framing is independent of the method used, content
received in a GET request has no generally defined semantics, cannot alter the
meaning or target of the request, and might lead some implementations to reject
the request and close the connection because of its potential as a request
smuggling attack (Section 11.2 of [HTTP/1.1]). A client SHOULD NOT generate
content in a GET request unless it is made directly to an origin server that has
previously indicated, in or out of band, that such a request has a purpose and
will be adequately supported. An origin server SHOULD NOT rely on private
agreements to receive content, since participants in HTTP communication are
often unaware of intermediaries along the request chain.¶
The response to a GET request is cacheable; a cache MAY use it to satisfy
subsequent GET and HEAD requests unless otherwise indicated by the Cache-Control
header field (Section 5.2 of [CACHING]).¶
When information retrieval is performed with a mechanism that constructs a
target URI from user-provided information, such as the query fields of a form
using GET, potentially sensitive data might be provided that would not be
appropriate for disclosure within a URI (see Section 17.9). In some cases, the
data can be filtered or transformed such that it would not reveal such
information. In others, particularly when there is no benefit from caching a
response, using the POST method (Section 9.3.3) instead of GET can transmit such
information in the request content rather than within the target URI.¶
9.3.2. HEAD
The HEAD method is identical to GET except that the server MUST NOT send content
in the response. HEAD is used to obtain metadata about the selected
representation without transferring its representation data, often for the sake
of testing hypertext links or finding recent modifications.¶
The server SHOULD send the same header fields in response to a HEAD request as
it would have sent if the request method had been GET. However, a server MAY
omit header fields for which a value is determined only while generating the
content. For example, some servers buffer a dynamic response to GET until a
minimum amount of data is generated so that they can more efficiently delimit
small responses or make late decisions with regard to content selection. Such a
response to GET might contain Content-Length and Vary fields, for example, that
are not generated within a HEAD response. These minor inconsistencies are
considered preferable to generating and discarding the content for a HEAD
request, since HEAD is usually requested for the sake of efficiency.¶
Although request message framing is independent of the method used, content
received in a HEAD request has no generally defined semantics, cannot alter the
meaning or target of the request, and might lead some implementations to reject
the request and close the connection because of its potential as a request
smuggling attack (Section 11.2 of [HTTP/1.1]). A client SHOULD NOT generate
content in a HEAD request unless it is made directly to an origin server that
has previously indicated, in or out of band, that such a request has a purpose
and will be adequately supported. An origin server SHOULD NOT rely on private
agreements to receive content, since participants in HTTP communication are
often unaware of intermediaries along the request chain.¶
The response to a HEAD request is cacheable; a cache MAY use it to satisfy
subsequent HEAD requests unless otherwise indicated by the Cache-Control header
field (Section 5.2 of [CACHING]). A HEAD response might also affect previously
cached responses to GET; see Section 4.3.5 of [CACHING].¶
9.3.3. POST
The POST method requests that the target resource process the representation
enclosed in the request according to the resource's own specific semantics. For
example, POST is used for the following functions (among others):¶
* Providing a block of data, such as the fields entered into an HTML form, to a
data-handling process;¶
* Posting a message to a bulletin board, newsgroup, mailing list, blog, or
similar group of articles;¶
* Creating a new resource that has yet to be identified by the origin server;
and¶
* Appending data to a resource's existing representation(s).¶
An origin server indicates response semantics by choosing an appropriate status
code depending on the result of processing the POST request; almost all of the
status codes defined by this specification could be received in a response to
POST (the exceptions being 206 (Partial Content), 304 (Not Modified), and 416
(Range Not Satisfiable)).¶
If one or more resources has been created on the origin server as a result of
successfully processing a POST request, the origin server SHOULD send a 201
(Created) response containing a Location header field that provides an
identifier for the primary resource created (Section 10.2.2) and a
representation that describes the status of the request while referring to the
new resource(s).¶
Responses to POST requests are only cacheable when they include explicit
freshness information (see Section 4.2.1 of [CACHING]) and a Content-Location
header field that has the same value as the POST's target URI (Section 8.7). A
cached POST response can be reused to satisfy a later GET or HEAD request. In
contrast, a POST request cannot be satisfied by a cached POST response because
POST is potentially unsafe; see Section 4 of [CACHING].¶
If the result of processing a POST would be equivalent to a representation of an
existing resource, an origin server MAY redirect the user agent to that resource
by sending a 303 (See Other) response with the existing resource's identifier in
the Location field. This has the benefits of providing the user agent a resource
identifier and transferring the representation via a method more amenable to
shared caching, though at the cost of an extra request if the user agent does
not already have the representation cached.¶
9.3.4. PUT
The PUT method requests that the state of the target resource be created or
replaced with the state defined by the representation enclosed in the request
message content. A successful PUT of a given representation would suggest that a
subsequent GET on that same target resource will result in an equivalent
representation being sent in a 200 (OK) response. However, there is no guarantee
that such a state change will be observable, since the target resource might be
acted upon by other user agents in parallel, or might be subject to dynamic
processing by the origin server, before any subsequent GET is received. A
successful response only implies that the user agent's intent was achieved at
the time of its processing by the origin server.¶
If the target resource does not have a current representation and the PUT
successfully creates one, then the origin server MUST inform the user agent by
sending a 201 (Created) response. If the target resource does have a current
representation and that representation is successfully modified in accordance
with the state of the enclosed representation, then the origin server MUST send
either a 200 (OK) or a 204 (No Content) response to indicate successful
completion of the request.¶
An origin server SHOULD verify that the PUT representation is consistent with
its configured constraints for the target resource. For example, if an origin
server determines a resource's representation metadata based on the URI, then
the origin server needs to ensure that the content received in a successful PUT
request is consistent with that metadata. When a PUT representation is
inconsistent with the target resource, the origin server SHOULD either make them
consistent, by transforming the representation or changing the resource
configuration, or respond with an appropriate error message containing
sufficient information to explain why the representation is unsuitable. The 409
(Conflict) or 415 (Unsupported Media Type) status codes are suggested, with the
latter being specific to constraints on Content-Type values.¶
For example, if the target resource is configured to always have a Content-Type
of "text/html" and the representation being PUT has a Content-Type of
"image/jpeg", the origin server ought to do one of:¶
a. reconfigure the target resource to reflect the new media type;¶
b. transform the PUT representation to a format consistent with that of the
resource before saving it as the new resource state; or,¶
c. reject the request with a 415 (Unsupported Media Type) response indicating
that the target resource is limited to "text/html", perhaps including a link
to a different resource that would be a suitable target for the new
representation.¶
HTTP does not define exactly how a PUT method affects the state of an origin
server beyond what can be expressed by the intent of the user agent request and
the semantics of the origin server response. It does not define what a resource
might be, in any sense of that word, beyond the interface provided via HTTP. It
does not define how resource state is "stored", nor how such storage might
change as a result of a change in resource state, nor how the origin server
translates resource state into representations. Generally speaking, all
implementation details behind the resource interface are intentionally hidden by
the server.¶
This extends to how header and trailer fields are stored; while common header
fields like Content-Type will typically be stored and returned upon subsequent
GET requests, header and trailer field handling is specific to the resource that
received the request. As a result, an origin server SHOULD ignore unrecognized
header and trailer fields received in a PUT request (i.e., not save them as part
of the resource state).¶
An origin server MUST NOT send a validator field (Section 8.8), such as an ETag
or Last-Modified field, in a successful response to PUT unless the request's
representation data was saved without any transformation applied to the content
(i.e., the resource's new representation data is identical to the content
received in the PUT request) and the validator field value reflects the new
representation. This requirement allows a user agent to know when the
representation it sent (and retains in memory) is the result of the PUT, and
thus it doesn't need to be retrieved again from the origin server. The new
validator(s) received in the response can be used for future conditional
requests in order to prevent accidental overwrites (Section 13.1).¶
The fundamental difference between the POST and PUT methods is highlighted by
the different intent for the enclosed representation. The target resource in a
POST request is intended to handle the enclosed representation according to the
resource's own semantics, whereas the enclosed representation in a PUT request
is defined as replacing the state of the target resource. Hence, the intent of
PUT is idempotent and visible to intermediaries, even though the exact effect is
only known by the origin server.¶
Proper interpretation of a PUT request presumes that the user agent knows which
target resource is desired. A service that selects a proper URI on behalf of the
client, after receiving a state-changing request, SHOULD be implemented using
the POST method rather than PUT. If the origin server will not make the
requested PUT state change to the target resource and instead wishes to have it
applied to a different resource, such as when the resource has been moved to a
different URI, then the origin server MUST send an appropriate 3xx (Redirection)
response; the user agent MAY then make its own decision regarding whether or not
to redirect the request.¶
A PUT request applied to the target resource can have side effects on other
resources. For example, an article might have a URI for identifying "the current
version" (a resource) that is separate from the URIs identifying each particular
version (different resources that at one point shared the same state as the
current version resource). A successful PUT request on "the current version" URI
might therefore create a new version resource in addition to changing the state
of the target resource, and might also cause links to be added between the
related resources.¶
Some origin servers support use of the Content-Range header field (Section 14.4)
as a request modifier to perform a partial PUT, as described in Section 14.5.¶
Responses to the PUT method are not cacheable. If a successful PUT request
passes through a cache that has one or more stored responses for the target URI,
those stored responses will be invalidated (see Section 4.4 of [CACHING]).¶
9.3.5. DELETE
The DELETE method requests that the origin server remove the association between
the target resource and its current functionality. In effect, this method is
similar to the "rm" command in UNIX: it expresses a deletion operation on the
URI mapping of the origin server rather than an expectation that the previously
associated information be deleted.¶
If the target resource has one or more current representations, they might or
might not be destroyed by the origin server, and the associated storage might or
might not be reclaimed, depending entirely on the nature of the resource and its
implementation by the origin server (which are beyond the scope of this
specification). Likewise, other implementation aspects of a resource might need
to be deactivated or archived as a result of a DELETE, such as database or
gateway connections. In general, it is assumed that the origin server will only
allow DELETE on resources for which it has a prescribed mechanism for
accomplishing the deletion.¶
Relatively few resources allow the DELETE method -- its primary use is for
remote authoring environments, where the user has some direction regarding its
effect. For example, a resource that was previously created using a PUT request,
or identified via the Location header field after a 201 (Created) response to a
POST request, might allow a corresponding DELETE request to undo those actions.
Similarly, custom user agent implementations that implement an authoring
function, such as revision control clients using HTTP for remote operations,
might use DELETE based on an assumption that the server's URI space has been
crafted to correspond to a version repository.¶
If a DELETE method is successfully applied, the origin server SHOULD send¶
* a 202 (Accepted) status code if the action will likely succeed but has not
yet been enacted,¶
* a 204 (No Content) status code if the action has been enacted and no further
information is to be supplied, or¶
* a 200 (OK) status code if the action has been enacted and the response
message includes a representation describing the status.¶
Although request message framing is independent of the method used, content
received in a DELETE request has no generally defined semantics, cannot alter
the meaning or target of the request, and might lead some implementations to
reject the request and close the connection because of its potential as a
request smuggling attack (Section 11.2 of [HTTP/1.1]). A client SHOULD NOT
generate content in a DELETE request unless it is made directly to an origin
server that has previously indicated, in or out of band, that such a request has
a purpose and will be adequately supported. An origin server SHOULD NOT rely on
private agreements to receive content, since participants in HTTP communication
are often unaware of intermediaries along the request chain.¶
Responses to the DELETE method are not cacheable. If a successful DELETE request
passes through a cache that has one or more stored responses for the target URI,
those stored responses will be invalidated (see Section 4.4 of [CACHING]).¶
9.3.6. CONNECT
The CONNECT method requests that the recipient establish a tunnel to the
destination origin server identified by the request target and, if successful,
thereafter restrict its behavior to blind forwarding of data, in both
directions, until the tunnel is closed. Tunnels are commonly used to create an
end-to-end virtual connection, through one or more proxies, which can then be
secured using TLS (Transport Layer Security, [TLS13]).¶
CONNECT uses a special form of request target, unique to this method, consisting
of only the host and port number of the tunnel destination, separated by a
colon. There is no default port; a client MUST send the port number even if the
CONNECT request is based on a URI reference that contains an authority component
with an elided port (Section 4.1). For example,¶
CONNECT server.example.com:80 HTTP/1.1
Host: server.example.com
¶
A server MUST reject a CONNECT request that targets an empty or invalid port
number, typically by responding with a 400 (Bad Request) status code.¶
Because CONNECT changes the request/response nature of an HTTP connection,
specific HTTP versions might have different ways of mapping its semantics into
the protocol's wire format.¶
CONNECT is intended for use in requests to a proxy. The recipient can establish
a tunnel either by directly connecting to the server identified by the request
target or, if configured to use another proxy, by forwarding the CONNECT request
to the next inbound proxy. An origin server MAY accept a CONNECT request, but
most origin servers do not implement CONNECT.¶
Any 2xx (Successful) response indicates that the sender (and all inbound
proxies) will switch to tunnel mode immediately after the response header
section; data received after that header section is from the server identified
by the request target. Any response other than a successful response indicates
that the tunnel has not yet been formed.¶
A tunnel is closed when a tunnel intermediary detects that either side has
closed its connection: the intermediary MUST attempt to send any outstanding
data that came from the closed side to the other side, close both connections,
and then discard any remaining data left undelivered.¶
Proxy authentication might be used to establish the authority to create a
tunnel. For example,¶
CONNECT server.example.com:443 HTTP/1.1
Host: server.example.com:443
Proxy-Authorization: basic aGVsbG86d29ybGQ=
¶
There are significant risks in establishing a tunnel to arbitrary servers,
particularly when the destination is a well-known or reserved TCP port that is
not intended for Web traffic. For example, a CONNECT to "example.com:25" would
suggest that the proxy connect to the reserved port for SMTP traffic; if
allowed, that could trick the proxy into relaying spam email. Proxies that
support CONNECT SHOULD restrict its use to a limited set of known ports or a
configurable list of safe request targets.¶
A server MUST NOT send any Transfer-Encoding or Content-Length header fields in
a 2xx (Successful) response to CONNECT. A client MUST ignore any Content-Length
or Transfer-Encoding header fields received in a successful response to
CONNECT.¶
A CONNECT request message does not have content. The interpretation of data sent
after the header section of the CONNECT request message is specific to the
version of HTTP in use.¶
Responses to the CONNECT method are not cacheable.¶
9.3.7. OPTIONS
The OPTIONS method requests information about the communication options
available for the target resource, at either the origin server or an intervening
intermediary. This method allows a client to determine the options and/or
requirements associated with a resource, or the capabilities of a server,
without implying a resource action.¶
An OPTIONS request with an asterisk ("*") as the request target (Section 7.1)
applies to the server in general rather than to a specific resource. Since a
server's communication options typically depend on the resource, the "*" request
is only useful as a "ping" or "no-op" type of method; it does nothing beyond
allowing the client to test the capabilities of the server. For example, this
can be used to test a proxy for HTTP/1.1 conformance (or lack thereof).¶
If the request target is not an asterisk, the OPTIONS request applies to the
options that are available when communicating with the target resource.¶
A server generating a successful response to OPTIONS SHOULD send any header that
might indicate optional features implemented by the server and applicable to the
target resource (e.g., Allow), including potential extensions not defined by
this specification. The response content, if any, might also describe the
communication options in a machine or human-readable representation. A standard
format for such a representation is not defined by this specification, but might
be defined by future extensions to HTTP.¶
A client MAY send a Max-Forwards header field in an OPTIONS request to target a
specific recipient in the request chain (see Section 7.6.2). A proxy MUST NOT
generate a Max-Forwards header field while forwarding a request unless that
request was received with a Max-Forwards field.¶
A client that generates an OPTIONS request containing content MUST send a valid
Content-Type header field describing the representation media type. Note that
this specification does not define any use for such content.¶
Responses to the OPTIONS method are not cacheable.¶
9.3.8. TRACE
The TRACE method requests a remote, application-level loop-back of the request
message. The final recipient of the request SHOULD reflect the message received,
excluding some fields described below, back to the client as the content of a
200 (OK) response. The "message/http" format (Section 10.1 of [HTTP/1.1]) is one
way to do so. The final recipient is either the origin server or the first
server to receive a Max-Forwards value of zero (0) in the request (Section
7.6.2).¶
A client MUST NOT generate fields in a TRACE request containing sensitive data
that might be disclosed by the response. For example, it would be foolish for a
user agent to send stored user credentials (Section 11) or cookies [COOKIE] in a
TRACE request. The final recipient of the request SHOULD exclude any request
fields that are likely to contain sensitive data when that recipient generates
the response content.¶
TRACE allows the client to see what is being received at the other end of the
request chain and use that data for testing or diagnostic information. The value
of the Via header field (Section 7.6.3) is of particular interest, since it acts
as a trace of the request chain. Use of the Max-Forwards header field allows the
client to limit the length of the request chain, which is useful for testing a
chain of proxies forwarding messages in an infinite loop.¶
A client MUST NOT send content in a TRACE request.¶
Responses to the TRACE method are not cacheable.¶
10. MESSAGE CONTEXT
10.1. REQUEST CONTEXT FIELDS
The request header fields below provide additional information about the request
context, including information about the user, user agent, and resource behind
the request.¶
10.1.1. EXPECT
The "Expect" header field in a request indicates a certain set of behaviors
(expectations) that need to be supported by the server in order to properly
handle this request.¶
Expect = #expectation
expectation = token [ "=" ( token / quoted-string ) parameters ]
¶
The Expect field value is case-insensitive.¶
The only expectation defined by this specification is "100-continue" (with no
defined parameters).¶
A server that receives an Expect field value containing a member other than
100-continue MAY respond with a 417 (Expectation Failed) status code to indicate
that the unexpected expectation cannot be met.¶
A "100-continue" expectation informs recipients that the client is about to send
(presumably large) content in this request and wishes to receive a 100
(Continue) interim response if the method, target URI, and header fields are not
sufficient to cause an immediate success, redirect, or error response. This
allows the client to wait for an indication that it is worthwhile to send the
content before actually doing so, which can improve efficiency when the data is
huge or when the client anticipates that an error is likely (e.g., when sending
a state-changing method, for the first time, without previously verified
authentication credentials).¶
For example, a request that begins with¶
PUT /somewhere/fun HTTP/1.1
Host: origin.example.com
Content-Type: video/h264
Content-Length: 1234567890987
Expect: 100-continue
¶
allows the origin server to immediately respond with an error message, such as
401 (Unauthorized) or 405 (Method Not Allowed), before the client starts filling
the pipes with an unnecessary data transfer.¶
Requirements for clients:¶
* A client MUST NOT generate a 100-continue expectation in a request that does
not include content.¶
* A client that will wait for a 100 (Continue) response before sending the
request content MUST send an Expect header field containing a 100-continue
expectation.¶
* A client that sends a 100-continue expectation is not required to wait for
any specific length of time; such a client MAY proceed to send the content
even if it has not yet received a response. Furthermore, since 100 (Continue)
responses cannot be sent through an HTTP/1.0 intermediary, such a client
SHOULD NOT wait for an indefinite period before sending the content.¶
* A client that receives a 417 (Expectation Failed) status code in response to
a request containing a 100-continue expectation SHOULD repeat that request
without a 100-continue expectation, since the 417 response merely indicates
that the response chain does not support expectations (e.g., it passes
through an HTTP/1.0 server).¶
Requirements for servers:¶
* A server that receives a 100-continue expectation in an HTTP/1.0 request MUST
ignore that expectation.¶
* A server MAY omit sending a 100 (Continue) response if it has already
received some or all of the content for the corresponding request, or if the
framing indicates that there is no content.¶
* A server that sends a 100 (Continue) response MUST ultimately send a final
status code, once it receives and processes the request content, unless the
connection is closed prematurely.¶
* A server that responds with a final status code before reading the entire
request content SHOULD indicate whether it intends to close the connection
(e.g., see Section 9.6 of [HTTP/1.1]) or continue reading the request
content.¶
Upon receiving an HTTP/1.1 (or later) request that has a method, target URI, and
complete header section that contains a 100-continue expectation and an
indication that request content will follow, an origin server MUST send either:¶
* an immediate response with a final status code, if that status can be
determined by examining just the method, target URI, and header fields, or¶
* an immediate 100 (Continue) response to encourage the client to send the
request content.¶
The origin server MUST NOT wait for the content before sending the 100
(Continue) response.¶
Upon receiving an HTTP/1.1 (or later) request that has a method, target URI, and
complete header section that contains a 100-continue expectation and indicates a
request content will follow, a proxy MUST either:¶
* send an immediate response with a final status code, if that status can be
determined by examining just the method, target URI, and header fields, or¶
* forward the request toward the origin server by sending a corresponding
request-line and header section to the next inbound server.¶
If the proxy believes (from configuration or past interaction) that the next
inbound server only supports HTTP/1.0, the proxy MAY generate an immediate 100
(Continue) response to encourage the client to begin sending the content.¶
10.1.2. FROM
The "From" header field contains an Internet email address for a human user who
controls the requesting user agent. The address ought to be machine-usable, as
defined by "mailbox" in Section 3.4 of [RFC5322]:¶
From = mailbox
mailbox = <mailbox, see [RFC5322], Section 3.4>
¶
An example is:¶
From: spider-admin@example.org
¶
The From header field is rarely sent by non-robotic user agents. A user agent
SHOULD NOT send a From header field without explicit configuration by the user,
since that might conflict with the user's privacy interests or their site's
security policy.¶
A robotic user agent SHOULD send a valid From header field so that the person
responsible for running the robot can be contacted if problems occur on servers,
such as if the robot is sending excessive, unwanted, or invalid requests.¶
A server SHOULD NOT use the From header field for access control or
authentication, since its value is expected to be visible to anyone receiving or
observing the request and is often recorded within logfiles and error reports
without any expectation of privacy.¶
10.1.3. REFERER
The "Referer" [sic] header field allows the user agent to specify a URI
reference for the resource from which the target URI was obtained (i.e., the
"referrer", though the field name is misspelled). A user agent MUST NOT include
the fragment and userinfo components of the URI reference [URI], if any, when
generating the Referer field value.¶
Referer = absolute-URI / partial-URI
¶
The field value is either an absolute-URI or a partial-URI. In the latter case
(Section 4), the referenced URI is relative to the target URI ([URI], Section
5).¶
The Referer header field allows servers to generate back-links to other
resources for simple analytics, logging, optimized caching, etc. It also allows
obsolete or mistyped links to be found for maintenance. Some servers use the
Referer header field as a means of denying links from other sites (so-called
"deep linking") or restricting cross-site request forgery (CSRF), but not all
requests contain it.¶
Example:¶
Referer: http://www.example.org/hypertext/Overview.html
¶
If the target URI was obtained from a source that does not have its own URI
(e.g., input from the user keyboard, or an entry within the user's
bookmarks/favorites), the user agent MUST either exclude the Referer header
field or send it with a value of "about:blank".¶
The Referer header field value need not convey the full URI of the referring
resource; a user agent MAY truncate parts other than the referring origin.¶
The Referer header field has the potential to reveal information about the
request context or browsing history of the user, which is a privacy concern if
the referring resource's identifier reveals personal information (such as an
account name) or a resource that is supposed to be confidential (such as behind
a firewall or internal to a secured service). Most general-purpose user agents
do not send the Referer header field when the referring resource is a local
"file" or "data" URI. A user agent SHOULD NOT send a Referer header field if the
referring resource was accessed with a secure protocol and the request target
has an origin differing from that of the referring resource, unless the
referring resource explicitly allows Referer to be sent. A user agent MUST NOT
send a Referer header field in an unsecured HTTP request if the referring
resource was accessed with a secure protocol. See Section 17.9 for additional
security considerations.¶
Some intermediaries have been known to indiscriminately remove Referer header
fields from outgoing requests. This has the unfortunate side effect of
interfering with protection against CSRF attacks, which can be far more harmful
to their users. Intermediaries and user agent extensions that wish to limit
information disclosure in Referer ought to restrict their changes to specific
edits, such as replacing internal domain names with pseudonyms or truncating the
query and/or path components. An intermediary SHOULD NOT modify or delete the
Referer header field when the field value shares the same scheme and host as the
target URI.¶
10.1.4. TE
The "TE" header field describes capabilities of the client with regard to
transfer codings and trailer sections.¶
As described in Section 6.5, a TE field with a "trailers" member sent in a
request indicates that the client will not discard trailer fields.¶
TE is also used within HTTP/1.1 to advise servers about which transfer codings
the client is able to accept in a response. As of publication, only HTTP/1.1
uses transfer codings (see Section 7 of [HTTP/1.1]).¶
The TE field value is a list of members, with each member (aside from
"trailers") consisting of a transfer coding name token with an optional weight
indicating the client's relative preference for that transfer coding (Section
12.4.2) and optional parameters for that transfer coding.¶
TE = #t-codings
t-codings = "trailers" / ( transfer-coding [ weight ] )
transfer-coding = token *( OWS ";" OWS transfer-parameter )
transfer-parameter = token BWS "=" BWS ( token / quoted-string )
¶
A sender of TE MUST also send a "TE" connection option within the Connection
header field (Section 7.6.1) to inform intermediaries not to forward this
field.¶
10.1.5. USER-AGENT
The "User-Agent" header field contains information about the user agent
originating the request, which is often used by servers to help identify the
scope of reported interoperability problems, to work around or tailor responses
to avoid particular user agent limitations, and for analytics regarding browser
or operating system use. A user agent SHOULD send a User-Agent header field in
each request unless specifically configured not to do so.¶
User-Agent = product *( RWS ( product / comment ) )
¶
The User-Agent field value consists of one or more product identifiers, each
followed by zero or more comments (Section 5.6.5), which together identify the
user agent software and its significant subproducts. By convention, the product
identifiers are listed in decreasing order of their significance for identifying
the user agent software. Each product identifier consists of a name and optional
version.¶
product = token ["/" product-version]
product-version = token
¶
A sender SHOULD limit generated product identifiers to what is necessary to
identify the product; a sender MUST NOT generate advertising or other
nonessential information within the product identifier. A sender SHOULD NOT
generate information in product-version that is not a version identifier (i.e.,
successive versions of the same product name ought to differ only in the
product-version portion of the product identifier).¶
Example:¶
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
¶
A user agent SHOULD NOT generate a User-Agent header field containing needlessly
fine-grained detail and SHOULD limit the addition of subproducts by third
parties. Overly long and detailed User-Agent field values increase request
latency and the risk of a user being identified against their wishes
("fingerprinting").¶
Likewise, implementations are encouraged not to use the product tokens of other
implementations in order to declare compatibility with them, as this circumvents
the purpose of the field. If a user agent masquerades as a different user agent,
recipients can assume that the user intentionally desires to see responses
tailored for that identified user agent, even if they might not work as well for
the actual user agent being used.¶
10.2. RESPONSE CONTEXT FIELDS
The response header fields below provide additional information about the
response, beyond what is implied by the status code, including information about
the server, about the target resource, or about related resources.¶
10.2.1. ALLOW
The "Allow" header field lists the set of methods advertised as supported by the
target resource. The purpose of this field is strictly to inform the recipient
of valid request methods associated with the resource.¶
Allow = #method
¶
Example of use:¶
Allow: GET, HEAD, PUT
¶
The actual set of allowed methods is defined by the origin server at the time of
each request. An origin server MUST generate an Allow header field in a 405
(Method Not Allowed) response and MAY do so in any other response. An empty
Allow field value indicates that the resource allows no methods, which might
occur in a 405 response if the resource has been temporarily disabled by
configuration.¶
A proxy MUST NOT modify the Allow header field -- it does not need to understand
all of the indicated methods in order to handle them according to the generic
message handling rules.¶
10.2.2. LOCATION
The "Location" header field is used in some responses to refer to a specific
resource in relation to the response. The type of relationship is defined by the
combination of request method and status code semantics.¶
Location = URI-reference
¶
The field value consists of a single URI-reference. When it has the form of a
relative reference ([URI], Section 4.2), the final value is computed by
resolving it against the target URI ([URI], Section 5).¶
For 201 (Created) responses, the Location value refers to the primary resource
created by the request. For 3xx (Redirection) responses, the Location value
refers to the preferred target resource for automatically redirecting the
request.¶
If the Location value provided in a 3xx (Redirection) response does not have a
fragment component, a user agent MUST process the redirection as if the value
inherits the fragment component of the URI reference used to generate the target
URI (i.e., the redirection inherits the original reference's fragment, if any).¶
For example, a GET request generated for the URI reference
"http://www.example.org/~tim" might result in a 303 (See Other) response
containing the header field:¶
Location: /People.html#tim
¶
which suggests that the user agent redirect to
"http://www.example.org/People.html#tim"¶
Likewise, a GET request generated for the URI reference
"http://www.example.org/index.html#larry" might result in a 301 (Moved
Permanently) response containing the header field:¶
Location: http://www.example.net/index.html
¶
which suggests that the user agent redirect to
"http://www.example.net/index.html#larry", preserving the original fragment
identifier.¶
There are circumstances in which a fragment identifier in a Location value would
not be appropriate. For example, the Location header field in a 201 (Created)
response is supposed to provide a URI that is specific to the created resource.¶
Note: Some recipients attempt to recover from Location header fields that are
not valid URI references. This specification does not mandate or define such
processing, but does allow it for the sake of robustness. A Location field value
cannot allow a list of members because the comma list separator is a valid data
character within a URI-reference. If an invalid message is sent with multiple
Location field lines, a recipient along the path might combine those field lines
into one value. Recovery of a valid Location field value from that situation is
difficult and not interoperable across implementations.¶
Note: The Content-Location header field (Section 8.7) differs from Location in
that the Content-Location refers to the most specific resource corresponding to
the enclosed representation. It is therefore possible for a response to contain
both the Location and Content-Location header fields.¶
10.2.3. RETRY-AFTER
Servers send the "Retry-After" header field to indicate how long the user agent
ought to wait before making a follow-up request. When sent with a 503 (Service
Unavailable) response, Retry-After indicates how long the service is expected to
be unavailable to the client. When sent with any 3xx (Redirection) response,
Retry-After indicates the minimum time that the user agent is asked to wait
before issuing the redirected request.¶
The Retry-After field value can be either an HTTP-date or a number of seconds to
delay after receiving the response.¶
Retry-After = HTTP-date / delay-seconds
¶
A delay-seconds value is a non-negative decimal integer, representing time in
seconds.¶
delay-seconds = 1*DIGIT
¶
Two examples of its use are¶
Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
Retry-After: 120
¶
In the latter example, the delay is 2 minutes.¶
10.2.4. SERVER
The "Server" header field contains information about the software used by the
origin server to handle the request, which is often used by clients to help
identify the scope of reported interoperability problems, to work around or
tailor requests to avoid particular server limitations, and for analytics
regarding server or operating system use. An origin server MAY generate a Server
header field in its responses.¶
Server = product *( RWS ( product / comment ) )
¶
The Server header field value consists of one or more product identifiers, each
followed by zero or more comments (Section 5.6.5), which together identify the
origin server software and its significant subproducts. By convention, the
product identifiers are listed in decreasing order of their significance for
identifying the origin server software. Each product identifier consists of a
name and optional version, as defined in Section 10.1.5.¶
Example:¶
Server: CERN/3.0 libwww/2.17
¶
An origin server SHOULD NOT generate a Server header field containing needlessly
fine-grained detail and SHOULD limit the addition of subproducts by third
parties. Overly long and detailed Server field values increase response latency
and potentially reveal internal implementation details that might make it
(slightly) easier for attackers to find and exploit known security holes.¶
11. HTTP AUTHENTICATION
11.1. AUTHENTICATION SCHEME
HTTP provides a general framework for access control and authentication, via an
extensible set of challenge-response authentication schemes, which can be used
by a server to challenge a client request and by a client to provide
authentication information. It uses a case-insensitive token to identify the
authentication scheme:¶
auth-scheme = token
¶
Aside from the general framework, this document does not specify any
authentication schemes. New and existing authentication schemes are specified
independently and ought to be registered within the "Hypertext Transfer Protocol
(HTTP) Authentication Scheme Registry". For example, the "basic" and "digest"
authentication schemes are defined by [RFC7617] and [RFC7616], respectively.¶
11.2. AUTHENTICATION PARAMETERS
The authentication scheme is followed by additional information necessary for
achieving authentication via that scheme as either a comma-separated list of
parameters or a single sequence of characters capable of holding base64-encoded
information.¶
token68 = 1*( ALPHA / DIGIT /
"-" / "." / "_" / "~" / "+" / "/" ) *"="
¶
The token68 syntax allows the 66 unreserved URI characters ([URI]), plus a few
others, so that it can hold a base64, base64url (URL and filename safe
alphabet), base32, or base16 (hex) encoding, with or without padding, but
excluding whitespace ([RFC4648]).¶
Authentication parameters are name/value pairs, where the name token is matched
case-insensitively and each parameter name MUST only occur once per challenge.¶
auth-param = token BWS "=" BWS ( token / quoted-string )
¶
Parameter values can be expressed either as "token" or as "quoted-string"
(Section 5.6). Authentication scheme definitions need to accept both notations,
both for senders and recipients, to allow recipients to use generic parsing
components regardless of the authentication scheme.¶
For backwards compatibility, authentication scheme definitions can restrict the
format for senders to one of the two variants. This can be important when it is
known that deployed implementations will fail when encountering one of the two
formats.¶
11.3. CHALLENGE AND RESPONSE
A 401 (Unauthorized) response message is used by an origin server to challenge
the authorization of a user agent, including a WWW-Authenticate header field
containing at least one challenge applicable to the requested resource.¶
A 407 (Proxy Authentication Required) response message is used by a proxy to
challenge the authorization of a client, including a Proxy-Authenticate header
field containing at least one challenge applicable to the proxy for the
requested resource.¶
challenge = auth-scheme [ 1*SP ( token68 / #auth-param ) ]
¶
Note: Many clients fail to parse a challenge that contains an unknown scheme. A
workaround for this problem is to list well-supported schemes (such as "basic")
first.¶
A user agent that wishes to authenticate itself with an origin server --
usually, but not necessarily, after receiving a 401 (Unauthorized) -- can do so
by including an Authorization header field with the request.¶
A client that wishes to authenticate itself with a proxy -- usually, but not
necessarily, after receiving a 407 (Proxy Authentication Required) -- can do so
by including a Proxy-Authorization header field with the request.¶
11.4. CREDENTIALS
Both the Authorization field value and the Proxy-Authorization field value
contain the client's credentials for the realm of the resource being requested,
based upon a challenge received in a response (possibly at some point in the
past). When creating their values, the user agent ought to do so by selecting
the challenge with what it considers to be the most secure auth-scheme that it
understands, obtaining credentials from the user as appropriate. Transmission of
credentials within header field values implies significant security
considerations regarding the confidentiality of the underlying connection, as
described in Section 17.16.1.¶
credentials = auth-scheme [ 1*SP ( token68 / #auth-param ) ]
¶
Upon receipt of a request for a protected resource that omits credentials,
contains invalid credentials (e.g., a bad password) or partial credentials
(e.g., when the authentication scheme requires more than one round trip), an
origin server SHOULD send a 401 (Unauthorized) response that contains a
WWW-Authenticate header field with at least one (possibly new) challenge
applicable to the requested resource.¶
Likewise, upon receipt of a request that omits proxy credentials or contains
invalid or partial proxy credentials, a proxy that requires authentication
SHOULD generate a 407 (Proxy Authentication Required) response that contains a
Proxy-Authenticate header field with at least one (possibly new) challenge
applicable to the proxy.¶
A server that receives valid credentials that are not adequate to gain access
ought to respond with the 403 (Forbidden) status code (Section 15.5.4).¶
HTTP does not restrict applications to this simple challenge-response framework
for access authentication. Additional mechanisms can be used, such as
authentication at the transport level or via message encapsulation, and with
additional header fields specifying authentication information. However, such
additional mechanisms are not defined by this specification.¶
Note that various custom mechanisms for user authentication use the Set-Cookie
and Cookie header fields, defined in [COOKIE], for passing tokens related to
authentication.¶
11.5. ESTABLISHING A PROTECTION SPACE (REALM)
The "realm" authentication parameter is reserved for use by authentication
schemes that wish to indicate a scope of protection.¶
A "protection space" is defined by the origin (see Section 4.3.1) of the server
being accessed, in combination with the realm value if present. These realms
allow the protected resources on a server to be partitioned into a set of
protection spaces, each with its own authentication scheme and/or authorization
database. The realm value is a string, generally assigned by the origin server,
that can have additional semantics specific to the authentication scheme. Note
that a response can have multiple challenges with the same auth-scheme but with
different realms.¶
The protection space determines the domain over which credentials can be
automatically applied. If a prior request has been authorized, the user agent
MAY reuse the same credentials for all other requests within that protection
space for a period of time determined by the authentication scheme, parameters,
and/or user preferences (such as a configurable inactivity timeout).¶
The extent of a protection space, and therefore the requests to which
credentials might be automatically applied, is not necessarily known to clients
without additional information. An authentication scheme might define parameters
that describe the extent of a protection space. Unless specifically allowed by
the authentication scheme, a single protection space cannot extend outside the
scope of its server.¶
For historical reasons, a sender MUST only generate the quoted-string syntax.
