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diff --git a/third_party/libwebrtc/docs/native-code/rtp-hdrext/abs-send-time/README.md b/third_party/libwebrtc/docs/native-code/rtp-hdrext/abs-send-time/README.md new file mode 100644 index 0000000000..86c3c733dc --- /dev/null +++ b/third_party/libwebrtc/docs/native-code/rtp-hdrext/abs-send-time/README.md @@ -0,0 +1,31 @@ +# 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](http://tools.ietf.org/html/draft-ietf-avt-tfrc-profile-10#section-5) + +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. |