Using ATM services for (in)efficient support of TCP

We study the performance of TCP/ABR and TCP/UBR as a function of the number of bottlenecks in an IP/ATM inter-networking system. We define an efficiency metric that captures the amount of badput generated per unit of goodput. We define a gain metric to be the ratio of the efficiencies of these two services. With these new metrics, we demonstrate that the bandwidth efficiency of TCP/ABR is scalable in the number of bottlenecks, whereas TCP/UBR is scalable only if there are no greedy sources in the traffic mix. We examine the influence of ABR and UBR on TCP factors such as packet loss, round trip time delays (RTTs), and the fraction of lost packets detected via fast retransmit events. We show that TCP/ABR is more efficient than TCP/UBR because the ABR control loop has favorable effects on the TCP control loop via its its influence on RTTs and loss behavior. We demonstrate that fairness has far reaching consequences beyond throughput fairness because the improvements to TCP in efficiency and scalability are a ramification of fairness.