Recipients might have to support both token and quoted-string syntax for maximum
interoperability with existing clients that have been accepting both notations
for a long time.¶
11.6. AUTHENTICATING USERS TO ORIGIN SERVERS
11.6.1. WWW-AUTHENTICATE
The "WWW-Authenticate" response header field indicates the authentication
scheme(s) and parameters applicable to the target resource.¶
WWW-Authenticate = #challenge
¶
A server generating a 401 (Unauthorized) response MUST send a WWW-Authenticate
header field containing at least one challenge. A server MAY generate a
WWW-Authenticate header field in other response messages to indicate that
supplying credentials (or different credentials) might affect the response.¶
A proxy forwarding a response MUST NOT modify any WWW-Authenticate header fields
in that response.¶
User agents are advised to take special care in parsing the field value, as it
might contain more than one challenge, and each challenge can contain a
comma-separated list of authentication parameters. Furthermore, the header field
itself can occur multiple times.¶
For instance:¶
WWW-Authenticate: Basic realm="simple", Newauth realm="apps",
type=1, title="Login to \"apps\""
¶
This header field contains two challenges, one for the "Basic" scheme with a
realm value of "simple" and another for the "Newauth" scheme with a realm value
of "apps". It also contains two additional parameters, "type" and "title".¶
Some user agents do not recognize this form, however. As a result, sending a
WWW-Authenticate field value with more than one member on the same field line
might not be interoperable.¶
Note: The challenge grammar production uses the list syntax as well. Therefore,
a sequence of comma, whitespace, and comma can be considered either as applying
to the preceding challenge, or to be an empty entry in the list of challenges.
In practice, this ambiguity does not affect the semantics of the header field
value and thus is harmless.¶
11.6.2. AUTHORIZATION
The "Authorization" header field allows a user agent to authenticate itself with
an origin server -- usually, but not necessarily, after receiving a 401
(Unauthorized) response. Its value consists of credentials containing the
authentication information of the user agent for the realm of the resource being
requested.¶
Authorization = credentials
¶
If a request is authenticated and a realm specified, the same credentials are
presumed to be valid for all other requests within this realm (assuming that the
authentication scheme itself does not require otherwise, such as credentials
that vary according to a challenge value or using synchronized clocks).¶
A proxy forwarding a request MUST NOT modify any Authorization header fields in
that request. See Section 3.5 of [CACHING] for details of and requirements
pertaining to handling of the Authorization header field by HTTP caches.¶
11.6.3. AUTHENTICATION-INFO
HTTP authentication schemes can use the "Authentication-Info" response field to
communicate information after the client's authentication credentials have been
accepted. This information can include a finalization message from the server
(e.g., it can contain the server authentication).¶
The field value is a list of parameters (name/value pairs), using the
"auth-param" syntax defined in Section 11.3. This specification only describes
the generic format; authentication schemes using Authentication-Info will define
the individual parameters. The "Digest" Authentication Scheme, for instance,
defines multiple parameters in Section 3.5 of [RFC7616].¶
Authentication-Info = #auth-param
¶
The Authentication-Info field can be used in any HTTP response, independently of
request method and status code. Its semantics are defined by the authentication
scheme indicated by the Authorization header field (Section 11.6.2) of the
corresponding request.¶
A proxy forwarding a response is not allowed to modify the field value in any
way.¶
Authentication-Info can be sent as a trailer field (Section 6.5) when the
authentication scheme explicitly allows this.¶
11.7. AUTHENTICATING CLIENTS TO PROXIES
11.7.1. PROXY-AUTHENTICATE
The "Proxy-Authenticate" header field consists of at least one challenge that
indicates the authentication scheme(s) and parameters applicable to the proxy
for this request. A proxy MUST send at least one Proxy-Authenticate header field
in each 407 (Proxy Authentication Required) response that it generates.¶
Proxy-Authenticate = #challenge
¶
Unlike WWW-Authenticate, the Proxy-Authenticate header field applies only to the
next outbound client on the response chain. This is because only the client that
chose a given proxy is likely to have the credentials necessary for
authentication. However, when multiple proxies are used within the same
administrative domain, such as office and regional caching proxies within a
large corporate network, it is common for credentials to be generated by the
user agent and passed through the hierarchy until consumed. Hence, in such a
configuration, it will appear as if Proxy-Authenticate is being forwarded
because each proxy will send the same challenge set.¶
Note that the parsing considerations for WWW-Authenticate apply to this header
field as well; see Section 11.6.1 for details.¶
11.7.2. PROXY-AUTHORIZATION
The "Proxy-Authorization" header field allows the client to identify itself (or
its user) to a proxy that requires authentication. Its value consists of
credentials containing the authentication information of the client for the
proxy and/or realm of the resource being requested.¶
Proxy-Authorization = credentials
¶
Unlike Authorization, the Proxy-Authorization header field applies only to the
next inbound proxy that demanded authentication using the Proxy-Authenticate
header field. When multiple proxies are used in a chain, the Proxy-Authorization
header field is consumed by the first inbound proxy that was expecting to
receive credentials. A proxy MAY relay the credentials from the client request
to the next proxy if that is the mechanism by which the proxies cooperatively
authenticate a given request.¶
11.7.3. PROXY-AUTHENTICATION-INFO
The "Proxy-Authentication-Info" response header field is equivalent to
Authentication-Info, except that it applies to proxy authentication (Section
11.3) and its semantics are defined by the authentication scheme indicated by
the Proxy-Authorization header field (Section 11.7.2) of the corresponding
request:¶
Proxy-Authentication-Info = #auth-param
¶
However, unlike Authentication-Info, the Proxy-Authentication-Info header field
applies only to the next outbound client on the response chain. This is because
only the client that chose a given proxy is likely to have the credentials
necessary for authentication. However, when multiple proxies are used within the
same administrative domain, such as office and regional caching proxies within a
large corporate network, it is common for credentials to be generated by the
user agent and passed through the hierarchy until consumed. Hence, in such a
configuration, it will appear as if Proxy-Authentication-Info is being forwarded
because each proxy will send the same field value.¶
Proxy-Authentication-Info can be sent as a trailer field (Section 6.5) when the
authentication scheme explicitly allows this.¶
12. CONTENT NEGOTIATION
When responses convey content, whether indicating a success or an error, the
origin server often has different ways of representing that information; for
example, in different formats, languages, or encodings. Likewise, different
users or user agents might have differing capabilities, characteristics, or
preferences that could influence which representation, among those available,
would be best to deliver. For this reason, HTTP provides mechanisms for content
negotiation.¶
This specification defines three patterns of content negotiation that can be
made visible within the protocol: "proactive" negotiation, where the server
selects the representation based upon the user agent's stated preferences;
"reactive" negotiation, where the server provides a list of representations for
the user agent to choose from; and "request content" negotiation, where the user
agent selects the representation for a future request based upon the server's
stated preferences in past responses.¶
Other patterns of content negotiation include "conditional content", where the
representation consists of multiple parts that are selectively rendered based on
user agent parameters, "active content", where the representation contains a
script that makes additional (more specific) requests based on the user agent
characteristics, and "Transparent Content Negotiation" ([RFC2295]), where
content selection is performed by an intermediary. These patterns are not
mutually exclusive, and each has trade-offs in applicability and practicality.¶
Note that, in all cases, HTTP is not aware of the resource semantics. The
consistency with which an origin server responds to requests, over time and over
the varying dimensions of content negotiation, and thus the "sameness" of a
resource's observed representations over time, is determined entirely by
whatever entity or algorithm selects or generates those responses.¶
12.1. PROACTIVE NEGOTIATION
When content negotiation preferences are sent by the user agent in a request to
encourage an algorithm located at the server to select the preferred
representation, it is called "proactive negotiation" (a.k.a., "server-driven
negotiation"). Selection is based on the available representations for a
response (the dimensions over which it might vary, such as language, content
coding, etc.) compared to various information supplied in the request, including
both the explicit negotiation header fields below and implicit characteristics,
such as the client's network address or parts of the User-Agent field.¶
Proactive negotiation is advantageous when the algorithm for selecting from
among the available representations is difficult to describe to a user agent, or
when the server desires to send its "best guess" to the user agent along with
the first response (when that "best guess" is good enough for the user, this
avoids the round-trip delay of a subsequent request). In order to improve the
server's guess, a user agent MAY send request header fields that describe its
preferences.¶
Proactive negotiation has serious disadvantages:¶
* It is impossible for the server to accurately determine what might be "best"
for any given user, since that would require complete knowledge of both the
capabilities of the user agent and the intended use for the response (e.g.,
does the user want to view it on screen or print it on paper?);¶
* Having the user agent describe its capabilities in every request can be both
very inefficient (given that only a small percentage of responses have
multiple representations) and a potential risk to the user's privacy;¶
* It complicates the implementation of an origin server and the algorithms for
generating responses to a request; and,¶
* It limits the reusability of responses for shared caching.¶
A user agent cannot rely on proactive negotiation preferences being consistently
honored, since the origin server might not implement proactive negotiation for
the requested resource or might decide that sending a response that doesn't
conform to the user agent's preferences is better than sending a 406 (Not
Acceptable) response.¶
A Vary header field (Section 12.5.5) is often sent in a response subject to
proactive negotiation to indicate what parts of the request information were
used in the selection algorithm.¶
The request header fields Accept, Accept-Charset, Accept-Encoding, and
Accept-Language are defined below for a user agent to engage in proactive
negotiation of the response content. The preferences sent in these fields apply
to any content in the response, including representations of the target
resource, representations of error or processing status, and potentially even
the miscellaneous text strings that might appear within the protocol.¶
12.2. REACTIVE NEGOTIATION
With "reactive negotiation" (a.k.a., "agent-driven negotiation"), selection of
content (regardless of the status code) is performed by the user agent after
receiving an initial response. The mechanism for reactive negotiation might be
as simple as a list of references to alternative representations.¶
If the user agent is not satisfied by the initial response content, it can
perform a GET request on one or more of the alternative resources to obtain a
different representation. Selection of such alternatives might be performed
automatically (by the user agent) or manually (e.g., by the user selecting from
a hypertext menu).¶
A server might choose not to send an initial representation, other than the list
of alternatives, and thereby indicate that reactive negotiation by the user
agent is preferred. For example, the alternatives listed in responses with the
300 (Multiple Choices) and 406 (Not Acceptable) status codes include information
about available representations so that the user or user agent can react by
making a selection.¶
Reactive negotiation is advantageous when the response would vary over commonly
used dimensions (such as type, language, or encoding), when the origin server is
unable to determine a user agent's capabilities from examining the request, and
generally when public caches are used to distribute server load and reduce
network usage.¶
Reactive negotiation suffers from the disadvantages of transmitting a list of
alternatives to the user agent, which degrades user-perceived latency if
transmitted in the header section, and needing a second request to obtain an
alternate representation. Furthermore, this specification does not define a
mechanism for supporting automatic selection, though it does not prevent such a
mechanism from being developed.¶
12.3. REQUEST CONTENT NEGOTIATION
When content negotiation preferences are sent in a server's response, the listed
preferences are called "request content negotiation" because they intend to
influence selection of an appropriate content for subsequent requests to that
resource. For example, the Accept (Section 12.5.1) and Accept-Encoding (Section
12.5.3) header fields can be sent in a response to indicate preferred media
types and content codings for subsequent requests to that resource.¶
Similarly, Section 3.1 of [RFC5789] defines the "Accept-Patch" response header
field, which allows discovery of which content types are accepted in PATCH
requests.¶
12.4. CONTENT NEGOTIATION FIELD FEATURES
12.4.1. ABSENCE
For each of the content negotiation fields, a request that does not contain the
field implies that the sender has no preference on that dimension of
negotiation.¶
If a content negotiation header field is present in a request and none of the
available representations for the response can be considered acceptable
according to it, the origin server can either honor the header field by sending
a 406 (Not Acceptable) response or disregard the header field by treating the
response as if it is not subject to content negotiation for that request header
field. This does not imply, however, that the client will be able to use the
representation.¶
Note: A user agent sending these header fields makes it easier for a server to
identify an individual by virtue of the user agent's request characteristics
(Section 17.13).¶
12.4.2. QUALITY VALUES
The content negotiation fields defined by this specification use a common
parameter, named "q" (case-insensitive), to assign a relative "weight" to the
preference for that associated kind of content. This weight is referred to as a
"quality value" (or "qvalue") because the same parameter name is often used
within server configurations to assign a weight to the relative quality of the
various representations that can be selected for a resource.¶
The weight is normalized to a real number in the range 0 through 1, where 0.001
is the least preferred and 1 is the most preferred; a value of 0 means "not
acceptable". If no "q" parameter is present, the default weight is 1.¶
weight = OWS ";" OWS "q=" qvalue
qvalue = ( "0" [ "." 0*3DIGIT ] )
/ ( "1" [ "." 0*3("0") ] )
¶
A sender of qvalue MUST NOT generate more than three digits after the decimal
point. User configuration of these values ought to be limited in the same
fashion.¶
12.4.3. WILDCARD VALUES
Most of these header fields, where indicated, define a wildcard value ("*") to
select unspecified values. If no wildcard is present, values that are not
explicitly mentioned in the field are considered unacceptable. Within Vary, the
wildcard value means that the variance is unlimited.¶
Note: In practice, using wildcards in content negotiation has limited practical
value because it is seldom useful to say, for example, "I prefer image/* more or
less than (some other specific value)". By sending Accept: */*;q=0, clients can
explicitly request a 406 (Not Acceptable) response if a more preferred format is
not available, but they still need to be able to handle a different response
since the server is allowed to ignore their preference.¶
12.5. CONTENT NEGOTIATION FIELDS
12.5.1. ACCEPT
The "Accept" header field can be used by user agents to specify their
preferences regarding response media types. For example, Accept header fields
can be used to indicate that the request is specifically limited to a small set
of desired types, as in the case of a request for an in-line image.¶
When sent by a server in a response, Accept provides information about which
content types are preferred in the content of a subsequent request to the same
resource.¶
Accept = #( media-range [ weight ] )
media-range = ( "*/*"
/ ( type "/" "*" )
/ ( type "/" subtype )
) parameters
¶
The asterisk "*" character is used to group media types into ranges, with "*/*"
indicating all media types and "type/*" indicating all subtypes of that type.
The media-range can include media type parameters that are applicable to that
range.¶
Each media-range might be followed by optional applicable media type parameters
(e.g., charset), followed by an optional "q" parameter for indicating a relative
weight (Section 12.4.2).¶
Previous specifications allowed additional extension parameters to appear after
the weight parameter. The accept extension grammar (accept-params, accept-ext)
has been removed because it had a complicated definition, was not being used in
practice, and is more easily deployed through new header fields. Senders using
weights SHOULD send "q" last (after all media-range parameters). Recipients
SHOULD process any parameter named "q" as weight, regardless of parameter
ordering.¶
Note: Use of the "q" parameter name to control content negotiation would
interfere with any media type parameter having the same name. Hence, the media
type registry disallows parameters named "q".¶
The example¶
Accept: audio/*; q=0.2, audio/basic
¶
is interpreted as "I prefer audio/basic, but send me any audio type if it is the
best available after an 80% markdown in quality".¶
A more elaborate example is¶
Accept: text/plain; q=0.5, text/html,
text/x-dvi; q=0.8, text/x-c
¶
Verbally, this would be interpreted as "text/html and text/x-c are the equally
preferred media types, but if they do not exist, then send the text/x-dvi
representation, and if that does not exist, send the text/plain
representation".¶
Media ranges can be overridden by more specific media ranges or specific media
types. If more than one media range applies to a given type, the most specific
reference has precedence. For example,¶
Accept: text/*, text/plain, text/plain;format=flowed, */*
¶
have the following precedence:¶
1. text/plain;format=flowed¶
2. text/plain¶
3. text/*¶
4. */*¶
The media type quality factor associated with a given type is determined by
finding the media range with the highest precedence that matches the type. For
example,¶
Accept: text/*;q=0.3, text/plain;q=0.7, text/plain;format=flowed,
text/plain;format=fixed;q=0.4, */*;q=0.5
¶
would cause the following values to be associated:¶
Table 5 Media Type Quality Value text/plain;format=flowed 1 text/plain 0.7
text/html 0.3 image/jpeg 0.5 text/plain;format=fixed 0.4 text/html;level=3 0.7
Note: A user agent might be provided with a default set of quality values for
certain media ranges. However, unless the user agent is a closed system that
cannot interact with other rendering agents, this default set ought to be
configurable by the user.¶
12.5.2. ACCEPT-CHARSET
The "Accept-Charset" header field can be sent by a user agent to indicate its
preferences for charsets in textual response content. For example, this field
allows user agents capable of understanding more comprehensive or
special-purpose charsets to signal that capability to an origin server that is
capable of representing information in those charsets.¶
Accept-Charset = #( ( token / "*" ) [ weight ] )
¶
Charset names are defined in Section 8.3.2. A user agent MAY associate a quality
value with each charset to indicate the user's relative preference for that
charset, as defined in Section 12.4.2. An example is¶
Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
¶
The special value "*", if present in the Accept-Charset header field, matches
every charset that is not mentioned elsewhere in the field.¶
Note: Accept-Charset is deprecated because UTF-8 has become nearly ubiquitous
and sending a detailed list of user-preferred charsets wastes bandwidth,
increases latency, and makes passive fingerprinting far too easy (Section
17.13). Most general-purpose user agents do not send Accept-Charset unless
specifically configured to do so.¶
12.5.3. ACCEPT-ENCODING
The "Accept-Encoding" header field can be used to indicate preferences regarding
the use of content codings (Section 8.4.1).¶
When sent by a user agent in a request, Accept-Encoding indicates the content
codings acceptable in a response.¶
When sent by a server in a response, Accept-Encoding provides information about
which content codings are preferred in the content of a subsequent request to
the same resource.¶
An "identity" token is used as a synonym for "no encoding" in order to
communicate when no encoding is preferred.¶
Accept-Encoding = #( codings [ weight ] )
codings = content-coding / "identity" / "*"
¶
Each codings value MAY be given an associated quality value (weight)
representing the preference for that encoding, as defined in Section 12.4.2. The
asterisk "*" symbol in an Accept-Encoding field matches any available content
coding not explicitly listed in the field.¶
Examples:¶
Accept-Encoding: compress, gzip
Accept-Encoding:
Accept-Encoding: *
Accept-Encoding: compress;q=0.5, gzip;q=1.0
Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
¶
A server tests whether a content coding for a given representation is acceptable
using these rules:¶
1. If no Accept-Encoding header field is in the request, any content coding is
considered acceptable by the user agent.¶
2. If the representation has no content coding, then it is acceptable by
default unless specifically excluded by the Accept-Encoding header field
stating either "identity;q=0" or "*;q=0" without a more specific entry for
"identity".¶
3. If the representation's content coding is one of the content codings listed
in the Accept-Encoding field value, then it is acceptable unless it is
accompanied by a qvalue of 0. (As defined in Section 12.4.2, a qvalue of 0
means "not acceptable".)¶
A representation could be encoded with multiple content codings. However, most
content codings are alternative ways to accomplish the same purpose (e.g., data
compression). When selecting between multiple content codings that have the same
purpose, the acceptable content coding with the highest non-zero qvalue is
preferred.¶
An Accept-Encoding header field with a field value that is empty implies that
the user agent does not want any content coding in response. If a non-empty
Accept-Encoding header field is present in a request and none of the available
representations for the response have a content coding that is listed as
acceptable, the origin server SHOULD send a response without any content coding
unless the identity coding is indicated as unacceptable.¶
When the Accept-Encoding header field is present in a response, it indicates
what content codings the resource was willing to accept in the associated
request. The field value is evaluated the same way as in a request.¶
Note that this information is specific to the associated request; the set of
supported encodings might be different for other resources on the same server
and could change over time or depend on other aspects of the request (such as
the request method).¶
Servers that fail a request due to an unsupported content coding ought to
respond with a 415 (Unsupported Media Type) status and include an
Accept-Encoding header field in that response, allowing clients to distinguish
between issues related to content codings and media types. In order to avoid
confusion with issues related to media types, servers that fail a request with a
415 status for reasons unrelated to content codings MUST NOT include the
Accept-Encoding header field.¶
The most common use of Accept-Encoding is in responses with a 415 (Unsupported
Media Type) status code, in response to optimistic use of a content coding by
clients. However, the header field can also be used to indicate to clients that
content codings are supported in order to optimize future interactions. For
example, a resource might include it in a 2xx (Successful) response when the
request content was big enough to justify use of a compression coding but the
client failed do so.¶
12.5.4. ACCEPT-LANGUAGE
The "Accept-Language" header field can be used by user agents to indicate the
set of natural languages that are preferred in the response. Language tags are
defined in Section 8.5.1.¶
Accept-Language = #( language-range [ weight ] )
language-range =
<language-range, see [RFC4647], Section 2.1>
¶
Each language-range can be given an associated quality value representing an
estimate of the user's preference for the languages specified by that range, as
defined in Section 12.4.2. For example,¶
Accept-Language: da, en-gb;q=0.8, en;q=0.7
¶
would mean: "I prefer Danish, but will accept British English and other types of
English".¶
Note that some recipients treat the order in which language tags are listed as
an indication of descending priority, particularly for tags that are assigned
equal quality values (no value is the same as q=1). However, this behavior
cannot be relied upon. For consistency and to maximize interoperability, many
user agents assign each language tag a unique quality value while also listing
them in order of decreasing quality. Additional discussion of language priority
lists can be found in Section 2.3 of [RFC4647].¶
For matching, Section 3 of [RFC4647] defines several matching schemes.
Implementations can offer the most appropriate matching scheme for their
requirements. The "Basic Filtering" scheme ([RFC4647], Section 3.3.1) is
identical to the matching scheme that was previously defined for HTTP in Section
14.4 of [RFC2616].¶
It might be contrary to the privacy expectations of the user to send an
Accept-Language header field with the complete linguistic preferences of the
user in every request (Section 17.13).¶
Since intelligibility is highly dependent on the individual user, user agents
need to allow user control over the linguistic preference (either through
configuration of the user agent itself or by defaulting to a user controllable
system setting). A user agent that does not provide such control to the user
MUST NOT send an Accept-Language header field.¶
Note: User agents ought to provide guidance to users when setting a preference,
since users are rarely familiar with the details of language matching as
described above. For example, users might assume that on selecting "en-gb", they
will be served any kind of English document if British English is not available.
A user agent might suggest, in such a case, to add "en" to the list for better
matching behavior.¶
12.5.5. VARY
The "Vary" header field in a response describes what parts of a request message,
aside from the method and target URI, might have influenced the origin server's
process for selecting the content of this response.¶
Vary = #( "*" / field-name )
¶
A Vary field value is either the wildcard member "*" or a list of request field
names, known as the selecting header fields, that might have had a role in
selecting the representation for this response. Potential selecting header
fields are not limited to fields defined by this specification.¶
A list containing the member "*" signals that other aspects of the request might
have played a role in selecting the response representation, possibly including
aspects outside the message syntax (e.g., the client's network address). A
recipient will not be able to determine whether this response is appropriate for
a later request without forwarding the request to the origin server. A proxy
MUST NOT generate "*" in a Vary field value.¶
For example, a response that contains¶
Vary: accept-encoding, accept-language
¶
indicates that the origin server might have used the request's Accept-Encoding
and Accept-Language header fields (or lack thereof) as determining factors while
choosing the content for this response.¶
A Vary field containing a list of field names has two purposes:¶
1. To inform cache recipients that they MUST NOT use this response to satisfy a
later request unless the later request has the same values for the listed
header fields as the original request (Section 4.1 of [CACHING]) or reuse of
the response has been validated by the origin server. In other words, Vary
expands the cache key required to match a new request to the stored cache
entry.¶
2. To inform user agent recipients that this response was subject to content
negotiation (Section 12) and a different representation might be sent in a
subsequent request if other values are provided in the listed header fields
(proactive negotiation).¶
An origin server SHOULD generate a Vary header field on a cacheable response
when it wishes that response to be selectively reused for subsequent requests.
