/mcpQuality of Service for Voice over IP Resource Reservation Protocol 22 QoSVoIP.mif RSVP for CAC Overview Using RSVP for VoIP CAC requires the synchronization of the call setup signaling and the RSVP signaling. This synchronization guarantees that the called-party phone rings only after the resources for the call have been reserved. This synchronization also gives voice gateways the control of what action to take before the call setup moves to the alerting stage if the reservation fails or cannot be completed within a predefined period of time. A voice call will trigger two RSVP reservations because the reservation and admission control mechanisms provided by RSVP are unidirectional. Each voice gateway is responsible for initiating and maintaining one reservation toward the other voice gateway. CAC for a VoIP call fails if at least one of the reservations fails. Figure 6 shows the sequence of packets exchanged between the gateways during a call setup if RSVP is used for resource reservation. Figure 6 Call Setup with RSVP Enabled In Figure 6, an originating gateway (OGW) initiates a call toward a terminating gateway (TGW). The originating gateway sends an H.323 SETUP message to the terminating gateway to initiate the call. That SETUP message carries the QoS that the originating gateway considers acceptable for the call. The terminating gateway responds with an H.323 CALL PROCEEDING message. Both the originating gateway and the terminating gateway initiate a reservation request by sending an RSVP PATH message. The packet flows of both reservations are independent of each other unless one of them fails. The terminating gateway blocks the call setup process waiting for the reservation results. The terminating gateway controls the admission decision for the call and needs to be notified that the reservations in both directions were successful. The terminating gateway discovers that its reservation was successful when it receives the RSVP RESV message. The terminating gateway detects that the originating gateway reservation was successful when it receives an RSVP RESV CONFIRMATION message from the originating gateway. At this point, the terminating gateway lets the call setup continue and sends an H.323 ALERTING message to the originating gateway once it is notified that the called side is in SETUP Call Setup Initiated OGW TGW Reservation Initiated Call Connected Normal Call Disconnect Initiated OGW TGW Reservation Teardown Initiated OGW TGW Reservation Initiated OGW TGW Reservation Teardown Initiated OGW Originating Gateway TGW Terminating Gateway H.323 Call Setup Signaling RSVP Reservation Initiated by TGW RSVP Reservation Initiated by OGW OGW TGW PATH RESV RESV CONFIRMATION RELEASE COMPLETE PATH TEAR PATH TEAR CALL PROCEEDING PATH RESV ALERTING PATH TEAR PATH TEAR CONNECT 60063