Chapter 4 Policy-enabled networks 277Using the Business Policy Switch 2000 Version 1.2DiffServ ConceptsDiffServ is described in IETF RFCs 2474 and 2475. This architecture is flexibleand allows for either end-to-end QoS or intradomain QoS by implementingcomplex classification and mapping functions at the network boundary or accesspoints. Within a DiffServ domain, the packet treatment is regulated by thisclassification and mapping.The DiffServ basic elements are implemented within the network and include:• Packet classification functions• A small set of per-hop forwarding behaviors• Traffic metering, marking, and policingTraffic is classified as it enters the DS network and is then assigned theappropriate PHB based on that classification. Within the IP packet, the 6 bits inthe DSCP are marked to identify how the packet should be treated at eachsubsequent network node. This mapping of DS codepoints to per-hop behavior(PHB) is configurable, and the DSCP may be re-marked as it passes through aDiffServ network. Re-marking the DSCP allows for the treatment of packets to bereset based on new network specifications or desired levels of service.DiffServ assumes the existence of a Service Level Agreement (SLA) between DSdomains that share a border. The SLA defines the profile for the aggregate trafficflowing from one network to the other based on policy criteria. As the trafficmoves within the DiffServ network, policies ensure that traffic marked by thedifferent DSCPs is treated according to that marking.QoS classesThe BPS 2000 supports the following Nortel Networks QoS classes:• Critical and Network classes have the highest priority over all other traffic.• Premium class is an end-to-end service functioning similarly to a virtualleased line. Traffic in this service class is normally guaranteed an agreed-uponpeak bandwidth. This service class is suitable for real-time applications likevideo and voice over IP. The recommended PHB for this service is theExpedited Forwarding (EF) PHB.
