ATM switches respond to UBR congestion by dropping cells when their buffers become full. TCP connections using the UBR service experience low throughput and low fairness. 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 consists of enhancements to UBR for intelligent drop. We found that Early Packet Discard (EPD) improves throughput but does not 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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