IP multicast allows sources to send to a multicast group without being a receiver of that group. However, for many conferencing purposes it is useful to know who is listening to the conference, and whether the media flows are reaching receivers properly. Accurately performing both these tasks restricts the scaling of the conference. IP multicast means that no-one knows the precise membership of a conference at a specific time, and this information cannot be discovered, as to try to do so would cause an implosion of messages, many of which would be lost This is not to say that we cannot know the bounds of a conference membership, a subset of whom might be present at any time - this can be done using encryption and restricted distribution of encryption keys, of which more later. Instead, RTCP provides approximate membership information through periodic multicast of session messages which, in addition to information about the recipient, also give information about the reception quality at that receiver. RTCP session messages are restricted in rate, so that as a conference grows, the rate of session messages remains constant, and each receiver reports less often. A member of the conference can never know exactly who is present at a particular time from RTCP reports, but does have a good approximation to the conference membership.
Reception quality information is primarily intended for debugging purposes, as debugging of IP multicast problems is a difficult task. However, it is possible to use reception quality information for rate adaptive senders, although it is not clear whether this information is sufficiently timely to be able to adapt fast enough to transient congestion. However, it is certainly sufficient for providing adaptation to a ``share'' of the current capacity.
The principle reason that it is hard to use this approach for general congestion avoidance is the fact that feedback messages are necessarily delayed and each receiver only reports at a rate which is inversely proportional to the membership of the conference (otherwise the network is flooded with these reports. While this means that the larger and more densely the receiver population permeates the network, the more places are sampled periodically, for modest size groups, this may mean that network conditions as perceived by a source are quite out of date. Some other aspects of RTCP reports are discussed in chapter eight.