h subnetwork should be able to provide QoS-guaranteed packet delivery with appropriate resource allocation and CAC [4]. In IEEE 802.3 Fast/Gigabit Ethernet, the legacy dummy hubs cannot pro- vide guaranteed QoS; while some recent Fast/Gigabit Ethernet switches, such as Cisco Catalyst 3550 and 2950, can provide QoS scheduling and queuing capability with strict priority queue and weighted random early detection (WRED) functions [14,15]. In Cisco Catalyst 3550, each port has four differ- ent output queues, and one of the queues can be configured as a strict priority queue, while the remaining queues are configured as non-strict priority queues and are serviced with the use of weighted round-robin (WRR). The strict pri- ority queue is specially designed for delay/jitter-sensitive traffic, such as voice © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Q-SIP/SDP Location Gigabits QSTA-Caller ethernet QSTA-Callee Proxy server server switches INVITE SDP1 Request location of the callee Response of location request Check SIP type for QoS request Create a QoS session Invite SDP1 183 Session Progress SDP2 Request resource allocation Find QoS path Resource reservation QoS-connectivity between QSTA-Caller Check the available establishment in Check the available bandwidth and QAP using TSPEC bandwidth Fast/gigabit ethernet Response of network resource Establish allocation QoS Connectivity 183 Session progress with modified SDP2 Check the result and modify SDP2 PRACK PRACK 200 OK (PRACK) 200 OK (PRACK) UPDATE SDP3 UPDATE SDP3 200 OK (UPDATE) SDP4 200 OK (UPDATE) SDP4 180 Ringing 180 Ringing PRACK PRACK 200 OK (PRACK) 200 OK (PRACK) 200 OK (INVITE) 200 OK (INVITE) Multimedia session with QoS FIGURE 15.6 Interactions between a Q-SIP/SDP proxy server and Q-CNM for QoS- connectivity establishment. and real-time video. Since the strict priority queue is always emptied first regardless of the status of other queues, it may eventually cause starvation of the other WRR queues. Since the currently 