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Home > Learning Center > AppSec > OSI Model

OSI MODEL

654.4k views
App SecurityEssentialsProtocols

WHAT IS THE OSI MODEL

The Open Systems Interconnection (OSI) model describes seven layers that
computer systems use to communicate over a network. It was the first standard
model for network communications, adopted by all major computer and
telecommunication companies in the early 1980s

The modern Internet is not based on OSI, but on the simpler TCP/IP model.
However, the OSI 7-layer model is still widely used, as it helps visualize and
communicate how networks operate, and helps isolate and troubleshoot networking
problems.

OSI was introduced in 1983 by representatives of the major computer and telecom
companies, and was adopted by ISO as an international standard in 1984.

OSI MODEL EXPLAINED: THE OSI 7 LAYERS

We’ll describe OSI layers “top down” from the application layer that directly
serves the end user, down to the physical layer.

7. Application Layer

The application layer is used by end-user software such as web browsers and
email clients. It provides protocols that allow software to send and receive
information and present meaningful data to users. A few examples of application
layer protocols are the Hypertext Transfer Protocol (HTTP), File Transfer
Protocol (FTP), Post Office Protocol (POP), Simple Mail Transfer Protocol
(SMTP), and Domain Name System (DNS).

6. Presentation Layer

The presentation layer prepares data for the application layer. It defines how
two devices should encode, encrypt, and compress data so it is received
correctly on the other end. The presentation layer takes any data transmitted by
the application layer and prepares it for transmission over the session layer.

5. Session Layer

The session layer creates communication channels, called sessions, between
devices. It is responsible for opening sessions, ensuring they remain open and
functional while data is being transferred, and closing them when communication
ends. The session layer can also set checkpoints during a data transfer—if the
session is interrupted, devices can resume data transfer from the last
checkpoint.

4. Transport Layer

The transport layer takes data transferred in the session layer and breaks it
into “segments” on the transmitting end. It is responsible for reassembling the
segments on the receiving end, turning it back into data that can be used by the
session layer. The transport layer carries out flow control, sending data at a
rate that matches the connection speed of the receiving device, and error
control, checking if data was received incorrectly and if not, requesting it
again.

3. Network Layer

The network layer has two main functions. One is breaking up segments into
network packets, and reassembling the packets on the receiving end. The other is
routing packets by discovering the best path across a physical network. The
network layer uses network addresses (typically Internet Protocol addresses) to
route packets to a destination node.

2. Data Link Layer

The data link layer establishes and terminates a connection between two
physically-connected nodes on a network. It breaks up packets into frames and
sends them from source to destination. This layer is composed of two
parts—Logical Link Control (LLC), which identifies network protocols, performs
error checking and synchronizes frames, and Media Access Control (MAC) which
uses MAC addresses to connect devices and define permissions to transmit and
receive data.

1. Physical Layer

The physical layer is responsible for the physical cable or wireless connection
between network nodes. It defines the connector, the electrical cable or
wireless technology connecting the devices, and is responsible for transmission
of the raw data, which is simply a series of 0s and 1s, while taking care of bit
rate control.

ADVANTAGES OF OSI MODEL

The OSI model helps users and operators of computer networks:

* Determine the required hardware and software to build their network.
* Understand and communicate the process followed by components communicating
across a network.
* Perform troubleshooting, by identifying which network layer is causing an
issue and focusing efforts on that layer.

The OSI model helps network device manufacturers and networking software
vendors:

* Create devices and software that can communicate with products from any other
vendor, allowing open interoperability
* Define which parts of the network their products should work with.
* Communicate to users at which network layers their product operates – for
example, only at the application layer, or across the stack.

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OSI VS. TCP/IP MODEL

The Transfer Control Protocol/Internet Protocol (TCP/IP) is older than the OSI
model and was created by the US Department of Defense (DoD). A key difference
between the models is that TCP/IP is simpler, collapsing several OSI layers into
one:

* OSI layers 5, 6, 7 are combined into one Application Layer in TCP/IP
* OSI layers 1, 2 are combined into one Network Access Layer in TCP/IP –
however TCP/IP does not take responsibility for sequencing and
acknowledgement functions, leaving these to the underlying transport layer.

Other important differences:

* TCP/IP is a functional model designed to solve specific communication
problems, and which is based on specific, standard protocols. OSI is a
generic, protocol-independent model intended to describe all forms of network
communication.
* In TCP/IP, most applications use all the layers, while in OSI simple
applications do not use all seven layers. Only layers 1, 2 and 3 are
mandatory to enable any data communication.

See how Imperva Web Application Firewall can help you with application security.

Request demo Learn more

IMPERVA APPLICATION SECURITY

Imperva security solutions secure your applications across multiple layers of
the OSI model, from the network layer, protected by Imperva DDoS mitigation, to
Imperva’s web application firewall (WAF), bot management and API security
technology that safeguards the application layer.

To secure applications and networks across the OSI stack, Imperva provides
multi-layered protection to make sure websites and applications are available,
easily accessible and safe. The Imperva application security solution includes:

* DDoS Protection—maintain uptime in all situations. Prevent any type of DDoS
attack, of any size, from preventing access to your website and network
infrastructure.
* CDN—enhance website performance and reduce bandwidth costs with a CDN
designed for developers. Cache static resources at the edge while
accelerating APIs and dynamic websites.
* WAF—cloud-based solution permits legitimate traffic and prevents bad traffic,
safeguarding applications at the edge. Gateway WAF keeps applications and
APIs inside your network safe.
* Bot protection—analyzes your bot traffic to pinpoint anomalies, identifies
bad bot behavior and validates it via challenge mechanisms that do not impact
user traffic.
* API security—protects APIs by ensuring only desired traffic can access your
API endpoint, as well as detecting and blocking exploits of vulnerabilities.
* Account takeover protection—uses an intent-based detection process to
identify and defends against attempts to take over users’ accounts for
malicious purposes.
* RASP—keep your applications safe from within against known and zero‑day
attacks. Fast and accurate protection with no signature or learning mode.
* Attack analytics—mitigate and respond to real cyber security threats
efficiently and accurately with actionable intelligence across all your
layers of defense.

ARTICLE'S CONTENT

* What Is the OSI Model
* OSI Model Explained: The OSI 7 Layers
* OSI vs. TCP/IP Model
* Imperva Application Security

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