Generally, that is the case when the response content has been tailored to
better fit the preferences expressed by those selecting header fields, such as
when an origin server has selected the response's language based on the
request's Accept-Language header field.¶
Vary might be elided when an origin server considers variance in content
selection to be less significant than Vary's performance impact on caching,
particularly when reuse is already limited by cache response directives (Section
5.2 of [CACHING]).¶
There is no need to send the Authorization field name in Vary because reuse of
that response for a different user is prohibited by the field definition
(Section 11.6.2). Likewise, if the response content has been selected or
influenced by network region, but the origin server wants the cached response to
be reused even if recipients move from one region to another, then there is no
need for the origin server to indicate such variance in Vary.¶
13. CONDITIONAL REQUESTS
A conditional request is an HTTP request with one or more request header fields
that indicate a precondition to be tested before applying the request method to
the target resource. Section 13.2 defines when to evaluate preconditions and
their order of precedence when more than one precondition is present.¶
Conditional GET requests are the most efficient mechanism for HTTP cache updates
[CACHING]. Conditionals can also be applied to state-changing methods, such as
PUT and DELETE, to prevent the "lost update" problem: one client accidentally
overwriting the work of another client that has been acting in parallel.¶
13.1. PRECONDITIONS
Preconditions are usually defined with respect to a state of the target resource
as a whole (its current value set) or the state as observed in a previously
obtained representation (one value in that set). If a resource has multiple
current representations, each with its own observable state, a precondition will
assume that the mapping of each request to a selected representation (Section
3.2) is consistent over time. Regardless, if the mapping is inconsistent or the
server is unable to select an appropriate representation, then no harm will
result when the precondition evaluates to false.¶
Each precondition defined below consists of a comparison between a set of
validators obtained from prior representations of the target resource to the
current state of validators for the selected representation (Section 8.8).
Hence, these preconditions evaluate whether the state of the target resource has
changed since a given state known by the client. The effect of such an
evaluation depends on the method semantics and choice of conditional, as defined
in Section 13.2.¶
Other preconditions, defined by other specifications as extension fields, might
place conditions on all recipients, on the state of the target resource in
general, or on a group of resources. For instance, the "If" header field in
WebDAV can make a request conditional on various aspects of multiple resources,
such as locks, if the recipient understands and implements that field ([WEBDAV],
Section 10.4).¶
Extensibility of preconditions is only possible when the precondition can be
safely ignored if unknown (like If-Modified-Since), when deployment can be
assumed for a given use case, or when implementation is signaled by some other
property of the target resource. This encourages a focus on mutually agreed
deployment of common standards.¶
13.1.1. IF-MATCH
The "If-Match" header field makes the request method conditional on the
recipient origin server either having at least one current representation of the
target resource, when the field value is "*", or having a current representation
of the target resource that has an entity tag matching a member of the list of
entity tags provided in the field value.¶
An origin server MUST use the strong comparison function when comparing entity
tags for If-Match (Section 8.8.3.2), since the client intends this precondition
to prevent the method from being applied if there have been any changes to the
representation data.¶
If-Match = "*" / #entity-tag
¶
Examples:¶
If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-Match: *
¶
If-Match is most often used with state-changing methods (e.g., POST, PUT,
DELETE) to prevent accidental overwrites when multiple user agents might be
acting in parallel on the same resource (i.e., to prevent the "lost update"
problem). In general, it can be used with any method that involves the selection
or modification of a representation to abort the request if the selected
representation's current entity tag is not a member within the If-Match field
value.¶
When an origin server receives a request that selects a representation and that
request includes an If-Match header field, the origin server MUST evaluate the
If-Match condition per Section 13.2 prior to performing the method.¶
To evaluate a received If-Match header field:¶
1. If the field value is "*", the condition is true if the origin server has a
current representation for the target resource.¶
2. If the field value is a list of entity tags, the condition is true if any of
the listed tags match the entity tag of the selected representation.¶
3. Otherwise, the condition is false.¶
An origin server that evaluates an If-Match condition MUST NOT perform the
requested method if the condition evaluates to false. Instead, the origin server
MAY indicate that the conditional request failed by responding with a 412
(Precondition Failed) status code. Alternatively, if the request is a
state-changing operation that appears to have already been applied to the
selected representation, the origin server MAY respond with a 2xx (Successful)
status code (i.e., the change requested by the user agent has already succeeded,
but the user agent might not be aware of it, perhaps because the prior response
was lost or an equivalent change was made by some other user agent).¶
Allowing an origin server to send a success response when a change request
appears to have already been applied is more efficient for many authoring use
cases, but comes with some risk if multiple user agents are making change
requests that are very similar but not cooperative. For example, multiple user
agents writing to a common resource as a semaphore (e.g., a nonatomic increment)
are likely to collide and potentially lose important state transitions. For
those kinds of resources, an origin server is better off being stringent in
sending 412 for every failed precondition on an unsafe method. In other cases,
excluding the ETag field from a success response might encourage the user agent
to perform a GET as its next request to eliminate confusion about the resource's
current state.¶
A client MAY send an If-Match header field in a GET request to indicate that it
would prefer a 412 (Precondition Failed) response if the selected representation
does not match. However, this is only useful in range requests (Section 14) for
completing a previously received partial representation when there is no desire
for a new representation. If-Range (Section 13.1.5) is better suited for range
requests when the client prefers to receive a new representation.¶
A cache or intermediary MAY ignore If-Match because its interoperability
features are only necessary for an origin server.¶
Note that an If-Match header field with a list value containing "*" and other
values (including other instances of "*") is syntactically invalid (therefore
not allowed to be generated) and furthermore is unlikely to be interoperable.¶
13.1.2. IF-NONE-MATCH
The "If-None-Match" header field makes the request method conditional on a
recipient cache or origin server either not having any current representation of
the target resource, when the field value is "*", or having a selected
representation with an entity tag that does not match any of those listed in the
field value.¶
A recipient MUST use the weak comparison function when comparing entity tags for
If-None-Match (Section 8.8.3.2), since weak entity tags can be used for cache
validation even if there have been changes to the representation data.¶
If-None-Match = "*" / #entity-tag
¶
Examples:¶
If-None-Match: "xyzzy"
If-None-Match: W/"xyzzy"
If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
If-None-Match: *
¶
If-None-Match is primarily used in conditional GET requests to enable efficient
updates of cached information with a minimum amount of transaction overhead.
When a client desires to update one or more stored responses that have entity
tags, the client SHOULD generate an If-None-Match header field containing a list
of those entity tags when making a GET request; this allows recipient servers to
send a 304 (Not Modified) response to indicate when one of those stored
responses matches the selected representation.¶
If-None-Match can also be used with a value of "*" to prevent an unsafe request
method (e.g., PUT) from inadvertently modifying an existing representation of
the target resource when the client believes that the resource does not have a
current representation (Section 9.2.1). This is a variation on the "lost update"
problem that might arise if more than one client attempts to create an initial
representation for the target resource.¶
When an origin server receives a request that selects a representation and that
request includes an If-None-Match header field, the origin server MUST evaluate
the If-None-Match condition per Section 13.2 prior to performing the method.¶
To evaluate a received If-None-Match header field:¶
1. If the field value is "*", the condition is false if the origin server has a
current representation for the target resource.¶
2. If the field value is a list of entity tags, the condition is false if one
of the listed tags matches the entity tag of the selected representation.¶
3. Otherwise, the condition is true.¶
An origin server that evaluates an If-None-Match condition MUST NOT perform the
requested method if the condition evaluates to false; instead, the origin server
MUST respond with either a) the 304 (Not Modified) status code if the request
method is GET or HEAD or b) the 412 (Precondition Failed) status code for all
other request methods.¶
Requirements on cache handling of a received If-None-Match header field are
defined in Section 4.3.2 of [CACHING].¶
Note that an If-None-Match header field with a list value containing "*" and
other values (including other instances of "*") is syntactically invalid
(therefore not allowed to be generated) and furthermore is unlikely to be
interoperable.¶
13.1.3. IF-MODIFIED-SINCE
The "If-Modified-Since" header field makes a GET or HEAD request method
conditional on the selected representation's modification date being more recent
than the date provided in the field value. Transfer of the selected
representation's data is avoided if that data has not changed.¶
If-Modified-Since = HTTP-date
¶
An example of the field is:¶
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
¶
A recipient MUST ignore If-Modified-Since if the request contains an
If-None-Match header field; the condition in If-None-Match is considered to be a
more accurate replacement for the condition in If-Modified-Since, and the two
are only combined for the sake of interoperating with older intermediaries that
might not implement If-None-Match.¶
A recipient MUST ignore the If-Modified-Since header field if the received field
value is not a valid HTTP-date, the field value has more than one member, or if
the request method is neither GET nor HEAD.¶
A recipient MUST ignore the If-Modified-Since header field if the resource does
not have a modification date available.¶
A recipient MUST interpret an If-Modified-Since field value's timestamp in terms
of the origin server's clock.¶
If-Modified-Since is typically used for two distinct purposes: 1) to allow
efficient updates of a cached representation that does not have an entity tag
and 2) to limit the scope of a web traversal to resources that have recently
changed.¶
When used for cache updates, a cache will typically use the value of the cached
message's Last-Modified header field to generate the field value of
If-Modified-Since. This behavior is most interoperable for cases where clocks
are poorly synchronized or when the server has chosen to only honor exact
timestamp matches (due to a problem with Last-Modified dates that appear to go
"back in time" when the origin server's clock is corrected or a representation
is restored from an archived backup). However, caches occasionally generate the
field value based on other data, such as the Date header field of the cached
message or the clock time at which the message was received, particularly when
the cached message does not contain a Last-Modified header field.¶
When used for limiting the scope of retrieval to a recent time window, a user
agent will generate an If-Modified-Since field value based on either its own
clock or a Date header field received from the server in a prior response.
Origin servers that choose an exact timestamp match based on the selected
representation's Last-Modified header field will not be able to help the user
agent limit its data transfers to only those changed during the specified
window.¶
When an origin server receives a request that selects a representation and that
request includes an If-Modified-Since header field without an If-None-Match
header field, the origin server SHOULD evaluate the If-Modified-Since condition
per Section 13.2 prior to performing the method.¶
To evaluate a received If-Modified-Since header field:¶
1. If the selected representation's last modification date is earlier or equal
to the date provided in the field value, the condition is false.¶
2. Otherwise, the condition is true.¶
An origin server that evaluates an If-Modified-Since condition SHOULD NOT
perform the requested method if the condition evaluates to false; instead, the
origin server SHOULD generate a 304 (Not Modified) response, including only
those metadata that are useful for identifying or updating a previously cached
response.¶
Requirements on cache handling of a received If-Modified-Since header field are
defined in Section 4.3.2 of [CACHING].¶
13.1.4. IF-UNMODIFIED-SINCE
The "If-Unmodified-Since" header field makes the request method conditional on
the selected representation's last modification date being earlier than or equal
to the date provided in the field value. This field accomplishes the same
purpose as If-Match for cases where the user agent does not have an entity tag
for the representation.¶
If-Unmodified-Since = HTTP-date
¶
An example of the field is:¶
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
¶
A recipient MUST ignore If-Unmodified-Since if the request contains an If-Match
header field; the condition in If-Match is considered to be a more accurate
replacement for the condition in If-Unmodified-Since, and the two are only
combined for the sake of interoperating with older intermediaries that might not
implement If-Match.¶
A recipient MUST ignore the If-Unmodified-Since header field if the received
field value is not a valid HTTP-date (including when the field value appears to
be a list of dates).¶
A recipient MUST ignore the If-Unmodified-Since header field if the resource
does not have a modification date available.¶
A recipient MUST interpret an If-Unmodified-Since field value's timestamp in
terms of the origin server's clock.¶
If-Unmodified-Since is most often used with state-changing methods (e.g., POST,
PUT, DELETE) to prevent accidental overwrites when multiple user agents might be
acting in parallel on a resource that does not supply entity tags with its
representations (i.e., to prevent the "lost update" problem). In general, it can
be used with any method that involves the selection or modification of a
representation to abort the request if the selected representation's last
modification date has changed since the date provided in the If-Unmodified-Since
field value.¶
When an origin server receives a request that selects a representation and that
request includes an If-Unmodified-Since header field without an If-Match header
field, the origin server MUST evaluate the If-Unmodified-Since condition per
Section 13.2 prior to performing the method.¶
To evaluate a received If-Unmodified-Since header field:¶
1. If the selected representation's last modification date is earlier than or
equal to the date provided in the field value, the condition is true.¶
2. Otherwise, the condition is false.¶
An origin server that evaluates an If-Unmodified-Since condition MUST NOT
perform the requested method if the condition evaluates to false. Instead, the
origin server MAY indicate that the conditional request failed by responding
with a 412 (Precondition Failed) status code. Alternatively, if the request is a
state-changing operation that appears to have already been applied to the
selected representation, the origin server MAY respond with a 2xx (Successful)
status code (i.e., the change requested by the user agent has already succeeded,
but the user agent might not be aware of it, perhaps because the prior response
was lost or an equivalent change was made by some other user agent).¶
Allowing an origin server to send a success response when a change request
appears to have already been applied is more efficient for many authoring use
cases, but comes with some risk if multiple user agents are making change
requests that are very similar but not cooperative. In those cases, an origin
server is better off being stringent in sending 412 for every failed
precondition on an unsafe method.¶
A client MAY send an If-Unmodified-Since header field in a GET request to
indicate that it would prefer a 412 (Precondition Failed) response if the
selected representation has been modified. However, this is only useful in range
requests (Section 14) for completing a previously received partial
representation when there is no desire for a new representation. If-Range
(Section 13.1.5) is better suited for range requests when the client prefers to
receive a new representation.¶
A cache or intermediary MAY ignore If-Unmodified-Since because its
interoperability features are only necessary for an origin server.¶
13.1.5. IF-RANGE
The "If-Range" header field provides a special conditional request mechanism
that is similar to the If-Match and If-Unmodified-Since header fields but that
instructs the recipient to ignore the Range header field if the validator
doesn't match, resulting in transfer of the new selected representation instead
of a 412 (Precondition Failed) response.¶
If a client has a partial copy of a representation and wishes to have an
up-to-date copy of the entire representation, it could use the Range header
field with a conditional GET (using either or both of If-Unmodified-Since and
If-Match.) However, if the precondition fails because the representation has
been modified, the client would then have to make a second request to obtain the
entire current representation.¶
The "If-Range" header field allows a client to "short-circuit" the second
request. Informally, its meaning is as follows: if the representation is
unchanged, send me the part(s) that I am requesting in Range; otherwise, send me
the entire representation.¶
If-Range = entity-tag / HTTP-date
¶
A valid entity-tag can be distinguished from a valid HTTP-date by examining the
first three characters for a DQUOTE.¶
A client MUST NOT generate an If-Range header field in a request that does not
contain a Range header field. A server MUST ignore an If-Range header field
received in a request that does not contain a Range header field. An origin
server MUST ignore an If-Range header field received in a request for a target
resource that does not support Range requests.¶
A client MUST NOT generate an If-Range header field containing an entity tag
that is marked as weak. A client MUST NOT generate an If-Range header field
containing an HTTP-date unless the client has no entity tag for the
corresponding representation and the date is a strong validator in the sense
defined by Section 8.8.2.2.¶
A server that receives an If-Range header field on a Range request MUST evaluate
the condition per Section 13.2 prior to performing the method.¶
To evaluate a received If-Range header field containing an HTTP-date:¶
1. If the HTTP-date validator provided is not a strong validator in the sense
defined by Section 8.8.2.2, the condition is false.¶
2. If the HTTP-date validator provided exactly matches the Last-Modified field
value for the selected representation, the condition is true.¶
3. Otherwise, the condition is false.¶
To evaluate a received If-Range header field containing an entity-tag:¶
1. If the entity-tag validator provided exactly matches the ETag field value
for the selected representation using the strong comparison function
(Section 8.8.3.2), the condition is true.¶
2. Otherwise, the condition is false.¶
A recipient of an If-Range header field MUST ignore the Range header field if
the If-Range condition evaluates to false. Otherwise, the recipient SHOULD
process the Range header field as requested.¶
Note that the If-Range comparison is by exact match, including when the
validator is an HTTP-date, and so it differs from the "earlier than or equal to"
comparison used when evaluating an If-Unmodified-Since conditional.¶
13.2. EVALUATION OF PRECONDITIONS
13.2.1. WHEN TO EVALUATE
Except when excluded below, a recipient cache or origin server MUST evaluate
received request preconditions after it has successfully performed its normal
request checks and just before it would process the request content (if any) or
perform the action associated with the request method. A server MUST ignore all
received preconditions if its response to the same request without those
conditions, prior to processing the request content, would have been a status
code other than a 2xx (Successful) or 412 (Precondition Failed). In other words,
redirects and failures that can be detected before significant processing occurs
take precedence over the evaluation of preconditions.¶
A server that is not the origin server for the target resource and cannot act as
a cache for requests on the target resource MUST NOT evaluate the conditional
request header fields defined by this specification, and it MUST forward them if
the request is forwarded, since the generating client intends that they be
evaluated by a server that can provide a current representation. Likewise, a
server MUST ignore the conditional request header fields defined by this
specification when received with a request method that does not involve the
selection or modification of a selected representation, such as CONNECT,
OPTIONS, or TRACE.¶
Note that protocol extensions can modify the conditions under which
preconditions are evaluated or the consequences of their evaluation. For
example, the immutable cache directive (defined by [RFC8246]) instructs caches
to forgo forwarding conditional requests when they hold a fresh response.¶
Although conditional request header fields are defined as being usable with the
HEAD method (to keep HEAD's semantics consistent with those of GET), there is no
point in sending a conditional HEAD because a successful response is around the
same size as a 304 (Not Modified) response and more useful than a 412
(Precondition Failed) response.¶
13.2.2. PRECEDENCE OF PRECONDITIONS
When more than one conditional request header field is present in a request, the
order in which the fields are evaluated becomes important. In practice, the
fields defined in this document are consistently implemented in a single,
logical order, since "lost update" preconditions have more strict requirements
than cache validation, a validated cache is more efficient than a partial
response, and entity tags are presumed to be more accurate than date
validators.¶
A recipient cache or origin server MUST evaluate the request preconditions
defined by this specification in the following order:¶
1. When recipient is the origin server and If-Match is present, evaluate the
If-Match precondition:¶
* if true, continue to step 3¶
* if false, respond 412 (Precondition Failed) unless it can be determined
that the state-changing request has already succeeded (see Section
13.1.1)¶
2. When recipient is the origin server, If-Match is not present, and
If-Unmodified-Since is present, evaluate the If-Unmodified-Since
precondition:¶
* if true, continue to step 3¶
* if false, respond 412 (Precondition Failed) unless it can be determined
that the state-changing request has already succeeded (see Section
13.1.4)¶
3. When If-None-Match is present, evaluate the If-None-Match precondition:¶
* if true, continue to step 5¶
* if false for GET/HEAD, respond 304 (Not Modified)¶
* if false for other methods, respond 412 (Precondition Failed)¶
4. When the method is GET or HEAD, If-None-Match is not present, and
If-Modified-Since is present, evaluate the If-Modified-Since precondition:¶
* if true, continue to step 5¶
* if false, respond 304 (Not Modified)¶
5. When the method is GET and both Range and If-Range are present, evaluate the
If-Range precondition:¶
* if true and the Range is applicable to the selected representation,
respond 206 (Partial Content)¶
* otherwise, ignore the Range header field and respond 200 (OK)¶
6. Otherwise,¶
* perform the requested method and respond according to its success or
failure.¶
Any extension to HTTP that defines additional conditional request header fields
ought to define the order for evaluating such fields in relation to those
defined in this document and other conditionals that might be found in
practice.¶
14. RANGE REQUESTS
Clients often encounter interrupted data transfers as a result of canceled
requests or dropped connections. When a client has stored a partial
representation, it is desirable to request the remainder of that representation
in a subsequent request rather than transfer the entire representation.
Likewise, devices with limited local storage might benefit from being able to
request only a subset of a larger representation, such as a single page of a
very large document, or the dimensions of an embedded image.¶
Range requests are an OPTIONAL feature of HTTP, designed so that recipients not
implementing this feature (or not supporting it for the target resource) can
respond as if it is a normal GET request without impacting interoperability.
Partial responses are indicated by a distinct status code to not be mistaken for
full responses by caches that might not implement the feature.¶
14.1. RANGE UNITS
Representation data can be partitioned into subranges when there are addressable
structural units inherent to that data's content coding or media type. For
example, octet (a.k.a. byte) boundaries are a structural unit common to all
representation data, allowing partitions of the data to be identified as a range
of bytes at some offset from the start or end of that data.¶
This general notion of a "range unit" is used in the Accept-Ranges (Section
14.3) response header field to advertise support for range requests, the Range
(Section 14.2) request header field to delineate the parts of a representation
that are requested, and the Content-Range (Section 14.4) header field to
describe which part of a representation is being transferred.¶
range-unit = token
¶
All range unit names are case-insensitive and ought to be registered within the
"HTTP Range Unit Registry", as defined in Section 16.5.1.¶
Range units are intended to be extensible, as described in Section 16.5.¶
14.1.1. RANGE SPECIFIERS
Ranges are expressed in terms of a range unit paired with a set of range
specifiers. The range unit name determines what kinds of range-spec are
applicable to its own specifiers. Hence, the following grammar is generic: each
range unit is expected to specify requirements on when int-range, suffix-range,
and other-range are allowed.¶
A range request can specify a single range or a set of ranges within a single
representation.¶
ranges-specifier = range-unit "=" range-set
range-set = 1#range-spec
range-spec = int-range
/ suffix-range
/ other-range
¶
An int-range is a range expressed as two non-negative integers or as one
non-negative integer through to the end of the representation data. The range
unit specifies what the integers mean (e.g., they might indicate unit offsets
from the beginning, inclusive numbered parts, etc.).¶
int-range = first-pos "-" [ last-pos ]
first-pos = 1*DIGIT
last-pos = 1*DIGIT
¶
An int-range is invalid if the last-pos value is present and less than the
first-pos.¶
A suffix-range is a range expressed as a suffix of the representation data with
the provided non-negative integer maximum length (in range units). In other
words, the last N units of the representation data.¶
suffix-range = "-" suffix-length
suffix-length = 1*DIGIT
¶
To provide for extensibility, the other-range rule is a mostly unconstrained
grammar that allows application-specific or future range units to define
additional range specifiers.¶
other-range = 1*( %x21-2B / %x2D-7E )
; 1*(VCHAR excluding comma)
¶
A ranges-specifier is invalid if it contains any range-spec that is invalid or
undefined for the indicated range-unit.¶
A valid ranges-specifier is "satisfiable" if it contains at least one range-spec
that is satisfiable, as defined by the indicated range-unit. Otherwise, the
ranges-specifier is "unsatisfiable".¶
14.1.2. BYTE RANGES
The "bytes" range unit is used to express subranges of a representation data's
octet sequence. Each byte range is expressed as an integer range at some offset,
relative to either the beginning (int-range) or end (suffix-range) of the
representation data. Byte ranges do not use the other-range specifier.¶
The first-pos value in a bytes int-range gives the offset of the first byte in a
range. The last-pos value gives the offset of the last byte in the range; that
is, the byte positions specified are inclusive. Byte offsets start at zero.¶
If the representation data has a content coding applied, each byte range is
calculated with respect to the encoded sequence of bytes, not the sequence of
underlying bytes that would be obtained after decoding.¶
Examples of bytes range specifiers:¶
* The first 500 bytes (byte offsets 0-499, inclusive):¶
bytes=0-499
¶
* The second 500 bytes (byte offsets 500-999, inclusive):¶
bytes=500-999
¶
A client can limit the number of bytes requested without knowing the size of the
selected representation. If the last-pos value is absent, or if the value is
greater than or equal to the current length of the representation data, the byte
range is interpreted as the remainder of the representation (i.e., the server
replaces the value of last-pos with a value that is one less than the current
length of the selected representation).¶
A client can refer to the last N bytes (N > 0) of the selected representation
using a suffix-range. If the selected representation is shorter than the
specified suffix-length, the entire representation is used.¶
Additional examples, assuming a representation of length 10000:¶
* The final 500 bytes (byte offsets 9500-9999, inclusive):¶
bytes=-500
¶
Or:¶
bytes=9500-
¶
* The first and last bytes only (bytes 0 and 9999):¶
bytes=0-0,-1
¶
* The first, middle, and last 1000 bytes:¶
bytes= 0-999, 4500-5499, -1000
¶
* Other valid (but not canonical) specifications of the second 500 bytes (byte
offsets 500-999, inclusive):¶
bytes=500-600,601-999
bytes=500-700,601-999
¶
For a GET request, a valid bytes range-spec is satisfiable if it is either:¶
* an int-range with a first-pos that is less than the current length of the
selected representation or¶
* a suffix-range with a non-zero suffix-length.¶
When a selected representation has zero length, the only satisfiable form of
range-spec in a GET request is a suffix-range with a non-zero suffix-length.¶
In the byte-range syntax, first-pos, last-pos, and suffix-length are expressed
as decimal number of octets. Since there is no predefined limit to the length of
content, recipients MUST anticipate potentially large decimal numerals and
prevent parsing errors due to integer conversion overflows.¶
14.2. RANGE
The "Range" header field on a GET request modifies the method semantics to
request transfer of only one or more subranges of the selected representation
data (Section 8.1), rather than the entire selected representation.¶
Range = ranges-specifier
¶
A server MAY ignore the Range header field. However, origin servers and
intermediate caches ought to support byte ranges when possible, since they
support efficient recovery from partially failed transfers and partial retrieval
of large representations.¶
A server MUST ignore a Range header field received with a request method that is
unrecognized or for which range handling is not defined. For this specification,
GET is the only method for which range handling is defined.¶
An origin server MUST ignore a Range header field that contains a range unit it
does not understand. A proxy MAY discard a Range header field that contains a
range unit it does not understand.¶
A server that supports range requests MAY ignore or reject a Range header field
that contains an invalid ranges-specifier (Section 14.1.1), a ranges-specifier
with more than two overlapping ranges, or a set of many small ranges that are
not listed in ascending order, since these are indications of either a broken
client or a deliberate denial-of-service attack (Section 17.15). A client SHOULD
NOT request multiple ranges that are inherently less efficient to process and
transfer than a single range that encompasses the same data.¶
A server that supports range requests MAY ignore a Range header field when the
selected representation has no content (i.e., the selected representation's data
is of zero length).¶
A client that is requesting multiple ranges SHOULD list those ranges in
ascending order (the order in which they would typically be received in a
complete representation) unless there is a specific need to request a later part
earlier. For example, a user agent processing a large representation with an
internal catalog of parts might need to request later parts first, particularly
if the representation consists of pages stored in reverse order and the user
agent wishes to transfer one page at a time.¶
The Range header field is evaluated after evaluating the precondition header
fields defined in Section 13.1, and only if the result in absence of the Range
header field would be a 200 (OK) response. In other words, Range is ignored when
a conditional GET would result in a 304 (Not Modified) response.¶
The If-Range header field (Section 13.1.5) can be used as a precondition to
applying the Range header field.¶
If all of the preconditions are true, the server supports the Range header field
for the target resource, the received Range field-value contains a valid
ranges-specifier with a range-unit supported for that target resource, and that
ranges-specifier is satisfiable with respect to the selected representation, the
server SHOULD send a 206 (Partial Content) response with content containing one
or more partial representations that correspond to the satisfiable range-spec(s)
requested.¶
The above does not imply that a server will send all requested ranges. In some
cases, it may only be possible (or efficient) to send a portion of the requested
ranges first, while expecting the client to re-request the remaining portions
later if they are still desired (see Section 15.3.7).¶
If all of the preconditions are true, the server supports the Range header field
for the target resource, the received Range field-value contains a valid
ranges-specifier, and either the range-unit is not supported for that target
resource or the ranges-specifier is unsatisfiable with respect to the selected
representation, the server SHOULD send a 416 (Range Not Satisfiable) response.¶
14.3. ACCEPT-RANGES
The "Accept-Ranges" field in a response indicates whether an upstream server
supports range requests for the target resource.¶
Accept-Ranges = acceptable-ranges
acceptable-ranges = 1#range-unit
¶
For example, a server that supports byte-range requests (Section 14.1.2) can
send the field¶
Accept-Ranges: bytes
¶
to indicate that it supports byte range requests for that target resource,
thereby encouraging its use by the client for future partial requests on the
same request path. Range units are defined in Section 14.1.¶
A client MAY generate range requests regardless of having received an
Accept-Ranges field. The information only provides advice for the sake of
improving performance and reducing unnecessary network transfers.¶
Conversely, a client MUST NOT assume that receiving an Accept-Ranges field means
that future range requests will return partial responses. The content might
change, the server might only support range requests at certain times or under
certain conditions, or a different intermediary might process the next request.¶
A server that does not support any kind of range request for the target resource
MAY send¶
Accept-Ranges: none
¶
to advise the client not to attempt a range request on the same request path.
