Introduction

Sometimes, in a network, there is the need to classify traffic. The reasons for classifying traffic vary from network to network but can range from marking packets with a flag to make them relatively more or less important than other packets on the network to identifying which packets to drop. This chapter will introduce you to several different theories of traffic classification and will discuss the mechanics of how these flags are set on a packet.

There are several different ways in which these flags are set, and the levels of classification depend on which method is used. Pay particular attention to the ideas covered in this chapter because the marking of packets will be a recurring theme throughout this book since many QoS mechanisms use these markings to classify traffic and perform QoS on the packets that have them.

Classification may be viewed as infusing data packets with a directive intelligence in regard to network devices. The use of prioritization schemes such as Random Early Detection (RED) and Adaptive Bit Rate (ABR) force the router to analyze data streams and congestion characteristics and then apply congestion controls to the data streams. These applications may involve the utilization of the TCP sliding window or back-off algorithms, the utilization of leaky or token bucket queuing mechanisms, or a number of other strategies. The use of traffic classification flags within the packet removes decision functionality from the router and establishes what service levels are required for the packet s particular traffic flow.