ZyXEL Communications ES-3148 Series - Manual (Page 155)

ES-3148 User’s Guide
155
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H A P T E R
19
Policy Rule
This chapter shows you how to configure policy rules.
19.1 Policy Rules Overview
A classifier distinguishes traffic into flows based on the configured criteria (refer to Chapter
18 on page 149 for more information). A policy rule ensures that a traffic flow gets the
requested treatment in the network.
19.1.1 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.
19.1.2 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.
DSCP (6 bits) Unused (2 bits)