The quest for LEDBAT fairness

TL;DR

This paper investigates the fairness issues of the LEDBAT congestion control protocol, identifies the root causes of late-comer advantage, and proposes solutions through simulations and analysis to improve fairness.

Contribution

It provides a detailed analysis of LEDBAT's unfairness problem and introduces effective countermeasures to address late-comer advantage.

Findings

Identified the root cause of LEDBAT unfairness.

Proposed solutions effectively mitigate late-comer advantage.

Validated solutions through packet-level simulations.

Abstract

BitTorrent developers have recently introduced a new application layer congestion control algorithm based on UDP framing at transport layer and currently under definition at the IETF LEDBAT Working Group. LEDBAT is a delay-based protocol which aims at offering a "lower than Best Effort" data transfer service, with a lower priority with respect to elastic TCP and interactive traffic (e.g., VoIP, game). However, in its current specification, LEDBAT is affected by a late-comer advantage: indeed the last flow arriving at the bottleneck is more aggressive due to a wrong estimation of the base delay and finally takes over all resources. In this work, we study several solutions to the late-comer problem by means of packet level simulations and simple analysis: in the investigation process, we individuate the root cause for LEDBAT unfairness and propose effective countermeasures.