Alternative Backoff with ECN (ABE)

CoDel and PIE are recently proposed Active Queue Management (AQM) mechanisms that minimize the time packets spend enqueued at a bottleneck, instantiating shallow, 5ms to 20ms buffers with tolerance to short-term packet bursts. However, shallow buffering causes noticeable TCP performance degradation when a path's underlying round trip time (RTT) heads above 60ms to 80ms (not uncommon with cross-continental and inter-continental traffic). Using less-aggressive multiplicative backoffs is known to compensate for shallow bottleneck buffering. We therefore propose ABE: "Alternative Backoff with ECN", which consists of enabling Explicit Congestion Notification (ECN) and letting individual TCP senders use a larger multiplicative decrease factor in reaction to ECN-marks from AQM-enabled bottlenecks. Using a mix of experiments, theory and simulations with standard NewReno and CUBIC flows, we show significant performance gains in lightly-multiplexed scenarios, without losing the delay-reduction benefits of deploying CoDel or PIE. ABE is a sender-side-only modification that can be deployed incrementally (requiring no flag-day) and offers a compelling reason to deploy and enable ECN across the Internet.

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Last update: 19. December 2018
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