) FIGURE 15.3 Q-SIP/SDP messages for QoS-guaranteed multimedia session establishment. receives the INVITE request from the caller, the callee checks the requested QoS parameters in the SDP message, checks the availability of resource and capacity, and sends a response message (183 Session Progress Response) to the caller. In this response message, the callee specifies the acceptable media types, QoS parameters, and bandwidth and traffic parameters. When the caller receives the 183 message, it checks the parameters that have been selected by the callee, and sends a Provisional Response ACKnowledge (PRACK) mes- sage to the callee as the answer to the 183 message. When the callee receives the PRACK message, it sends a 200 OK message as the acknowledgement to the the PRACK. Table 15.1 presents the Q-SIP messages used in multimedia session establishment. After receiving the 200 OK message, the caller initiates the connection establishment procedure using network layer signaling, such as RSVP-TE UNI signaling in IP/MPLS subnetwork. Details of the connection establishment procedure in the IP/MPLS subnetwork using RSVP-TE UNI/NNI signaling are explained in the next section. When the two connections are successfully established for each direction, the caller then sends an UPDATE message to inform that the multimedia stream may be turned on to deliver the multimedia audio/video data. By receiving the UPDATE message, the called party finally accepts the multi- media call/session by replying a 200 OK (INVITE) message to the caller. The caller sends an ACK message that informs the successfully established call/session. Bidirectional QoS-guaranteed multimedia communication can be provided at this moment. When either the caller or the called party sends © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security TABLE 15.1 Q-SIP Messages Using Multimedia Session Establishment SIP Message Direction Major Contents INVITE A (caller) B (callee) Media information of the caller, QoS or security informatio