Real-time Transport Control Protocol (RTCP) is a sister protocol of the Real-time Transport Protocol (RTP).
RTCP 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.
RTCP 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 using a different codec.
There are several type of RTCP packets: Sender report packet, Receiver report packet, Source Description RTCP Packet, Goodbye RTCP Packet and Application Specific RTCP packets.
RTCP itself does not provide any flow encryption or authentication means. SRTCP protocol can be used for that purpose.