276 Chapter 4 Policy-enabled networks208700-BFigure 101 Schematic of QoS policyDifferentiated Services (DiffServ) overviewDifferentiated services (DiffServ) is a Quality of Service (QoS) networkarchitecture that offers varied levels of service for different types of data traffic.DiffServ lets you designate a specific level of performance on a packet-by-packetbasis instead of using the “best-effort” model for your data delivery. You can givepreferential treatment (prioritization) to applications that require highperformance and reliable service, such as voice and video over IP.To differentiate between traffic flows, the DiffServ (DS) field, as defined inRFCs 2474 and 2475, is marked. The DS field in the IP header is an octet, andthe first six bits, called the DS codepoint (DSCP), are used in the DiffServarchitecture. The DSCP marking dictates the forwarding treatment given to thepacket at each hop. This marking occurs at the edge of the DiffServ domain and isbased on the policy or filter for the particular microflow or an aggregate flow.Within the DiffServ network, the marked packets are placed in a queue accordingto their marking, which in turn determines the per-hop-behavior (PHB) of thatpacket. For example, if a video stream is marked so that it receives the highestpriority, then it is placed in a high-priority queue. As those packets traverse theDiffServ network, the video stream is forwarded before any other packets.To ensure that the traffic stream conforms to the bandwidth assigned, policingwithin the network is necessary.ClassifierPacketsPorts Filter Action QueuesMeter Marker10451EADropper802.1pDSCPLoss-SensitivityPort