The range unit "none" is reserved for this purpose.¶
The Accept-Ranges field MAY be sent in a trailer section, but is preferred to be
sent as a header field because the information is particularly useful for
restarting large information transfers that have failed in mid-content (before
the trailer section is received).¶
14.4. CONTENT-RANGE
The "Content-Range" header field is sent in a single part 206 (Partial Content)
response to indicate the partial range of the selected representation enclosed
as the message content, sent in each part of a multipart 206 response to
indicate the range enclosed within each body part (Section 14.6), and sent in
416 (Range Not Satisfiable) responses to provide information about the selected
representation.¶
Content-Range = range-unit SP
( range-resp / unsatisfied-range )
range-resp = incl-range "/" ( complete-length / "*" )
incl-range = first-pos "-" last-pos
unsatisfied-range = "*/" complete-length
complete-length = 1*DIGIT
¶
If a 206 (Partial Content) response contains a Content-Range header field with a
range unit (Section 14.1) that the recipient does not understand, the recipient
MUST NOT attempt to recombine it with a stored representation. A proxy that
receives such a message SHOULD forward it downstream.¶
Content-Range might also be sent as a request modifier to request a partial PUT,
as described in Section 14.5, based on private agreements between client and
origin server. A server MUST ignore a Content-Range header field received in a
request with a method for which Content-Range support is not defined.¶
For byte ranges, a sender SHOULD indicate the complete length of the
representation from which the range has been extracted, unless the complete
length is unknown or difficult to determine. An asterisk character ("*") in
place of the complete-length indicates that the representation length was
unknown when the header field was generated.¶
The following example illustrates when the complete length of the selected
representation is known by the sender to be 1234 bytes:¶
Content-Range: bytes 42-1233/1234
¶
and this second example illustrates when the complete length is unknown:¶
Content-Range: bytes 42-1233/*
¶
A Content-Range field value is invalid if it contains a range-resp that has a
last-pos value less than its first-pos value, or a complete-length value less
than or equal to its last-pos value. The recipient of an invalid Content-Range
MUST NOT attempt to recombine the received content with a stored
representation.¶
A server generating a 416 (Range Not Satisfiable) response to a byte-range
request SHOULD send a Content-Range header field with an unsatisfied-range
value, as in the following example:¶
Content-Range: bytes */1234
¶
The complete-length in a 416 response indicates the current length of the
selected representation.¶
The Content-Range header field has no meaning for status codes that do not
explicitly describe its semantic. For this specification, only the 206 (Partial
Content) and 416 (Range Not Satisfiable) status codes describe a meaning for
Content-Range.¶
The following are examples of Content-Range values in which the selected
representation contains a total of 1234 bytes:¶
* The first 500 bytes:¶
Content-Range: bytes 0-499/1234
¶
* The second 500 bytes:¶
Content-Range: bytes 500-999/1234
¶
* All except for the first 500 bytes:¶
Content-Range: bytes 500-1233/1234
¶
* The last 500 bytes:¶
Content-Range: bytes 734-1233/1234
¶
14.5. PARTIAL PUT
Some origin servers support PUT of a partial representation when the user agent
sends a Content-Range header field (Section 14.4) in the request, though such
support is inconsistent and depends on private agreements with user agents. In
general, it requests that the state of the target resource be partly replaced
with the enclosed content at an offset and length indicated by the Content-Range
value, where the offset is relative to the current selected representation.¶
An origin server SHOULD respond with a 400 (Bad Request) status code if it
receives Content-Range on a PUT for a target resource that does not support
partial PUT requests.¶
Partial PUT is not backwards compatible with the original definition of PUT. It
may result in the content being written as a complete replacement for the
current representation.¶
Partial resource updates are also possible by targeting a separately identified
resource with state that overlaps or extends a portion of the larger resource,
or by using a different method that has been specifically defined for partial
updates (for example, the PATCH method defined in [RFC5789]).¶
14.6. MEDIA TYPE MULTIPART/BYTERANGES
When a 206 (Partial Content) response message includes the content of multiple
ranges, they are transmitted as body parts in a multipart message body
([RFC2046], Section 5.1) with the media type of "multipart/byteranges".¶
The "multipart/byteranges" media type includes one or more body parts, each with
its own Content-Type and Content-Range fields. The required boundary parameter
specifies the boundary string used to separate each body part.¶
Implementation Notes:¶
1. Additional CRLFs might precede the first boundary string in the body.¶
2. Although [RFC2046] permits the boundary string to be quoted, some existing
implementations handle a quoted boundary string incorrectly.¶
3. A number of clients and servers were coded to an early draft of the
byteranges specification that used a media type of "multipart/x-byteranges",
which is almost (but not quite) compatible with this type.¶
Despite the name, the "multipart/byteranges" media type is not limited to byte
ranges. The following example uses an "exampleunit" range unit:¶
HTTP/1.1 206 Partial Content
Date: Tue, 14 Nov 1995 06:25:24 GMT
Last-Modified: Tue, 14 July 04:58:08 GMT
Content-Length: 2331785
Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES
--THIS_STRING_SEPARATES
Content-Type: video/example
Content-Range: exampleunit 1.2-4.3/25
...the first range...
--THIS_STRING_SEPARATES
Content-Type: video/example
Content-Range: exampleunit 11.2-14.3/25
...the second range
--THIS_STRING_SEPARATES--
¶
The following information serves as the registration form for the
"multipart/byteranges" media type.¶
Type name: multipart¶ Subtype name: byteranges¶ Required parameters: boundary¶
Optional parameters: N/A¶ Encoding considerations: only "7bit", "8bit", or
"binary" are permitted¶ Security considerations: see Section 17¶
Interoperability considerations: N/A¶ Published specification: RFC 9110 (see
Section 14.6)¶ Applications that use this media type: HTTP components supporting
multiple ranges in a single request¶ Fragment identifier considerations: N/A¶
Additional information: Deprecated alias names for this type: N/A¶ Magic
number(s): N/A¶ File extension(s): N/A¶ Macintosh file type code(s): N/A¶ Person
and email address to contact for further information: See Authors' Addresses
section.¶ Intended usage: COMMON¶ Restrictions on usage: N/A¶ Author: See
Authors' Addresses section.¶ Change controller: IESG¶
15. STATUS CODES
The status code of a response is a three-digit integer code that describes the
result of the request and the semantics of the response, including whether the
request was successful and what content is enclosed (if any). All valid status
codes are within the range of 100 to 599, inclusive.¶
The first digit of the status code defines the class of response. The last two
digits do not have any categorization role. There are five values for the first
digit:¶
* 1xx (Informational): The request was received, continuing process¶
* 2xx (Successful): The request was successfully received, understood, and
accepted¶
* 3xx (Redirection): Further action needs to be taken in order to complete the
request¶
* 4xx (Client Error): The request contains bad syntax or cannot be fulfilled¶
* 5xx (Server Error): The server failed to fulfill an apparently valid request¶
HTTP status codes are extensible. A client is not required to understand the
meaning of all registered status codes, though such understanding is obviously
desirable. However, a client MUST understand the class of any status code, as
indicated by the first digit, and treat an unrecognized status code as being
equivalent to the x00 status code of that class.¶
For example, if a client receives an unrecognized status code of 471, it can see
from the first digit that there was something wrong with its request and treat
the response as if it had received a 400 (Bad Request) status code. The response
message will usually contain a representation that explains the status.¶
Values outside the range 100..599 are invalid. Implementations often use
three-digit integer values outside of that range (i.e., 600..999) for internal
communication of non-HTTP status (e.g., library errors). A client that receives
a response with an invalid status code SHOULD process the response as if it had
a 5xx (Server Error) status code.¶
A single request can have multiple associated responses: zero or more "interim"
(non-final) responses with status codes in the "informational" (1xx) range,
followed by exactly one "final" response with a status code in one of the other
ranges.¶
15.1. OVERVIEW OF STATUS CODES
The status codes listed below are defined in this specification. The reason
phrases listed here are only recommendations -- they can be replaced by local
equivalents or left out altogether without affecting the protocol.¶
Responses with status codes that are defined as heuristically cacheable (e.g.,
200, 203, 204, 206, 300, 301, 308, 404, 405, 410, 414, and 501 in this
specification) can be reused by a cache with heuristic expiration unless
otherwise indicated by the method definition or explicit cache controls
[CACHING]; all other status codes are not heuristically cacheable.¶
Additional status codes, outside the scope of this specification, have been
specified for use in HTTP. All such status codes ought to be registered within
the "Hypertext Transfer Protocol (HTTP) Status Code Registry", as described in
Section 16.2.¶
15.2. INFORMATIONAL 1XX
The 1xx (Informational) class of status code indicates an interim response for
communicating connection status or request progress prior to completing the
requested action and sending a final response. Since HTTP/1.0 did not define any
1xx status codes, a server MUST NOT send a 1xx response to an HTTP/1.0 client.¶
A 1xx response is terminated by the end of the header section; it cannot contain
content or trailers.¶
A client MUST be able to parse one or more 1xx responses received prior to a
final response, even if the client does not expect one. A user agent MAY ignore
unexpected 1xx responses.¶
A proxy MUST forward 1xx responses unless the proxy itself requested the
generation of the 1xx response. For example, if a proxy adds an "Expect:
100-continue" header field when it forwards a request, then it need not forward
the corresponding 100 (Continue) response(s).¶
15.2.1. 100 CONTINUE
The 100 (Continue) status code indicates that the initial part of a request has
been received and has not yet been rejected by the server. The server intends to
send a final response after the request has been fully received and acted upon.¶
When the request contains an Expect header field that includes a 100-continue
expectation, the 100 response indicates that the server wishes to receive the
request content, as described in Section 10.1.1. The client ought to continue
sending the request and discard the 100 response.¶
If the request did not contain an Expect header field containing the
100-continue expectation, the client can simply discard this interim response.¶
15.2.2. 101 SWITCHING PROTOCOLS
The 101 (Switching Protocols) status code indicates that the server understands
and is willing to comply with the client's request, via the Upgrade header field
(Section 7.8), for a change in the application protocol being used on this
connection. The server MUST generate an Upgrade header field in the response
that indicates which protocol(s) will be in effect after this response.¶
It is assumed that the server will only agree to switch protocols when it is
advantageous to do so. For example, switching to a newer version of HTTP might
be advantageous over older versions, and switching to a real-time, synchronous
protocol might be advantageous when delivering resources that use such
features.¶
15.3. SUCCESSFUL 2XX
The 2xx (Successful) class of status code indicates that the client's request
was successfully received, understood, and accepted.¶
15.3.1. 200 OK
The 200 (OK) status code indicates that the request has succeeded. The content
sent in a 200 response depends on the request method. For the methods defined by
this specification, the intended meaning of the content can be summarized as:¶
Table 6 Request Method Response content is a representation of: GET the target
resource HEAD the target resource, like GET, but without transferring the
representation data POST the status of, or results obtained from, the action
PUT, DELETE the status of the action OPTIONS communication options for the
target resource TRACE the request message as received by the server returning
the trace
Aside from responses to CONNECT, a 200 response is expected to contain message
content unless the message framing explicitly indicates that the content has
zero length. If some aspect of the request indicates a preference for no content
upon success, the origin server ought to send a 204 (No Content) response
instead. For CONNECT, there is no content because the successful result is a
tunnel, which begins immediately after the 200 response header section.¶
A 200 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of
[CACHING]).¶
In 200 responses to GET or HEAD, an origin server SHOULD send any available
validator fields (Section 8.8) for the selected representation, with both a
strong entity tag and a Last-Modified date being preferred.¶
In 200 responses to state-changing methods, any validator fields (Section 8.8)
sent in the response convey the current validators for the new representation
formed as a result of successfully applying the request semantics. Note that the
PUT method (Section 9.3.4) has additional requirements that might preclude
sending such validators.¶
15.3.2. 201 CREATED
The 201 (Created) status code indicates that the request has been fulfilled and
has resulted in one or more new resources being created. The primary resource
created by the request is identified by either a Location header field in the
response or, if no Location header field is received, by the target URI.¶
The 201 response content typically describes and links to the resource(s)
created. Any validator fields (Section 8.8) sent in the response convey the
current validators for a new representation created by the request. Note that
the PUT method (Section 9.3.4) has additional requirements that might preclude
sending such validators.¶
15.3.3. 202 ACCEPTED
The 202 (Accepted) status code indicates that the request has been accepted for
processing, but the processing has not been completed. The request might or
might not eventually be acted upon, as it might be disallowed when processing
actually takes place. There is no facility in HTTP for re-sending a status code
from an asynchronous operation.¶
The 202 response is intentionally noncommittal. Its purpose is to allow a server
to accept a request for some other process (perhaps a batch-oriented process
that is only run once per day) without requiring that the user agent's
connection to the server persist until the process is completed. The
representation sent with this response ought to describe the request's current
status and point to (or embed) a status monitor that can provide the user with
an estimate of when the request will be fulfilled.¶
15.3.4. 203 NON-AUTHORITATIVE INFORMATION
The 203 (Non-Authoritative Information) status code indicates that the request
was successful but the enclosed content has been modified from that of the
origin server's 200 (OK) response by a transforming proxy (Section 7.7). This
status code allows the proxy to notify recipients when a transformation has been
applied, since that knowledge might impact later decisions regarding the
content. For example, future cache validation requests for the content might
only be applicable along the same request path (through the same proxies).¶
A 203 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of
[CACHING]).¶
15.3.5. 204 NO CONTENT
The 204 (No Content) status code indicates that the server has successfully
fulfilled the request and that there is no additional content to send in the
response content. Metadata in the response header fields refer to the target
resource and its selected representation after the requested action was
applied.¶
For example, if a 204 status code is received in response to a PUT request and
the response contains an ETag field, then the PUT was successful and the ETag
field value contains the entity tag for the new representation of that target
resource.¶
The 204 response allows a server to indicate that the action has been
successfully applied to the target resource, while implying that the user agent
does not need to traverse away from its current "document view" (if any). The
server assumes that the user agent will provide some indication of the success
to its user, in accord with its own interface, and apply any new or updated
metadata in the response to its active representation.¶
For example, a 204 status code is commonly used with document editing interfaces
corresponding to a "save" action, such that the document being saved remains
available to the user for editing. It is also frequently used with interfaces
that expect automated data transfers to be prevalent, such as within distributed
version control systems.¶
A 204 response is terminated by the end of the header section; it cannot contain
content or trailers.¶
A 204 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of
[CACHING]).¶
15.3.6. 205 RESET CONTENT
The 205 (Reset Content) status code indicates that the server has fulfilled the
request and desires that the user agent reset the "document view", which caused
the request to be sent, to its original state as received from the origin
server.¶
This response is intended to support a common data entry use case where the user
receives content that supports data entry (a form, notepad, canvas, etc.),
enters or manipulates data in that space, causes the entered data to be
submitted in a request, and then the data entry mechanism is reset for the next
entry so that the user can easily initiate another input action.¶
Since the 205 status code implies that no additional content will be provided, a
server MUST NOT generate content in a 205 response.¶
15.3.7. 206 PARTIAL CONTENT
The 206 (Partial Content) status code indicates that the server is successfully
fulfilling a range request for the target resource by transferring one or more
parts of the selected representation.¶
A server that supports range requests (Section 14) will usually attempt to
satisfy all of the requested ranges, since sending less data will likely result
in another client request for the remainder. However, a server might want to
send only a subset of the data requested for reasons of its own, such as
temporary unavailability, cache efficiency, load balancing, etc. Since a 206
response is self-descriptive, the client can still understand a response that
only partially satisfies its range request.¶
A client MUST inspect a 206 response's Content-Type and Content-Range field(s)
to determine what parts are enclosed and whether additional requests are
needed.¶
A server that generates a 206 response MUST generate the following header
fields, in addition to those required in the subsections below, if the field
would have been sent in a 200 (OK) response to the same request: Date,
Cache-Control, ETag, Expires, Content-Location, and Vary.¶
A Content-Length header field present in a 206 response indicates the number of
octets in the content of this message, which is usually not the complete length
of the selected representation. Each Content-Range header field includes
information about the selected representation's complete length.¶
A sender that generates a 206 response to a request with an If-Range header
field SHOULD NOT generate other representation header fields beyond those
required because the client already has a prior response containing those header
fields. Otherwise, a sender MUST generate all of the representation header
fields that would have been sent in a 200 (OK) response to the same request.¶
A 206 response is heuristically cacheable; i.e., unless otherwise indicated by
explicit cache controls (see Section 4.2.2 of [CACHING]).¶
15.3.7.1. SINGLE PART
If a single part is being transferred, the server generating the 206 response
MUST generate a Content-Range header field, describing what range of the
selected representation is enclosed, and a content consisting of the range. For
example:¶
HTTP/1.1 206 Partial Content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-Range: bytes 21010-47021/47022
Content-Length: 26012
Content-Type: image/gif
... 26012 bytes of partial image data ...
¶
15.3.7.2. MULTIPLE PARTS
If multiple parts are being transferred, the server generating the 206 response
MUST generate "multipart/byteranges" content, as defined in Section 14.6, and a
Content-Type header field containing the "multipart/byteranges" media type and
its required boundary parameter. To avoid confusion with single-part responses,
a server MUST NOT generate a Content-Range header field in the HTTP header
section of a multiple part response (this field will be sent in each part
instead).¶
Within the header area of each body part in the multipart content, the server
MUST generate a Content-Range header field corresponding to the range being
enclosed in that body part. If the selected representation would have had a
Content-Type header field in a 200 (OK) response, the server SHOULD generate
that same Content-Type header field in the header area of each body part. For
example:¶
HTTP/1.1 206 Partial Content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-Length: 1741
Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES
--THIS_STRING_SEPARATES
Content-Type: application/pdf
Content-Range: bytes 500-999/8000
...the first range...
--THIS_STRING_SEPARATES
Content-Type: application/pdf
Content-Range: bytes 7000-7999/8000
...the second range
--THIS_STRING_SEPARATES--
¶
When multiple ranges are requested, a server MAY coalesce any of the ranges that
overlap, or that are separated by a gap that is smaller than the overhead of
sending multiple parts, regardless of the order in which the corresponding
range-spec appeared in the received Range header field. Since the typical
overhead between each part of a "multipart/byteranges" is around 80 bytes,
depending on the selected representation's media type and the chosen boundary
parameter length, it can be less efficient to transfer many small disjoint parts
than it is to transfer the entire selected representation.¶
A server MUST NOT generate a multipart response to a request for a single range,
since a client that does not request multiple parts might not support multipart
responses. However, a server MAY generate a "multipart/byteranges" response with
only a single body part if multiple ranges were requested and only one range was
found to be satisfiable or only one range remained after coalescing. A client
that cannot process a "multipart/byteranges" response MUST NOT generate a
request that asks for multiple ranges.¶
A server that generates a multipart response SHOULD send the parts in the same
order that the corresponding range-spec appeared in the received Range header
field, excluding those ranges that were deemed unsatisfiable or that were
coalesced into other ranges. A client that receives a multipart response MUST
inspect the Content-Range header field present in each body part in order to
determine which range is contained in that body part; a client cannot rely on
receiving the same ranges that it requested, nor the same order that it
requested.¶
15.3.7.3. COMBINING PARTS
A response might transfer only a subrange of a representation if the connection
closed prematurely or if the request used one or more Range specifications.
After several such transfers, a client might have received several ranges of the
same representation. These ranges can only be safely combined if they all have
in common the same strong validator (Section 8.8.1).¶
A client that has received multiple partial responses to GET requests on a
target resource MAY combine those responses into a larger continuous range if
they share the same strong validator.¶
If the most recent response is an incomplete 200 (OK) response, then the header
fields of that response are used for any combined response and replace those of
the matching stored responses.¶
If the most recent response is a 206 (Partial Content) response and at least one
of the matching stored responses is a 200 (OK), then the combined response
header fields consist of the most recent 200 response's header fields. If all of
the matching stored responses are 206 responses, then the stored response with
the most recent header fields is used as the source of header fields for the
combined response, except that the client MUST use other header fields provided
in the new response, aside from Content-Range, to replace all instances of the
corresponding header fields in the stored response.¶
The combined response content consists of the union of partial content ranges
within the new response and all of the matching stored responses. If the union
consists of the entire range of the representation, then the client MUST process
the combined response as if it were a complete 200 (OK) response, including a
Content-Length header field that reflects the complete length. Otherwise, the
client MUST process the set of continuous ranges as one of the following: an
incomplete 200 (OK) response if the combined response is a prefix of the
representation, a single 206 (Partial Content) response containing
"multipart/byteranges" content, or multiple 206 (Partial Content) responses,
each with one continuous range that is indicated by a Content-Range header
field.¶
15.4. REDIRECTION 3XX
The 3xx (Redirection) class of status code indicates that further action needs
to be taken by the user agent in order to fulfill the request. There are several
types of redirects:¶
1. Redirects that indicate this resource might be available at a different URI,
as provided by the Location header field, as in the status codes 301 (Moved
Permanently), 302 (Found), 307 (Temporary Redirect), and 308 (Permanent
Redirect).¶
2. Redirection that offers a choice among matching resources capable of
representing this resource, as in the 300 (Multiple Choices) status code.¶
3. Redirection to a different resource, identified by the Location header
field, that can represent an indirect response to the request, as in the 303
(See Other) status code.¶
4. Redirection to a previously stored result, as in the 304 (Not Modified)
status code.¶
Note: In HTTP/1.0, the status codes 301 (Moved Permanently) and 302 (Found) were
originally defined as method-preserving ([HTTP/1.0], Section 9.3) to match their
implementation at CERN; 303 (See Other) was defined for a redirection that
changed its method to GET. However, early user agents split on whether to
redirect POST requests as POST (according to then-current specification) or as
GET (the safer alternative when redirected to a different site). Prevailing
practice eventually converged on changing the method to GET. 307 (Temporary
Redirect) and 308 (Permanent Redirect) [RFC7538] were later added to
unambiguously indicate method-preserving redirects, and status codes 301 and 302
have been adjusted to allow a POST request to be redirected as GET.¶
If a Location header field (Section 10.2.2) is provided, the user agent MAY
automatically redirect its request to the URI referenced by the Location field
value, even if the specific status code is not understood. Automatic redirection
needs to be done with care for methods not known to be safe, as defined in
Section 9.2.1, since the user might not wish to redirect an unsafe request.¶
When automatically following a redirected request, the user agent SHOULD resend
the original request message with the following modifications:¶
1. Replace the target URI with the URI referenced by the redirection response's
Location header field value after resolving it relative to the original
request's target URI.¶
2. Remove header fields that were automatically generated by the
implementation, replacing them with updated values as appropriate to the new
request. This includes:¶
1. Connection-specific header fields (see Section 7.6.1),¶
2. Header fields specific to the client's proxy configuration, including
(but not limited to) Proxy-Authorization,¶
3. Origin-specific header fields (if any), including (but not limited to)
Host,¶
4. Validating header fields that were added by the implementation's cache
(e.g., If-None-Match, If-Modified-Since), and¶
5. Resource-specific header fields, including (but not limited to) Referer,
Origin, Authorization, and Cookie.¶
3. Consider removing header fields that were not automatically generated by the
implementation (i.e., those present in the request because they were added
by the calling context) where there are security implications; this includes
but is not limited to Authorization and Cookie.¶
4. Change the request method according to the redirecting status code's
semantics, if applicable.¶
5. If the request method has been changed to GET or HEAD, remove
content-specific header fields, including (but not limited to)
Content-Encoding, Content-Language, Content-Location, Content-Type,
Content-Length, Digest, Last-Modified.¶
A client SHOULD detect and intervene in cyclical redirections (i.e., "infinite"
redirection loops).¶
Note: An earlier version of this specification recommended a maximum of five
redirections ([RFC2068], Section 10.3). Content developers need to be aware that
some clients might implement such a fixed limitation.¶
15.4.1. 300 MULTIPLE CHOICES
The 300 (Multiple Choices) status code indicates that the target resource has
more than one representation, each with its own more specific identifier, and
information about the alternatives is being provided so that the user (or user
agent) can select a preferred representation by redirecting its request to one
or more of those identifiers. In other words, the server desires that the user
agent engage in reactive negotiation to select the most appropriate
representation(s) for its needs (Section 12).¶
If the server has a preferred choice, the server SHOULD generate a Location
header field containing a preferred choice's URI reference. The user agent MAY
use the Location field value for automatic redirection.¶
For request methods other than HEAD, the server SHOULD generate content in the
300 response containing a list of representation metadata and URI reference(s)
from which the user or user agent can choose the one most preferred. The user
agent MAY make a selection from that list automatically if it understands the
provided media type. A specific format for automatic selection is not defined by
this specification because HTTP tries to remain orthogonal to the definition of
its content. In practice, the representation is provided in some easily parsed
format believed to be acceptable to the user agent, as determined by shared
design or content negotiation, or in some commonly accepted hypertext format.¶
A 300 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of
[CACHING]).¶
Note: The original proposal for the 300 status code defined the URI header field
as providing a list of alternative representations, such that it would be usable
for 200, 300, and 406 responses and be transferred in responses to the HEAD
method. However, lack of deployment and disagreement over syntax led to both URI
and Alternates (a subsequent proposal) being dropped from this specification. It
is possible to communicate the list as a Link header field value [RFC8288] whose
members have a relationship of "alternate", though deployment is a
chicken-and-egg problem.¶
15.4.2. 301 MOVED PERMANENTLY
The 301 (Moved Permanently) status code indicates that the target resource has
been assigned a new permanent URI and any future references to this resource
ought to use one of the enclosed URIs. The server is suggesting that a user
agent with link-editing capability can permanently replace references to the
target URI with one of the new references sent by the server. However, this
suggestion is usually ignored unless the user agent is actively editing
references (e.g., engaged in authoring content), the connection is secured, and
the origin server is a trusted authority for the content being edited.¶
The server SHOULD generate a Location header field in the response containing a
preferred URI reference for the new permanent URI. The user agent MAY use the
Location field value for automatic redirection. The server's response content
usually contains a short hypertext note with a hyperlink to the new URI(s).¶
Note: For historical reasons, a user agent MAY change the request method from
POST to GET for the subsequent request. If this behavior is undesired, the 308
(Permanent Redirect) status code can be used instead.¶
A 301 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of
[CACHING]).¶
15.4.3. 302 FOUND
The 302 (Found) status code indicates that the target resource resides
temporarily under a different URI. Since the redirection might be altered on
occasion, the client ought to continue to use the target URI for future
requests.¶
The server SHOULD generate a Location header field in the response containing a
URI reference for the different URI. The user agent MAY use the Location field
value for automatic redirection. The server's response content usually contains
a short hypertext note with a hyperlink to the different URI(s).¶
Note: For historical reasons, a user agent MAY change the request method from
POST to GET for the subsequent request. If this behavior is undesired, the 307
(Temporary Redirect) status code can be used instead.¶
15.4.4. 303 SEE OTHER
The 303 (See Other) status code indicates that the server is redirecting the
user agent to a different resource, as indicated by a URI in the Location header
field, which is intended to provide an indirect response to the original
request. A user agent can perform a retrieval request targeting that URI (a GET
or HEAD request if using HTTP), which might also be redirected, and present the
eventual result as an answer to the original request. Note that the new URI in
the Location header field is not considered equivalent to the target URI.¶
This status code is applicable to any HTTP method. It is primarily used to allow
the output of a POST action to redirect the user agent to a different resource,
since doing so provides the information corresponding to the POST response as a
resource that can be separately identified, bookmarked, and cached.¶
A 303 response to a GET request indicates that the origin server does not have a
representation of the target resource that can be transferred by the server over
HTTP. However, the Location field value refers to a resource that is descriptive
of the target resource, such that making a retrieval request on that other
resource might result in a representation that is useful to recipients without
implying that it represents the original target resource. Note that answers to
the questions of what can be represented, what representations are adequate, and
what might be a useful description are outside the scope of HTTP.¶
Except for responses to a HEAD request, the representation of a 303 response
ought to contain a short hypertext note with a hyperlink to the same URI
reference provided in the Location header field.¶
15.4.5. 304 NOT MODIFIED
The 304 (Not Modified) status code indicates that a conditional GET or HEAD
request has been received and would have resulted in a 200 (OK) response if it
were not for the fact that the condition evaluated to false. In other words,
there is no need for the server to transfer a representation of the target
resource because the request indicates that the client, which made the request
conditional, already has a valid representation; the server is therefore
redirecting the client to make use of that stored representation as if it were
the content of a 200 (OK) response.¶
The server generating a 304 response MUST generate any of the following header
fields that would have been sent in a 200 (OK) response to the same request:¶
* Content-Location, Date, ETag, and Vary¶
* Cache-Control and Expires (see [CACHING])¶
Since the goal of a 304 response is to minimize information transfer when the
recipient already has one or more cached representations, a sender SHOULD NOT
generate representation metadata other than the above listed fields unless said
metadata exists for the purpose of guiding cache updates (e.g., Last-Modified
might be useful if the response does not have an ETag field).¶
Requirements on a cache that receives a 304 response are defined in Section
4.3.4 of [CACHING]. If the conditional request originated with an outbound
client, such as a user agent with its own cache sending a conditional GET to a
shared proxy, then the proxy SHOULD forward the 304 response to that client.¶
A 304 response is terminated by the end of the header section; it cannot contain
content or trailers.¶
15.4.6. 305 USE PROXY
The 305 (Use Proxy) status code was defined in a previous version of this
specification and is now deprecated (Appendix B of [RFC7231]).¶
15.4.7. 306 (UNUSED)
The 306 status code was defined in a previous version of this specification, is
no longer used, and the code is reserved.¶
15.4.8. 307 TEMPORARY REDIRECT
The 307 (Temporary Redirect) status code indicates that the target resource
resides temporarily under a different URI and the user agent MUST NOT change the
request method if it performs an automatic redirection to that URI. Since the
redirection can change over time, the client ought to continue using the
original target URI for future requests.¶
The server SHOULD generate a Location header field in the response containing a
URI reference for the different URI. The user agent MAY use the Location field
value for automatic redirection. The server's response content usually contains
a short hypertext note with a hyperlink to the different URI(s).¶
15.4.9. 308 PERMANENT REDIRECT
The 308 (Permanent Redirect) status code indicates that the target resource has
been assigned a new permanent URI and any future references to this resource
ought to use one of the enclosed URIs. The server is suggesting that a user
agent with link-editing capability can permanently replace references to the
target URI with one of the new references sent by the server. However, this
suggestion is usually ignored unless the user agent is actively editing
references (e.g., engaged in authoring content), the connection is secured, and
the origin server is a trusted authority for the content being edited.¶
The server SHOULD generate a Location header field in the response containing a
preferred URI reference for the new permanent URI. The user agent MAY use the
Location field value for automatic redirection. The server's response content
usually contains a short hypertext note with a hyperlink to the new URI(s).¶
A 308 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of
[CACHING]).¶
Note: This status code is much younger (June 2014) than its sibling codes and
thus might not be recognized everywhere. See Section 4 of [RFC7538] for
deployment considerations.¶
15.5. CLIENT ERROR 4XX
The 4xx (Client Error) class of status code indicates that the client seems to
have erred. Except when responding to a HEAD request, the server SHOULD send a
representation containing an explanation of the error situation, and whether it
is a temporary or permanent condition. These status codes are applicable to any
request method. User agents SHOULD display any included representation to the
user.¶
15.5.1. 400 BAD REQUEST
The 400 (Bad Request) status code indicates that the server cannot or will not
process the request due to something that is perceived to be a client error
(e.g., malformed request syntax, invalid request message framing, or deceptive
request routing).¶
15.5.2. 401 UNAUTHORIZED
The 401 (Unauthorized) status code indicates that the request has not been
applied because it lacks valid authentication credentials for the target
resource. The server generating a 401 response MUST send a WWW-Authenticate
header field (Section 11.6.1) containing at least one challenge applicable to
the target resource.¶
If the request included authentication credentials, then the 401 response
indicates that authorization has been refused for those credentials. The user
agent MAY repeat the request with a new or replaced Authorization header field
(Section 11.6.2). If the 401 response contains the same challenge as the prior
response, and the user agent has already attempted authentication at least once,
then the user agent SHOULD present the enclosed representation to the user,
since it usually contains relevant diagnostic information.¶
15.5.3. 402 PAYMENT REQUIRED
The 402 (Payment Required) status code is reserved for future use.¶
15.5.4. 403 FORBIDDEN
The 403 (Forbidden) status code indicates that the server understood the request
but refuses to fulfill it. A server that wishes to make public why the request
has been forbidden can describe that reason in the response content (if any).¶
If authentication credentials were provided in the request, the server considers
them insufficient to grant access. The client SHOULD NOT automatically repeat
the request with the same credentials. The client MAY repeat the request with
new or different credentials. However, a request might be forbidden for reasons
unrelated to the credentials.¶
An origin server that wishes to "hide" the current existence of a forbidden
target resource MAY instead respond with a status code of 404 (Not Found).¶
15.5.5. 404 NOT FOUND
The 404 (Not Found) status code indicates that the origin server did not find a
current representation for the target resource or is not willing to disclose
that one exists. A 404 status code does not indicate whether this lack of
representation is temporary or permanent; the 410 (Gone) status code is
preferred over 404 if the origin server knows, presumably through some
configurable means, that the condition is likely to be permanent.¶
A 404 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of
[CACHING]).¶
15.5.6. 405 METHOD NOT ALLOWED
The 405 (Method Not Allowed) status code indicates that the method received in
the request-line is known by the origin server but not supported by the target
resource. The origin server MUST generate an Allow header field in a 405
response containing a list of the target resource's currently supported
methods.¶
A 405 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of
[CACHING]).¶
15.5.7. 406 NOT ACCEPTABLE
The 406 (Not Acceptable) status code indicates that the target resource does not
have a current representation that would be acceptable to the user agent,
according to the proactive negotiation header fields received in the request
(Section 12.1), and the server is unwilling to supply a default representation.¶
The server SHOULD generate content containing a list of available representation
characteristics and corresponding resource identifiers from which the user or
user agent can choose the one most appropriate. A user agent MAY automatically
select the most appropriate choice from that list. However, this specification
does not define any standard for such automatic selection, as described in
Section 15.4.1.¶
15.5.8. 407 PROXY AUTHENTICATION REQUIRED
The 407 (Proxy Authentication Required) status code is similar to 401
(Unauthorized), but it indicates that the client needs to authenticate itself in
order to use a proxy for this request. The proxy MUST send a Proxy-Authenticate
header field (Section 11.7.1) containing a challenge applicable to that proxy
for the request. The client MAY repeat the request with a new or replaced
Proxy-Authorization header field (Section 11.7.2).¶
15.5.9. 408 REQUEST TIMEOUT
The 408 (Request Timeout) status code indicates that the server did not receive
a complete request message within the time that it was prepared to wait.¶
If the client has an outstanding request in transit, it MAY repeat that request.
