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32 lines
1.4 KiB
32 lines
1.4 KiB
7 months ago
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# Absolute Send Time
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The Absolute Send Time extension is used to stamp RTP packets with a timestamp
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showing the departure time from the system that put this packet on the wire
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(or as close to this as we can manage). Contact <solenberg@google.com> for
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more info.
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Name: "Absolute Sender Time" ; "RTP Header Extension for Absolute Sender Time"
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Formal name: <http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time>
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SDP "a= name": "abs-send-time" ; this is also used in client/cloud signaling.
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Not unlike [RTP with TFRC](http://tools.ietf.org/html/draft-ietf-avt-tfrc-profile-10#section-5)
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Wire format: 1-byte extension, 3 bytes of data. total 4 bytes extra per packet
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(plus shared 4 bytes for all extensions present: 2 byte magic word 0xBEDE, 2
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byte # of extensions). Will in practice replace the "toffset" extension so we
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should see no long term increase in traffic as a result.
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Encoding: Timestamp is in seconds, 24 bit 6.18 fixed point, yielding 64s
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wraparound and 3.8us resolution (one increment for each 477 bytes going out on
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a 1Gbps interface).
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Relation to NTP timestamps: abs_send_time_24 = (ntp_timestamp_64 >> 14) &
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0x00ffffff ; NTP timestamp is 32 bits for whole seconds, 32 bits fraction of
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second.
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Notes: Packets are time stamped when going out, preferably close to metal.
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Intermediate RTP relays (entities possibly altering the stream) should remove
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the extension or set its own timestamp.
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