  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.
