ses in a session, each of which has varying message formats or types. The six phases are registration, session initiation, session establishment, joining a session, controlling the session and session updates, and finally, terminating a session. 13.3.1 Registration During registration, the user notifies the local MSIP server at which terminal he is located. Users can also change their passwords or change their status after registration. 13.3.2 Session Initiation In this phase, the user (chairman) requests to create a new session. 13.3.3 Session Establishment In this phase, the MSIP server sends an invitation message to all users chosen by the chairman to be involved in the session. IP datagram TCP segment MSIP segment IP header TCP header MSIP header MSIP message MSIP data 20 Bytes 20 Bytes 1 Byte 1 Byte FIGURE 13.2 MSIP general message format. © 2009 by Taylor & Francis Group, LLC Multipoint Extensions for SIP 13.3.4 Joining a Session A user can join a session when he receives (through a MSIP client) an invi- tation message by responding to it. He can also join a session by sending a message to the MSIP server requesting the list of sessions he has been invited to. 13.3.5 Controlling the Session and Session Updates This phase continually occurs while a session is running. A control criterion is used to control the flow of the session. In this phase, a participant can request to be an active site (the MSIP server will then put him in the first empty cell in the session queue), release the active site status, withdraw from the session queue, or logout from the session. In this phase, the chairman can invite new users, remove users from the active site, change user status (from participant to observer or vice versa) or drop a user completely from the session. 13.3.6 Terminating a Session In this phase, the chairman client terminates the session by sending a termi- nate message to the MSIP server. The MSIP server informs all MSIP clients about the termination of the session by sending an update message to all MSIP c