ATM-UBR switches respond to congestion by dropping cells when their buffers become full. TCP connections running over UBR experience low throughput and high unfairness. For 100\% TCP throughput each switch needs buffers equal to the sum of the window sizes of all the TCP connections. Intelligent drop policies can improve the performance of TCP over UBR with limited buffers. The UBR+ service proposes enhancements to UBR for intelligent drop. Early Packet Discard improves throughput but does not attempt to improve fairness. Selective packet drop based on per-connection buffer occupancy improves fairness. The Fair Buffer Allocation scheme further improves both throughput and fairness.
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