Congestion Control in the Wild—Investigating Content Provider Fairness

Congestion control (CC) is an indispensable component of transport protocols to prevent congestion collapse as it distributes the available bandwidth among all competing flows, ideally in a fair manner. It thus has a large impact on performance and there exists a constantly evolving set of CC algorithms, each addressing different performance needs. While the algorithms are commonly tested regarding the problems underlying their implementation, the interaction with existing algorithms is often not considered. Additionally considering the fact that content providers (CPs) such as content distribution networks (CDNs) are known to tune TCP stacks for performance gains, the large assortment of algorithms opens the door for custom parametrization and potentially unfair bandwidth sharing. In this paper, we thus empirically investigate if current Internet traffic generated by CPs still adheres to the conventional understanding of fairness. For this, we compare fairness properties of testbed hosts to actual traffic of six major CPs subject to different queue sizes and queueing disciplines in a home-user setting. Additionally, we investigate how mice and elephant flows from the different CPs interact. We find that some employed CC algorithms lead to significantly asymmetric bandwidth shares and very poor flow completion times for mice flows. Fortunately, AQMs such as FQ_CoDel are able to alleviate such unfairness.