Nortel business policy switch 2000 User Manual page 289

To ensure that the traffic stream conforms to the bandwidth assigned, policing
within the network is necessary. Traffic shaping may also be used to temporarily
delay traffic to ensure that the flows conform to downstream bandwidth limits.
DiffServ Concepts
DiffServ is described in IETF RFCs 2474 and 2475. This architecture is flexible
and allows for either end-to-end QoS or intradomain QoS by implementing
complex classification and mapping functions at the network boundary or access
points. Within a DiffServ domain, the packet treatment is regulated by this
classification 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 shaping
Traffic is classified as it enters the DS network and is then assigned the
appropriate PHB based on that classification. Within the IP packet, the 6 bits in
the DSCP are marked to identify how the packet should be treated at each
subsequent 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 a
DiffServ network. Re-marking the DSCP allows for the treatment of packets to be
reset based on new network specifications or desired levels of service.
DiffServ assumes the existence of a Service Level Agreement (SLA) between DS
domains that share a border. The SLA defines the profile for the aggregate traffic
flowing from one network to the other based on policy criteria. In a given traffic
direction, the traffic is expected to be shaped at the egress point of the upstream
network and metered at the ingress point of the downstream network.
As the traffic moves within the DiffServ network, policies ensure that traffic
marked by the different DSCPs is treated according to that marking. Traffic
metering and shaping ensures that the traffic flow conforms to an SLA to provide
certain levels of service in terms of bandwidth for different types of network
traffic.
Chapter 4 Policy-enabled networks 289
Using the Business Policy Switch 2000 Version 2.0