 which have varying message formats or types. 13.4.1.1 Registration Phase The first message that should be received from the MSIP client is C_USER LOGIN. This message contains the user information. The MSIP server will check the user authentication and update user table by adding a new record with the user information (if successful). The MSIP server then sends back an USER_LOGIN message to the MSIP client with login successful or not. It also sends S_USER_UPDATE to other clients (if they log into the MSIP server). After a user logs into the MSIP server, the user can change his password by sending a C_PASS_CHG message to the MSIP server. The message format is as shown in Figure 13.4. The VER field is for MSIP version. The type field is © 2009 by Taylor & Francis Group, LLC Multipoint Extensions for SIP User ID MSIP header MSIP message MSIP data Byte 1 Byte 1 Byte 2 Bytes 10 Bytes 10 Bytes 10 Bytes FIGURE 13.4 Change password message format. for message type; in this case it is either C_PASS_CHG if it is a request sent by the MSIP client, or S_PASS_CHG if it is a response from the MSIP server. The flag field is used to return the operation's result to the client. The user ID, user name, user old password, and user new password constitute the rest of the fields. When the MSIP server receives C_PASS_CHG from a client, it compares the old password with the one in the user table stored in the server. After that, the MSIP server will change the password in the table with the new password (if correct), then send back an S_PASS_CHG message to the client with a flag. The flag will explain the operation. The flag can be one of the following: PASS_WRG 0 X 11 if a wrong password is received from the client PASS_OK 0x 10 if a correct password is received and change password is successful FILE ERROR 0 X 12 if writing the new password in the user's file could not be done for any reason Figure 13.5 shows an unsuccessful attempt to change the password followed by a successful one. The Figure shows the communications between the M