or, Interaction of Active Queue Management (AQM) and Low Priority Congestion Control (LPCC) protocols

Nowadays, due to excessive queuing, delays on the Internet can grow longer than several round trips between the Moon and the Earth -- for which the ``bufferbloat'' term was recently coined. Some point to active queue management (AQM) as the solution. Others propose end-to-end low-priority congestion control techniques (LPCC). Under both approaches, promising advances have been made in recent times: notable examples are CoDel for AQM, and LEDBAT for LPCC.

However, a potentially fateful interaction arise when AQM and LPCC techniques are combined [CONEXT-12]: namely (i) AQM resets the relative level of priority between best effort and low-priority congestion control protocols; (ii) while reprioritization generally equalizes the priority of LPCC and TCP, we also find that some AQM settings may actually lead best effort TCP to starvation.

By an extended set of experiments [TMA-13] conducted on both controlled testbeds and on the Internet, we show the problem to hold in the real world for any tested combination of AQM policies and LPCC protocols. To further validate the generality of our findings, we complement our experiments with packet-level simulation, to cover cases of other popular AQM and LPCC that are not available in the Linux kernel [COMNET-14b].

To gather fundamental insights on the problem we resort to a control theoretic modeling and analysis of the system dynamics [ITC-13]. Finally, we propose and implement a solution able to (i) reinstate the relative levels of priority between heterogeneous congestion control (ii) while avoiding the bufferbloat problem at the same time.

To promote cross-comparison, we make our scripts and dataset available to the research community.


Note: PhD-grade skills are required to run the experiments and tweak the experimental settings


Note: PhD-grade ns2 skills are required to modify and build the ns2 source tree; modification includes our own implementation of the LEDBAT and TCP-NICE low priority congestion control (LPCC), plus Choke and CoDel (AQM) from the respective authors, packaged here for commodity.

  • tarball of needed modification to ns2
  • scripts for launching simulations and post-processing results described in [COMNET-14b], including a short howto on the workflow
  • scripts of the proof-of-concept solution to the reprioritization sketched in [ITC-13] and that we implemented for a journal extension currently under review
  • For historical reference, we also provide a copy of the initial student project of YiXi on the topic


  1. [COMNET-14b] YiXi Gong, Dario Rossi, Claudio Testa, Silvio Valenti and Dave Taht, Fighting the bufferbloat: on the coexistence of AQM and low priority congestion control (extended version) . Computer Networks, 65:255 - 267, 2014.
  2. [ITC-13] Gong, YiXi, Rossi, Dario and Leonardi, Emilio, Modeling the interdependency of low-prioritycongestion control and active queue management . In The 25th International Teletraffic Congress (ITC25), Runner-up for Best Paper Award, september 2013.
  3. [TMA-13] Y. Gong, D. Rossi, C. Testa, S. Valenti and D. Taht , Fighting the bufferbloat: on the coexistence of AQM and low priority congestion control . In IEEE INFOCOM Workshop on Traffic Measurement and Analysis (TMA'13),, April 2013.
  4. [CONEXT-12] Y. Gong, D. Rossi, C. Testa, S. Valenti and D. Taht , Interaction or Interference: can AQM and Low PriorityCongestion Control Successfully Collaborate . In ACM CoNEXT'12, Extended Abstract,, December 2012.