SIP server and the MSIP client. The MSIP client can also send a C_STS_CHG message to the MSIP server if one of his flags has changed (i.e., video flag, audio flag). The MSIP client can also use a C_STS_CHG message to announce the change of his status (in a session, available, busy, etc.) The message format is as shown in Figure 13.6. The VER field is for the MSIP version. The type field is for message type, in this case it is C_STS_CHG. The flag field is used to determine which device is changed (video, audio, client status, etc.) Other fields are the user ID and user status. When the MSIP server receives C_STS_CHG from a client, it updates the user table based on the information received, and then it sends an S_USER UPDATE message to the current user and all other involved users. Involved users are users communicating with the user who changes his status. Fig- ure 13.7 shows the message flows between the MSIP server and three MSIP clients involved in the session. The messages showed in Figure 13.7 are from the MSIP server start-up and before starting a session. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security MSIP Client 1 MSIP Server 1 C_PASS_CHG S_PASS_CHG Flag = PASS_WRG C_PASS_CHG S_PASS_CHG Flag = PASS_OK FIGURE 13.5 Change password messages flow. User ID User status 1 Byte 1 Byte 1 Byte 2 Bytes 1 Byte FIGURE 13.6 Change status message format. 13.4.1.2 Session Initiation Phase In this phase, the user (chairman) requests to initiate a new session. Before the MSIP client starts a session, it sends a C_GET_USERLIST to the MSIP server, the MSIP server replies with S_START_USERLIST, followed by S_CONT_ USERLIST if required. The S_CONT_USERLIST is required if the number of users is very big and their information cannot fit in one message (message buffer size = 202 byte). The MSIP server will also send a SLUSER_UPDATE message to the MSIP client (one for each user in the user list). The S_USER_UPDATE message contains the latest updates for the users. The MSIP client will then