| # 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. |