In Part One of this book, we discussed the concept of per-hop behavior (PHB), in which each router in the network independently implements its own QOS settings because no signaling is possible either between neighbors or end to end. Consistency is achieved by applying a QOS configuration on each router in the network that applies the same PHB to traffic belonging to each class of service at each hop along the path that the traffic takes through the network.
In Part One, we also described the classifier tool, which assigns a class of service to each packet. This assignment is a crucial and fundamental step in the QOS realm because identifying traffic is the key factor in knowing the class of service to which it belongs. It is the class of service that determines the PHB that is applied to the traffic.
As previously discussed, the classifier tool has one input and N possible outputs. The input is the packet itself, and the N possible outputs are the number of different classes of service into which the packet can be classified. Classification can be considered to be an operation implemented using a set of IF/THEN rules, applied in the form of “IF (match conditions), THEN (class-of-service assignment).”
In this chapter, we focus on the types of classification processes that are currently commonly used, and we examine the differences between them in terms of the options available for the “IF (match conditions)” set of rules applied by the classifier. We also compare ...