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Feedback Control in a Distributed Scheduling Architecture

01 January 2000

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Multiple-module packet switching systems are gaining in popularity to address the ever increasing demand in aggregate switching throughput. They differ in their behavior from a single-module output-buffered switch primarily due to the presence of additional contention points between the modules. These systems usually employ some form of selective feedback, also known as back-pressure, in order to communicate the state of the queues in different stages. The existing feedback control schemes are devoted to increasing the throughput of the switching system. On the other hand, a number of sophisticated scheduling algorithms have been defined to provide diverse Quality of Service (QoS) guarantees to individual flows and traffic classes on the context of a single contention point. We address the problem of emulating the QoS properties of an output-buffered switch in a multiple-module switch through the use of intelligent feedback control. We discuss a credit-based feedback mechanism that allows to merge different non-delay-constrained flows in a down-stream stage of a multi-stage switch without compromising the QoS guarantees of the individual flows. This enables to decrease the number of queues and, therefore, the complexity of the downstream stage, while keeping all the per-flow scheduling information in the upstream-stage modules. We use the properties of the credit-based mechanism to construct a novel back-pressure scheme that allows to efficiently handle Guaranteed Bandwidth and Best Effort traffic in the same FIFO queue in the switching fabric of a multimodule packet switch. We also present the analysis of the scheme and the simulation results.