1. MPEG-4 SL Payload Format
IETF-46
MPEG-4 SL Payload Format
draft-ietf-avt-rtp-mpeg4-02.txt
Andrea Basso
AT&T Labs - Research
IETF-46 Nov. 99

2. What is New
A descriptive paragraph about timestamps in RTCP SR reports has been added.
IETF-46 Nov. 99

3. RTP Timestamps in RTCP SR pkts
“The RTP timestamp value is calculated from the NTP timestamp for the current time which also goes in the RTCP SR packet. To perform that calculation, an implementation needs to periodically establish a correspondence between the CTS (Composition Time Stamp) value of a data packet and the NTP time at which that data was sampled.”
IETF-46 Nov. 99

4. RTP Timestamps in RTCP SR pkts
When the RTCP SR packet is sent:
The MPEG-4 sender knows the offset between composition time and capture time for that media.
There might not be a media sample which starts being sampled at that time. Thus the RTP timestamp that goes in the SR may not be equal to any CTS of any RTP data packet.
Therefore:
Application has to establish this correspondence and not just use the CTS of the RTP data packet.
Sender should factor the offset in the SR RTP timestamp calculation to comply with RTP timing model.
IETF-46 Nov. 99

5. Implementation status
At least 2 payload format implementations
AT&T
NOKIA
Interoperability test presented at the last MPEG-4 meeting in Melbourne (AU).
IETF-46 Nov. 99