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Rapid Synchronization of RTP Multicast Sessions


Zlatan Begi?, Mladen Kos


Vol. 10  No. 9  pp. 42-47


Quality of Service (QoS) is an important, and admittedly overloaded, concept for emerging services based on the Real-Time Transport Protocol (RTP) to deliver multimedia content. In the context of the Internet Protocol Television (IPTV) services, the synchronization of multimedia components plays an important role. In order to synchronize audio and video components of multimedia content, RTP timestamps have to be related. Periodic Real-Time Transport Control Protocol (RTCP) Sender Report (SR) packets, which associate the RTP timestamps in the stream with a common clock at the transmitter, are associated with each stream. Thus, the frequency and the timing of the RTCP SR packets often contribute to the delay before audio and video are rendered, not just to their synchronization, because many clients will not render anything before the synchronization has been established. Whereas the synchronization delay is problematic in case of network congestion, the retransmission technique is used for the RTP receiver to request the rapid synchronization of RTP multicast sessions from the retransmission server (RS). This paper investigates the concept of isolation of the RTCP from the RTP retransmission packets by assigning them to higher priority class which ensures the synchronization delay of the multicast multimedia sessions sufficient for lip-synchronization without excessive delay regardless of network congestion, with accurate statistics measured by RTCP. A primary use case for this concept is to reduce the channel-change times in IPTV networks where compressed video streams are multicast in different Source Specific Multicast (SSM) sessions and viewers randomly join these sessions.


DiffServ, multicast, retransmission, RTP, RTCP, synchronization