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In this paper we study the design issues for improving TCP performance over the ATM UBR service. ATM-UBR switches respond to congestion by dropping cells when their buffers become full. TCP connections running over UBR can experience low throughput and high unfairness. Intelligent switch drop policies and end-system policies can improve the performance of TCP over UBR with limited buffers. We describe the various design options available to the network as well as to the end systems to improve TCP performance over UBR. We study the effects of early packet discard, and present a per-VC accounting-based buffer management policy. We analyze the performance of the buffer management policies with various TCP end system congestion control policies, including slow start and congestion avoidance, fast retransmit and recovery and selective acknowledgments. We present simulation results for …