Real-time Transport Protocol (RTP) allows only that type of message, which carries data from the source to the destination. But in some cases, we need some other type of messages in a session. The messages that can control the transmission and quality of data as well as also allow the recipients so that they can send feedback to the source or sources. A protocol designed for this purpose, which is known as Real-time Transport Control Protocol (RTCP).
The above diagram shows the flow structure of RTP and RTCP protocol.
RTCP has five types of messages that are given below:
Sender Report :
The sender report is sent after a fixed interval by the active sender in a conference to report transmission as well as statistics of reception for all RTP packets transmitted during the time period. The report sent by the sender contains the detail of absolute time-stamp, that is the number of seconds elapsed since midnight on January 1, 1970. After receiving the RTP messages by the receiver, these details of absolute timestamps helps the receiver for synchronization process. And this is very much important in audio video transmission for fing the relative timestamp.
Receiver Report :
Passive participants are those participants that do not send RTP packets, and for them the Receiver report is used. This report is used to informs the sender and other receivers about the quality of service.
Source Description Message :
The source sends a source description message within a fixed interval to give some extra information about itself. It contains the details about the name of the source, its mail ID, contact number or source controller.
Bye Message :
To shut down a stream, a source sends a type of message which is known as Bye message. It is used by the source to announcing for leaving the conference. This message is a direct announcement for other sources about the absence of a source. It can be used for combining different media file.
Application-Specific Message :
If we want to make our application extensible then RTCP allows application-specific RTCP packets which is introduced by RTC 3611. It can be used to extend the type of application.
UDP Port of RTCP :
RTP use a well known UDP Port, but RTCP does not. RTCP uses a temporary port. It must be an odd-numbered port. It uses UDP port number which is the next higher odd number and that follows the port number which selected for RTP.
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