Differential congestion notification: taming the elephants

Active queue management (AQM) in routers has been proposed as a solution to some of the scalability issues associated with TCP\´s pure end-to-end approach to congestion control. A recent study of AQM demonstrated its effectiveness in reducing the response times of Web request/response exchanges as well as increasing link throughput and reducing loss rates [L. Le et al., 2003]. However, use of the ECN (explicit congestion notification) signaling protocol was required to outperform drop-tail queuing. Since ECN is not currently widely deployed on end-systems, we investigate an alternative to ECN, namely applying AQM differentially to flows based on a heuristic classification of the flow\´s transmission rate. Our approach, called differential congestion notification (DCN), distinguishes between "small" flows and "large" high-bandwidth flows and only provides congestion notification to large high-bandwidth flows. We compare DCN to other prominent AQM schemes and demonstrate that for Web and general TCP traffic, DCN outperforms all the other AQM designs, including those previously designed to differentiate between flows based on their size and rate.