Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 216
Application-Specific Message

This type of message allows for experimentation of the RTCP protocol. It allows for application developers to place their own messages in a packet to be used by a receiver or sender of their application. This is another example of how this protocol is really not finished and never will be, which allows for uniqueness and extensibility.