**RTP Control Protocol** (RTCP) is a helper protocol for *Real-time Transport Protocol* (RTP).
It is used to gather statistics about RTP sessions and to control the RTP flow. It does not transport multimedia data itself.
Example control message (which names show RTCP purpose): Sender Report (SR), Receiver Report (RR), Source Description (SDES), End of participation (BYE).
Detailed documentation can be found in part of RFC 3550 (RTP: A Transport Protocol for Real-Time Applications) which describes RTCP.
More information at http://en.wikipedia.org/wiki/RTP_Control_Protocol