This memo outlines how RTP sessions are synchronised@ and discusses how rapidly such synchronisation can occur. We show that most RTP sessions can be synchronised immediately@ but that the use of video switching multipoint conference units (MCUs) or large source-specific multicast (SSM) groups can greatly increase the synchronisation delay. This increase in delay can be unacceptable to some applications that use layered and/or multi-description codecs. This memo introduces three mechanisms to reduce the synchronisation delay for such sessions. First@ it updates the RTP Control Protocol (RTCP) timing rules to reduce the initial synchronisation delay for SSM sessions. Second@ a new feedback packet is defined for use with the extended RTP profile for RTCP-based feedback (RTP/AVPF)@ allowing video switching MCUs to rapidly request resynchronisation. Finally@ new RTP header extensions are defined to allow rapid synchronisation of late joiners@ and guarantee correct timestamp-based decoding order recovery for layered codecs in the presence of clock skew.
RFC 6051-2010 history
2010RFC 6051-2010 Rapid Synchronisation of RTP Flows