In this chapter, we look at how multimedia sessions are set up. In principle, no formal session set up mechanisms are needed for multicast communication - senders send to a group address, receivers subscribe to the same address, and communication ensures. However, in practice, we normally need a means for users that wish to communicate to discover which multicast address to use, to discover which protocols and codecs to transmit and receive with, and just to discover that a session is going to take place at all. Sometimes email has served these purposes, but typically we require more integrated mechanisms that are designed specifically with session initiation in mind, and which hide most of the details from the users.
There are several complementary mechanisms for initiating sessions, depending on the purpose of the session, but they essentially can be divided into invitation and announcement mechanisms. A traditional example of an invitation mechanism would be making a telephone call, which is essentially an invitation to participate in a private session. A traditional example of an announcement mechanism is the television guide in a newspaper, which announces the time and channel that each program is broadcast. In the Internet, in addition to these two extremes, there are also sessions that fall in the middle, such as an invitation to listen to a public session, and announcements of private sessions to restricted groups.