Dual Queue Coupled AQM: Deployable Very Low Queuing Delay for All

TL;DR

This paper presents a deployable Active Queue Management system that achieves very low, consistent latency and high throughput for all traffic types by balancing scalable and classic congestion controls without flow inspection.

Contribution

It introduces Dual Queue Coupled AQM, enabling coexistence of scalable and classic flows with low latency and high efficiency, validated through extensive testbed experiments.

Findings

Median delay 100-300 microseconds

99th percentile delay below 2 milliseconds

Zero congestion loss and near-full utilization

Abstract

On the Internet, sub-millisecond queueing delay and capacity-seeking have traditionally been considered mutually exclusive. We introduce a service that offers both: Low Latency Low Loss Scalable throughput (L4S). When tested under a wide range of conditions emulated on a testbed using real residential broadband equipment, queue delay remained both low (median 100--300 $\mu$s) and consistent (99th percentile below 2 ms even under highly dynamic workloads), without compromising other metrics (zero congestion loss and close to full utilization). L4S exploits the properties of `Scalable' congestion controls (e.g., DCTCP, TCP Prague). Flows using such congestion control are however very aggressive, which causes a deployment challenge as L4S has to coexist with so-called `Classic' flows (e.g., Reno, CUBIC). This paper introduces an architectural solution: `Dual Queue Coupled Active Queue Management', which enables balance between Scalable and Classic flows. It counterbalances the more aggressive response of Scalable flows with more aggressive marking, without having to inspect flow identifiers. The Dual Queue structure has been implemented as a Linux queuing discipline. It acts like a semi-permeable membrane, isolating the latency of Scalable and `Classic' traffic, but coupling their capacity into a single bandwidth pool. This paper justifies the design and implementation choices, and visualizes a representative selection of hundreds of thousands of experiment runs to test our claims.