Performance of TCP over Multi-Hop ATM Networks: A Comparative Study of ATM-Layer Congestion Control Schemes

this paper is to study the dynamic behavior of TCP over ATM networks and internetworks consisting of multiple hops of ATM switches, in an effort to identify the influence of various ATM-layer congestion control approaches on TCP performance. Of particular interest is to investigate the interaction between connections with different round-trip delays to determine how the fairness characteristics vary for the different schemes. To achieve these objectives, we simulated a network configuration with 8 nodes, 4 switches, and a total of 10 TCP sessions sharing network resources. In addition to the baseline case with no congestion control, we considered two distinct congestion control approaches at the ATM layer --- the ATM Early Packet Discard (ATM-EPD) proposed by Romanow and Floyd [15] and link-level flow control proposed by Kung and Chapman [10] and Varghese, et al. [17]. To compare with the performance of TCP over a datagram network, we also simulated a network configuration in which the ATM switches were replaced by IP gateways. The rest of this paper is organized as follows: In Section 2, we outline the ATM-layer congestion control approaches simulated. In Section 3 we describe the network model and tools used in the simulations. We discuss the simulation results in Section 4 and compare the congestion-control approaches in terms of their effective throughput, fairness in bandwidth usage, number of retransmissions, and delay characteristics. In Section 5 we discuss the problem of setting the threshold in the ATM Early Packet Discard scheme. We conclude the paper in Section 6 with a discussion of the lessons learned and directions for future research. Appendix A provides details of computing the buffer sizes for the link-level flow control scheme we simulated. 2 Congesti..