ES-2048 User’s Guide

CHAPTER 22

DiffServ Code Point

This chapter shows you how to configure Differentiated Services (DiffServ) on the switch.

22.1 DiffServ Overview

Quality of Service (QoS) is used to prioritize source-to-destination traffic flows. All packets in the flow are given the same priority. You can use CoS (class of service) to give different priorities to different packet types.

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.

22.1.1 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 71 DiffServ: Differentiated Service Field

DSCP (6 bits)

DS (2 bits)

 

 

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.

22.1.2 DiffServ Network Example

The following figure depicts a simple DiffServ network consisting of a group of contiguous DiffServ-compliant network devices.

Chapter 22 DiffServ Code Point

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