Performance of TCP over multi-hop ATM networks: a comparative study of ATM-layer congestion control schemes

We study the performance of TCP/IP on a multi-hop ATM network by simulation in an effort to evaluate the effectiveness of various congestion-control schemes at the ATM layer. The congestion control schemes considered are the ATM-early packet discard (ATM-EPD) and link-level flow control. The performance of these schemes are compared against that of an ATM layer with no congestion control, as well as TCP/IP over a datagram network. We compare the schemes in terms of the throughputs obtained by long and short connections, number of retransmissions, and fairness in bandwidth allocation between connections with unequal round-trip delays. Our results show that significant unfairness in the amount of bandwidth allocated to connections may result in a large ATM network if no congestion control policies are used at the ATM layer. The ATM-EPD scheme is able to remove some of the unfairness, providing performance close to that of a datagram network, but can still provide unacceptable performance if the buffer sizes in the switches are small. Link-level flow control provided the best performance among the schemes studied.