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Dimension ES-4024 Ethernet Switch
Differentiated Services 19-1
This chapter shows you how to configure Differentiated Services (DiffServ) on the ES.
19.1 Introduction to DiffServ
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.
19.1.1 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 19-1 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.
19.1.2 DiffServ Network Example
The following figure depicts a simple DiffServ network consisting of a group of contiguous DiffServ-compliant
network devices.
Chapter 19
Differentiated
Services
DSCP Unused