Excerpt from MPLS Fundamentals.
By Luc De Ghein Published by Cisco Press ISBN-10: 1-58705-197-4 ISBN-13: 978-1-58705-197-5

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What You Will Learn

By the end of this chapter, you should know and be able to explain the following:

• How QoS information is propagated in MPLS networks

• Which kind of DiffServ tunneling models are available

• How to implement the different DiffServ tunneling models in Cisco IOS

Quality of service (QoS) has become popular the past few years. Few networks have unlimited bandwidth, so congestion is always a possibility in the network. QoS is a means to prioritize important traffic over less important traffic and make sure it is delivered.

The Internet Engineering Task Force (IETF) has designated two ways to implement QoS in an IP network: Integrated Services (IntServ) and Differentiated Services (DiffServ). IntServ uses the signaling protocol Resource Reservation Protocol (RSVP). The hosts signal to the network via RSVP what the QoS needs are for the flows of traffic that they send. DiffServ uses the DiffServ bits in the IP header to qualify the IP packet to be of a certain QoS. The routers look at these bits to mark, queue, shape, and set the drop precedence of the packet. The big advantage of DiffServ over IntServ is that the DiffServ model needs no signaling protocol. The IntServ model uses a signaling protocol that must run on the hosts and routers. If the network has many thousands of flows, the routers must keep state information for each flow passing through it. This is a serious scalability issue, which is why IntServ has not proven to be popular.

A good example where QoS is needed is VoIP traffic. VoIP traffic needs to be delivered within a certain time to the destination, or it becomes obsolete. Therefore, QoS should prioritize the VoIP traffic to ensure that it is delivered within a certain time constraint. To achieve this, it is possible within Cisco IOS to queue VoIP with a higher priority than FTP or HTTP traffic and to make sure that if congestion occurs, FTP or HTTP traffic is dropped ahead of the VoIP traffic. Cisco IOS has several mechanisms to do this on a router. Refer to Table 12-1 for examples of QoS functions and features in Cisco IOS.

Table 12-1 QoS Functions and Corresponding Cisco IOS-Enabling Features

You can set the priority of an IP packet either in the IP Precedence field (three bits) or in the six bits of the DiffServ Codepoint field (DSCP). Originally, only three bits of the Type of Service field (TOS) in the IP header were reserved for QoS. The number of bits in the IP header that could be used for QoS was later increased to six with the introduction of DiffServ QoS.