st of possible flows is endless. So what do we mean by the phrase "process properly"? Every flow in the modern network realm, where QoS and guarantees are required, must be processed in a specific way to ensure compliance with an SLA. For example, all packets conforming to a specific flow may be required to be delivered with a maximum delay stipulated in the agreement that the car- rier has signed with the customer. Or packets arriving from a specific router may need to be accounted for with comprehensive statistics for subsequent network management decisions. Or packets of a cific type with a specific destination host may need to be blocked for security reasons. And SO forth. The classification problem here is formulated as the process that a switch/router must perform in order to distinguish to which flow it must assign every new packet for subsequent processing. In order for this to happen, several fields inside the packet must usually be looked up and a decision based on several criteria must be made. This is the origin of the name deep packet classification, which is also known as classification based on multiple fields. To better visualize the problem (in pursuit of a rigorous mathematical formulation), imagine two axes with the values of two fields along them. Multiple regions can exist based on the policy rules. For instance, Figure 13.5 shows a conceptual example of classification along two fields and the com- binations of these fields may very well create three, four, five, or more regions. Each region is COV- ered by one set of actions to be taken by the classifier. Every incoming packet in this case will have its two specific fields inspected. Based on the policy rules, it will be assigned into one of these regions for subsequent action. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. CLASSIFICATION PROCESSORS CLASSIFICATION PROC