Differentiated Services (DiffServ) is a multiple-service model for implementing quality of service (QoS) in the network. With DiffServ, the network tries to deliver a particular kind of service that is based on the QoS specified by each packet. This specification can occur in different ways, such as using the differentiated services code point (DSCP) in IP packets or source and destination addresses. The network uses the QoS specification of each packet to classify, shape, and police traffic and to perform intelligent queuing.


Upon completing this lesson, you will be able to describe DiffServ and explain how it can be used to implement QoS in the network. This ability includes being able to meet these objectives:

■ Describe the purpose and function of DiffServ

■ Describe the basic format and explain the purpose of the DSCP field in the IP header

■ Describe the different per-hop behaviors used in DSCP

■ Describe the interoperability between DSCP-based and IP Precedence-based devices in a network

Differentiated Services Model

This topic describes the purpose and function of the DiffServ model.

This topic describes the purpose and function of the DiffServ model.

The DiffServ architecture is based on a simple model in which traffic entering a network is classified and possibly conditioned at the boundaries of the network. The traffic class is then identified with a DSCP or bit marking in the IP header.

DSCP values are used to mark packets to select a per-hop behavior (PHB). Within the core of the network, packets are forwarded according to the PHB that is associated with the DSCP. The PHB is defined as an externally observable forwarding behavior applied at a DiffServ-compliant node to a collection of packets with the same DSCP value.

One of the primary principles of DiffServ is that you should mark packets as close to the edge of the network as possible. It is often a difficult and time-consuming task to determine the traffic class for a data packet. You should classify the data as few times as possible. By marking the traffic at the network edge, core network devices and other devices along the forwarding path will be able to quickly determine the proper class of service (CoS) to apply to a given traffic flow.

The primary advantage of DiffServ is scalability.

The introduction of DSCPs replaces IP Precedence, a 3-bit field in the type of service (ToS) byte of the IP header originally used to classify and prioritize types of traffic. However, DiffServ maintains interoperability with non-DiffServ-compliant devices (those that still use IP Precedence). Because of this backward compatibility, DiffServ can be deployed gradually in large networks.

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