08/01/1996 09:00 AM
Computer Engineering
We examine the performance of bidirectional TCP/IP connections over a network which uses rate-based flow and congestion control mechanisms. An example of such a network is an Asynchronous Transfer Mode (ATM) network using the Available Bit Rate (ABR) service. The sharing of a common buffer by TCP packets and acknowledgements has been known to result in an effect called ack compression, where acks of a connection arrive at the source bunched together, resulting in unfairness and degraded throughput. It has been the expectation that maintaining a smooth flow of data using rate-based flow control would mitigate, if not eliminate, the various forms of burstiness experienced with the TCP window flow control. However, we show that the problem of TCP ack compression re-appears even while operating over a rate-controlled channel, although the queues are primarily at the end systems now. By analyzing the periodic bursty behavior of the source IP queue, we are able to predict the peak values for the queue and arrive at a simple robust predictor for the degraded throughput, applicable for relatively general situations. The degradation in throughput due to bidirectional traffic can be significant. For example, even in the simple case of symmetrical connections with adequate window sizes, the throughput of each connection is only 66.67% of that under one-way traffic. We validate our analysis using simulation on an ATM network using the Explicit Rate option of the ABR service. We show that the analysis predicts the behavior of the queue and the throughput degradation not only in simple configurations, but also in more general situations, such as multiple synchronized connections between a pair of end-systems, multiple end-systems having different round-trip delays, and in configurations with significant amounts of cross- traffic. We observe the need to separate the flow of acknowledgements and data for the bidirectional TCP connection and for interleaving their processing at the end-systems to overcome the problem of ack compression. We show that the rate-controlled channel localizes the effect of TCP burstiness and the accompanying dynamics to the outgoing queue at the source, requiring source-based approaches to reduce ack compression.
