 queue and the session table accordingly, and then send an S_CONF_UPDATE message with updated information to all clients involved. By doing this, all involved clients are up to date. Figure 13.12 shows message flows for requesting and releasing the active site from MSIP client 2. The S_CONF_UPDATE message contains the update type (Update_Opera- tion Field), user ID, and session ID, SO when the current active participant releases, the participant who is next at the top of the session queue will know that he is now an active site. Participants are able to recognize themselves since each user has a unique ID in the MSIP server. The S_CONF_UPDATE message is also sent to all clients when a new user joins a session. © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security MSIP Server 1 MSIP Client 1 MSIP Client 2 MSIP Client 3 C_CONF_UPDATE (REQ) S_CONF_UPDATE OPR = REQ S_CONF_UPDATE OPR = REQ Client 1 active S_CONF_UPDATE C_CONF_UPDATE (REL) S_CONF_UPDATE OPR = REL S_CONF_UPDATE OPR = REL Client 1 passive S_CONF_UPDATE FIGURE 13.12 Messages flow for request and release active. If a participant wants to logout from a session (hang up), his client will send a CLEAVE_CONF message to the MSIP server. Once the MSIP server receives this message, it will update the session table by changing the user's Join Status field in the session table to LEFT_CONF. Then it sends an S_CONF_UPDATE message to all MSIP clients involved in the session with the Update_Operation field = LEAVE. During this phase, the chairman can upgrade observers to participants or conversely, using the C_CONF_UPDATE message. The chairman will send the message to the MSIP server with the Update_Operation field = CHGSTS. When the MSIP server receives the message, it will check and confirm if the message is received from the session chairman or not. If so, the MSIP server will update the user information in the session table and send an S_CONF_UPDATE message to all MSIP clients involved in the session with The Update_Operation fie