1# Absolute Capture Time 2 3The Absolute Capture Time extension is used to stamp RTP packets with a NTP 4timestamp showing when the first audio or video frame in a packet was originally 5captured. The intent of this extension is to provide a way to accomplish 6audio-to-video synchronization when RTCP-terminating intermediate systems (e.g. 7mixers) are involved. 8 9**Name:** 10"Absolute Capture Time"; "RTP Header Extension for Absolute Capture Time" 11 12**Formal name:** 13<http://www.webrtc.org/experiments/rtp-hdrext/abs-capture-time> 14 15**Status:** 16This extension is defined here to allow for experimentation. Once experience has 17shown that it is useful, we intend to make a proposal based on it for 18standardization in the IETF. 19 20Contact <[email protected]> for more info. 21 22## RTP header extension format 23 24### Data layout overview 25Data layout of the shortened version of `abs-capture-time` with a 1-byte header 26\+ 8 bytes of data: 27 28 0 1 2 3 29 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 30 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 31 | ID | len=7 | absolute capture timestamp (bit 0-23) | 32 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 33 | absolute capture timestamp (bit 24-55) | 34 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 35 | ... (56-63) | 36 +-+-+-+-+-+-+-+-+ 37 38Data layout of the extended version of `abs-capture-time` with a 1-byte header + 3916 bytes of data: 40 41 0 1 2 3 42 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 43 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 44 | ID | len=15| absolute capture timestamp (bit 0-23) | 45 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 46 | absolute capture timestamp (bit 24-55) | 47 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 48 | ... (56-63) | estimated capture clock offset (bit 0-23) | 49 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 50 | estimated capture clock offset (bit 24-55) | 51 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 52 | ... (56-63) | 53 +-+-+-+-+-+-+-+-+ 54 55### Data layout details 56#### Absolute capture timestamp 57 58Absolute capture timestamp is the NTP timestamp of when the first frame in a 59packet was originally captured. This timestamp MUST be based on the same clock 60as the clock used to generate NTP timestamps for RTCP sender reports on the 61capture system. 62 63It's not always possible to do an NTP clock readout at the exact moment of when 64a media frame is captured. A capture system MAY postpone the readout until a 65more convenient time. A capture system SHOULD have known delays (e.g. from 66hardware buffers) subtracted from the readout to make the final timestamp as 67close to the actual capture time as possible. 68 69This field is encoded as a 64-bit unsigned fixed-point number with the high 32 70bits for the timestamp in seconds and low 32 bits for the fractional part. This 71is also known as the UQ32.32 format and is what the RTP specification defines as 72the canonical format to represent NTP timestamps. 73 74#### Estimated capture clock offset 75 76Estimated capture clock offset is the sender's estimate of the offset between 77its own NTP clock and the capture system's NTP clock. The sender is here defined 78as the system that owns the NTP clock used to generate the NTP timestamps for 79the RTCP sender reports on this stream. The sender system is typically either 80the capture system or a mixer. 81 82This field is encoded as a 64-bit two’s complement **signed** fixed-point number 83with the high 32 bits for the seconds and low 32 bits for the fractional part. 84It’s intended to make it easy for a receiver, that knows how to estimate the 85sender system’s NTP clock, to also estimate the capture system’s NTP clock: 86 87 Capture NTP Clock = Sender NTP Clock + Capture Clock Offset 88 89### Further details 90 91#### Capture system 92 93A receiver MUST treat the first CSRC in the CSRC list of a received packet as if 94it belongs to the capture system. If the CSRC list is empty, then the receiver 95MUST treat the SSRC as if it belongs to the capture system. Mixers SHOULD put 96the most prominent CSRC as the first CSRC in a packet’s CSRC list. 97 98#### Intermediate systems 99 100An intermediate system (e.g. mixer) MAY adjust these timestamps as needed. It 101MAY also choose to rewrite the timestamps completely, using its own NTP clock as 102reference clock, if it wants to present itself as a capture system for A/V-sync 103purposes. 104 105#### Timestamp interpolation 106 107A sender SHOULD save bandwidth by not sending `abs-capture-time` with every 108RTP packet. It SHOULD still send them at regular intervals (e.g. every second) 109to help mitigate the impact of clock drift and packet loss. Mixers SHOULD always 110send `abs-capture-time` with the first RTP packet after changing capture system. 111 112A receiver SHOULD memorize the capture system (i.e. CSRC/SSRC), capture 113timestamp, and RTP timestamp of the most recently received `abs-capture-time` 114packet on each received stream. It can then use that information, in combination 115with RTP timestamps of packets without `abs-capture-time`, to extrapolate 116missing capture timestamps. 117 118Timestamp interpolation works fine as long as there’s reasonably low NTP/RTP 119clock drift. This is not always true. Senders that detect "jumps" between its 120NTP and RTP clock mappings SHOULD send `abs-capture-time` with the first RTP 121packet after such a thing happening. 122