Nortel 2000, Using Manual

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276 Chapter 4 Policy-enabled networks
208700-B
Figure 101 Schematic of QoS policy
Differentiated Services (DiffServ) overview
Differentiated services (DiffServ) is a Quality of Service (QoS) network
architecture 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-packet
basis instead of using the “best-effort” model for your data delivery. You can give
preferential treatment (prioritization) to applications that require high
performance and reliable service, such as voice and video over IP.
To differentiate between traffic flows, the DiffServ (DS) field, as defined in
RFCs 2474 and 2475, is marked. The DS field in the IP header is an octet, and
the first six bits, called the DS codepoint (DSCP), are used in the DiffServ
architecture. The DSCP marking dictates the forwarding treatment given to the
packet at each hop. This marking occurs at the edge of the DiffServ domain and is
based 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 according
to their marking, which in turn determines the per-hop-behavior (PHB) of that
packet. For example, if a video stream is marked so that it receives the highest
priority, then it is placed in a high-priority queue. As those packets traverse the
DiffServ network, the video stream is forwarded before any other packets.
To ensure that the traffic stream conforms to the bandwidth assigned, policing
within the network is necessary.
Classifier Packets
Ports Filter Action Queues
Meter Marker
10451EA
Dropper
802.1p
DSCP
Loss-Sensitivity
P
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