lasses are good? This is an open and ongoing debate, but the widespread consensus in the industry is that four classes of traffic seem sufficient. Some net- work equipment vendors (NEVs) offer the possibility of more, but this seems superfluous since the industry still cannot agree on what levels of granularity make sense for two levels of markets that positioned downstream from NEVs-that is, the carriers and the carriers' customers (enterprises and end-user organizations). Network applications may at some point require (and perhaps be formally entitled to) better treat- ment for their packets. For example, consider the Transmission Control Protocol (TCP). TCP requires the reliable delivery of its packets to their respective destination. It not only requests acknowledge- ments of reception, but based on feedback that it obtains on congestion conditions from the network, readapts the specifics of its parameters to optimize its behavior, and SO on. In some cases, special sig- naling techniques must be implemented that are ideally understood across the wider network and that allow the dynamic reallocation of portions of shared resources. This is similar to what happens bandwidth provisioning. Sometimes this may not be possible or even desirable. In these cases, an acceptable QoS archi- tecture may require that individual packets be marked with specific bit fields set appropriately, such as the Differentiated Services (DS) field of the IP header. This would enable packets to be immedi- ately visible when they traverse the network and, more specifically, when they enter and exit its indi- vidual systems at network nodes, such as routers, switches, and hubs. It would make it straightforward in principle to ensure that each packet received the treatment to which it is entitled. In a multiservice switching or routing system, the hardware can use such flagging bits to detect the presence of specific types of packets either at an ingress port or at an egress port and act accord- ingly. If packets are not handled with the a