RTP Control Protocol (RTCP) is a sister protocol of the Real-time Transport Protocol (RTP). It is defined in RFC 3550 (which obsoletes RFC 1889).
RTCP stands for Real-time Transport Control Protocol, provides out-of-band control information for an RTP flow. It partners RTP in the delivery and packaging of multimedia data, but does not transport any data itself. It is used periodically to transmit control packets to participants in a streaming multimedia session. The primary function of RTCP is to provide feedback on the quality of service being provided by RTP.
It gathers statistics on a media connection and information such as bytes sent, packets sent, lost packets, jitter, feedback and round trip delay. An application may use this information to increase the quality of service perhaps by limiting flow, or maybe using a low compression codec instead of a high compression codec. RTCP is used for QoS reporting.
RTCP itself does not provide any flow encryption or authentication means. SRTCP protocol can be used for that purpose.
http://en.wikipedia.org/wiki/Real_time_control_protocol
The Secure Real-time Transport Protocol (or SRTP) defines a profile of RTP (Real-time Transport Protocol), intended to provide encryption, message authentication and integrity, and replay protection to the RTP data in both unicast and multicast applications. It was developed by David Oran (Cisco) and Rolf Blom (Ericsson) and first published by IETF in March 2004 as RFC 3711.
Since RTP is closely related to RTCP (RTP control protocol) which can be used to control the RTP session, SRTP also has a sister protocol, called Secure RTCP (or SRTCP); SRTCP provides the same security-related features to RTCP, as the ones provided by SRTP to RTP.
Utilization of SRTP or SRTCP is optional to utilization of RTP or RTCP; but even if SRTP/SRTCP are used, all provided features (such as encryption and authentication) are optional and can be separately enabled or disabled. The only exception is the message authentication feature which is indispensably required when using SRTCP.
http://en.wikipedia.org/wiki/Secure_Real-time_Transport_Protocol
