Removing the Clock Machinery Lag from DCTCP/Prague

TL;DR

This paper proposes per-ACK mechanisms for DCTCP to eliminate the 2-3 round lag caused by clocking machinery, enabling faster congestion response with minimal delay.

Contribution

It introduces a novel per-ACK feedback mechanism that reduces congestion response lag in DCTCP, improving responsiveness without increasing feedback noise.

Findings

Per-ACK mechanisms significantly reduce congestion response delay.

Evaluation of v02 revealed design errors, leading to the improved v04 version.

The proposed approach maintains smoothing of feedback variations over the same number of round trips.

Abstract

This report explains how DCTCP takes 2--3 rounds before it even starts to respond to congestion. This is due to the clocking machinery in its moving average of congestion feedback. Instead, per-ACK mechanisms are proposed, which cut out all the extra lag, leaving just the inherent single round of feedback delay. Even though clocking per ACK updates the average much more frequently, it is arranged to inherently smooth out variations over the same number of round trips, independent of the number of ACKs per round. Evaluation of the v02 algorithm found design errors. This version (v04) is published prior to evaluation, in order to elicit early feedback on the design.