ZyXEL Communications ES-2024A, User Manual

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ES-2024A User’s Guide
Chapter 20 Differentiated Services 109
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H A P T E R
20
Differentiated Services
This chapter shows you how to configure Differentiated Services (DiffServ) on the switch.
20.1 Overview
Quality of Service (QoS) mechanisms provide the best service on a per-flow guarantee. To
fine-tune the levels of services on the priority of the traffic flow using QoS places a heavy
burden on the network infrastructure.
DiffServ 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.
20.1.1 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.
Figure 53 DiffServ: Differentiated Service Field
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 priorities of forwarding. Resources can then be allocated
according to the DSCP values and the configured policies.
20.1.2 DiffServ Network Example
The following figure depicts a simple DiffServ network consisting of a group of contiguous
DiffServ-compliant network devices.
DSCP (6 bits)
DS (2 bits)