Schulzrinne, et al Standards Track [Page 15]
RFC 1889 RTP January 1996
multicasting have shown that it is also critical to get
feedback from the receivers to diagnose faults in the
distribution. Sending reception feedback reports to all
participants allows one who is observing problems to
evaluate whether those problems are local or global. With a
distribution mechanism like IP multicast, it is also
possible for an entity such as a network service provider
who is not otherwise involved in the session to receive the
feedback information and act as a third-party monitor to
diagnose network problems. This feedback function is
performed by the RTCP sender and receiver reports,
described below in Section 6.3.
2. RTCP carries a persistent transport-level identifier for an
RTP source called the canonical name or CNAME, Section
6.4.1. Since the SSRC identifier may change if a conflict
is discovered or a program is restarted, receivers require
the CNAME to keep track of each participant. Receivers also
require the CNAME to associate multiple data streams from a
given participant in a set of related RTP sessions, for
example to synchronize audio and video.
3. The first two functions require that all participants send
RTCP packets, therefore the rate must be controlled in
order for RTP to scale up to a large number of
participants. By having each participant send its control
packets to all the others, each can independently observe
the number of participants. This number is used to
calculate the rate at which the packets are sent, as
explained in Section 6.2.
