The Internet is facing two problems simultaneously: there is a need for a faster switching/routing infrastructure, and a need to introduce guaranteed qualities of service (QoS). Each problem can be solved independently: switches and routers can be made faster by using input-queued crossbars, instead of shared memory systems; and QoS can be provided using WFQ-based packet scheduling. However, until now, the two solutions have been mutually exclusive-all of the work on WFQ-based scheduling algorithms has required that switches/routers use output-queueing, or centralized shared memory. This paper demonstrates that a combined input output queueing (CIOQ) switch running twice as fast as an input-queued switch can provide precise emulation of a broad class of packet scheduling algorithms, including WFQ and strict priorities. More precisely, we show that a "speedup" of 2 is sufficient, and a speedup of 2-1/N is necessary, for this exact emulation. We introduce a variety of algorithms that configure the crossbar so that emulation is achieved with a speedup of two, and consider their running time and implementation complexity. An interesting feature of our work is that the exact emulation holds for all input traffic patterns. We believe that, in the future, these results will make possible the support of QoS in very high bandwidth routers.
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