The IMS: IP Multimedia Concepts And Services, Second Edition

5.4. Session-based messaging

Session-based messaging relates to a familiar paradigm of messaging already in use in the Internet: Internet Relay Chat (IRC) [RFC2810]. In this mode of messaging the user takes part in a session in which the main media component often consists of short textual messages. As in any other session a message session has a well-defined lifetime: a message session starts when the participants begin the session and stops when the participants close the session. After the session is set up – using SIP and SDP between the participants – media then flows directly from peer to peer. Figure 5.2 illustrates the typical message flow of a message session.

Figure 5.2. Session-based messaging flow.

Session-based messaging can be peer to peer, in which case the experience closely mimics that of a normal voice call. An ordinary invitation to a session is received, the only difference being that the main media component is a session of messages. However, this is not an actual limitation to session-based messaging, since it is, of course, possible to combine other media sessions with message sessions. In fact, many useful and exciting applications are enabled by this functionality: for example, video calls with a text side channel might be a valuable application for hearing-impaired people.

The actual protocol for conveying the messages within a session is called the ...