Design and evaluation of schemes for more accurate ECN feedback

Explicit Congestion Notification (ECN) is a mechanism in the Internet that allows network nodes to mark packets instead of (early) dropping them in case of overload. A receiver will detect this signal and inform the sender about the congestion situation. The current ECN specification does only allow one feedback signal from the receiver to the sender per Round-Trip Time (RTT). In case a network node is marking more than one packet per RTT, the sender will not notice. Up to now, congestion control using this signal as an input parameter was designed to not react more than once per RTT time. However, new mechanisms like Congestion Exposure (ConEx) or Data Center TCP (DCTCP) need a more accurate feedback than one signal per RTT. This paper proposes two approaches to address this problem by reusing the ECN TCP header bits without allocating further option space. Both approaches are developed based on very different protocol design choices, which makes it hard to evaluate both approaches against each other. This paper takes a first approach for evaluation based on simple simulations. Further evaluations in more realistic scenarios and a Linux kernel implementation are currently work in progress.