274 Chapter4 Policy-enabled networks
208700-B
Policy-enabled networks allow system administrators to prioritize the network
traffic, thereby providing better service for selected appl ications . Using Quality o f
Service (QoS), the system administrators can establish service level agreements
(SLAs) with customers of the network.
In general, QoS helps with two network problems: bandwidth and time -sensitivity.
QoS can help you allocate guaranteed bandwidth to the critical applications, and
you can limit bandwidth for less critical applications. Applications such as video
and voice must have a certain amount of bandwidth to work correctly; using QoS,
you can provide that bandwidth when necessary. Also, you can put a hi gh pri or it y
on applications that are sensitive to timing out or cannot tolerate delay by
assigning that traffic to a high-priority queue.
Nortel Networks uses Differentiated Services (DiffServ) to provide QoS
functionality. A DiffServ architecture enables service discrimination of traffic
flows or microflows by offering network resources to higher c lasses at the expense
of lower classes of service. This architecture allows you to prioritize microflows
or aggregate flows and provides Quality of Service (QoS) that is scalable
Briefly, with DiffServ, you use policies to direct traffic by assigning packets to
certain queues. The system marks the DiffServ (DS) field of IP packets to define
how the packet is treated as it moves through the network. You classify traffic so
that, together, the policies and the DS fields direct the traffic prioritization. You
can specify a number of policies, and each policy can match one or many flows—
supporting complex classification scenarios.
Summary of packet classifiers
The BPS 2000 classifies packets based on various parameters:
•IP packets
—source address/mask
—destination address/mask
—IP protocol type (such as TCP/UDP)
—DSCP value
—Layer 4 source port number