RTCP

The RTP Control Protocol (RTCP) is a sister protocol of the (RTP). Its basic functionality and packet structure is defined in RFC 3550. RTCP provides out-of-band statistics and control information for an RTP session. It partners with RTP in the delivery and packaging of multimedia data, but does not transport any media data itself. The primary function of RTCP is to provide feedback on the quality of service (QoS) in media distribution by periodically sending statistics information to participants in a streaming multimedia session.

RTCP transports statistics for a media connection and information such as transmitted octet and packet counts, packet loss, packet delay variation, and round-trip delay time. An application may use this information to control quality of service parameters, perhaps by limiting flow, or using a different codec.

Typically RTP will be sent on an even-numbered port, with RTCP messages being sent over the next higher odd-numbered port.

RTCP itself does not provide any flow encryption or authentication methods. Such mechanisms may be implemented, for example, with the (SRTP) defined in RFC 3711.

RTCP provides basic functions expected to be implemented in all RTP sessions:

RTCP reports are expected to be sent by all participants, even in a multicast session which may involve thousands of recipients. Such traffic will increase proportionally with the number of participants. Thus, to avoid network congestion, the protocol must include session bandwidth management. This is achieved by dynamically controlling the frequency of report transmissions. RTCP bandwidth usage should generally not exceed 5% of total session bandwidth. Furthermore, 25% of the RTCP bandwidth should be reserved to media sources at all times, so that in large conferences new participants can receive the CNAME identifiers of the senders without excessive delay.

The RTCP reporting interval is randomized to prevent unintended synchronization of reporting. The recommended minimum RTCP report interval per station is 5 seconds. Stations should not transmit RTCP reports more often than once every 5 seconds.

RTCP distinguishes several types of packets: sender report, receiver report, source description, and goodbye. In addition, the protocol is extensible and allows application-specific RTCP packets. A standards-based extension of RTCP is the extended report packet type introduced by RFC 3611.

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