Unicast-Based Rapid Synchronization with RTP Multicast Sessions
When a receiver joins a multicast session, it may need to acquire and parse certain key information before it can process any data sent in the multicast session. Depending on the join time, length of the key information repetition interval, size of the key information as well as the application and transport properties, the time lag before a receiver can usefully consume the multicast data, which we refer to as the synchronization delay, varies and may be large. This is an undesirable phenomenon for receivers that frequently switch among different multicast sessions, such as video broadcasts. In this document, we describe a method using existing RTP and RTCP protocol machinery that reduces the synchronization delay. In this method, an auxiliary unicast RTP session carrying the key information to the receiver precedes/accompanies the multicast flow. This unicast flow may be transmitted at a faster than natural rate to further accelerate the synchronization. The motivating use case for this capability is multicast applications that carry real-time compressed audio and video. However, the proposed method can also be used in other types of multicast applications where the synchronization delay is long enough to be a problem.