RTP allows only one type of message, one that carries data from the source to the destination.
In many cases, there is a need for other messages in a session. These messages
control the flow and quality of data and allow the recipient to send feedback to the source
or sources. Real-Time Transport Control Protocol (RTCP) is a protocol designed for
this purpose. RTCP has five types of messages, as shown in Figure 25.20. The number
next to each box defines the type of the message.