plementation is provided in Chapter 7, "Agere PayloadPlus™ Family Network Processors." The principle of the dual bucket approach is also used in policing. Two good examples can found in RFC 2698 and RFC 2697, which define the trTCM and srTCM algorithms, respectively. CONGESTION MANAGEMENT We briefly mentioned the need to decide when to simply discard (drop) one or more packets. This rad- ical decision is often the unfortunate result of a highly undesirable state called congestion. The rea- sons for congestion can be numerous and unpredictable. Fundamental textbooks on network traffic engineering discuss the problem in length. However, at a time when actual guarantees for specific lev- els of QoS are required and paid for by the customer, it is important to have robust mechanisms that ensure a fair approach toward making the decision of what gets dropped, when, and under what cir- cumstances. In fact, the traffic manager is responsible for action when it detects conditions of traffic conges- tion. Two choices of systems engineering are available: Decelerate the traffic arriving from upstream (this is similar to backpressure signaling, which we discussed in Chapter 14, "Switch Fabrics"). Start dropping some packets locally. Originally, the brute-force method was used and only the lat- est arrivals were sacrificed. Because countless TCP sessions were by overwhelming proportions the direct victim of such an approach, the industry soon moved to levels of choice that reflect some choice based on a degree of fairness. The term fairness is not used here lightly or subjectively. Given the limited resources of a network, what is fair for one party may be completely unfair for the oth- ers. Therefore, some objectively defined criteria and agreed-upon techniques were required in order for the industry to enable such a regime. 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 gi