If the current connection is not usable (e.g., as it would be in HTTP/1.1
because request delimitation is lost), a new connection will be used.¶
15.5.10. 409 CONFLICT
The 409 (Conflict) status code indicates that the request could not be completed
due to a conflict with the current state of the target resource. This code is
used in situations where the user might be able to resolve the conflict and
resubmit the request. The server SHOULD generate content that includes enough
information for a user to recognize the source of the conflict.¶
Conflicts are most likely to occur in response to a PUT request. For example, if
versioning were being used and the representation being PUT included changes to
a resource that conflict with those made by an earlier (third-party) request,
the origin server might use a 409 response to indicate that it can't complete
the request. In this case, the response representation would likely contain
information useful for merging the differences based on the revision history.¶
15.5.11. 410 GONE
The 410 (Gone) status code indicates that access to the target resource is no
longer available at the origin server and that this condition is likely to be
permanent. If the origin server does not know, or has no facility to determine,
whether or not the condition is permanent, the status code 404 (Not Found) ought
to be used instead.¶
The 410 response is primarily intended to assist the task of web maintenance by
notifying the recipient that the resource is intentionally unavailable and that
the server owners desire that remote links to that resource be removed. Such an
event is common for limited-time, promotional services and for resources
belonging to individuals no longer associated with the origin server's site. It
is not necessary to mark all permanently unavailable resources as "gone" or to
keep the mark for any length of time -- that is left to the discretion of the
server owner.¶
A 410 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of
[CACHING]).¶
15.5.12. 411 LENGTH REQUIRED
The 411 (Length Required) status code indicates that the server refuses to
accept the request without a defined Content-Length (Section 8.6). The client
MAY repeat the request if it adds a valid Content-Length header field containing
the length of the request content.¶
15.5.13. 412 PRECONDITION FAILED
The 412 (Precondition Failed) status code indicates that one or more conditions
given in the request header fields evaluated to false when tested on the server
(Section 13). This response status code allows the client to place preconditions
on the current resource state (its current representations and metadata) and,
thus, prevent the request method from being applied if the target resource is in
an unexpected state.¶
15.5.14. 413 CONTENT TOO LARGE
The 413 (Content Too Large) status code indicates that the server is refusing to
process a request because the request content is larger than the server is
willing or able to process. The server MAY terminate the request, if the
protocol version in use allows it; otherwise, the server MAY close the
connection.¶
If the condition is temporary, the server SHOULD generate a Retry-After header
field to indicate that it is temporary and after what time the client MAY try
again.¶
15.5.15. 414 URI TOO LONG
The 414 (URI Too Long) status code indicates that the server is refusing to
service the request because the target URI is longer than the server is willing
to interpret. This rare condition is only likely to occur when a client has
improperly converted a POST request to a GET request with long query
information, when the client has descended into an infinite loop of redirection
(e.g., a redirected URI prefix that points to a suffix of itself) or when the
server is under attack by a client attempting to exploit potential security
holes.¶
A 414 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of
[CACHING]).¶
15.5.16. 415 UNSUPPORTED MEDIA TYPE
The 415 (Unsupported Media Type) status code indicates that the origin server is
refusing to service the request because the content is in a format not supported
by this method on the target resource.¶
The format problem might be due to the request's indicated Content-Type or
Content-Encoding, or as a result of inspecting the data directly.¶
If the problem was caused by an unsupported content coding, the Accept-Encoding
response header field (Section 12.5.3) ought to be used to indicate which (if
any) content codings would have been accepted in the request.¶
On the other hand, if the cause was an unsupported media type, the Accept
response header field (Section 12.5.1) can be used to indicate which media types
would have been accepted in the request.¶
15.5.17. 416 RANGE NOT SATISFIABLE
The 416 (Range Not Satisfiable) status code indicates that the set of ranges in
the request's Range header field (Section 14.2) has been rejected either because
none of the requested ranges are satisfiable or because the client has requested
an excessive number of small or overlapping ranges (a potential denial of
service attack).¶
Each range unit defines what is required for its own range sets to be
satisfiable. For example, Section 14.1.2 defines what makes a bytes range set
satisfiable.¶
A server that generates a 416 response to a byte-range request SHOULD generate a
Content-Range header field specifying the current length of the selected
representation (Section 14.4).¶
For example:¶
HTTP/1.1 416 Range Not Satisfiable
Date: Fri, 20 Jan 2012 15:41:54 GMT
Content-Range: bytes */47022
¶
Note: Because servers are free to ignore Range, many implementations will
respond with the entire selected representation in a 200 (OK) response. That is
partly because most clients are prepared to receive a 200 (OK) to complete the
task (albeit less efficiently) and partly because clients might not stop making
an invalid range request until they have received a complete representation.
Thus, clients cannot depend on receiving a 416 (Range Not Satisfiable) response
even when it is most appropriate.¶
15.5.18. 417 EXPECTATION FAILED
The 417 (Expectation Failed) status code indicates that the expectation given in
the request's Expect header field (Section 10.1.1) could not be met by at least
one of the inbound servers.¶
15.5.19. 418 (UNUSED)
[RFC2324] was an April 1 RFC that lampooned the various ways HTTP was abused;
one such abuse was the definition of an application-specific 418 status code,
which has been deployed as a joke often enough for the code to be unusable for
any future use.¶
Therefore, the 418 status code is reserved in the IANA HTTP Status Code
Registry. This indicates that the status code cannot be assigned to other
applications currently. If future circumstances require its use (e.g.,
exhaustion of 4NN status codes), it can be re-assigned to another use.¶
15.5.20. 421 MISDIRECTED REQUEST
The 421 (Misdirected Request) status code indicates that the request was
directed at a server that is unable or unwilling to produce an authoritative
response for the target URI. An origin server (or gateway acting on behalf of
the origin server) sends 421 to reject a target URI that does not match an
origin for which the server has been configured (Section 4.3.1) or does not
match the connection context over which the request was received (Section 7.4).¶
A client that receives a 421 (Misdirected Request) response MAY retry the
request, whether or not the request method is idempotent, over a different
connection, such as a fresh connection specific to the target resource's origin,
or via an alternative service [ALTSVC].¶
A proxy MUST NOT generate a 421 response.¶
15.5.21. 422 UNPROCESSABLE CONTENT
The 422 (Unprocessable Content) status code indicates that the server
understands the content type of the request content (hence a 415 (Unsupported
Media Type) status code is inappropriate), and the syntax of the request content
is correct, but it was unable to process the contained instructions. For
example, this status code can be sent if an XML request content contains
well-formed (i.e., syntactically correct), but semantically erroneous XML
instructions.¶
15.5.22. 426 UPGRADE REQUIRED
The 426 (Upgrade Required) status code indicates that the server refuses to
perform the request using the current protocol but might be willing to do so
after the client upgrades to a different protocol. The server MUST send an
Upgrade header field in a 426 response to indicate the required protocol(s)
(Section 7.8).¶
Example:¶
HTTP/1.1 426 Upgrade Required
Upgrade: HTTP/3.0
Connection: Upgrade
Content-Length: 53
Content-Type: text/plain
This service requires use of the HTTP/3.0 protocol.
¶
15.6. SERVER ERROR 5XX
The 5xx (Server Error) class of status code indicates that the server is aware
that it has erred or is incapable of performing the requested method. Except
when responding to a HEAD request, the server SHOULD send a representation
containing an explanation of the error situation, and whether it is a temporary
or permanent condition. A user agent SHOULD display any included representation
to the user. These status codes are applicable to any request method.¶
15.6.1. 500 INTERNAL SERVER ERROR
The 500 (Internal Server Error) status code indicates that the server
encountered an unexpected condition that prevented it from fulfilling the
request.¶
15.6.2. 501 NOT IMPLEMENTED
The 501 (Not Implemented) status code indicates that the server does not support
the functionality required to fulfill the request. This is the appropriate
response when the server does not recognize the request method and is not
capable of supporting it for any resource.¶
A 501 response is heuristically cacheable; i.e., unless otherwise indicated by
the method definition or explicit cache controls (see Section 4.2.2 of
[CACHING]).¶
15.6.3. 502 BAD GATEWAY
The 502 (Bad Gateway) status code indicates that the server, while acting as a
gateway or proxy, received an invalid response from an inbound server it
accessed while attempting to fulfill the request.¶
15.6.4. 503 SERVICE UNAVAILABLE
The 503 (Service Unavailable) status code indicates that the server is currently
unable to handle the request due to a temporary overload or scheduled
maintenance, which will likely be alleviated after some delay. The server MAY
send a Retry-After header field (Section 10.2.3) to suggest an appropriate
amount of time for the client to wait before retrying the request.¶
Note: The existence of the 503 status code does not imply that a server has to
use it when becoming overloaded. Some servers might simply refuse the
connection.¶
15.6.5. 504 GATEWAY TIMEOUT
The 504 (Gateway Timeout) status code indicates that the server, while acting as
a gateway or proxy, did not receive a timely response from an upstream server it
needed to access in order to complete the request.¶
15.6.6. 505 HTTP VERSION NOT SUPPORTED
The 505 (HTTP Version Not Supported) status code indicates that the server does
not support, or refuses to support, the major version of HTTP that was used in
the request message. The server is indicating that it is unable or unwilling to
complete the request using the same major version as the client, as described in
Section 2.5, other than with this error message. The server SHOULD generate a
representation for the 505 response that describes why that version is not
supported and what other protocols are supported by that server.¶
16. EXTENDING HTTP
HTTP defines a number of generic extension points that can be used to introduce
capabilities to the protocol without introducing a new version, including
methods, status codes, field names, and further extensibility points within
defined fields, such as authentication schemes and cache directives (see
Cache-Control extensions in Section 5.2.3 of [CACHING]). Because the semantics
of HTTP are not versioned, these extension points are persistent; the version of
the protocol in use does not affect their semantics.¶
Version-independent extensions are discouraged from depending on or interacting
with the specific version of the protocol in use. When this is unavoidable,
careful consideration needs to be given to how the extension can interoperate
across versions.¶
Additionally, specific versions of HTTP might have their own extensibility
points, such as transfer codings in HTTP/1.1 (Section 6.1 of [HTTP/1.1]) and
HTTP/2 SETTINGS or frame types ([HTTP/2]). These extension points are specific
to the version of the protocol they occur within.¶
Version-specific extensions cannot override or modify the semantics of a
version-independent mechanism or extension point (like a method or header field)
without explicitly being allowed by that protocol element. For example, the
CONNECT method (Section 9.3.6) allows this.¶
These guidelines assure that the protocol operates correctly and predictably,
even when parts of the path implement different versions of HTTP.¶
16.1. METHOD EXTENSIBILITY
16.1.1. METHOD REGISTRY
The "Hypertext Transfer Protocol (HTTP) Method Registry", maintained by IANA at
<https://www.iana.org/assignments/http-methods>, registers method names.¶
HTTP method registrations MUST include the following fields:¶
* Method Name (see Section 9)¶
* Safe ("yes" or "no", see Section 9.2.1)¶
* Idempotent ("yes" or "no", see Section 9.2.2)¶
* Pointer to specification text¶
Values to be added to this namespace require IETF Review (see [RFC8126], Section
4.8).¶
16.1.2. CONSIDERATIONS FOR NEW METHODS
Standardized methods are generic; that is, they are potentially applicable to
any resource, not just one particular media type, kind of resource, or
application. As such, it is preferred that new methods be registered in a
document that isn't specific to a single application or data format, since
orthogonal technologies deserve orthogonal specification.¶
Since message parsing (Section 6) needs to be independent of method semantics
(aside from responses to HEAD), definitions of new methods cannot change the
parsing algorithm or prohibit the presence of content on either the request or
the response message. Definitions of new methods can specify that only a
zero-length content is allowed by requiring a Content-Length header field with a
value of "0".¶
Likewise, new methods cannot use the special host:port and asterisk forms of
request target that are allowed for CONNECT and OPTIONS, respectively (Section
7.1). A full URI in absolute form is needed for the target URI, which means
either the request target needs to be sent in absolute form or the target URI
will be reconstructed from the request context in the same way it is for other
methods.¶
A new method definition needs to indicate whether it is safe (Section 9.2.1),
idempotent (Section 9.2.2), cacheable (Section 9.2.3), what semantics are to be
associated with the request content (if any), and what refinements the method
makes to header field or status code semantics. If the new method is cacheable,
its definition ought to describe how, and under what conditions, a cache can
store a response and use it to satisfy a subsequent request. The new method
ought to describe whether it can be made conditional (Section 13.1) and, if so,
how a server responds when the condition is false. Likewise, if the new method
might have some use for partial response semantics (Section 14.2), it ought to
document this, too.¶
Note: Avoid defining a method name that starts with "M-", since that prefix
might be misinterpreted as having the semantics assigned to it by [RFC2774].¶
16.2. STATUS CODE EXTENSIBILITY
16.2.1. STATUS CODE REGISTRY
The "Hypertext Transfer Protocol (HTTP) Status Code Registry", maintained by
IANA at <https://www.iana.org/assignments/http-status-codes>, registers status
code numbers.¶
A registration MUST include the following fields:¶
* Status Code (3 digits)¶
* Short Description¶
* Pointer to specification text¶
Values to be added to the HTTP status code namespace require IETF Review (see
[RFC8126], Section 4.8).¶
16.2.2. CONSIDERATIONS FOR NEW STATUS CODES
When it is necessary to express semantics for a response that are not defined by
current status codes, a new status code can be registered. Status codes are
generic; they are potentially applicable to any resource, not just one
particular media type, kind of resource, or application of HTTP. As such, it is
preferred that new status codes be registered in a document that isn't specific
to a single application.¶
New status codes are required to fall under one of the categories defined in
Section 15. To allow existing parsers to process the response message, new
status codes cannot disallow content, although they can mandate a zero-length
content.¶
Proposals for new status codes that are not yet widely deployed ought to avoid
allocating a specific number for the code until there is clear consensus that it
will be registered; instead, early drafts can use a notation such as "4NN", or
"3N0" .. "3N9", to indicate the class of the proposed status code(s) without
consuming a number prematurely.¶
The definition of a new status code ought to explain the request conditions that
would cause a response containing that status code (e.g., combinations of
request header fields and/or method(s)) along with any dependencies on response
header fields (e.g., what fields are required, what fields can modify the
semantics, and what field semantics are further refined when used with the new
status code).¶
By default, a status code applies only to the request corresponding to the
response it occurs within. If a status code applies to a larger scope of
applicability -- for example, all requests to the resource in question or all
requests to a server -- this must be explicitly specified. When doing so, it
should be noted that not all clients can be expected to consistently apply a
larger scope because they might not understand the new status code.¶
The definition of a new final status code ought to specify whether or not it is
heuristically cacheable. Note that any response with a final status code can be
cached if the response has explicit freshness information. A status code defined
as heuristically cacheable is allowed to be cached without explicit freshness
information. Likewise, the definition of a status code can place constraints
upon cache behavior if the must-understand cache directive is used. See
[CACHING] for more information.¶
Finally, the definition of a new status code ought to indicate whether the
content has any implied association with an identified resource (Section
6.4.2).¶
16.3. FIELD EXTENSIBILITY
HTTP's most widely used extensibility point is the definition of new header and
trailer fields.¶
New fields can be defined such that, when they are understood by a recipient,
they override or enhance the interpretation of previously defined fields, define
preconditions on request evaluation, or refine the meaning of responses.¶
However, defining a field doesn't guarantee its deployment or recognition by
recipients. Most fields are designed with the expectation that a recipient can
safely ignore (but forward downstream) any field not recognized. In other cases,
the sender's ability to understand a given field might be indicated by its prior
communication, perhaps in the protocol version or fields that it sent in prior
messages, or its use of a specific media type. Likewise, direct inspection of
support might be possible through an OPTIONS request or by interacting with a
defined well-known URI [RFC8615] if such inspection is defined along with the
field being introduced.¶
16.3.1. FIELD NAME REGISTRY
The "Hypertext Transfer Protocol (HTTP) Field Name Registry" defines the
namespace for HTTP field names.¶
Any party can request registration of an HTTP field. See Section 16.3.2 for
considerations to take into account when creating a new HTTP field.¶
The "Hypertext Transfer Protocol (HTTP) Field Name Registry" is located at
<https://www.iana.org/assignments/http-fields/>. Registration requests can be
made by following the instructions located there or by sending an email to the
"ietf-http-wg@w3.org" mailing list.¶
Field names are registered on the advice of a designated expert (appointed by
the IESG or their delegate). Fields with the status 'permanent' are
Specification Required ([RFC8126], Section 4.6).¶
Registration requests consist of the following information:¶
Field name: The requested field name. It MUST conform to the field-name syntax
defined in Section 5.1, and it SHOULD be restricted to just letters, digits, and
hyphen ('-') characters, with the first character being a letter.¶ Status:
"permanent", "provisional", "deprecated", or "obsoleted".¶ Specification
document(s): Reference to the document that specifies the field, preferably
including a URI that can be used to retrieve a copy of the document. Optional
but encouraged for provisional registrations. An indication of the relevant
section(s) can also be included, but is not required.¶
And optionally:¶
Comments: Additional information, such as about reserved entries.¶
The expert(s) can define additional fields to be collected in the registry, in
consultation with the community.¶
Standards-defined names have a status of "permanent". Other names can also be
registered as permanent if the expert(s) finds that they are in use, in
consultation with the community. Other names should be registered as
"provisional".¶
Provisional entries can be removed by the expert(s) if -- in consultation with
the community -- the expert(s) find that they are not in use. The expert(s) can
change a provisional entry's status to permanent at any time.¶
Note that names can be registered by third parties (including the expert(s)) if
the expert(s) determines that an unregistered name is widely deployed and not
likely to be registered in a timely manner otherwise.¶
16.3.2. CONSIDERATIONS FOR NEW FIELDS
HTTP header and trailer fields are a widely used extension point for the
protocol. While they can be used in an ad hoc fashion, fields that are intended
for wider use need to be carefully documented to ensure interoperability.¶
In particular, authors of specifications defining new fields are advised to
consider and, where appropriate, document the following aspects:¶
* Under what conditions the field can be used; e.g., only in responses or
requests, in all messages, only on responses to a particular request method,
etc.¶
* Whether the field semantics are further refined by their context, such as
their use with certain request methods or status codes.¶
* The scope of applicability for the information conveyed. By default, fields
apply only to the message they are associated with, but some response fields
are designed to apply to all representations of a resource, the resource
itself, or an even broader scope. Specifications that expand the scope of a
response field will need to carefully consider issues such as content
negotiation, the time period of applicability, and (in some cases)
multi-tenant server deployments.¶
* Under what conditions intermediaries are allowed to insert, delete, or modify
the field's value.¶
* If the field is allowable in trailers; by default, it will not be (see
Section 6.5.1).¶
* Whether it is appropriate or even required to list the field name in the
Connection header field (i.e., if the field is to be hop-by-hop; see Section
7.6.1).¶
* Whether the field introduces any additional security considerations, such as
disclosure of privacy-related data.¶
Request header fields have additional considerations that need to be documented
if the default behavior is not appropriate:¶
* If it is appropriate to list the field name in a Vary response header field
(e.g., when the request header field is used by an origin server's content
selection algorithm; see Section 12.5.5).¶
* If the field is intended to be stored when received in a PUT request (see
Section 9.3.4).¶
* If the field ought to be removed when automatically redirecting a request due
to security concerns (see Section 15.4).¶
16.3.2.1. CONSIDERATIONS FOR NEW FIELD NAMES
Authors of specifications defining new fields are advised to choose a short but
descriptive field name. Short names avoid needless data transmission;
descriptive names avoid confusion and "squatting" on names that might have
broader uses.¶
To that end, limited-use fields (such as a header confined to a single
application or use case) are encouraged to use a name that includes that use (or
an abbreviation) as a prefix; for example, if the Foo Application needs a
Description field, it might use "Foo-Desc"; "Description" is too generic, and
"Foo-Description" is needlessly long.¶
While the field-name syntax is defined to allow any token character, in practice
some implementations place limits on the characters they accept in field-names.
To be interoperable, new field names SHOULD constrain themselves to alphanumeric
characters, "-", and ".", and SHOULD begin with a letter. For example, the
underscore ("_") character can be problematic when passed through non-HTTP
gateway interfaces (see Section 17.10).¶
Field names ought not be prefixed with "X-"; see [BCP178] for further
information.¶
Other prefixes are sometimes used in HTTP field names; for example, "Accept-" is
used in many content negotiation headers, and "Content-" is used as explained in
Section 6.4. These prefixes are only an aid to recognizing the purpose of a
field and do not trigger automatic processing.¶
16.3.2.2. CONSIDERATIONS FOR NEW FIELD VALUES
A major task in the definition of a new HTTP field is the specification of the
field value syntax: what senders should generate, and how recipients should
infer semantics from what is received.¶
Authors are encouraged (but not required) to use either the ABNF rules in this
specification or those in [RFC8941] to define the syntax of new field values.¶
Authors are advised to carefully consider how the combination of multiple field
lines will impact them (see Section 5.3). Because senders might erroneously send
multiple values, and both intermediaries and HTTP libraries can perform
combination automatically, this applies to all field values -- even when only a
single value is anticipated.¶
Therefore, authors are advised to delimit or encode values that contain commas
(e.g., with the quoted-string rule of Section 5.6.4, the String data type of
[RFC8941], or a field-specific encoding). This ensures that commas within field
data are not confused with the commas that delimit a list value.¶
For example, the Content-Type field value only allows commas inside quoted
strings, which can be reliably parsed even when multiple values are present. The
Location field value provides a counter-example that should not be emulated:
because URIs can include commas, it is not possible to reliably distinguish
between a single value that includes a comma from two values.¶
Authors of fields with a singleton value (see Section 5.5) are additionally
advised to document how to treat messages where the multiple members are present
(a sensible default would be to ignore the field, but this might not always be
the right choice).¶
16.4. AUTHENTICATION SCHEME EXTENSIBILITY
16.4.1. AUTHENTICATION SCHEME REGISTRY
The "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry" defines
the namespace for the authentication schemes in challenges and credentials. It
is maintained at <https://www.iana.org/assignments/http-authschemes>.¶
Registrations MUST include the following fields:¶
* Authentication Scheme Name¶
* Pointer to specification text¶
* Notes (optional)¶
Values to be added to this namespace require IETF Review (see [RFC8126], Section
4.8).¶
16.4.2. CONSIDERATIONS FOR NEW AUTHENTICATION SCHEMES
There are certain aspects of the HTTP Authentication framework that put
constraints on how new authentication schemes can work:¶
* HTTP authentication is presumed to be stateless: all of the information
necessary to authenticate a request MUST be provided in the request, rather
than be dependent on the server remembering prior requests. Authentication
based on, or bound to, the underlying connection is outside the scope of this
specification and inherently flawed unless steps are taken to ensure that the
connection cannot be used by any party other than the authenticated user (see
Section 3.3).¶
* The authentication parameter "realm" is reserved for defining protection
spaces as described in Section 11.5. New schemes MUST NOT use it in a way
incompatible with that definition.¶
* The "token68" notation was introduced for compatibility with existing
authentication schemes and can only be used once per challenge or credential.
Thus, new schemes ought to use the auth-param syntax instead, because
otherwise future extensions will be impossible.¶
* The parsing of challenges and credentials is defined by this specification
and cannot be modified by new authentication schemes. When the auth-param
syntax is used, all parameters ought to support both token and quoted-string
syntax, and syntactical constraints ought to be defined on the field value
after parsing (i.e., quoted-string processing). This is necessary so that
recipients can use a generic parser that applies to all authentication
schemes.¶
Note: The fact that the value syntax for the "realm" parameter is restricted
to quoted-string was a bad design choice not to be repeated for new
parameters.¶
* Definitions of new schemes ought to define the treatment of unknown extension
parameters. In general, a "must-ignore" rule is preferable to a
"must-understand" rule, because otherwise it will be hard to introduce new
parameters in the presence of legacy recipients. Furthermore, it's good to
describe the policy for defining new parameters (such as "update the
specification" or "use this registry").¶
* Authentication schemes need to document whether they are usable in
origin-server authentication (i.e., using WWW-Authenticate), and/or proxy
authentication (i.e., using Proxy-Authenticate).¶
* The credentials carried in an Authorization header field are specific to the
user agent and, therefore, have the same effect on HTTP caches as the
"private" cache response directive (Section 5.2.2.7 of [CACHING]), within the
scope of the request in which they appear.¶
Therefore, new authentication schemes that choose not to carry credentials in
the Authorization header field (e.g., using a newly defined header field)
will need to explicitly disallow caching, by mandating the use of cache
response directives (e.g., "private").¶
* Schemes using Authentication-Info, Proxy-Authentication-Info, or any other
authentication related response header field need to consider and document
the related security considerations (see Section 17.16.4).¶
16.5. RANGE UNIT EXTENSIBILITY
16.5.1. RANGE UNIT REGISTRY
The "HTTP Range Unit Registry" defines the namespace for the range unit names
and refers to their corresponding specifications. It is maintained at
<https://www.iana.org/assignments/http-parameters>.¶
Registration of an HTTP Range Unit MUST include the following fields:¶
* Name¶
* Description¶
* Pointer to specification text¶
Values to be added to this namespace require IETF Review (see [RFC8126], Section
4.8).¶
16.5.2. CONSIDERATIONS FOR NEW RANGE UNITS
Other range units, such as format-specific boundaries like pages, sections,
records, rows, or time, are potentially usable in HTTP for application-specific
purposes, but are not commonly used in practice. Implementors of alternative
range units ought to consider how they would work with content codings and
general-purpose intermediaries.¶
16.6. CONTENT CODING EXTENSIBILITY
16.6.1. CONTENT CODING REGISTRY
The "HTTP Content Coding Registry", maintained by IANA at
<https://www.iana.org/assignments/http-parameters/>, registers content-coding
names.¶
Content coding registrations MUST include the following fields:¶
* Name¶
* Description¶
* Pointer to specification text¶
Names of content codings MUST NOT overlap with names of transfer codings (per
the "HTTP Transfer Coding Registry" located at
<https://www.iana.org/assignments/http-parameters/>) unless the encoding
transformation is identical (as is the case for the compression codings defined
in Section 8.4.1).¶
Values to be added to this namespace require IETF Review (see Section 4.8 of
[RFC8126]) and MUST conform to the purpose of content coding defined in Section
8.4.1.¶
16.6.2. CONSIDERATIONS FOR NEW CONTENT CODINGS
New content codings ought to be self-descriptive whenever possible, with
optional parameters discoverable within the coding format itself, rather than
rely on external metadata that might be lost during transit.¶
16.7. UPGRADE TOKEN REGISTRY
The "Hypertext Transfer Protocol (HTTP) Upgrade Token Registry" defines the
namespace for protocol-name tokens used to identify protocols in the Upgrade
header field. The registry is maintained at
<https://www.iana.org/assignments/http-upgrade-tokens>.¶
Each registered protocol name is associated with contact information and an
optional set of specifications that details how the connection will be processed
after it has been upgraded.¶
Registrations happen on a "First Come First Served" basis (see Section 4.4 of
[RFC8126]) and are subject to the following rules:¶
1. A protocol-name token, once registered, stays registered forever.¶
2. A protocol-name token is case-insensitive and registered with the preferred
case to be generated by senders.¶
3. The registration MUST name a responsible party for the registration.¶
4. The registration MUST name a point of contact.¶
5. The registration MAY name a set of specifications associated with that
token. Such specifications need not be publicly available.¶
6. The registration SHOULD name a set of expected "protocol-version" tokens
associated with that token at the time of registration.¶
7. The responsible party MAY change the registration at any time. The IANA will
keep a record of all such changes, and make them available upon request.¶
8. The IESG MAY reassign responsibility for a protocol token. This will
normally only be used in the case when a responsible party cannot be
contacted.¶
17. SECURITY CONSIDERATIONS
This section is meant to inform developers, information providers, and users of
known security concerns relevant to HTTP semantics and its use for transferring
information over the Internet. Considerations related to caching are discussed
in Section 7 of [CACHING], and considerations related to HTTP/1.1 message syntax
and parsing are discussed in Section 11 of [HTTP/1.1].¶
The list of considerations below is not exhaustive. Most security concerns
related to HTTP semantics are about securing server-side applications (code
behind the HTTP interface), securing user agent processing of content received
via HTTP, or secure use of the Internet in general, rather than security of the
protocol. The security considerations for URIs, which are fundamental to HTTP
operation, are discussed in Section 7 of [URI]. Various organizations maintain
topical information and links to current research on Web application security
(e.g., [OWASP]).¶
17.1. ESTABLISHING AUTHORITY
HTTP relies on the notion of an "authoritative response": a response that has
been determined by (or at the direction of) the origin server identified within
the target URI to be the most appropriate response for that request given the
state of the target resource at the time of response message origination.¶
When a registered name is used in the authority component, the "http" URI scheme
(Section 4.2.1) relies on the user's local name resolution service to determine
where it can find authoritative responses. This means that any attack on a
user's network host table, cached names, or name resolution libraries becomes an
avenue for attack on establishing authority for "http" URIs. Likewise, the
user's choice of server for Domain Name Service (DNS), and the hierarchy of
servers from which it obtains resolution results, could impact the authenticity
of address mappings; DNS Security Extensions (DNSSEC, [RFC4033]) are one way to
improve authenticity, as are the various mechanisms for making DNS requests over
more secure transfer protocols.¶
Furthermore, after an IP address is obtained, establishing authority for an
"http" URI is vulnerable to attacks on Internet Protocol routing.¶
The "https" scheme (Section 4.2.2) is intended to prevent (or at least reveal)
many of these potential attacks on establishing authority, provided that the
negotiated connection is secured and the client properly verifies that the
communicating server's identity matches the target URI's authority component
(Section 4.3.4). Correctly implementing such verification can be difficult (see
[Georgiev]).¶
Authority for a given origin server can be delegated through protocol
extensions; for example, [ALTSVC]. Likewise, the set of servers for which a
connection is considered authoritative can be changed with a protocol extension
like [RFC8336].¶
Providing a response from a non-authoritative source, such as a shared proxy
cache, is often useful to improve performance and availability, but only to the
extent that the source can be trusted or the distrusted response can be safely
used.¶
Unfortunately, communicating authority to users can be difficult. For example,
"phishing" is an attack on the user's perception of authority, where that
perception can be misled by presenting similar branding in hypertext, possibly
aided by userinfo obfuscating the authority component (see Section 4.2.1). User
agents can reduce the impact of phishing attacks by enabling users to easily
inspect a target URI prior to making an action, by prominently distinguishing
(or rejecting) userinfo when present, and by not sending stored credentials and
cookies when the referring document is from an unknown or untrusted source.¶
17.2. RISKS OF INTERMEDIARIES
HTTP intermediaries are inherently situated for on-path attacks. Compromise of
the systems on which the intermediaries run can result in serious security and
privacy problems. Intermediaries might have access to security-related
information, personal information about individual users and organizations, and
proprietary information belonging to users and content providers. A compromised
intermediary, or an intermediary implemented or configured without regard to
security and privacy considerations, might be used in the commission of a wide
range of potential attacks.¶
Intermediaries that contain a shared cache are especially vulnerable to cache
poisoning attacks, as described in Section 7 of [CACHING].¶
Implementers need to consider the privacy and security implications of their
design and coding decisions, and of the configuration options they provide to
operators (especially the default configuration).¶
Intermediaries are no more trustworthy than the people and policies under which
they operate; HTTP cannot solve this problem.¶
17.3. ATTACKS BASED ON FILE AND PATH NAMES
Origin servers frequently make use of their local file system to manage the
mapping from target URI to resource representations. Most file systems are not
designed to protect against malicious file or path names. Therefore, an origin
server needs to avoid accessing names that have a special significance to the
system when mapping the target resource to files, folders, or directories.¶
For example, UNIX, Microsoft Windows, and other operating systems use ".." as a
path component to indicate a directory level above the current one, and they use
specially named paths or file names to send data to system devices. Similar
naming conventions might exist within other types of storage systems. Likewise,
local storage systems have an annoying tendency to prefer user-friendliness over
security when handling invalid or unexpected characters, recomposition of
decomposed characters, and case-normalization of case-insensitive names.¶
Attacks based on such special names tend to focus on either denial-of-service
(e.g., telling the server to read from a COM port) or disclosure of
configuration and source files that are not meant to be served.¶
17.4. ATTACKS BASED ON COMMAND, CODE, OR QUERY INJECTION
Origin servers often use parameters within the URI as a means of identifying
system services, selecting database entries, or choosing a data source. However,
data received in a request cannot be trusted. An attacker could construct any of
the request data elements (method, target URI, header fields, or content) to
contain data that might be misinterpreted as a command, code, or query when
passed through a command invocation, language interpreter, or database
interface.¶
For example, SQL injection is a common attack wherein additional query language
is inserted within some part of the target URI or header fields (e.g., Host,
Referer, etc.). If the received data is used directly within a SELECT statement,
the query language might be interpreted as a database command instead of a
simple string value. This type of implementation vulnerability is extremely
common, in spite of being easy to prevent.¶
In general, resource implementations ought to avoid use of request data in
contexts that are processed or interpreted as instructions. Parameters ought to
be compared to fixed strings and acted upon as a result of that comparison,
rather than passed through an interface that is not prepared for untrusted data.
