RTCP stands for Real-time Transport Control Protocol and is defined in RFC 3550. RTCP works hand in hand with RTP. RTP does the delivery of the actual data, whereas RTCP is used to send control packets to participants in a call. The primary function is to provide feedback on the quality of service being provided by RTP.
RTP is originated and received on even port numbers, and the associated RTCP communication uses the next higher odd port number. It transports statistics and information such as octet and packet counts, jitter, and round-trip time. An application can use this information to control QoS parameters and choose, for example, to use a different codec.
RTCP does not provide any flow encryption or authentication methods, but such mechanisms can be implemented with the use of the Secure Real-time Transport Protocol (SRTP) .