Two-way TCP traffic over rate controlled channels: Effects and analysis

We study the performance of bidirectional TCP/IP connections over a network that uses rate-based dow and congestion control. An example of such a netw ork is an asynchronous transfer mode (ATM) network using the available bit rate (ABR) service, The sharing of a common buffer by TCP packets and ackno wledgment tacks) has been known to result in an effect called ack compressi on, where acks of a connection arrive at the source bunched together, resul ting in unfairness and degraded throughput. It has been the expectation tha t maintaining a smooth flow of data using rate-based how control would miti gate, if not eliminate, the various forms of burstiness experienced with th e TCP window flow control. However, we show that the problem of TCP ack com pression appears even while operating over a rate-controlled channel, altho ugh the queues are primarily at the end systems now. The degradation in thr oughput due to bidirectional traffic can be significant. For example, even in the simple case of symmetrical connections with adequate window sizes, t he throughput of each connection is only 66.67% of that under one-way traff ic. By analyzing the periodic bursty behavior of the source LP queue, we derive estimates for the maximum queue size and arrive at a simple predictor for the degraded throughput, applicable for relatively general situations. We v alidate our analysis using simulation on an ATM network using the explicit rate option of the ABR service. We show that the analysis predicts the beha vior of the queue and the throughput degradation not only in simple configu rations, but also in more general situations, such as multiple synchronized connections between a pair of end systems, multiple end systems having dif ferent round-trip delays, and in configurations with significant amounts of cross-traffic.