ld = CHGSTS. The session chairman can also cut short a lengthy active site, that is to "kill" a currently active site by sending a C_CONF_UPDATE message to the MSIP server with the Update_Operation field = KILL, with which the MSIP server will then follow the procedure for releasing the active site. During the session, the chairman can invite new participants or drop an existing participant. To invite a new participant, the chairman sends aC. CONF_ADDUSER message to the MSIP server. Please refer to Section 13.4.1.3 (session establishment phase) for message contents and MSIP server behavior C 2009 by Taylor & Francis Group, LLC Multipoint Extensions for SIP MSIP Server 1 MSIP Client 1 MSIP Client 2 MSIP Client 3 CONF_UPDATE OPR = CHGSTS (Client 1) S CONF_UPDATE OPR=CHGSTS S_CONF_UPDATE OPR = CHGSTS Client 1 status changed S CONF_UPDATE OPR = CHGSTS (Client 1) C_CONF_DROPUSER Client 1 dropped S_CONF_UPDATE from session OPR=DROP S CONF_UPDATE OPR = DROP S CONF_UPDATE OPR = DROP C_LEAVE_CONF Client 3 left the session S CONF_UPDATE OPR = LEAVE FIGURE 13.13 Messages flow for change status, drop client, and leave session. upon receiving it. The session chairman could also drop any participant at any time during the session. To drop a participant, the chairman sends a C_CONF FROPUSER mes- sage to the MSIP server. The server will remove the dropped participant from the session invited users, update the session table, and then send an S_CONF_UPDATE message to all involved clients with Update_Operation field = DROP. Once a user is removed from the invited participant list, he would not be able to join the session again even if the session is still running. Figure 13.13 shows the message flow for some of the session update messages between three clients. Client 2 is considered the session chairman. The chair- man changes client 1's status first, then drops it completely from the session. After that, Client 3 leaves the session. 13.4.1.6 Terminating a Session Phase In this phase, the chairman's MSIP client terminates the sess