Absolute Capture Timestamp RTP header extension

Absolute Capture Time The Absolute Capture Time extension is used to stamp RTP packets with a NTP timestamp showing when the first audio or video frame in a packet was originally captured. The intent of this extension is to provide a way to accomplish audio-to-video synchronization when RTCP-terminating intermediate systems (e.g. mixers) are involved. Name: "Absolute Capture Time"; "RTP Header Extension for Absolute Capture Time" Formal name: http://www.webrtc.org/experiments/rtp-hdrext/abs-capture-time Status: This extension is defined here to allow for experimentation. Experience with the experiment has shown that it is useful; this draft therefore presents it to the IETF for consideration of whether to standardize it or leave it as a proprietary extension.

RTP header extension format

Data layout overview Data layout of the shortened version of abs-capture-time with a 1-byte header + 8 bytes of data: Data layout of the extended version of abs-capture-time with a 1-byte header + 16 bytes of data:

Data layout details

Absolute capture timestamp Absolute capture timestamp is the NTP timestamp of when the first frame in a packet was originally captured. This timestamp MUST be based on the same clock as the clock used to generate NTP timestamps for RTCP sender reports on the capture system. It's not always possible to do an NTP clock readout at the exact moment of when a media frame is captured. A capture system MAY postpone the readout until a more convenient time. A capture system SHOULD have known delays (e.g. from hardware buffers) subtracted from the readout to make the final timestamp as close to the actual capture time as possible. This field is encoded as a 64-bit unsigned fixed-point number with the high 32 bits for the timestamp in seconds and low 32 bits for the fractional part. This is also known as the UQ32.32 format and is what the RTP specification defines as the canonical format to represent NTP timestamps.

Estimated capture clock offset Estimated capture clock offset is the sender's estimate of the offset between its own NTP clock and the capture system's NTP clock. The sender is here defined as the system that owns the NTP clock used to generate the NTP timestamps for the RTCP sender reports on this stream. The sender system is typically either the capture system or a mixer. This field is encoded as a 64-bit two's complement signed fixed-point number with the high 32 bits for the seconds and low 32 bits for the fractional part. It's intended to make it easy for a receiver, that knows how to estimate the sender system's NTP clock, to also estimate the capture system's NTP clock: If the estimated capture clock offset is not present (short format), it means that the sending system does not have enough data to compute a clock offset.

Further details

Capture system The capture system generates the timestamp as close as possible to the true capture time. This may involve subtracting known delays in the capture pipeline from the time at which the system clock is read. The capture time SHOULD be from the same clock as used to generate the NTP timestamp in RTP Sender Reports (SR) ( section 6.4.1), and indicate this by setting the "estimated capture clock offset" to zero; if this is not possible, the "estimated capture clock offset" MUST indicate the offset between the clock used for the capture timestamp and the clock used for RTP Sender Reports.

Intermediate systems An intermediate system MAY compute the outgoing capture clock offset as follows:

Start with the "estimated capture clock offset" from the incoming packet
Add the estimated offset between the sender's NTP clock and the intermediate's NTP clock (see )

This should give a reasonable estimate of the offset between the capture system's clock and the NTP timestamps sent in SR blocks by the intermediate system. An intermediate system (e.g. mixer) MAY adjust these timestamps as needed. It MAY also choose to rewrite the timestamps completely, using its own NTP clock as reference clock, if it wants to present itself as a capture system for A/V-sync purposes.

End systems A receiver can use the same algorithm as intermediate systems in order to compute the approximate time in the receiver's NTP clock at which the packet was generated. This should be more comparable between source systems with different clocks than just using the raw timestamp. A receiver MUST treat the first CSRC in the CSRC list of a received packet as if it belongs to the capture system. If the CSRC list is empty, then the receiver MUST treat the SSRC as if it belongs to the capture system. Mixers SHOULD put the most prominent CSRC as the first CSRC in a packet's CSRC list.

Estimating the NTP clock offset The NTP clock offset can be calculated from an SR packet in the following way:

Take the NTP timestamp from the SR packet
Subtract the arrival time of the SR packet, in the receiver's NTP clock
Add half the estimated RTT between the sender and the receiver

The resulting number should be a reasonable approximation of the offset between the two clocks, with positive numbers indicating that the sender's clock is running ahead, and negative numbers indicate that the sender's clock is running behind. Note that this method is sensitive to a number of issues:

Clock drift means that you have to continuously monitor and update the offset
RTT variance will cause variation in offset; a smoothed value should be used

This document is not normative about how the NTP clock offset is estimated.

Timestamp interpolation A sender SHOULD save bandwidth by not sending abs-capture-time with every RTP packet. It SHOULD still send them at regular intervals (e.g. every second) to help mitigate the impact of clock drift and packet loss. Mixers SHOULD always send abs-capture-time with the first RTP packet after changing capture system. A receiver SHOULD memorize the capture system (i.e. CSRC/SSRC), capture timestamp, and RTP timestamp of the most recently received abs-capture-time packet on each received stream. It can then use that information, in combination with RTP timestamps of packets without abs-capture-time, to extrapolate missing capture timestamps. Timestamp interpolation works fine as long as there's reasonably low NTP/RTP clock drift. This is not always true. Senders that detect "jumps" between its NTP and RTP clock mappings SHOULD send abs-capture-time with the first RTP packet after such a thing happening.