AuthorsK. De Schepper, O. Bondarenko, I. Tsang and B. Briscoe
TitlePI2 : A Linearized AQM for both Classic and Scalable TCP
AfilliationCommunication Systems
Project(s)RITE: Reducing Internet Transport Latency, TimeIn: Traffic behaviour of interactive time-dependent thin streams on the modern Internet
Publication TypeProceedings, refereed
Year of Publication2016
Conference NameProc. ACM CoNEXT 2016
Date Published12/2016
Place PublishedNew York, NY, USA
ISBN Number978-1-4503-4297-1
KeywordsAlgorithms, Analysis, AQM, Congestion Avoidance, congestion control, Data Communication, Design, Evaluation, Internet, latency, networks, Performance, QoS, Scaling, tcp

This paper concerns the use of Active Queue Management (AQM) to reduce queuing delay. It offers insight into why it has proved hard for a Proportional Integral (PI) controller to remain both responsive and stable while controlling `Classic' TCP flows, such as TCP Reno and Cubic. Due to their non-linearity, the controller's adjustments have to be smaller when the target drop probability is lower. The PI Enhanced (PIE) algorithm attempts to solve this problem by scaling down the adjustments of the controller using a look-up table. Instead, we control an internal variable that is by definition linearly proportional to the load, then post-process it into the required Classic drop probability---in fact we show that the output simply needs to be squared. This allows tighter control, giving responsiveness and stability better or no worse than PIE achieves, but without all its corrective heuristics.

With suitable packet classification, it becomes simple to extend this PI2 AQM to support coexistence between Classic and Scalable congestion controls in the public Internet. A Scalable congestion control ensures sufficient feedback at any flow rate, an example being Data Centre TCP (DCTCP). A Scalable control is linear, so we can use the internal variable directly without any squaring, by omitting the post-processing stage.

We implemented PI2 as a Linux qdisc to extensively test our claims using Classic and Scalable TCPs.


Citation KeyDeSchepper16a:PI2