_GET_GROUPLIST S_START_GROUPLIST S_CONT_GROUPLIST If required FIGURE 13.8 Messages flow for the pre-create session stage. MSIP Server 1 MSIP Client 1 MSIP Client 2 MSIP Client 3 Pre join session C_GET_CONFLIST S_START_CONFLIST S_CONT_CONFLIST If required FIGURE 13.9 Messages flow for pre-join session stages. S_CREATE_CONF message with session priority field = CREATE_CONF_ - MAX (0x21). If the maximum number of sessions is not reached, the MSIP server will initi- ate all session parameters based on the information received in a C_CREATE. CONF message, and then add a new session record. After that, the MSIP server sends back an S_CREATE_CONF message with session priority field = CREATE_CONF_OK (0x20). C 2009 by Taylor & Francis Group, LLC Multipoint Extensions for SIP MSIP Server 1 MSIP Client 1 MSIP Client 2 MSIP Client 3 C_CREATE_CONF S_CREATE_CON CREATE_CONF_OK (Client 1) C_CONF_ADDUSER S_CONF_UPDATE (Client 3) C_CONF_ADDUSER S_CONF_UPDATE S_CONF_UPDATE FIGURE 13.10 Messages flow for establishing a session. 13.4.1.3 Session Establishment Phase After the MSIP client receives CREATE_CONF_OK, it will send multiple C_CONF_ADDUSER messages (one for each user invited to the session). This message contains the session ID, session operation, user ID, user group ID, user MSIP server ID, operation type, and user name. Once the MSIP server receives the message, it adds the user to the session invited list, then sends an S_CONF_UPDATE message to all users involved in the session and to the user invited with Update_Operation field = ADDPAR. Figure 13.10 shows the messages flow for session establishment between three clients. Client 2 is considered the session chairman. 13.4.1.4 Joining a Session Phase A user can join a session when he receives an invitation message by responding to it. A user should know that he is invited to a session when he receives an S_CONF_UPDATE message that contains session operation field = add participant (ADDPAR), and the participant ID is equal to his ID. A user can also join a session by sending