RTP Control Protocol (RTCP)

[upward]	enables endpoints to provide meta-information to the source - this enables the sources to be adaptive to the endpoints. For example, by using an adaptive coding algorithm the source can accommodate the actually data rate of packets arriving at the endpoint.
[downward]	enables sources to send the endpoints information about a session

VER - version number (currently 2)

P - whether padding follows the payload (last octet indicates how much was added)

RC - Report Count - specifies the number of reports in this packet^†

PTYPE - Type of payload

^† RTCP uses compound packets with multiple RTCP messages in a single packet.

RTCP payload types
Name		Type	Meaning
Sender Report	SR	200	Time information for each synchronization source and a count of data octets sent
Receiver Report	RR	201	Report of packet loss and jitter, information for timing and round-trip estimation
Source Description	SDES	202	Description of who owns the source
Goodbye	BYE	203	Receiver leaving the session
Application	APP	204	Application-specific report

Transcript

[slide97] Now, the companion protocol to RTP in the next port number is the real-time protocol, control protocol, called RTCP, and its messages look like this. A version, padding, and now a report count, a payload type, a length. And what are these reports? Well, they're sender reports, receiver reports, source descriptions, goodbye, or application-specific messages. And here the idea is that a sender can say, in the last interval of time, this is the number of packets that I sent. Then a receiver can send back in a report, hey, in this period of time, this is how many packets I received. So now we can actually compute packet loss and jitter information.