  In this paper we study the design issues in 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 two per-VC accounting based buffer
management policies. We also study the effects of 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 various small and large latency configurations with varying buffer
sizes and number of sources.
