Real-Time Transport Control Protocol (RTCP)

Real-Time Transport Control Protocol (RTCP) is a protocol that works with Real-Time Protocol (RTP) to monitor data delivery on large Multicast networks. 

The purpose of monitoring delivery is to determine whether RTP is providing the necessary Quality of Service (QoS) and to compensate for delays, if needed.  RTCP is used in voice over IP (VoIP) and Internet Protocol Television (IPTV), streaming media and video conferencing.

RTCP carries statistical and control data, while RTP delivers the data. RTCP statistics typically include the number of bytes sent, packets sent, lost packets and round trip delay between endpoints. RTCP also carries the canomical name (CNAME), which is a unique identifier for a participant during a session.

RTCP can use five different packet types to carry statistical and control data. The packets are RR (receiver report), SR (sender report), SDES (source description items), BYE (indicates end of participation) and APP (application specific functions).

RTCP was originally defined in RFC 1889, which was superseded by RFC 3550.