Received data that isn't based on fixed parameters ought to be carefully
filtered or encoded to avoid being misinterpreted.¶
Similar considerations apply to request data when it is stored and later
processed, such as within log files, monitoring tools, or when included within a
data format that allows embedded scripts.¶
17.5. ATTACKS VIA PROTOCOL ELEMENT LENGTH
Because HTTP uses mostly textual, character-delimited fields, parsers are often
vulnerable to attacks based on sending very long (or very slow) streams of data,
particularly where an implementation is expecting a protocol element with no
predefined length (Section 2.3).¶
To promote interoperability, specific recommendations are made for minimum size
limits on fields (Section 5.4). These are minimum recommendations, chosen to be
supportable even by implementations with limited resources; it is expected that
most implementations will choose substantially higher limits.¶
A server can reject a message that has a target URI that is too long (Section
15.5.15) or request content that is too large (Section 15.5.14). Additional
status codes related to capacity limits have been defined by extensions to HTTP
[RFC6585].¶
Recipients ought to carefully limit the extent to which they process other
protocol elements, including (but not limited to) request methods, response
status phrases, field names, numeric values, and chunk lengths. Failure to limit
such processing can result in arbitrary code execution due to buffer or
arithmetic overflows, and increased vulnerability to denial-of-service attacks.¶
17.6. ATTACKS USING SHARED-DICTIONARY COMPRESSION
Some attacks on encrypted protocols use the differences in size created by
dynamic compression to reveal confidential information; for example, [BREACH].
These attacks rely on creating a redundancy between attacker-controlled content
and the confidential information, such that a dynamic compression algorithm
using the same dictionary for both content will compress more efficiently when
the attacker-controlled content matches parts of the confidential content.¶
HTTP messages can be compressed in a number of ways, including using TLS
compression, content codings, transfer codings, and other extension or
version-specific mechanisms.¶
The most effective mitigation for this risk is to disable compression on
sensitive data, or to strictly separate sensitive data from attacker-controlled
data so that they cannot share the same compression dictionary. With careful
design, a compression scheme can be designed in a way that is not considered
exploitable in limited use cases, such as HPACK ([HPACK]).¶
17.7. DISCLOSURE OF PERSONAL INFORMATION
Clients are often privy to large amounts of personal information, including both
information provided by the user to interact with resources (e.g., the user's
name, location, mail address, passwords, encryption keys, etc.) and information
about the user's browsing activity over time (e.g., history, bookmarks, etc.).
Implementations need to prevent unintentional disclosure of personal
information.¶
17.8. PRIVACY OF SERVER LOG INFORMATION
A server is in the position to save personal data about a user's requests over
time, which might identify their reading patterns or subjects of interest. In
particular, log information gathered at an intermediary often contains a history
of user agent interaction, across a multitude of sites, that can be traced to
individual users.¶
HTTP log information is confidential in nature; its handling is often
constrained by laws and regulations. Log information needs to be securely stored
and appropriate guidelines followed for its analysis. Anonymization of personal
information within individual entries helps, but it is generally not sufficient
to prevent real log traces from being re-identified based on correlation with
other access characteristics. As such, access traces that are keyed to a
specific client are unsafe to publish even if the key is pseudonymous.¶
To minimize the risk of theft or accidental publication, log information ought
to be purged of personally identifiable information, including user identifiers,
IP addresses, and user-provided query parameters, as soon as that information is
no longer necessary to support operational needs for security, auditing, or
fraud control.¶
17.9. DISCLOSURE OF SENSITIVE INFORMATION IN URIS
URIs are intended to be shared, not secured, even when they identify secure
resources. URIs are often shown on displays, added to templates when a page is
printed, and stored in a variety of unprotected bookmark lists. Many servers,
proxies, and user agents log or display the target URI in places where it might
be visible to third parties. It is therefore unwise to include information
within a URI that is sensitive, personally identifiable, or a risk to disclose.¶
When an application uses client-side mechanisms to construct a target URI out of
user-provided information, such as the query fields of a form using GET,
potentially sensitive data might be provided that would not be appropriate for
disclosure within a URI. POST is often preferred in such cases because it
usually doesn't construct a URI; instead, POST of a form transmits the
potentially sensitive data in the request content. However, this hinders caching
and uses an unsafe method for what would otherwise be a safe request.
Alternative workarounds include transforming the user-provided data prior to
constructing the URI or filtering the data to only include common values that
are not sensitive. Likewise, redirecting the result of a query to a different
(server-generated) URI can remove potentially sensitive data from later links
and provide a cacheable response for later reuse.¶
Since the Referer header field tells a target site about the context that
resulted in a request, it has the potential to reveal information about the
user's immediate browsing history and any personal information that might be
found in the referring resource's URI. Limitations on the Referer header field
are described in Section 10.1.3 to address some of its security considerations.¶
17.10. APPLICATION HANDLING OF FIELD NAMES
Servers often use non-HTTP gateway interfaces and frameworks to process a
received request and produce content for the response. For historical reasons,
such interfaces often pass received field names as external variable names,
using a name mapping suitable for environment variables.¶
For example, the Common Gateway Interface (CGI) mapping of protocol-specific
meta-variables, defined by Section 4.1.18 of [RFC3875], is applied to received
header fields that do not correspond to one of CGI's standard variables; the
mapping consists of prepending "HTTP_" to each name and changing all instances
of hyphen ("-") to underscore ("_"). This same mapping has been inherited by
many other application frameworks in order to simplify moving applications from
one platform to the next.¶
In CGI, a received Content-Length field would be passed as the meta-variable
"CONTENT_LENGTH" with a string value matching the received field's value. In
contrast, a received "Content_Length" header field would be passed as the
protocol-specific meta-variable "HTTP_CONTENT_LENGTH", which might lead to some
confusion if an application mistakenly reads the protocol-specific meta-variable
instead of the default one. (This historical practice is why Section 16.3.2.1
discourages the creation of new field names that contain an underscore.)¶
Unfortunately, mapping field names to different interface names can lead to
security vulnerabilities if the mapping is incomplete or ambiguous. For example,
if an attacker were to send a field named "Transfer_Encoding", a naive interface
might map that to the same variable name as the "Transfer-Encoding" field,
resulting in a potential request smuggling vulnerability (Section 11.2 of
[HTTP/1.1]).¶
To mitigate the associated risks, implementations that perform such mappings are
advised to make the mapping unambiguous and complete for the full range of
potential octets received as a name (including those that are discouraged or
forbidden by the HTTP grammar). For example, a field with an unusual name
character might result in the request being blocked, the specific field being
removed, or the name being passed with a different prefix to distinguish it from
other fields.¶
17.11. DISCLOSURE OF FRAGMENT AFTER REDIRECTS
Although fragment identifiers used within URI references are not sent in
requests, implementers ought to be aware that they will be visible to the user
agent and any extensions or scripts running as a result of the response. In
particular, when a redirect occurs and the original request's fragment
identifier is inherited by the new reference in Location (Section 10.2.2), this
might have the effect of disclosing one site's fragment to another site. If the
first site uses personal information in fragments, it ought to ensure that
redirects to other sites include a (possibly empty) fragment component in order
to block that inheritance.¶
17.12. DISCLOSURE OF PRODUCT INFORMATION
The User-Agent (Section 10.1.5), Via (Section 7.6.3), and Server (Section
10.2.4) header fields often reveal information about the respective sender's
software systems. In theory, this can make it easier for an attacker to exploit
known security holes; in practice, attackers tend to try all potential holes
regardless of the apparent software versions being used.¶
Proxies that serve as a portal through a network firewall ought to take special
precautions regarding the transfer of header information that might identify
hosts behind the firewall. The Via header field allows intermediaries to replace
sensitive machine names with pseudonyms.¶
17.13. BROWSER FINGERPRINTING
Browser fingerprinting is a set of techniques for identifying a specific user
agent over time through its unique set of characteristics. These characteristics
might include information related to how it uses the underlying transport
protocol, feature capabilities, and scripting environment, though of particular
interest here is the set of unique characteristics that might be communicated
via HTTP. Fingerprinting is considered a privacy concern because it enables
tracking of a user agent's behavior over time ([Bujlow]) without the
corresponding controls that the user might have over other forms of data
collection (e.g., cookies). Many general-purpose user agents (i.e., Web
browsers) have taken steps to reduce their fingerprints.¶
There are a number of request header fields that might reveal information to
servers that is sufficiently unique to enable fingerprinting. The From header
field is the most obvious, though it is expected that From will only be sent
when self-identification is desired by the user. Likewise, Cookie header fields
are deliberately designed to enable re-identification, so fingerprinting
concerns only apply to situations where cookies are disabled or restricted by
the user agent's configuration.¶
The User-Agent header field might contain enough information to uniquely
identify a specific device, usually when combined with other characteristics,
particularly if the user agent sends excessive details about the user's system
or extensions. However, the source of unique information that is least expected
by users is proactive negotiation (Section 12.1), including the Accept,
Accept-Charset, Accept-Encoding, and Accept-Language header fields.¶
In addition to the fingerprinting concern, detailed use of the Accept-Language
header field can reveal information the user might consider to be of a private
nature. For example, understanding a given language set might be strongly
correlated to membership in a particular ethnic group. An approach that limits
such loss of privacy would be for a user agent to omit the sending of
Accept-Language except for sites that have been explicitly permitted, perhaps
via interaction after detecting a Vary header field that indicates language
negotiation might be useful.¶
In environments where proxies are used to enhance privacy, user agents ought to
be conservative in sending proactive negotiation header fields. General-purpose
user agents that provide a high degree of header field configurability ought to
inform users about the loss of privacy that might result if too much detail is
provided. As an extreme privacy measure, proxies could filter the proactive
negotiation header fields in relayed requests.¶
17.14. VALIDATOR RETENTION
The validators defined by this specification are not intended to ensure the
validity of a representation, guard against malicious changes, or detect on-path
attacks. At best, they enable more efficient cache updates and optimistic
concurrent writes when all participants are behaving nicely. At worst, the
conditions will fail and the client will receive a response that is no more
harmful than an HTTP exchange without conditional requests.¶
An entity tag can be abused in ways that create privacy risks. For example, a
site might deliberately construct a semantically invalid entity tag that is
unique to the user or user agent, send it in a cacheable response with a long
freshness time, and then read that entity tag in later conditional requests as a
means of re-identifying that user or user agent. Such an identifying tag would
become a persistent identifier for as long as the user agent retained the
original cache entry. User agents that cache representations ought to ensure
that the cache is cleared or replaced whenever the user performs
privacy-maintaining actions, such as clearing stored cookies or changing to a
private browsing mode.¶
17.15. DENIAL-OF-SERVICE ATTACKS USING RANGE
Unconstrained multiple range requests are susceptible to denial-of-service
attacks because the effort required to request many overlapping ranges of the
same data is tiny compared to the time, memory, and bandwidth consumed by
attempting to serve the requested data in many parts. Servers ought to ignore,
coalesce, or reject egregious range requests, such as requests for more than two
overlapping ranges or for many small ranges in a single set, particularly when
the ranges are requested out of order for no apparent reason. Multipart range
requests are not designed to support random access.¶
17.16. AUTHENTICATION CONSIDERATIONS
Everything about the topic of HTTP authentication is a security consideration,
so the list of considerations below is not exhaustive. Furthermore, it is
limited to security considerations regarding the authentication framework, in
general, rather than discussing all of the potential considerations for specific
authentication schemes (which ought to be documented in the specifications that
define those schemes). Various organizations maintain topical information and
links to current research on Web application security (e.g., [OWASP]), including
common pitfalls for implementing and using the authentication schemes found in
practice.¶
17.16.1. CONFIDENTIALITY OF CREDENTIALS
The HTTP authentication framework does not define a single mechanism for
maintaining the confidentiality of credentials; instead, each authentication
scheme defines how the credentials are encoded prior to transmission. While this
provides flexibility for the development of future authentication schemes, it is
inadequate for the protection of existing schemes that provide no
confidentiality on their own, or that do not sufficiently protect against replay
attacks. Furthermore, if the server expects credentials that are specific to
each individual user, the exchange of those credentials will have the effect of
identifying that user even if the content within credentials remains
confidential.¶
HTTP depends on the security properties of the underlying transport- or
session-level connection to provide confidential transmission of fields.
Services that depend on individual user authentication require a secured
connection prior to exchanging credentials (Section 4.2.2).¶
17.16.2. CREDENTIALS AND IDLE CLIENTS
Existing HTTP clients and user agents typically retain authentication
information indefinitely. HTTP does not provide a mechanism for the origin
server to direct clients to discard these cached credentials, since the protocol
has no awareness of how credentials are obtained or managed by the user agent.
The mechanisms for expiring or revoking credentials can be specified as part of
an authentication scheme definition.¶
Circumstances under which credential caching can interfere with the
application's security model include but are not limited to:¶
* Clients that have been idle for an extended period, following which the
server might wish to cause the client to re-prompt the user for credentials.¶
* Applications that include a session termination indication (such as a
"logout" or "commit" button on a page) after which the server side of the
application "knows" that there is no further reason for the client to retain
the credentials.¶
User agents that cache credentials are encouraged to provide a readily
accessible mechanism for discarding cached credentials under user control.¶
17.16.3. PROTECTION SPACES
Authentication schemes that solely rely on the "realm" mechanism for
establishing a protection space will expose credentials to all resources on an
origin server. Clients that have successfully made authenticated requests with a
resource can use the same authentication credentials for other resources on the
same origin server. This makes it possible for a different resource to harvest
authentication credentials for other resources.¶
This is of particular concern when an origin server hosts resources for multiple
parties under the same origin (Section 11.5). Possible mitigation strategies
include restricting direct access to authentication credentials (i.e., not
making the content of the Authorization request header field available), and
separating protection spaces by using a different host name (or port number) for
each party.¶
17.16.4. ADDITIONAL RESPONSE FIELDS
Adding information to responses that are sent over an unencrypted channel can
affect security and privacy. The presence of the Authentication-Info and
Proxy-Authentication-Info header fields alone indicates that HTTP authentication
is in use. Additional information could be exposed by the contents of the
authentication-scheme specific parameters; this will have to be considered in
the definitions of these schemes.¶
18. IANA CONSIDERATIONS
The change controller for the following registrations is: "IETF (iesg@ietf.org)
- Internet Engineering Task Force".¶
18.1. URI SCHEME REGISTRATION
IANA has updated the "Uniform Resource Identifier (URI) Schemes" registry
[BCP35] at <https://www.iana.org/assignments/uri-schemes/> with the permanent
schemes listed in Table 2 in Section 4.2.¶
18.2. METHOD REGISTRATION
IANA has updated the "Hypertext Transfer Protocol (HTTP) Method Registry" at
<https://www.iana.org/assignments/http-methods> with the registration procedure
of Section 16.1.1 and the method names summarized in the following table.¶
Table 7 Method Safe Idempotent Section CONNECT no no 9.3.6 DELETE no yes 9.3.5
GET yes yes 9.3.1 HEAD yes yes 9.3.2 OPTIONS yes yes 9.3.7 POST no no 9.3.3 PUT
no yes 9.3.4 TRACE yes yes 9.3.8 * no no 18.2
The method name "*" is reserved because using "*" as a method name would
conflict with its usage as a wildcard in some fields (e.g.,
"Access-Control-Request-Method").¶
18.3. STATUS CODE REGISTRATION
IANA has updated the "Hypertext Transfer Protocol (HTTP) Status Code Registry"
at <https://www.iana.org/assignments/http-status-codes> with the registration
procedure of Section 16.2.1 and the status code values summarized in the
following table.¶
Table 8 Value Description Section 100 Continue 15.2.1 101 Switching Protocols
15.2.2 200 OK 15.3.1 201 Created 15.3.2 202 Accepted 15.3.3 203
Non-Authoritative Information 15.3.4 204 No Content 15.3.5 205 Reset Content
15.3.6 206 Partial Content 15.3.7 300 Multiple Choices 15.4.1 301 Moved
Permanently 15.4.2 302 Found 15.4.3 303 See Other 15.4.4 304 Not Modified 15.4.5
305 Use Proxy 15.4.6 306 (Unused) 15.4.7 307 Temporary Redirect 15.4.8 308
Permanent Redirect 15.4.9 400 Bad Request 15.5.1 401 Unauthorized 15.5.2 402
Payment Required 15.5.3 403 Forbidden 15.5.4 404 Not Found 15.5.5 405 Method Not
Allowed 15.5.6 406 Not Acceptable 15.5.7 407 Proxy Authentication Required
15.5.8 408 Request Timeout 15.5.9 409 Conflict 15.5.10 410 Gone 15.5.11 411
Length Required 15.5.12 412 Precondition Failed 15.5.13 413 Content Too Large
15.5.14 414 URI Too Long 15.5.15 415 Unsupported Media Type 15.5.16 416 Range
Not Satisfiable 15.5.17 417 Expectation Failed 15.5.18 418 (Unused) 15.5.19 421
Misdirected Request 15.5.20 422 Unprocessable Content 15.5.21 426 Upgrade
Required 15.5.22 500 Internal Server Error 15.6.1 501 Not Implemented 15.6.2 502
Bad Gateway 15.6.3 503 Service Unavailable 15.6.4 504 Gateway Timeout 15.6.5 505
HTTP Version Not Supported 15.6.6
18.4. FIELD NAME REGISTRATION
This specification updates the HTTP-related aspects of the existing registration
procedures for message header fields defined in [RFC3864]. It replaces the old
procedures as they relate to HTTP by defining a new registration procedure and
moving HTTP field definitions into a separate registry.¶
IANA has created a new registry titled "Hypertext Transfer Protocol (HTTP) Field
Name Registry" as outlined in Section 16.3.1.¶
IANA has moved all entries in the "Permanent Message Header Field Names" and
"Provisional Message Header Field Names" registries (see
<https://www.iana.org/assignments/message-headers/>) with the protocol 'http' to
this registry and has applied the following changes:¶
1. The 'Applicable Protocol' field has been omitted.¶
2. Entries that had a status of 'standard', 'experimental', 'reserved', or
'informational' have been made to have a status of 'permanent'.¶
3. Provisional entries without a status have been made to have a status of
'provisional'.¶
4. Permanent entries without a status (after confirmation that the registration
document did not define one) have been made to have a status of
'provisional'. The expert(s) can choose to update the entries' status if
there is evidence that another is more appropriate.¶
IANA has annotated the "Permanent Message Header Field Names" and "Provisional
Message Header Field Names" registries with the following note to indicate that
HTTP field name registrations have moved:¶
Note¶
HTTP field name registrations have been moved to
[https://www.iana.org/assignments/http-fields] per [RFC9110].¶
IANA has updated the "Hypertext Transfer Protocol (HTTP) Field Name Registry"
with the field names listed in the following table.¶
Table 9 Field Name Status Section Comments Accept permanent 12.5.1
Accept-Charset deprecated 12.5.2 Accept-Encoding permanent 12.5.3
Accept-Language permanent 12.5.4 Accept-Ranges permanent 14.3 Allow permanent
10.2.1 Authentication-Info permanent 11.6.3 Authorization permanent 11.6.2
Connection permanent 7.6.1 Content-Encoding permanent 8.4 Content-Language
permanent 8.5 Content-Length permanent 8.6 Content-Location permanent 8.7
Content-Range permanent 14.4 Content-Type permanent 8.3 Date permanent 6.6.1
ETag permanent 8.8.3 Expect permanent 10.1.1 From permanent 10.1.2 Host
permanent 7.2 If-Match permanent 13.1.1 If-Modified-Since permanent 13.1.3
If-None-Match permanent 13.1.2 If-Range permanent 13.1.5 If-Unmodified-Since
permanent 13.1.4 Last-Modified permanent 8.8.2 Location permanent 10.2.2
Max-Forwards permanent 7.6.2 Proxy-Authenticate permanent 11.7.1
Proxy-Authentication-Info permanent 11.7.3 Proxy-Authorization permanent 11.7.2
Range permanent 14.2 Referer permanent 10.1.3 Retry-After permanent 10.2.3
Server permanent 10.2.4 TE permanent 10.1.4 Trailer permanent 6.6.2 Upgrade
permanent 7.8 User-Agent permanent 10.1.5 Vary permanent 12.5.5 Via permanent
7.6.3 WWW-Authenticate permanent 11.6.1 * permanent 12.5.5 (reserved)
The field name "*" is reserved because using that name as an HTTP header field
might conflict with its special semantics in the Vary header field (Section
12.5.5).¶
IANA has updated the "Content-MD5" entry in the new registry to have a status of
'obsoleted' with references to Section 14.15 of [RFC2616] (for the definition of
the header field) and Appendix B of [RFC7231] (which removed the field
definition from the updated specification).¶
18.5. AUTHENTICATION SCHEME REGISTRATION
IANA has updated the "Hypertext Transfer Protocol (HTTP) Authentication Scheme
Registry" at <https://www.iana.org/assignments/http-authschemes> with the
registration procedure of Section 16.4.1. No authentication schemes are defined
in this document.¶
18.6. CONTENT CODING REGISTRATION
IANA has updated the "HTTP Content Coding Registry" at
<https://www.iana.org/assignments/http-parameters/> with the registration
procedure of Section 16.6.1 and the content coding names summarized in the table
below.¶
Table 10 Name Description Section compress UNIX "compress" data format [Welch]
8.4.1.1 deflate "deflate" compressed data ([RFC1951]) inside the "zlib" data
format ([RFC1950]) 8.4.1.2 gzip GZIP file format [RFC1952] 8.4.1.3 identity
Reserved 12.5.3 x-compress Deprecated (alias for compress) 8.4.1.1 x-gzip
Deprecated (alias for gzip) 8.4.1.3
18.7. RANGE UNIT REGISTRATION
IANA has updated the "HTTP Range Unit Registry" at
<https://www.iana.org/assignments/http-parameters/> with the registration
procedure of Section 16.5.1 and the range unit names summarized in the table
below.¶
Table 11 Range Unit Name Description Section bytes a range of octets 14.1.2 none
reserved as keyword to indicate range requests are not supported 14.3
18.8. MEDIA TYPE REGISTRATION
IANA has updated the "Media Types" registry at
<https://www.iana.org/assignments/media-types> with the registration information
in Section 14.6 for the media type "multipart/byteranges".¶
IANA has updated the registry note about "q" parameters with a link to Section
12.5.1 of this document.¶
18.9. PORT REGISTRATION
IANA has updated the "Service Name and Transport Protocol Port Number Registry"
at <https://www.iana.org/assignments/service-names-port-numbers/> for the
services on ports 80 and 443 that use UDP or TCP to:¶
1. use this document as "Reference", and¶
2. when currently unspecified, set "Assignee" to "IESG" and "Contact" to
"IETF_Chair".¶
18.10. UPGRADE TOKEN REGISTRATION
IANA has updated the "Hypertext Transfer Protocol (HTTP) Upgrade Token Registry"
at <https://www.iana.org/assignments/http-upgrade-tokens> with the registration
procedure described in Section 16.7 and the upgrade token names summarized in
the following table.¶
Table 12 Name Description Expected Version Tokens Section HTTP Hypertext
Transfer Protocol any DIGIT.DIGIT (e.g., "2.0") 2.5
19. REFERENCES
19.1. NORMATIVE REFERENCES
[CACHING] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP
Caching", STD 98, RFC 9111, DOI 10.17487/RFC9111, June 2022,
<https://www.rfc-editor.org/info/rfc9111>. [RFC1950] Deutsch, P. and J-L.
Gailly, "ZLIB Compressed Data Format Specification version 3.3", RFC 1950, DOI
10.17487/RFC1950, May 1996, <https://www.rfc-editor.org/info/rfc1950>. [RFC1951]
Deutsch, P., "DEFLATE Compressed Data Format Specification version 1.3", RFC
1951, DOI 10.17487/RFC1951, May 1996, <https://www.rfc-editor.org/info/rfc1951>.
[RFC1952] Deutsch, P., "GZIP file format specification version 4.3", RFC 1952,
DOI 10.17487/RFC1952, May 1996, <https://www.rfc-editor.org/info/rfc1952>.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions
(MIME) Part Two: Media Types", RFC 2046, DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/info/rfc2046>. [RFC2119] Bradner, S., "Key words for
use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI
10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC4647] Phillips, A., Ed. and M. Davis, Ed., "Matching of Language Tags", BCP
47, RFC 4647, DOI 10.17487/RFC4647, September 2006,
<https://www.rfc-editor.org/info/rfc4647>. [RFC4648] Josefsson, S., "The Base16,
Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October
2006, <https://www.rfc-editor.org/info/rfc4648>. [RFC5234] Crocker, D., Ed. and
P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008, <https://www.rfc-editor.org/info/rfc5234>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and
W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>. [RFC5322] Resnick, P., Ed., "Internet
Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>. [RFC5646] Phillips, A., Ed. and M.
Davis, Ed., "Tags for Identifying Languages", BCP 47, RFC 5646, DOI
10.17487/RFC5646, September 2009, <https://www.rfc-editor.org/info/rfc5646>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of
Domain-Based Application Service Identity within Internet Public Key
Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 2011,
<https://www.rfc-editor.org/info/rfc6125>. [RFC6365] Hoffman, P. and J. Klensin,
"Terminology Used in Internationalization in the IETF", BCP 166, RFC 6365, DOI
10.17487/RFC6365, September 2011, <https://www.rfc-editor.org/info/rfc6365>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC 7405, DOI
10.17487/RFC7405, December 2014, <https://www.rfc-editor.org/info/rfc7405>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key
Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017,
<https://www.rfc-editor.org/info/rfc8174>. [TCP] Postel, J., "Transmission
Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>. [TLS13] Rescorla, E., "The Transport
Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446,
August 2018, <https://www.rfc-editor.org/info/rfc8446>. [URI] Berners-Lee, T.,
Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic
Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>. [USASCII] American National Standards
Institute, "Coded Character Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986. [Welch] Welch, T., "A Technique for
High-Performance Data Compression", IEEE Computer 17(6), DOI
10.1109/MC.1984.1659158, June 1984,
<https://ieeexplore.ieee.org/document/1659158/>.
19.2. INFORMATIVE REFERENCES
[ALTSVC] Nottingham, M., McManus, P., and J. Reschke, "HTTP Alternative
Services", RFC 7838, DOI 10.17487/RFC7838, April 2016,
<https://www.rfc-editor.org/info/rfc7838>. [BCP13]
Freed, N. and J. Klensin, "Multipurpose Internet Mail Extensions (MIME) Part
Four: Registration Procedures", BCP 13, RFC 4289, December 2005.
Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 6838, January 2013.
