Overview

To many people, quality of service (QoS) is the driving factor behind MPLS. This is actually something of a misconception. Compared to other factors, such as traffic engineering and Virtual Private Network support, QoS is not a very strong reason to deploy MPLS. As we will see in this chapter, most of the work that has been done in the realm of MPLS QoS has focused on supporting the features of IP QoS in a network that happens to be using MPLS. In other words, the goal has been to establish parity between the QoS features of IP and MPLS, not to make MPLS QoS somehow superior to IP QoS.

One of the main reasons that MPLS supports, rather than extends, the IP QoS model is that MPLS, unlike IP, is not an end-to-end protocol. MPLS does not currently run in hosts, and for the foreseeable future, many IP networks that do not run MPLS will exist. QoS, on the other hand, usually is an end-to-end characteristic of the communication between peers. For example, if one link of an end-to-end path has high delay, high loss, or low bandwidth, that limits the QoS that can be provided along that end-to-end path. Another way to think of this is that MPLS does not fundamentally change the IP service model. Service providers do not sell MPLS service; they sell IP service (or Frame Relay service or a variety of other services), and thus, if they offer any QoS at all, they...