MES3500 Series User’s Guide
174
CHAPTER 22
Policy Rule
This chapter shows you how to configure policy rules.
22.1 Policy Rules Overview
A classifier distinguishes traffic into flows based on the configured criteria (refer to Chapter 21 on
page 168 for more information). A policy rule ensures that a traffic flow gets the requested
treatment in the network.
22.1.1 DiffServ
DiffServ (Differentiated Services) is a class of service (CoS) model that marks packets so that they
receive specific per-hop treatment at DiffServ-compliant network devices along the route based on
the application types and traffic flow. Packets are marked with DiffServ Code Points (DSCPs)
indicating the level of service desired. This allows the intermediary DiffServ-compliant network
devices to handle the packets differently depending on the code points without the need to
negotiate paths or remember state information for every flow. In addition, applications do not have
to request a particular service or give advanced notice of where the traffic is going.
22.1.2 DSCP and Per-Hop Behavior
DiffServ defines a new DS (Differentiated Services) field to replace the Type of Service (TOS) field
in the IP header. The DS field contains a 2-bit unused field and a 6-bit DSCP field which can define
up to 64 service levels. The following figure illustrates the DS field.
DSCP is backward compatible with the three precedence bits in the ToS octet so that non-DiffServ
compliant, ToS-enabled network device will not conflict with the DSCP mapping.
The DSCP value determines the forwarding behavior, the PHB (Per-Hop Behavior), that each packet
gets across the DiffServ network. Based on the marking rule, different kinds of traffic can be
marked for different kinds of forwarding. Resources can then be allocated according to the DSCP
values and the configured policies.
22.2 Configuring Policy Rules
You must first configure a classifier in the Classifier screen. Refer to Section 21.2 on page 168 for
more information.
DSCP (6 bits) Unused (2 bits)
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