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webrtc / src / refs/heads/main / . / docs / native-code / rtp-hdrext /
abs-send-time
tree: 913d86cce5b7dbaac5fa0cd8ebbe399939654e1e [path history] [tgz]
 1. README.md

docs/native-code/rtp-hdrext/abs-send-time/README.md


ABSOLUTE SEND TIME

The Absolute Send Time extension is used to stamp RTP packets with a timestamp
showing the departure time from the system that put this packet on the wire (or
as close to this as we can manage). Contact solenberg@google.com for more info.

Name: “Absolute Sender Time” ; “RTP Header Extension for Absolute Sender Time”

Formal name: http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time

SDP “a= name”: “abs-send-time” ; this is also used in client/cloud signaling.

Not unlike RTP with TFRC

Wire format: 1-byte extension, 3 bytes of data. total 4 bytes extra per packet
(plus shared 4 bytes for all extensions present: 2 byte magic word 0xBEDE, 2
byte # of extensions). Will in practice replace the “toffset” extension so we
should see no long term increase in traffic as a result.

Encoding: Timestamp is in seconds, 24 bit 6.18 fixed point, yielding 64s
wraparound and 3.8us resolution (one increment for each 477 bytes going out on a
1Gbps interface).

Relation to NTP timestamps: abs_send_time_24 = (ntp_timestamp_64 >> 14) &
0x00ffffff ; NTP timestamp is 32 bits for whole seconds, 32 bits fraction of
second.

Notes: Packets are time stamped when going out, preferably close to metal.
Intermediate RTP relays (entities possibly altering the stream) should remove
the extension or set its own timestamp.

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