RTP, the Real-time Transport Protocol is used for transmitting
multimedia data over the internet. It provides sequence numbers, time
stamps, the used coding format (such as PCM, GSM, H.263) and other information
that are useful for many multimedia applications. RTP is not limited
to unicast, instead RTP packets can be sent to over an IP multicast address
to more than one receiver.
RTCP, the RTP Control Protocol, is a control protocol used in
conjunction with RTP. The primary function of RTCP is to provide feedback
on the quality of service (e.g. the number of sent packets, the number
of errors, delay values) being provided by RTP. RTCP packets are transmitted
periodically from each participant in a RTP session to all other participants
by IP multicast.
Scaling in large groups RTP packets are sent by multicast to all
receivers, but the amount of packets sent does not change if the number
of receivers increases. In contrast, each participant sends RTCP packets
and therefore, the amount of RTCP traffic grows linearly with the number
of receivers. To solve this scaling problem, RTCP adopts the interval
at which packets are sent as a function of the number of participants.
RTCP traffic is limited to 5 percent of the session bandwidth. .
Have a look at the scaling problem by choosing different group sizes!