in the session queue (if any) who are wait- ing for their turn to become active. The joining client uses all the information to build up its session table. The MSIP server sends an S_CONF_UPDATE message to the other clients involved in the session to update them on newly joined clients. Figure 13.11 shows the messages flow for joining a session between three clients. Client 2 is considered the session chairman, and he invited client 1 and client 3 to a session. Client 1 joined immediately, while client 3 joined later, and found out that the session has already ended. 13.4.1.5 Controlling the Session and Session Updates Phase 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 by sending C_CONF_UPDATE message to the MSIP server with Update_Operation field = REQ. If one of the active site cells is empty, the participant will be active, if not, the MSIP server will put the participant ID in the first empty cell in the session queue then send an S_CONF_UPDATE message with updated information to C 2009 by Taylor & Francis Group, LLC Multipoint Extensions for SIP MSIP Server 1 MSIP Client 1 MSIP Client 2 MSIP Client 3 S_CONF_UPDATE S_CONF_UPDATE S_REQJOIN_CONF S_REQJOIN_CONF Flag = JOIN_PAR C_JOIN_CONF S_START_PARLIST If required S_CONT_PARLIST S_ACTIVE_LIST START QLIST S_CONT_QLIST If required S_CONF_UPDATE S_CONF_UPDATE C_GET_CONFLIST S_START_CONFLIST S_CONT_CONFLIST If required C_REQJOIN_CONF S_REQJOIN_CONF Flag = CONF_ENDED FIGURE 13.11 Messages flow for joining a session. all clients involved SO that the clients can also update their session table. The number of active cells can be one, two, three, or more (based on the control criteria used for session control). Participants can also release the active site status or withdraw from the session queue by sending C_CONF_UPDATE message to the MSIP server with the Update_Operation field = REL or UNQ. The MSIP server will update the session