Implementing AQM in FreeBSD


Recent years have seen a resurgence of interest in better managing the depth of bottleneck queues in routers, switches and other places that get congested. Solutions include transport protocol enhancements at the end-hosts (such as delay-based or hybrid congestion control schemes) and active queue management (AQM) schemes applied within bottleneck queues.

The notion of AQM has been around since at least the late 1990s (e.g. RFC 2309). In recent years the proliferation of oversized buffers in all sorts of network devices (aka bufferbloat) has stimulated keen community interest in four new AQM schemes -- CoDel, FQ-CoDel, PIE and FQ-PIE.

The IETF AQM working group is looking to document these schemes, and independent implementations are a corner-stone of the IETF's process for confirming the clarity of publicly available protocol descriptions. While significant development work on all three schemes has occured in the Linux kernel, there is very little in FreeBSD.

Note: Dummynet AQM v0.2.1 has been committed to the official FreeBSD 11 source tree as of r300779 (26 May 2016), and backported to FreeBSD 10.3 as of r301772 (10 Jun 2016).

Project Goals

This project began in late 2015, and aims to design and implement functionally-correct versions of CoDel, FQ-CoDel, PIE and FQ_PIE in FreeBSD (with code BSD-licensed as much as practical). We have chosen to do this as extensions to FreeBSD's ipfw/dummynet firewall and traffic shaper. Implementation of these AQM schemes in FreeBSD will:

  • Demonstrate whether the publicly available documentation is sufficient to enable independent, functionally equivalent implementations
  • Provide a broader suite of AQM options for sections the networking community that rely on FreeBSD platforms

Program Members


This project has been made possible in part by a gift from the Comcast Innovation Fund.

Last Updated: Friday 28-Oct-2016 12:28:07 EST | Maintained by: Rasool Al-Saadi ( | Authorised by: Grenville Armitage (