ZyXEL Communications XGS-4526 User Manual

XGS-4526/4528F/4728F User’s Guide

This chapter shows you how to configure policy rules. 

A classifier distinguishes traffic into flows based on the configured criteria (refer to 

 for more information). A policy rule ensures that a traffic 

flow gets the requested treatment in the network. 

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.

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.

DSCP (6 bits)

Unused (2 bits)