 header. In the IPv6 header, it is the traffic class octet that composes the so-called Differentiated Services (DS) field where traffic classes are encoded. At origination, a packet's DS field can be marked by a customer for whatever service is desired. It can also be left to the first DS-compliant router (a leaf router in the routing tree) to decide how to modify the DS field based on the multifield classifi- cation of the packet, which we covered in length in Chapter 12, "Search Engines," and Chapter "Classification Coprocessors." The scheme requires that all ordinary network-application flows will be consolidated into one of these aggregate flows. 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. TRAFFIC MANAGERS TRAFFIC MANAGERS The idea has merits from several angles. The number of bits in the DS field is very small-hence, the number of classes is not astronomical. Therefore, the classification that happens in the core of network must occur based on behavior aggregates (BAs) and not based on individual flows as would be expected on a campus or at the edge. Different levels of service will be assigned to these aggre- gates before they enter the network. This is the scalability advantage that DiffServ offers. A packet header also contains the DSCP field, which identifies the aggregates. The DSCP can also be passed through a network junction (node) and be handed over to a subsequent router/switch. As a result, the DiffServ protocol is quite flexible in the sense that it does not require that all possible nodes between a source device and a destination device be DiffServ enabled. This represents a serious advantage of DiffServ over IntServ. In order to run the protocol, all com- putational requirements on packet flows are handled by the local network equipment before the traf- fic is injected into the network. As a result, downstream routers 