Although switched LANs are usually over-provisioned, their characteristics (short RTT, link speed mismatches) lead to increased burstiness, and thus to the occurrence of transient congestion. In order to fully utilize the potential of large switched LANs, a link layer back-pressure mechanism may be used to complement the end-to-end flow control by handling the short term congestion. A simple such mechanism, as the one specified in IEEE 802.3x, is shown to improve network performance in some situations, but to lead to poor performance in others. We propose selective back-pressure schemes based on the destination MAC address and traffic class information, which overcome the limitations of the simpler scheme. These are shown to provide superior performance for a wide range of situations. The results obtained suggest the need to incorporate these enhancements in the 802.3x standard.
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