The power of explicit congestion notification

Despite the fact that Explicit Congestion Notification (ECN) demonstrated a clear potential to substantially improve network performance, recent network measurements reveal an extremely poor usage of this option in today's Internet. In this paper, we analyze the roots of this phenomenon and develop a set of novel incentives to encourage network providers, end-hosts, and web servers to apply ECN. Initially, we examine a fundamental drawback of the current ECN specification, and demonstrate that the absence of ECN indications in TCP control packets can dramatically hinder system performance. While security reasons primarily prevent the usage of ECN bits in TCP SYN packets, we show that applying ECN to TCP SYN ACK packets can significantly improve system performance without introducing any novel security or stability side-effects. Our network experiments on a cluster of web servers show a dramatic performance improvement over the existing ECN specification: throughput increases by more than 40%, while the average web response-time simultaneously decreases by nearly an order of magnitude. In light of the above finding, using large-scale simulations, modeling, and network experiments, we re-investigate the relevance of ECN, and provide a set of practical recommendations and insights: (i) ECN systematically improves the performance of all investigated AQM schemes; contrary to common belief, this particularly holds for RED. (ii) The impact of ECN is highest for web-only traffic mixes such that even a generic AQM algorithm with ECN support outperforms all non-ECN-enabled AQM schemes that we investigated. (iii) Primarily due to moderate queuing levels, the superiority of ECN over other AQM mechanisms largely holds for high-speed backbone routers, even in more general traffic scenarios. (iv) End-hosts that apply ECN can exercise the above performance benefits instantly, without waiting for the entire Internet community to support the option.