<https://www.rfc-editor.org/info/bcp13> [BCP178]
Saint-Andre, P., Crocker, D., and M. Nottingham, "Deprecating the "X-" Prefix
and Similar Constructs in Application Protocols", BCP 178, RFC 6648, June 2012.
<https://www.rfc-editor.org/info/bcp178> [BCP35]
Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines and Registration
Procedures for URI Schemes", BCP 35, RFC 7595, June 2015.
<https://www.rfc-editor.org/info/bcp35> [BREACH] Gluck, Y., Harris, N., and A.
Prado, "BREACH: Reviving the CRIME Attack", July 2013,
<http://breachattack.com/resources/BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf>.
[Bujlow] Bujlow, T., Carela-Español, V., Solé-Pareta, J., and P. Barlet-Ros, "A
Survey on Web Tracking: Mechanisms, Implications, and Defenses", In Proceedings
of the IEEE 105(8), DOI 10.1109/JPROC.2016.2637878, August 2017,
<https://doi.org/10.1109/JPROC.2016.2637878>. [COOKIE] Barth, A., "HTTP State
Management Mechanism", RFC 6265, DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>. [Err1912] RFC Errata, Erratum ID
1912, RFC 2978, <https://www.rfc-editor.org/errata/eid1912>. [Err5433] RFC
Errata, Erratum ID 5433, RFC 2978, <https://www.rfc-editor.org/errata/eid5433>.
[Georgiev] Georgiev, M., Iyengar, S., Jana, S., Anubhai, R., Boneh, D., and V.
Shmatikov, "The Most Dangerous Code in the World: Validating SSL Certificates in
Non-Browser Software", In Proceedings of the 2012 ACM Conference on Computer and
Communications Security (CCS '12), pp. 38-49, DOI 10.1145/2382196.2382204,
October 2012, <https://doi.org/10.1145/2382196.2382204>. [HPACK] Peon, R. and H.
Ruellan, "HPACK: Header Compression for HTTP/2", RFC 7541, DOI 10.17487/RFC7541,
May 2015, <https://www.rfc-editor.org/info/rfc7541>. [HTTP/1.0] Berners-Lee, T.,
Fielding, R., and H. Frystyk, "Hypertext Transfer Protocol -- HTTP/1.0", RFC
1945, DOI 10.17487/RFC1945, May 1996, <https://www.rfc-editor.org/info/rfc1945>.
[HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed.,
"HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112, June 2022,
<https://www.rfc-editor.org/info/rfc9112>. [HTTP/2] Thomson, M., Ed. and C.
Benfield, Ed., "HTTP/2", RFC 9113, DOI 10.17487/RFC9113, June 2022,
<https://www.rfc-editor.org/info/rfc9113>. [HTTP/3] Bishop, M., Ed., "HTTP/3",
RFC 9114, DOI 10.17487/RFC9114, June 2022,
<https://www.rfc-editor.org/info/rfc9114>. [ISO-8859-1] International
Organization for Standardization, "Information technology -- 8-bit single-byte
coded graphic character sets -- Part 1: Latin alphabet No. 1", ISO/IEC
8859-1:1998, 1998. [Kri2001] Kristol, D., "HTTP Cookies: Standards, Privacy, and
Politics", ACM Transactions on Internet Technology 1(2), November 2001,
<http://arxiv.org/abs/cs.SE/0105018>. [OWASP] The Open Web Application Security
Project, <https://www.owasp.org/>. [REST] Fielding, R.T., "Architectural Styles
and the Design of Network-based Software Architectures", Doctoral Dissertation,
University of California, Irvine, September 2000,
<https://roy.gbiv.com/pubs/dissertation/top.htm>. [RFC1919] Chatel, M.,
"Classical versus Transparent IP Proxies", RFC 1919, DOI 10.17487/RFC1919, March
1996, <https://www.rfc-editor.org/info/rfc1919>. [RFC2047] Moore, K., "MIME
(Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions
for Non-ASCII Text", RFC 2047, DOI 10.17487/RFC2047, November 1996,
<https://www.rfc-editor.org/info/rfc2047>. [RFC2068] Fielding, R., Gettys, J.,
Mogul, J., Frystyk, H., and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2068, DOI 10.17487/RFC2068, January 1997,
<https://www.rfc-editor.org/info/rfc2068>. [RFC2145] Mogul, J. C., Fielding, R.,
Gettys, J., and H. Frystyk, "Use and Interpretation of HTTP Version Numbers",
RFC 2145, DOI 10.17487/RFC2145, May 1997,
<https://www.rfc-editor.org/info/rfc2145>. [RFC2295] Holtman, K. and A. Mutz,
"Transparent Content Negotiation in HTTP", RFC 2295, DOI 10.17487/RFC2295, March
1998, <https://www.rfc-editor.org/info/rfc2295>. [RFC2324] Masinter, L., "Hyper
Text Coffee Pot Control Protocol (HTCPCP/1.0)", RFC 2324, DOI 10.17487/RFC2324,
1 April 1998, <https://www.rfc-editor.org/info/rfc2324>. [RFC2557] Palme, J.,
Hopmann, A., and N. Shelness, "MIME Encapsulation of Aggregate Documents, such
as HTML (MHTML)", RFC 2557, DOI 10.17487/RFC2557, March 1999,
<https://www.rfc-editor.org/info/rfc2557>. [RFC2616] Fielding, R., Gettys, J.,
Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, DOI 10.17487/RFC2616, June 1999,
<https://www.rfc-editor.org/info/rfc2616>. [RFC2617] Franks, J., Hallam-Baker,
P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication", RFC 2617, DOI
10.17487/RFC2617, June 1999, <https://www.rfc-editor.org/info/rfc2617>.
[RFC2774] Nielsen, H., Leach, P., and S. Lawrence, "An HTTP Extension
Framework", RFC 2774, DOI 10.17487/RFC2774, February 2000,
<https://www.rfc-editor.org/info/rfc2774>. [RFC2818] Rescorla, E., "HTTP Over
TLS", RFC 2818, DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>. [RFC2978] Freed, N. and J. Postel,
"IANA Charset Registration Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,
October 2000, <https://www.rfc-editor.org/info/rfc2978>. [RFC3040] Cooper, I.,
Melve, I., and G. Tomlinson, "Internet Web Replication and Caching Taxonomy",
RFC 3040, DOI 10.17487/RFC3040, January 2001,
<https://www.rfc-editor.org/info/rfc3040>. [RFC3864] Klyne, G., Nottingham, M.,
and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC
3864, DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>. [RFC3875] Robinson, D. and K. Coar,
"The Common Gateway Interface (CGI) Version 1.1", RFC 3875, DOI
10.17487/RFC3875, October 2004, <https://www.rfc-editor.org/info/rfc3875>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS
Security Introduction and Requirements", RFC 4033, DOI 10.17487/RFC4033, March
2005, <https://www.rfc-editor.org/info/rfc4033>. [RFC4559] Jaganathan, K., Zhu,
L., and J. Brezak, "SPNEGO-based Kerberos and NTLM HTTP Authentication in
Microsoft Windows", RFC 4559, DOI 10.17487/RFC4559, June 2006,
<https://www.rfc-editor.org/info/rfc4559>. [RFC5789] Dusseault, L. and J. Snell,
"PATCH Method for HTTP", RFC 5789, DOI 10.17487/RFC5789, March 2010,
<https://www.rfc-editor.org/info/rfc5789>. [RFC5905] Mills, D., Martin, J., Ed.,
Burbank, J., and W. Kasch, "Network Time Protocol Version 4: Protocol and
Algorithms Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>. [RFC6454] Barth, A., "The Web Origin
Concept", RFC 6454, DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>. [RFC6585] Nottingham, M. and R.
Fielding, "Additional HTTP Status Codes", RFC 6585, DOI 10.17487/RFC6585, April
2012, <https://www.rfc-editor.org/info/rfc6585>. [RFC7230] Fielding, R., Ed. and
J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and
Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>. [RFC7231] Fielding, R., Ed. and J.
Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content",
RFC 7231, DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>. [RFC7232] Fielding, R., Ed. and J.
Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests",
RFC 7232, DOI 10.17487/RFC7232, June 2014,
<https://www.rfc-editor.org/info/rfc7232>. [RFC7233] Fielding, R., Ed., Lafon,
Y., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Range
Requests", RFC 7233, DOI 10.17487/RFC7233, June 2014,
<https://www.rfc-editor.org/info/rfc7233>. [RFC7234] Fielding, R., Ed.,
Nottingham, M., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol
(HTTP/1.1): Caching", RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>. [RFC7235] Fielding, R., Ed. and J.
Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Authentication", RFC
7235, DOI 10.17487/RFC7235, June 2014,
<https://www.rfc-editor.org/info/rfc7235>. [RFC7538] Reschke, J., "The Hypertext
Transfer Protocol Status Code 308 (Permanent Redirect)", RFC 7538, DOI
10.17487/RFC7538, April 2015, <https://www.rfc-editor.org/info/rfc7538>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext Transfer
Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>. [RFC7578] Masinter, L., "Returning
Values from Forms: multipart/form-data", RFC 7578, DOI 10.17487/RFC7578, July
2015, <https://www.rfc-editor.org/info/rfc7578>. [RFC7615] Reschke, J., "HTTP
Authentication-Info and Proxy-Authentication-Info Response Header Fields", RFC
7615, DOI 10.17487/RFC7615, September 2015,
<https://www.rfc-editor.org/info/rfc7615>. [RFC7616] Shekh-Yusef, R., Ed.,
Ahrens, D., and S. Bremer, "HTTP Digest Access Authentication", RFC 7616, DOI
10.17487/RFC7616, September 2015, <https://www.rfc-editor.org/info/rfc7616>.
[RFC7617] Reschke, J., "The 'Basic' HTTP Authentication Scheme", RFC 7617, DOI
10.17487/RFC7617, September 2015, <https://www.rfc-editor.org/info/rfc7617>.
[RFC7694] Reschke, J., "Hypertext Transfer Protocol (HTTP) Client-Initiated
Content-Encoding", RFC 7694, DOI 10.17487/RFC7694, November 2015,
<https://www.rfc-editor.org/info/rfc7694>. [RFC8126] Cotton, M., Leiba, B., and
T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP
26, RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>. [RFC8187] Reschke, J., "Indicating
Character Encoding and Language for HTTP Header Field Parameters", RFC 8187, DOI
10.17487/RFC8187, September 2017, <https://www.rfc-editor.org/info/rfc8187>.
[RFC8246] McManus, P., "HTTP Immutable Responses", RFC 8246, DOI
10.17487/RFC8246, September 2017, <https://www.rfc-editor.org/info/rfc8246>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288, DOI 10.17487/RFC8288, October
2017, <https://www.rfc-editor.org/info/rfc8288>. [RFC8336] Nottingham, M. and E.
Nygren, "The ORIGIN HTTP/2 Frame", RFC 8336, DOI 10.17487/RFC8336, March 2018,
<https://www.rfc-editor.org/info/rfc8336>. [RFC8615] Nottingham, M., "Well-Known
Uniform Resource Identifiers (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/info/rfc8615>. [RFC8941] Nottingham, M. and P-H.
Kamp, "Structured Field Values for HTTP", RFC 8941, DOI 10.17487/RFC8941,
February 2021, <https://www.rfc-editor.org/info/rfc8941>. [Sniffing] WHATWG,
"MIME Sniffing", <https://mimesniff.spec.whatwg.org>. [WEBDAV] Dusseault, L.,
Ed., "HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)",
RFC 4918, DOI 10.17487/RFC4918, June 2007,
<https://www.rfc-editor.org/info/rfc4918>.
APPENDIX A. COLLECTED ABNF
In the collected ABNF below, list rules are expanded per Section 5.6.1.¶
Accept = [ ( media-range [ weight ] ) *( OWS "," OWS ( media-range [
weight ] ) ) ]
Accept-Charset = [ ( ( token / "*" ) [ weight ] ) *( OWS "," OWS ( (
token / "*" ) [ weight ] ) ) ]
Accept-Encoding = [ ( codings [ weight ] ) *( OWS "," OWS ( codings [
weight ] ) ) ]
Accept-Language = [ ( language-range [ weight ] ) *( OWS "," OWS (
language-range [ weight ] ) ) ]
Accept-Ranges = acceptable-ranges
Allow = [ method *( OWS "," OWS method ) ]
Authentication-Info = [ auth-param *( OWS "," OWS auth-param ) ]
Authorization = credentials
BWS = OWS
Connection = [ connection-option *( OWS "," OWS connection-option )
]
Content-Encoding = [ content-coding *( OWS "," OWS content-coding )
]
Content-Language = [ language-tag *( OWS "," OWS language-tag ) ]
Content-Length = 1*DIGIT
Content-Location = absolute-URI / partial-URI
Content-Range = range-unit SP ( range-resp / unsatisfied-range )
Content-Type = media-type
Date = HTTP-date
ETag = entity-tag
Expect = [ expectation *( OWS "," OWS expectation ) ]
From = mailbox
GMT = %x47.4D.54 ; GMT
HTTP-date = IMF-fixdate / obs-date
Host = uri-host [ ":" port ]
IMF-fixdate = day-name "," SP date1 SP time-of-day SP GMT
If-Match = "*" / [ entity-tag *( OWS "," OWS entity-tag ) ]
If-Modified-Since = HTTP-date
If-None-Match = "*" / [ entity-tag *( OWS "," OWS entity-tag ) ]
If-Range = entity-tag / HTTP-date
If-Unmodified-Since = HTTP-date
Last-Modified = HTTP-date
Location = URI-reference
Max-Forwards = 1*DIGIT
OWS = *( SP / HTAB )
Proxy-Authenticate = [ challenge *( OWS "," OWS challenge ) ]
Proxy-Authentication-Info = [ auth-param *( OWS "," OWS auth-param )
]
Proxy-Authorization = credentials
RWS = 1*( SP / HTAB )
Range = ranges-specifier
Referer = absolute-URI / partial-URI
Retry-After = HTTP-date / delay-seconds
Server = product *( RWS ( product / comment ) )
TE = [ t-codings *( OWS "," OWS t-codings ) ]
Trailer = [ field-name *( OWS "," OWS field-name ) ]
URI-reference = <URI-reference, see [URI], Section 4.1>
Upgrade = [ protocol *( OWS "," OWS protocol ) ]
User-Agent = product *( RWS ( product / comment ) )
Vary = [ ( "*" / field-name ) *( OWS "," OWS ( "*" / field-name ) )
]
Via = [ ( received-protocol RWS received-by [ RWS comment ] ) *( OWS
"," OWS ( received-protocol RWS received-by [ RWS comment ] ) ) ]
WWW-Authenticate = [ challenge *( OWS "," OWS challenge ) ]
absolute-URI = <absolute-URI, see [URI], Section 4.3>
absolute-path = 1*( "/" segment )
acceptable-ranges = range-unit *( OWS "," OWS range-unit )
asctime-date = day-name SP date3 SP time-of-day SP year
auth-param = token BWS "=" BWS ( token / quoted-string )
auth-scheme = token
authority = <authority, see [URI], Section 3.2>
challenge = auth-scheme [ 1*SP ( token68 / [ auth-param *( OWS ","
OWS auth-param ) ] ) ]
codings = content-coding / "identity" / "*"
comment = "(" *( ctext / quoted-pair / comment ) ")"
complete-length = 1*DIGIT
connection-option = token
content-coding = token
credentials = auth-scheme [ 1*SP ( token68 / [ auth-param *( OWS ","
OWS auth-param ) ] ) ]
ctext = HTAB / SP / %x21-27 ; '!'-'''
/ %x2A-5B ; '*'-'['
/ %x5D-7E ; ']'-'~'
/ obs-text
date1 = day SP month SP year
date2 = day "-" month "-" 2DIGIT
date3 = month SP ( 2DIGIT / ( SP DIGIT ) )
day = 2DIGIT
day-name = %x4D.6F.6E ; Mon
/ %x54.75.65 ; Tue
/ %x57.65.64 ; Wed
/ %x54.68.75 ; Thu
/ %x46.72.69 ; Fri
/ %x53.61.74 ; Sat
/ %x53.75.6E ; Sun
day-name-l = %x4D.6F.6E.64.61.79 ; Monday
/ %x54.75.65.73.64.61.79 ; Tuesday
/ %x57.65.64.6E.65.73.64.61.79 ; Wednesday
/ %x54.68.75.72.73.64.61.79 ; Thursday
/ %x46.72.69.64.61.79 ; Friday
/ %x53.61.74.75.72.64.61.79 ; Saturday
/ %x53.75.6E.64.61.79 ; Sunday
delay-seconds = 1*DIGIT
entity-tag = [ weak ] opaque-tag
etagc = "!" / %x23-7E ; '#'-'~'
/ obs-text
expectation = token [ "=" ( token / quoted-string ) parameters ]
field-content = field-vchar [ 1*( SP / HTAB / field-vchar )
field-vchar ]
field-name = token
field-value = *field-content
field-vchar = VCHAR / obs-text
first-pos = 1*DIGIT
hour = 2DIGIT
http-URI = "http://" authority path-abempty [ "?" query ]
https-URI = "https://" authority path-abempty [ "?" query ]
incl-range = first-pos "-" last-pos
int-range = first-pos "-" [ last-pos ]
language-range = <language-range, see [RFC4647], Section 2.1>
language-tag = <Language-Tag, see [RFC5646], Section 2.1>
last-pos = 1*DIGIT
mailbox = <mailbox, see [RFC5322], Section 3.4>
media-range = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) )
parameters
media-type = type "/" subtype parameters
method = token
minute = 2DIGIT
month = %x4A.61.6E ; Jan
/ %x46.65.62 ; Feb
/ %x4D.61.72 ; Mar
/ %x41.70.72 ; Apr
/ %x4D.61.79 ; May
/ %x4A.75.6E ; Jun
/ %x4A.75.6C ; Jul
/ %x41.75.67 ; Aug
/ %x53.65.70 ; Sep
/ %x4F.63.74 ; Oct
/ %x4E.6F.76 ; Nov
/ %x44.65.63 ; Dec
obs-date = rfc850-date / asctime-date
obs-text = %x80-FF
opaque-tag = DQUOTE *etagc DQUOTE
other-range = 1*( %x21-2B ; '!'-'+'
/ %x2D-7E ; '-'-'~'
)
parameter = parameter-name "=" parameter-value
parameter-name = token
parameter-value = ( token / quoted-string )
parameters = *( OWS ";" OWS [ parameter ] )
partial-URI = relative-part [ "?" query ]
path-abempty = <path-abempty, see [URI], Section 3.3>
port = <port, see [URI], Section 3.2.3>
product = token [ "/" product-version ]
product-version = token
protocol = protocol-name [ "/" protocol-version ]
protocol-name = token
protocol-version = token
pseudonym = token
qdtext = HTAB / SP / "!" / %x23-5B ; '#'-'['
/ %x5D-7E ; ']'-'~'
/ obs-text
query = <query, see [URI], Section 3.4>
quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )
quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE
qvalue = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
range-resp = incl-range "/" ( complete-length / "*" )
range-set = range-spec *( OWS "," OWS range-spec )
range-spec = int-range / suffix-range / other-range
range-unit = token
ranges-specifier = range-unit "=" range-set
received-by = pseudonym [ ":" port ]
received-protocol = [ protocol-name "/" ] protocol-version
relative-part = <relative-part, see [URI], Section 4.2>
rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT
second = 2DIGIT
segment = <segment, see [URI], Section 3.3>
subtype = token
suffix-length = 1*DIGIT
suffix-range = "-" suffix-length
t-codings = "trailers" / ( transfer-coding [ weight ] )
tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
"^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
time-of-day = hour ":" minute ":" second
token = 1*tchar
token68 = 1*( ALPHA / DIGIT / "-" / "." / "_" / "~" / "+" / "/" )
*"="
transfer-coding = token *( OWS ";" OWS transfer-parameter )
transfer-parameter = token BWS "=" BWS ( token / quoted-string )
type = token
unsatisfied-range = "*/" complete-length
uri-host = <host, see [URI], Section 3.2.2>
weak = %x57.2F ; W/
weight = OWS ";" OWS "q=" qvalue
year = 4DIGIT
¶
APPENDIX B. CHANGES FROM PREVIOUS RFCS
B.1. CHANGES FROM RFC 2818
None.¶
B.2. CHANGES FROM RFC 7230
The sections introducing HTTP's design goals, history, architecture, conformance
criteria, protocol versioning, URIs, message routing, and header fields have
been moved here.¶
The requirement on semantic conformance has been replaced with permission to
ignore or work around implementation-specific failures. (Section 2.2)¶
The description of an origin and authoritative access to origin servers has been
extended for both "http" and "https" URIs to account for alternative services
and secured connections that are not necessarily based on TCP. (Sections 4.2.1,
4.2.2, 4.3.1, and 7.3.3)¶
Explicit requirements have been added to check the target URI scheme's semantics
and reject requests that don't meet any associated requirements. (Section 7.4)¶
Parameters in media type, media range, and expectation can be empty via one or
more trailing semicolons. (Section 5.6.6)¶
"Field value" now refers to the value after multiple field lines are combined
with commas -- by far the most common use. To refer to a single header line's
value, use "field line value". (Section 6.3)¶
Trailer field semantics now transcend the specifics of chunked transfer coding.
The use of trailer fields has been further limited to allow generation as a
trailer field only when the sender knows the field defines that usage and to
allow merging into the header section only if the recipient knows the
corresponding field definition permits and defines how to merge. In all other
cases, implementations are encouraged either to store the trailer fields
separately or to discard them instead of merging. (Section 6.5.1)¶
The priority of the absolute form of the request URI over the Host header field
by origin servers has been made explicit to align with proxy handling. (Section
7.2)¶
The grammar definition for the Via field's "received-by" was expanded in RFC
7230 due to changes in the URI grammar for host [URI] that are not desirable for
Via. For simplicity, we have removed uri-host from the received-by production
because it can be encompassed by the existing grammar for pseudonym. In
particular, this change removed comma from the allowed set of characters for a
host name in received-by. (Section 7.6.3)¶
B.3. CHANGES FROM RFC 7231
Minimum URI lengths to be supported by implementations are now recommended.
(Section 4.1)¶
The following have been clarified: CR and NUL in field values are to be rejected
or mapped to SP, and leading and trailing whitespace needs to be stripped from
field values before they are consumed. (Section 5.5)¶
Parameters in media type, media range, and expectation can be empty via one or
more trailing semicolons. (Section 5.6.6)¶
An abstract data type for HTTP messages has been introduced to define the
components of a message and their semantics as an abstraction across multiple
HTTP versions, rather than in terms of the specific syntax form of HTTP/1.1 in
[HTTP/1.1], and reflect the contents after the message is parsed. This makes it
easier to distinguish between requirements on the content (what is conveyed)
versus requirements on the messaging syntax (how it is conveyed) and avoids
baking limitations of early protocol versions into the future of HTTP. (Section
6)¶
The terms "payload" and "payload body" have been replaced with "content", to
better align with its usage elsewhere (e.g., in field names) and to avoid
confusion with frame payloads in HTTP/2 and HTTP/3. (Section 6.4)¶
The term "effective request URI" has been replaced with "target URI". (Section
7.1)¶
Restrictions on client retries have been loosened to reflect implementation
behavior. (Section 9.2.2)¶
The fact that request bodies on GET, HEAD, and DELETE are not interoperable has
been clarified. (Sections 9.3.1, 9.3.2, and 9.3.5)¶
The use of the Content-Range header field (Section 14.4) as a request modifier
on PUT is allowed. (Section 9.3.4)¶
A superfluous requirement about setting Content-Length has been removed from the
description of the OPTIONS method. (Section 9.3.7)¶
The normative requirement to use the "message/http" media type in TRACE
responses has been removed. (Section 9.3.8)¶
List-based grammar for Expect has been restored for compatibility with RFC 2616.
(Section 10.1.1)¶
Accept and Accept-Encoding are allowed in response messages; the latter was
introduced by [RFC7694]. (Section 12.3)¶
"Accept Parameters" (accept-params and accept-ext ABNF production) have been
removed from the definition of the Accept field. (Section 12.5.1)¶
The Accept-Charset field is now deprecated. (Section 12.5.2)¶
The semantics of "*" in the Vary header field when other values are present was
clarified. (Section 12.5.5)¶
Range units are compared in a case-insensitive fashion. (Section 14.1)¶
The use of the Accept-Ranges field is not restricted to origin servers. (Section
14.3)¶
The process of creating a redirected request has been clarified. (Section 15.4)¶
Status code 308 (previously defined in [RFC7538]) has been added so that it's
defined closer to status codes 301, 302, and 307. (Section 15.4.9)¶
Status code 421 (previously defined in Section 9.1.2 of [RFC7540]) has been
added because of its general applicability. 421 is no longer defined as
heuristically cacheable since the response is specific to the connection (not
the target resource). (Section 15.5.20)¶
Status code 422 (previously defined in Section 11.2 of [WEBDAV]) has been added
because of its general applicability. (Section 15.5.21)¶
B.4. CHANGES FROM RFC 7232
Previous revisions of HTTP imposed an arbitrary 60-second limit on the
determination of whether Last-Modified was a strong validator to guard against
the possibility that the Date and Last-Modified values are generated from
different clocks or at somewhat different times during the preparation of the
response. This specification has relaxed that to allow reasonable discretion.
(Section 8.8.2.2)¶
An edge-case requirement on If-Match and If-Unmodified-Since has been removed
that required a validator not to be sent in a 2xx response if validation fails
because the change request has already been applied. (Sections 13.1.1 and
13.1.4)¶
The fact that If-Unmodified-Since does not apply to a resource without a concept
of modification time has been clarified. (Section 13.1.4)¶
Preconditions can now be evaluated before the request content is processed
rather than waiting until the response would otherwise be successful. (Section
13.2)¶
B.5. CHANGES FROM RFC 7233
Refactored the range-unit and ranges-specifier grammars to simplify and reduce
artificial distinctions between bytes and other (extension) range units,
removing the overlapping grammar of other-range-unit by defining range units
generically as a token and placing extensions within the scope of a range-spec
(other-range). This disambiguates the role of list syntax (commas) in all range
sets, including extension range units, for indicating a range-set of more than
one range. Moving the extension grammar into range specifiers also allows
protocol specific to byte ranges to be specified separately.¶
It is now possible to define Range handling on extension methods. (Section
14.2)¶
Described use of the Content-Range header field (Section 14.4) as a request
modifier to perform a partial PUT. (Section 14.5)¶
B.6. CHANGES FROM RFC 7235
None.¶
B.7. CHANGES FROM RFC 7538
None.¶
B.8. CHANGES FROM RFC 7615
None.¶
B.9. CHANGES FROM RFC 7694
This specification includes the extension defined in [RFC7694] but leaves out
examples and deployment considerations.¶
ACKNOWLEDGEMENTS
Aside from the current editors, the following individuals deserve special
recognition for their contributions to early aspects of HTTP and its core
specifications: Marc Andreessen, Tim Berners-Lee, Robert Cailliau, Daniel W.
Connolly, Bob Denny, John Franks, Jim Gettys, Jean-François Groff, Phillip M.
