RTCP packets do not encapsulate chunks of audi or video. Instead, RTCP packets are sent peridoically and contain
sender and/or receiver reports that announce statistics that can be useful to the application. These statistics include
number of packets sent, number of packets lost, and interarrival jutter. The RTP does not dictate what the application
should do with this feedback information; this is up to the application developer.
RTCP Packet Types
For each RTP stream that a receiver receives as part of a session, the receiver generates a reception report. The
receiver aggregates its reception reports into a single RTCP packet. The packet is then sent into the multicast tree
that connects all the session's participants. The reception reports includes several fields, the most important of which
are listed below.
- The SSRC of the RTP stream for which the reception report is being generated.
- The fraction of packets lost within the RTP stream. THe last sequence number received in the stream of RTP packets.
- The interarrival jitter, which is a smoothed estimate of the variation in the interarrival time between successive packets
in the RTP stream.
For each RTP stream that a sender is transmitting, the sender creates and transmitts RTCP sender report packets.
These packets include information about the RTP stream, including:
- The SSRC of the RTP stream
- The timestamp and wall clock time of the most recently generated RTP packet in the stream.
- The number of packets sent in the stream.
- The number of bytes sent in the stream.
RTCP Bandwidth Scaling
The number amount of RTP traffic sent into the multicast tree does not change as the number of receivers increases, whereas
the amount of RTCP traffic grows linearly with the number of receivers. To solve this scaling problem. RTCP modifies
the rate at which a participane sends RTCP packets into the multicast tree as a function of the number of participants in
the session. Since each participant sends control packets to everyone else, each participant can setimate the total
number of participants in teh session. RTCP attempts to limit its traffic to 5 percent of the session bandwidth.
