ZyXEL ES-3148 Guida Utente

ES-3148 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.

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)