Hallam-Baker, Koen Holtman, Jeffery L. Hostetler, Shel Kaphan, Dave Kristol,
Yves Lafon, Scott D. Lawrence, Paul J. Leach, Håkon W. Lie, Ari Luotonen, Larry
Masinter, Rob McCool, Jeffrey C. Mogul, Lou Montulli, David Morris, Henrik
Frystyk Nielsen, Dave Raggett, Eric Rescorla, Tony Sanders, Lawrence C. Stewart,
Marc VanHeyningen, and Steve Zilles.¶
This document builds on the many contributions that went into past
specifications of HTTP, including [HTTP/1.0], [RFC2068], [RFC2145], [RFC2616],
[RFC2617], [RFC2818], [RFC7230], [RFC7231], [RFC7232], [RFC7233], [RFC7234], and
[RFC7235]. The acknowledgements within those documents still apply.¶
Since 2014, the following contributors have helped improve this specification by
reporting bugs, asking smart questions, drafting or reviewing text, and
evaluating issues:¶
Alan Egerton, Alex Rousskov, Amichai Rothman, Amos Jeffries, Anders Kaseorg,
Andreas Gebhardt, Anne van Kesteren, Armin Abfalterer, Aron Duby, Asanka Herath,
Asbjørn Ulsberg, Asta Olofsson, Attila Gulyas, Austin Wright, Barry Pollard, Ben
Burkert, Benjamin Kaduk, Björn Höhrmann, Brad Fitzpatrick, Chris Pacejo, Colin
Bendell, Cory Benfield, Cory Nelson, Daisuke Miyakawa, Dale Worley, Daniel
Stenberg, Danil Suits, David Benjamin, David Matson, David Schinazi, Дилян
Палаузов (Dilyan Palauzov), Eric Anderson, Eric Rescorla, Éric Vyncke, Erik
Kline, Erwin Pe, Etan Kissling, Evert Pot, Evgeny Vrublevsky, Florian Best,
Francesca Palombini, Igor Lubashev, James Callahan, James Peach, Jeffrey
Yasskin, Kalin Gyokov, Kannan Goundan, 奥 一穂 (Kazuho Oku), Ken Murchison,
Krzysztof Maczyński, Lars Eggert, Lucas Pardue, Martin Duke, Martin Dürst,
Martin Thomson, Martynas Jusevičius, Matt Menke, Matthias Pigulla, Mattias
Grenfeldt, Michael Osipov, Mike Bishop, Mike Pennisi, Mike Taylor, Mike West,
Mohit Sethi, Murray Kucherawy, Nathaniel J. Smith, Nicholas Hurley, Nikita
Prokhorov, Patrick McManus, Piotr Sikora, Poul-Henning Kamp, Rick van Rein,
Robert Wilton, Roberto Polli, Roman Danyliw, Samuel Williams, Semyon Kholodnov,
Simon Pieters, Simon Schüppel, Stefan Eissing, Taylor Hunt, Todd Greer, Tommy
Pauly, Vasiliy Faronov, Vladimir Lashchev, Wenbo Zhu, William A. Rowe Jr., Willy
Tarreau, Xingwei Liu, Yishuai Li, and Zaheduzzaman Sarker.¶
INDEX
1 2 3 4 5 A B C D E F G H I L M N O P R S T U V W X¶
* 1¶
* 100 Continue (status code)
Section 15.2.1¶
100-continue (expect value)
Section 10.1.1¶
101 Switching Protocols (status code)
Section 15.2.2¶
1xx Informational (status code class)
Section 15.2¶
* 2¶
* 200 OK (status code)
Section 15.3.1¶
201 Created (status code)
Section 15.3.2¶
202 Accepted (status code)
Section 15.3.3¶
203 Non-Authoritative Information (status code)
Section 15.3.4¶
204 No Content (status code)
Section 15.3.5¶
205 Reset Content (status code)
Section 15.3.6¶
206 Partial Content (status code)
Section 15.3.7¶
2xx Successful (status code class)
Section 15.3¶
* 3¶
* 300 Multiple Choices (status code)
Section 15.4.1¶
301 Moved Permanently (status code)
Section 15.4.2¶
302 Found (status code)
Section 15.4.3¶
303 See Other (status code)
Section 15.4.4¶
304 Not Modified (status code)
Section 15.4.5¶
305 Use Proxy (status code)
Section 15.4.6¶
306 (Unused) (status code)
Section 15.4.7¶
307 Temporary Redirect (status code)
Section 15.4.8¶
308 Permanent Redirect (status code)
Section 15.4.9¶
3xx Redirection (status code class)
Section 15.4¶
* 4¶
* 400 Bad Request (status code)
Section 15.5.1¶
401 Unauthorized (status code)
Section 15.5.2¶
402 Payment Required (status code)
Section 15.5.3¶
403 Forbidden (status code)
Section 15.5.4¶
404 Not Found (status code)
Section 15.5.5¶
405 Method Not Allowed (status code)
Section 15.5.6¶
406 Not Acceptable (status code)
Section 15.5.7¶
407 Proxy Authentication Required (status code)
Section 15.5.8¶
408 Request Timeout (status code)
Section 15.5.9¶
409 Conflict (status code)
Section 15.5.10¶
410 Gone (status code)
Section 15.5.11¶
411 Length Required (status code)
Section 15.5.12¶
412 Precondition Failed (status code)
Section 15.5.13¶
413 Content Too Large (status code)
Section 15.5.14¶
414 URI Too Long (status code)
Section 15.5.15¶
415 Unsupported Media Type (status code)
Section 15.5.16¶
416 Range Not Satisfiable (status code)
Section 15.5.17¶
417 Expectation Failed (status code)
Section 15.5.18¶
418 (Unused) (status code)
Section 15.5.19¶
421 Misdirected Request (status code)
Section 15.5.20¶
422 Unprocessable Content (status code)
Section 15.5.21¶
426 Upgrade Required (status code)
Section 15.5.22¶
4xx Client Error (status code class)
Section 15.5¶
* 5¶
* 500 Internal Server Error (status code)
Section 15.6.1¶
501 Not Implemented (status code)
Section 15.6.2¶
502 Bad Gateway (status code)
Section 15.6.3¶
503 Service Unavailable (status code)
Section 15.6.4¶
504 Gateway Timeout (status code)
Section 15.6.5¶
505 HTTP Version Not Supported (status code)
Section 15.6.6¶
5xx Server Error (status code class)
Section 15.6¶
* A¶
* accelerator
Section 3.7, Paragraph 6¶
Accept header field
Section 12.5.1¶
Accept-Charset header field
Section 12.5.2¶
Accept-Encoding header field
Section 12.5.3¶
Accept-Language header field
Section 12.5.4¶
Accept-Ranges header field
Section 14.3¶
Allow header field
Section 10.2.1¶
Authentication-Info header field
Section 11.6.3¶
authoritative response
Section 17.1¶
Authorization header field
Section 11.6.2¶
* B¶
* browser
Section 3.5¶
* C¶
* cache
Section 3.8¶
cacheable
Section 3.8, Paragraph 4¶
client
Section 3.3¶
clock
Section 5.6.7¶
complete
Section 6.1¶
compress (Coding Format)
Section 8.4.1.1¶
compress (content coding)
Section 8.4.1¶
conditional request
Section 13¶
CONNECT method
Section 9.3.6¶
connection
Section 3.3¶
Connection header field
Section 7.6.1¶
content
Section 6.4¶
content coding
Section 8.4.1¶
content negotiation
Section 1.3, Paragraph 4¶
Content-Encoding header field
Section 8.4¶
Content-Language header field
Section 8.5¶
Content-Length header field
Section 8.6¶
Content-Location header field
Section 8.7¶
Content-MD5 header field
Section 18.4, Paragraph 10¶
Content-Range header field
Section 14.4; Section 14.5¶
Content-Type header field
Section 8.3¶
control data
Section 6.2¶
* D¶
* Date header field
Section 6.6.1¶
deflate (Coding Format)
Section 8.4.1.2¶
deflate (content coding)
Section 8.4.1¶
DELETE method
Section 9.3.5¶
Delimiters
Section 5.6.2, Paragraph 3¶
downstream
Section 3.7, Paragraph 4¶
* E¶
* effective request URI
Section 7.1, Paragraph 8.1¶
ETag field
Section 8.8.3¶
Expect header field
Section 10.1.1¶
* F¶
* field
Section 5; Section 6.3¶
field line
Section 5.2, Paragraph 1¶
field line value
Section 5.2, Paragraph 1¶
field name
Section 5.2, Paragraph 1¶
field value
Section 5.2, Paragraph 2¶
Fields *
Section 18.4, Paragraph 9¶
Accept
Section 12.5.1¶
Accept-Charset
Section 12.5.2¶
Accept-Encoding
Section 12.5.3¶
Accept-Language
Section 12.5.4¶
Accept-Ranges
Section 14.3¶
Allow
Section 10.2.1¶
Authentication-Info
Section 11.6.3¶
Authorization
Section 11.6.2¶
Connection
Section 7.6.1¶
Content-Encoding
Section 8.4¶
Content-Language
Section 8.5¶
Content-Length
Section 8.6¶
Content-Location
Section 8.7¶
Content-MD5
Section 18.4, Paragraph 10¶
Content-Range
Section 14.4; Section 14.5¶
Content-Type
Section 8.3¶
Date
Section 6.6.1¶
ETag
Section 8.8.3¶
Expect
Section 10.1.1¶
From
Section 10.1.2¶
Host
Section 7.2¶
If-Match
Section 13.1.1¶
If-Modified-Since
Section 13.1.3¶
If-None-Match
Section 13.1.2¶
If-Range
Section 13.1.5¶
If-Unmodified-Since
Section 13.1.4¶
Last-Modified
Section 8.8.2¶
Location
Section 10.2.2¶
Max-Forwards
Section 7.6.2¶
Proxy-Authenticate
Section 11.7.1¶
Proxy-Authentication-Info
Section 11.7.3¶
Proxy-Authorization
Section 11.7.2¶
Range
Section 14.2¶
Referer
Section 10.1.3¶
Retry-After
Section 10.2.3¶
Server
Section 10.2.4¶
TE
Section 10.1.4¶
Trailer
Section 6.6.2¶
Upgrade
Section 7.8¶
User-Agent
Section 10.1.5¶
Vary
Section 12.5.5¶
Via
Section 7.6.3¶
WWW-Authenticate
Section 11.6.1¶
Fragment Identifiers
Section 4.2.5¶
From header field
Section 10.1.2¶
* G¶
* gateway
Section 3.7, Paragraph 6¶
GET method
Section 9.3.1¶
Grammar ALPHA
Section 2.1¶
Accept
Section 12.5.1¶
Accept-Charset
Section 12.5.2¶
Accept-Encoding
Section 12.5.3¶
Accept-Language
Section 12.5.4¶
Accept-Ranges
Section 14.3¶
Allow
Section 10.2.1¶
Authentication-Info
Section 11.6.3¶
Authorization
Section 11.6.2¶
BWS
Section 5.6.3¶
CR
Section 2.1¶
CRLF
Section 2.1¶
CTL
Section 2.1¶
Connection
Section 7.6.1¶
Content-Encoding
Section 8.4¶
Content-Language
Section 8.5¶
Content-Length
Section 8.6¶
Content-Location
Section 8.7¶
Content-Range
Section 14.4¶
Content-Type
Section 8.3¶
DIGIT
Section 2.1¶
DQUOTE
Section 2.1¶
Date
Section 6.6.1¶
ETag
Section 8.8.3¶
Expect
Section 10.1.1¶
From
Section 10.1.2¶
GMT
Section 5.6.7¶
HEXDIG
Section 2.1¶
HTAB
Section 2.1¶
HTTP-date
Section 5.6.7¶
Host
Section 7.2¶
IMF-fixdate
Section 5.6.7¶
If-Match
Section 13.1.1¶
If-Modified-Since
Section 13.1.3¶
If-None-Match
Section 13.1.2¶
If-Range
Section 13.1.5¶
If-Unmodified-Since
Section 13.1.4¶
LF
Section 2.1¶
Last-Modified
Section 8.8.2¶
Location
Section 10.2.2¶
Max-Forwards
Section 7.6.2¶
OCTET
Section 2.1¶
OWS
Section 5.6.3¶
Proxy-Authenticate
Section 11.7.1¶
Proxy-Authentication-Info
Section 11.7.3¶
Proxy-Authorization
Section 11.7.2¶
RWS
Section 5.6.3¶
Range
Section 14.2¶
Referer
Section 10.1.3¶
Retry-After
Section 10.2.3¶
SP
Section 2.1¶
Server
Section 10.2.4¶
TE
Section 10.1.4¶
Trailer
Section 6.6.2¶
URI-reference
Section 4.1¶
Upgrade
Section 7.8¶
User-Agent
Section 10.1.5¶
VCHAR
Section 2.1¶
Vary
Section 12.5.5¶
Via
Section 7.6.3¶
WWW-Authenticate
Section 11.6.1¶
absolute-URI
Section 4.1¶
absolute-path
Section 4.1¶
acceptable-ranges
Section 14.3¶
asctime-date
Section 5.6.7¶
auth-param
Section 11.2¶
auth-scheme
Section 11.1¶
authority
Section 4.1¶
challenge
Section 11.3¶
codings
Section 12.5.3¶
comment
Section 5.6.5¶
complete-length
Section 14.4¶
connection-option
Section 7.6.1¶
content-coding
Section 8.4.1¶
credentials
Section 11.4¶
ctext
Section 5.6.5¶
date1
Section 5.6.7¶
day
Section 5.6.7¶
day-name
Section 5.6.7¶
day-name-l
Section 5.6.7¶
delay-seconds
Section 10.2.3¶
entity-tag
Section 8.8.3¶
etagc
Section 8.8.3¶
field-content
Section 5.5¶
field-name
Section 5.1; Section 6.6.2¶
field-value
Section 5.5¶
field-vchar
Section 5.5¶
first-pos
Section 14.1.1; Section 14.4¶
hour
Section 5.6.7¶
http-URI
Section 4.2.1¶
https-URI
Section 4.2.2¶
incl-range
Section 14.4¶
int-range
Section 14.1.1¶
language-range
Section 12.5.4¶
language-tag
Section 8.5.1¶
last-pos
Section 14.1.1; Section 14.4¶
media-range
Section 12.5.1¶
media-type
Section 8.3.1¶
method
Section 9.1¶
minute
Section 5.6.7¶
month
Section 5.6.7¶
obs-date
Section 5.6.7¶
obs-text
Section 5.5¶
opaque-tag
Section 8.8.3¶
other-range
Section 14.1.1¶
parameter
Section 5.6.6¶
parameter-name
Section 5.6.6¶
parameter-value
Section 5.6.6¶
parameters
Section 5.6.6¶
partial-URI
Section 4.1¶
port
Section 4.1¶
product
Section 10.1.5¶
product-version
Section 10.1.5¶
protocol-name
Section 7.6.3¶
protocol-version
Section 7.6.3¶
pseudonym
Section 7.6.3¶
qdtext
Section 5.6.4¶
query
Section 4.1¶
quoted-pair
Section 5.6.4¶
quoted-string
Section 5.6.4¶
qvalue
Section 12.4.2¶
range-resp
Section 14.4¶
range-set
Section 14.1.1¶
range-spec
Section 14.1.1¶
range-unit
Section 14.1¶
ranges-specifier
Section 14.1.1¶
received-by
Section 7.6.3¶
received-protocol
Section 7.6.3¶
rfc850-date
Section 5.6.7¶
second
Section 5.6.7¶
segment
Section 4.1¶
subtype
Section 8.3.1¶
suffix-length
Section 14.1.1¶
suffix-range
Section 14.1.1¶
t-codings
Section 10.1.4¶
tchar
Section 5.6.2¶
time-of-day
Section 5.6.7¶
token
Section 5.6.2¶
token68
Section 11.2¶
transfer-coding
Section 10.1.4¶
transfer-parameter
Section 10.1.4¶
type
Section 8.3.1¶
unsatisfied-range
Section 14.4¶
uri-host
Section 4.1¶
weak
Section 8.8.3¶
weight
Section 12.4.2¶
year
Section 5.6.7¶
gzip (Coding Format)
Section 8.4.1.3¶
gzip (content coding)
Section 8.4.1¶
* H¶
* HEAD method
Section 9.3.2¶
Header Fields Accept
Section 12.5.1¶
Accept-Charset
Section 12.5.2¶
Accept-Encoding
Section 12.5.3¶
Accept-Language
Section 12.5.4¶
Accept-Ranges
Section 14.3¶
Allow
Section 10.2.1¶
Authentication-Info
Section 11.6.3¶
Authorization
Section 11.6.2¶
Connection
Section 7.6.1¶
Content-Encoding
Section 8.4¶
Content-Language
Section 8.5¶
Content-Length
Section 8.6¶
Content-Location
Section 8.7¶
Content-MD5
Section 18.4, Paragraph 10¶
Content-Range
Section 14.4; Section 14.5¶
Content-Type
Section 8.3¶
Date
Section 6.6.1¶
ETag
Section 8.8.3¶
Expect
Section 10.1.1¶
From
Section 10.1.2¶
Host
Section 7.2¶
If-Match
Section 13.1.1¶
If-Modified-Since
Section 13.1.3¶
If-None-Match
Section 13.1.2¶
If-Range
Section 13.1.5¶
If-Unmodified-Since
Section 13.1.4¶
Last-Modified
Section 8.8.2¶
Location
Section 10.2.2¶
Max-Forwards
Section 7.6.2¶
Proxy-Authenticate
Section 11.7.1¶
Proxy-Authentication-Info
Section 11.7.3¶
Proxy-Authorization
Section 11.7.2¶
Range
Section 14.2¶
Referer
Section 10.1.3¶
Retry-After
Section 10.2.3¶
Server
Section 10.2.4¶
TE
Section 10.1.4¶
Trailer
Section 6.6.2¶
Upgrade
Section 7.8¶
User-Agent
Section 10.1.5¶
Vary
Section 12.5.5¶
Via
Section 7.6.3¶
WWW-Authenticate
Section 11.6.1¶
header section
Section 6.3¶
Host header field
Section 7.2¶
http URI scheme
Section 4.2.1¶
https URI scheme
Section 4.2.2¶
* I¶
* idempotent
Section 9.2.2¶
If-Match header field
Section 13.1.1¶
If-Modified-Since header field
Section 13.1.3¶
If-None-Match header field
Section 13.1.2¶
If-Range header field
Section 13.1.5¶
If-Unmodified-Since header field
Section 13.1.4¶
inbound
Section 3.7, Paragraph 4¶
incomplete
Section 6.1¶
interception proxy
Section 3.7, Paragraph 10¶
intermediary
Section 3.7¶
* L¶
* Last-Modified header field
Section 8.8.2¶
list-based field
Section 5.5, Paragraph 7¶
Location header field
Section 10.2.2¶
* M¶
* Max-Forwards header field
Section 7.6.2¶
Media Type multipart/byteranges
Section 14.6¶
multipart/x-byteranges
Section 14.6, Paragraph 4, Item 3¶
message
Section 3.4; Section 6¶
message abstraction
Section 6¶
messages
Section 3.4¶
metadata
Section 8.8¶
Method *
Section 18.2, Paragraph 3¶
CONNECT
Section 9.3.6¶
DELETE
Section 9.3.5¶
GET
Section 9.3.1¶
HEAD
Section 9.3.2¶
OPTIONS
Section 9.3.7¶
POST
Section 9.3.3¶
PUT
Section 9.3.4¶
TRACE
Section 9.3.8¶
multipart/byteranges Media Type
Section 14.6¶
multipart/x-byteranges Media Type
Section 14.6, Paragraph 4, Item 3¶
* N¶
* non-transforming proxy
Section 7.7¶
* O¶
* OPTIONS method
Section 9.3.7¶
origin
Section 4.3.1; Section 11.5¶
origin server
Section 3.6¶
outbound
Section 3.7, Paragraph 4¶
* P¶
* phishing
Section 17.1¶
POST method
Section 9.3.3¶
Protection Space
Section 11.5¶
proxy
Section 3.7, Paragraph 5¶
Proxy-Authenticate header field
Section 11.7.1¶
Proxy-Authentication-Info header field
Section 11.7.3¶
Proxy-Authorization header field
Section 11.7.2¶
PUT method
Section 9.3.4¶
* R¶
* Range header field
Section 14.2¶
Realm
Section 11.5¶
recipient
Section 3.4¶
Referer header field
Section 10.1.3¶
representation
Section 3.2¶
request
Section 3.4¶
request target
Section 7.1¶
resource
Section 3.1; Section 4¶
response
Section 3.4¶
Retry-After header field
Section 10.2.3¶
reverse proxy
Section 3.7, Paragraph 6¶
* S¶
* safe
Section 9.2.1¶
satisfiable range
Section 14.1.1¶
secured
Section 4.2.2¶
selected representation
Section 3.2, Paragraph 4; Section 8.8; Section 13.1¶
self-descriptive
Section 6¶
sender
Section 3.4¶
server
Section 3.3¶
Server header field
Section 10.2.4¶
singleton field
Section 5.5, Paragraph 6¶
spider
Section 3.5¶
Status Code
Section 15¶
Status Codes Final
Section 15, Paragraph 7¶
Informational
Section 15, Paragraph 7¶
Interim
Section 15, Paragraph 7¶
Status Codes Classes 1xx Informational
Section 15.2¶
2xx Successful
Section 15.3¶
3xx Redirection
Section 15.4¶
4xx Client Error
Section 15.5¶
5xx Server Error
Section 15.6¶
* T¶
* target resource
Section 7.1¶
target URI
Section 7.1¶
TE header field
Section 10.1.4¶
TRACE method
Section 9.3.8¶
Trailer Fields
Section 6.5¶
ETag
Section 8.8.3¶
Trailer header field
Section 6.6.2¶
trailer section
Section 6.5¶
trailers
Section 6.5¶
transforming proxy
Section 7.7¶
transparent proxy
Section 3.7, Paragraph 10¶
tunnel
Section 3.7, Paragraph 8¶
* U¶
* unsatisfiable range
Section 14.1.1¶
Upgrade header field
Section 7.8¶
upstream
Section 3.7, Paragraph 4¶
URI
Section 4¶
origin
Section 4.3.1¶
URI reference
Section 4.1¶
URI scheme http
Section 4.2.1¶
https
Section 4.2.2¶
user agent
Section 3.5¶
User-Agent header field
Section 10.1.5¶
* V¶
* validator
Section 8.8¶
strong
Section 8.8.1¶
weak
Section 8.8.1¶
Vary header field
Section 12.5.5¶
Via header field
Section 7.6.3¶
* W¶
* WWW-Authenticate header field
Section 11.6.1¶
* X¶
* x-compress (content coding)
Section 8.4.1¶
x-gzip (content coding)
Section 8.4.1¶
AUTHORS' ADDRESSES
Roy T. Fielding (editor)
Adobe
345 Park Ave
San Jose, CA 95110
United States of America
Email: fielding@gbiv.com
URI: https://roy.gbiv.com/
Mark Nottingham (editor)
Fastly
Prahran
Australia
Email: mnot@mnot.net
URI: https://www.mnot.net/
Julian Reschke (editor)
greenbytes GmbH
Hafenweg 16
48155 Münster
Germany
Email: julian.reschke@greenbytes.de
URI: https://greenbytes.de/tech/webdav/
Datatracker
RFC 9110
RFC - Internet Standard
* Info
* Contents
* Prefs
Document Document type RFC - Internet Standard
June 2022
View errata Report errata IPR
Obsoletes RFC 7538, RFC 7233, RFC 2818, RFC 7694, RFC 7232, RFC 7615, RFC 7230,
RFC 7235, RFC 7231
Updates RFC 3864
Was draft-ietf-httpbis-semantics (httpbis WG)
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Authors Roy T. Fielding , Mark Nottingham , Julian Reschke
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Abstract
Status of This Memo
Copyright Notice
Table of Contents
1.
Introduction
1.1.
Purpose
1.2.
History and Evolution
1.3.
Core Semantics
1.4.
Specifications Obsoleted by This Document
2.
Conformance
2.1.
Syntax Notation
2.2.
Requirements Notation
2.3.
Length Requirements
2.4.
Error Handling
2.5.
Protocol Version
3.
Terminology and Core Concepts
3.1.
Resources
3.2.
Representations
3.3.
Connections, Clients, and Servers
3.4.
Messages
3.5.
User Agents
3.6.
Origin Server
3.7.
Intermediaries
3.8.
Caches
3.9.
Example Message Exchange
4.
Identifiers in HTTP
4.1.
URI References
4.2.
HTTP-Related URI Schemes
4.2.1.
http URI Scheme
4.2.2.
https URI Scheme
4.2.3.
http(s) Normalization and Comparison
4.2.4.
Deprecation of userinfo in http(s) URIs
4.2.5.
http(s) References with Fragment Identifiers
4.3.
Authoritative Access
4.3.1.
URI Origin
4.3.2.
http Origins
4.3.3.
https Origins
4.3.4.
https Certificate Verification
4.3.5.
IP-ID Reference Identity
5.
Fields
5.1.
Field Names
5.2.
Field Lines and Combined Field Value
5.3.
Field Order
5.4.
Field Limits
5.5.
Field Values
5.6.
Common Rules for Defining Field Values
5.6.1.
Lists (#rule ABNF Extension)
5.6.1.1.
Sender Requirements
5.6.1.2.
Recipient Requirements
5.6.2.
Tokens
5.6.3.
Whitespace
5.6.4.
Quoted Strings
5.6.5.
Comments
5.6.6.
Parameters
5.6.7.
Date/Time Formats
6.
Message Abstraction
6.1.
Framing and Completeness
6.2.
Control Data
6.3.
Header Fields
6.4.
Content
6.4.1.
Content Semantics
6.4.2.
Identifying Content
6.5.
Trailer Fields
6.5.1.
Limitations on Use of Trailers
6.5.2.
Processing Trailer Fields
6.6.
Message Metadata
6.6.1.
Date
6.6.2.
Trailer
7.
Routing HTTP Messages
7.1.
Determining the Target Resource
7.2.
Host and :authority
7.3.
Routing Inbound Requests
7.3.1.
To a Cache
7.3.2.
To a Proxy
7.3.3.
To the Origin
7.4.
Rejecting Misdirected Requests
7.5.
Response Correlation
7.6.
Message Forwarding
7.6.1.
Connection
7.6.2.
Max-Forwards
7.6.3.
Via
7.7.
Message Transformations
7.8.
Upgrade
8.
Representation Data and Metadata
8.1.
Representation Data
8.2.
Representation Metadata
8.3.
Content-Type
8.3.1.
Media Type
8.3.2.
Charset
8.3.3.
Multipart Types
8.4.
Content-Encoding
8.4.1.
Content Codings
8.4.1.1.
Compress Coding
8.4.1.2.
Deflate Coding
8.4.1.3.
Gzip Coding
8.5.
Content-Language
8.5.1.
Language Tags
8.6.
Content-Length
8.7.
Content-Location
8.8.
Validator Fields
8.8.1.
Weak versus Strong
8.8.2.
Last-Modified
8.8.2.1.
Generation
8.8.2.2.
Comparison
8.8.3.
ETag
8.8.3.1.
Generation
8.8.3.2.
Comparison
8.8.3.3.
Example: Entity Tags Varying on Content-Negotiated Resources
9.
Methods
9.1.
Overview
9.2.
Common Method Properties
9.2.1.
Safe Methods
9.2.2.
Idempotent Methods
9.2.3.
Methods and Caching
9.3.
Method Definitions
9.3.1.
GET
9.3.2.
HEAD
9.3.3.
POST
9.3.4.
PUT
9.3.5.
DELETE
9.3.6.
CONNECT
9.3.7.
OPTIONS
9.3.8.
TRACE
10.
Message Context
10.1.
Request Context Fields
10.1.1.
Expect
10.1.2.
From
10.1.3.
Referer
10.1.4.
TE
10.1.5.
User-Agent
10.2.
Response Context Fields
10.2.1.
Allow
10.2.2.
Location
10.2.3.
Retry-After
10.2.4.
Server
11.
HTTP Authentication
11.1.
Authentication Scheme
11.2.
Authentication Parameters
11.3.
Challenge and Response
11.4.
Credentials
11.5.
Establishing a Protection Space (Realm)
11.6.
Authenticating Users to Origin Servers
11.6.1.
WWW-Authenticate
11.6.2.
Authorization
11.6.3.
Authentication-Info
11.7.
Authenticating Clients to Proxies
11.7.1.
Proxy-Authenticate
11.7.2.
Proxy-Authorization
11.7.3.
Proxy-Authentication-Info
12.
Content Negotiation
12.1.
Proactive Negotiation
12.2.
Reactive Negotiation
12.3.
Request Content Negotiation
12.4.
Content Negotiation Field Features
12.4.1.
Absence
12.4.2.
Quality Values
12.4.3.
Wildcard Values
12.5.
Content Negotiation Fields
12.5.1.
Accept
12.5.2.
Accept-Charset
12.5.3.
Accept-Encoding
12.5.4.
Accept-Language
12.5.5.
Vary
13.
Conditional Requests
13.1.
Preconditions
13.1.1.
If-Match
13.1.2.
If-None-Match
13.1.3.
If-Modified-Since
13.1.4.
If-Unmodified-Since
13.1.5.
If-Range
13.2.
Evaluation of Preconditions
13.2.1.
When to Evaluate
13.2.2.
Precedence of Preconditions
14.
Range Requests
14.1.
Range Units
14.1.1.
Range Specifiers
14.1.2.
Byte Ranges
14.2.
Range
14.3.
Accept-Ranges
14.4.
Content-Range
14.5.
Partial PUT
14.6.
Media Type multipart/byteranges
15.
Status Codes
15.1.
Overview of Status Codes
15.2.
Informational 1xx
15.2.1.
100 Continue
15.2.2.
101 Switching Protocols
15.3.
Successful 2xx
15.3.1.
200 OK
15.3.2.
201 Created
15.3.3.
202 Accepted
15.3.4.
203 Non-Authoritative Information
15.3.5.
204 No Content
15.3.6.
205 Reset Content
15.3.7.
206 Partial Content
15.3.7.1.
Single Part
15.3.7.2.
Multiple Parts
15.3.7.3.
Combining Parts
15.4.
Redirection 3xx
15.4.1.
300 Multiple Choices
15.4.2.
301 Moved Permanently
15.4.3.
302 Found
15.4.4.
303 See Other
15.4.5.
304 Not Modified
15.4.6.
305 Use Proxy
15.4.7.
306 (Unused)
15.4.8.
307 Temporary Redirect
15.4.9.
308 Permanent Redirect
15.5.
Client Error 4xx
15.5.1.
400 Bad Request
15.5.2.
401 Unauthorized
15.5.3.
402 Payment Required
15.5.4.
403 Forbidden
15.5.5.
404 Not Found
15.5.6.
405 Method Not Allowed
15.5.7.
406 Not Acceptable
15.5.8.
407 Proxy Authentication Required
15.5.9.
408 Request Timeout
15.5.10.
409 Conflict
15.5.11.
410 Gone
15.5.12.
411 Length Required
15.5.13.
412 Precondition Failed
15.5.14.
413 Content Too Large
15.5.15.
414 URI Too Long
15.5.16.
415 Unsupported Media Type
15.5.17.
416 Range Not Satisfiable
15.5.18.
417 Expectation Failed
15.5.19.
418 (Unused)
15.5.20.
421 Misdirected Request
15.5.21.
422 Unprocessable Content
15.5.22.
426 Upgrade Required
15.6.
Server Error 5xx
15.6.1.
500 Internal Server Error
15.6.2.
501 Not Implemented
15.6.3.
502 Bad Gateway
15.6.4.
503 Service Unavailable
15.6.5.
504 Gateway Timeout
15.6.6.
505 HTTP Version Not Supported
16.
Extending HTTP
16.1.
Method Extensibility
16.1.1.
Method Registry
16.1.2.
Considerations for New Methods
16.2.
Status Code Extensibility
16.2.1.
Status Code Registry
16.2.2.
Considerations for New Status Codes
16.3.
Field Extensibility
16.3.1.
Field Name Registry
16.3.2.
Considerations for New Fields
16.3.2.1.
Considerations for New Field Names
16.3.2.2.
Considerations for New Field Values
16.4.
Authentication Scheme Extensibility
16.4.1.
Authentication Scheme Registry
16.4.2.
Considerations for New Authentication Schemes
16.5.
Range Unit Extensibility
16.5.1.
Range Unit Registry
16.5.2.
Considerations for New Range Units
16.6.
Content Coding Extensibility
16.6.1.
Content Coding Registry
16.6.2.
Considerations for New Content Codings
16.7.
Upgrade Token Registry
17.
Security Considerations
17.1.
Establishing Authority
17.2.
Risks of Intermediaries
17.3.
Attacks Based on File and Path Names
17.4.
Attacks Based on Command, Code, or Query Injection
17.5.
Attacks via Protocol Element Length
17.6.
Attacks Using Shared-Dictionary Compression
17.7.
Disclosure of Personal Information
17.8.
Privacy of Server Log Information
17.9.
Disclosure of Sensitive Information in URIs
17.10.
Application Handling of Field Names
17.11.
Disclosure of Fragment after Redirects
17.12.
Disclosure of Product Information
17.13.
Browser Fingerprinting
17.14.
Validator Retention
17.15.
Denial-of-Service Attacks Using Range
17.16.
Authentication Considerations
17.16.1.
Confidentiality of Credentials
17.16.2.
Credentials and Idle Clients
17.16.3.
Protection Spaces
17.16.4.
Additional Response Fields
18.
IANA Considerations
18.1.
URI Scheme Registration
18.2.
Method Registration
18.3.
Status Code Registration
18.4.
Field Name Registration
18.5.
Authentication Scheme Registration
18.6.
Content Coding Registration
18.7.
Range Unit Registration
18.8.
Media Type Registration
18.9.
Port Registration
18.10.
Upgrade Token Registration
19.
References
19.1.
Normative References
19.2.
Informative References
A.
Collected ABNF
B.
Changes from Previous RFCs
B.1.
Changes from RFC 2818
B.2.
Changes from RFC 7230
B.3.
Changes from RFC 7231
B.4.
Changes from RFC 7232
B.5.
Changes from RFC 7233
B.6.
Changes from RFC 7235
B.7.
Changes from RFC 7538
B.8.
Changes from RFC 7615
B.9.
Changes from RFC 7694
Acknowledgements
Index
Authors' Addresses
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