RTP Control Protocol (RTCP) – the optional companion protocol to Real-time Transport Protocol (RTP) in a Voice Over Internet Protocol (VoIP) network. It is used to monitor the transmission and quality-of-service (QoS) of streaming media, however, it is not needed for RTP to work. The primary function of RTCP is to provide feedback on the quality of the data distribution being accomplished by RTP. This function is an integral part of RTP’s role as a transport protocol and is related to the flow and congestion control functions of the network. Although the feedback reports from RTCP do not describe where problems are occurring (only that they are), they can be used as a tool to locate problems. With the information generated from different media gateways in the network, RTCP feedback reports enable an administrator to evaluate where network performance might be degrading.
RTCP enables administrators to monitor the quality of a call session by tracking packet loss, latency (delay), jitter, and other key VoIP concerns. This information is provided on a periodic basis to both ends and is processed per call by the media gateways.
Some gateway devices might not employ RTCP because the facility to report such information is not applicable to the end user. For example, a single residential user (with an analog phone) might not have access to the gateway providing the service. Also, the media gateway vendor can use a more scalable approach of tracking call quality statistics. In this case, the storage, transport and presentation of statistical info are device dependent.
If using RTCP (or a vendor-specific implementation) in the network, the organization needs to take into account bandwidth calculations for the protocol. Administrators need to limit the control traffic of RTCP to a small and known fraction of the session bandwidth. It should be small so as not to impair the ability of the transport protocol to carry data. An organization should investigate the amount of bandwidth needed so that they can include the control traffic in the bandwidth specification. Remote fucntion call (RFC) specifications recommend that the fraction of the session bandwidth allocated to RTCP be fixed at five percent of RTP traffic.