| # 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 as close as possible to the time the packet is |
| sent on the wire. The current implementation allocates space before pacing |
| and stamps them after pacing and before encryption. |
| Intermediate RTP relays (entities possibly altering the stream) should remove |
| the extension or set its own timestamp. |
