1# Absolute Send Time 2 3The Absolute Send Time extension is used to stamp RTP packets with a timestamp 4showing the departure time from the system that put this packet on the wire 5(or as close to this as we can manage). Contact <[email protected]> for 6more info. 7 8Name: "Absolute Sender Time" ; "RTP Header Extension for Absolute Sender Time" 9 10Formal name: <http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time> 11 12SDP "a= name": "abs-send-time" ; this is also used in client/cloud signaling. 13 14Not unlike [RTP with TFRC](http://tools.ietf.org/html/draft-ietf-avt-tfrc-profile-10#section-5) 15 16Wire format: 1-byte extension, 3 bytes of data. total 4 bytes extra per packet 17(plus shared 4 bytes for all extensions present: 2 byte magic word 0xBEDE, 2 18byte # of extensions). Will in practice replace the "toffset" extension so we 19should see no long term increase in traffic as a result. 20 21Encoding: Timestamp is in seconds, 24 bit 6.18 fixed point, yielding 64s 22wraparound and 3.8us resolution (one increment for each 477 bytes going out on 23a 1Gbps interface). 24 25Relation to NTP timestamps: abs_send_time_24 = (ntp_timestamp_64 >> 14) & 260x00ffffff ; NTP timestamp is 32 bits for whole seconds, 32 bits fraction of 27second. 28 29Notes: Packets are time stamped when going out, preferably close to metal. 30Intermediate RTP relays (entities possibly altering the stream) should remove 31the extension or set its own timestamp. 32