15.2. RTCP
RTP Control Protocol (RTCP) packets are transmitted periodically to all participants in a session. There are four RTCP functions:
To provide feedback on the QoS of real-time data distribution.
To carry a persistent identifier of the RTP source (called a CNAME).
To permit an adjustable RTCP packet distribution interval (the report interval).
To convey session control information.
15.2.1. RTCP packet types
There are five types of RTCP packets:
SR – a sender report, providing transmission and reception statistics, sent by active media senders.
RR – a receiver report, providing reception statistics, sent by non-active senders.
SDES – source description items, such as CNAME.
BYE – indicates end of participation.
APP – application-specific functions (defined by a profile).
15.2.2. RTCP report transmission interval
Every participant is required to send RTCP packets. If there are many participants (say, a conference), then there is a scalability problem as further users join in. To control the scalability problem, the rate at which RTCP packets are sent must be scaled down by dynamically calculating the interval between RTCP packet transmissions.
A certain percentage of session bandwidth should be dedicated to RTCP; this percentage and the interval between report transmissions are profile-defined.
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