1*d9f75844SAndroid Build Coastguard Worker# Absolute Send Time 2*d9f75844SAndroid Build Coastguard Worker 3*d9f75844SAndroid Build Coastguard WorkerThe Absolute Send Time extension is used to stamp RTP packets with a timestamp 4*d9f75844SAndroid Build Coastguard Workershowing the departure time from the system that put this packet on the wire 5*d9f75844SAndroid Build Coastguard Worker(or as close to this as we can manage). Contact <[email protected]> for 6*d9f75844SAndroid Build Coastguard Workermore info. 7*d9f75844SAndroid Build Coastguard Worker 8*d9f75844SAndroid Build Coastguard WorkerName: "Absolute Sender Time" ; "RTP Header Extension for Absolute Sender Time" 9*d9f75844SAndroid Build Coastguard Worker 10*d9f75844SAndroid Build Coastguard WorkerFormal name: <http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time> 11*d9f75844SAndroid Build Coastguard Worker 12*d9f75844SAndroid Build Coastguard WorkerSDP "a= name": "abs-send-time" ; this is also used in client/cloud signaling. 13*d9f75844SAndroid Build Coastguard Worker 14*d9f75844SAndroid Build Coastguard WorkerNot unlike [RTP with TFRC](http://tools.ietf.org/html/draft-ietf-avt-tfrc-profile-10#section-5) 15*d9f75844SAndroid Build Coastguard Worker 16*d9f75844SAndroid Build Coastguard WorkerWire format: 1-byte extension, 3 bytes of data. total 4 bytes extra per packet 17*d9f75844SAndroid Build Coastguard Worker(plus shared 4 bytes for all extensions present: 2 byte magic word 0xBEDE, 2 18*d9f75844SAndroid Build Coastguard Workerbyte # of extensions). Will in practice replace the "toffset" extension so we 19*d9f75844SAndroid Build Coastguard Workershould see no long term increase in traffic as a result. 20*d9f75844SAndroid Build Coastguard Worker 21*d9f75844SAndroid Build Coastguard WorkerEncoding: Timestamp is in seconds, 24 bit 6.18 fixed point, yielding 64s 22*d9f75844SAndroid Build Coastguard Workerwraparound and 3.8us resolution (one increment for each 477 bytes going out on 23*d9f75844SAndroid Build Coastguard Workera 1Gbps interface). 24*d9f75844SAndroid Build Coastguard Worker 25*d9f75844SAndroid Build Coastguard WorkerRelation to NTP timestamps: abs_send_time_24 = (ntp_timestamp_64 >> 14) & 26*d9f75844SAndroid Build Coastguard Worker0x00ffffff ; NTP timestamp is 32 bits for whole seconds, 32 bits fraction of 27*d9f75844SAndroid Build Coastguard Workersecond. 28*d9f75844SAndroid Build Coastguard Worker 29*d9f75844SAndroid Build Coastguard WorkerNotes: Packets are time stamped when going out, preferably close to metal. 30*d9f75844SAndroid Build Coastguard WorkerIntermediate RTP relays (entities possibly altering the stream) should remove 31*d9f75844SAndroid Build Coastguard Workerthe extension or set its own timestamp. 32