The Effects of Active Queue Management and Explicit Congestion Notification on Web Performance

L. Le, J. Aikat, K. Jeffay, F.D. Smith

IEEE/ACM Transactions on Networking

Volume 15, Number 6, December 2007

pages 1217-1230.

ABSTRACT: We present an empirical study of the effects of active queue management (AQM) and explicit congestion notification (ECN) on the distribution of response times experienced by a population of users browsing the Web. Three prominent AQM schemes are considered: the Proportional Integral (PI) controller, the Random Exponential Marking (REM) controller, and Adaptive Random Early Detection (ARED). The effects of these AQM schemes were studied with and without ECN. Our primary measure of performance is the end-to-end response time for HTTP request-response exchanges. For this measure, our major results are:

• If ECN is not supported, ARED operating in byte-mode was the best performing AQM scheme, providing better response time performance than drop-tail FIFO queuing at offered loads above 90% of link capacity. However, ARED operating in packet-mode (with and without ECN) was the worst performing scheme, performing worse than drop-tail FIFO queuing.
• ECN support is beneficial to PI and REM. With ECN, PI and REM were the best performing overall schemes, providing significant response time improvement over ARED operating in byte-mode. In the case of REM, the benefit of ECN was dramatic. Without ECN, response time performance with REM was worse than drop-tail FIFO queuing at all loads considered.
• ECN was not beneficial to ARED. Under current ECN implementation guidelines, ECN had no effect on ARED performance. However, ARED performance with ECN improved significantly after reversing a guideline that was intended to police unresponsive flows. Nonetheless, overall, the best ARED performance was achieved without ECN.
• Whether or not the improvement in response times with AQM is significant (when compared to drop-tail FIFO), depends heavily on the range of round-trip times (RTTs) experienced by flows. As the variation in flows' RTT increases, the impact of AQM and ECN on response-time performance is reduced.