VoIP Voice and Fax Signal Processing

10.2 RTP CONTROL PROTOCOL (RTCP)

RTCP is used to convey the end-to-end quality of the data stream in an RTP session. Statistics such as delay, jitter, number of packets sent, and packet loss rate allows the session participants to monitor the health of the connection. Timing parameters like network time protocol (NTP) and the parameters given in RFC1305 [Mills (1992)] establish a possible timing relation between gateways. The protocol also provides limited control functions such as BYE packets to control the status of the session. RTCP packets are transmitted on a different port than that of RTP. RTCP packets are exchanged between the end points in a periodic fashion. RTCP packets are sent at less than 5% of the RTP packets, which makes RTCP statistics to be used as steady parameter values. Packet-to-packet parameters have to be taken from RTP only. RTCP consists of five packet types:

1. Sender Report (SR)—Conveys the statistics of the active RTP sender.
2. Receiver Report (RR)—Conveys the statistics of the RTP receiver.
3. Source Description (SDES)—Description of the source regardless of whether it is a sender or a receiver.
4. BYE—Used to hang up from a session and indicates end of participation.
5. APP—Application-specific packet for experimental use.

RTCP packets are normally compound packets, which consist of a combination of several packet types from the above list. A compound packet always starts with either a sender or a receiver report, followed by the other packet types. A BYE packet ...