CoDel Overview

CoDel is a novel “no knobs”, “just works”, “handles variable bandwidth and RTT”, and simple AQM algorithm.

  • It is parameterless — no knobs are required for operators, users, or implementers to adjust.
  • It treats good queue and bad queue differently - that is, it keeps the delays low while permitting bursts of traffic.
  • It controls delay, while insensitive to round-trip delays, link rates, and traffic loads.
  • It adapts to dynamically changing link rates with no negative impact on utilization.

CoDel (the name comes from “controlled delay”) was a fundamental advance in the state of the art of network of Active Queue Management in 2012.

It is pronounced “coddle”, because it handles network streams in a gentle way.

Immediately after codel, came “fq_codel” (Fair/Flow Queueing + Codel), invented by Eric Dumazet. The combination made it possible to reduce bottleneck delays by several orders of magnitude, and provide accurate RTT estimates to elephant TCP flows, while allowing shorter (sparser) flows like DNS, ARP, SYN, routing, etc packets priority access. And indeed it did. And from our small research project it then became the default queuing mechanism for all the world of Linux, and IoS and OSX.

We didn’t say then, several orders of magnitude lightly. We had the benchmarks to back it up. Benchmarks, from 2014, showed enormous improvements on cable systems, dsl, fiber and wireless technologies.


All Linux systems that use systemd, now default to fq_codel. That includes but is not limited to, debian, Ubuntu, redhat, fedora, and arch.

fq_codel is the default queuing mechanism in most third party router firmwares today, like OpenWrt, dd-wrt, and asus-wrt. IPfire, Firewalla, evenroute, ubnt, eero, Mikrotik and many others now use fq_codel heavily.’s revolution V6 router used it by default. It is a component of Qualcomm’s “streamboost” QoS system. It is in Netgear’s “Dynamic Qos” feature for their X4 product.\ And now in many places elsewhere.

We finished creating a successor to fq_codel and the “sqm” system, called cake in 2018, which we addresses a few edge cases fq_codel had, and is better all across the board.

Papers and Publications

The most up to date descriptions of codel and fq_codel are now the following IETF internet standards:

Codel - RFC8289

FlowQueueCodel - RFC8290

Older Papers

Controlling Queue Delay ACM Queue, Kathleen Nichols, Van Jacobson, May, 2012
Codel page at Pollere . Pollere does research on and analysis of network performance via modeling and simulation, measurement,and laboratory prototypes.
Kathie Nichol’s CoDel at the IETF-84 Transport Area Open Meeting, 30 July, 2012, Vancouver, Canada, by Van Jacobson.


Kathie Nichols’ and Van Jacobson’s original ns2 simulation of codel
Their most current ns2 code is now available via git

We worked on updating the codel and fq_codel ns3 simulations in a github repository - but that is now obsolete. Codel landed in ns3 mainline in sept, 2014, as part of the Google Summer of Code, and fq_codel (and variants) are slated for the next version.

There are now basically 6 slightly different variants of codel (see Reconciling codel variants), flying in loose formation. The code as published in the original paper is obsolete. Research is continuing. Come help!

Mailing list and chat room

There is a CoDel mailing list, and discussions that take place on in the #bufferbloat chat room.
Please go to the codel mailing list if you have questions.

Linux Code

CoDel - in order to run well at line rate - requires the Linux 3.3 Byte Queue Limits . It has proven too hard to backport BQL to Linux 3.2 or earlier (an attempt for 3.2 exists, but no driver support), so you will need to upgrade to Linux 3.5 or later, and have a driver that supports BQL (only about 24 as of the present writing).

If you are doing soft rate limiting (eg shaping with HTB or HFSC), BQL is not required at the driver level. Codel and fq_codel are in the Linux 3.5 release - no patches are required, but BQL support is limited, as noted. There were multiple bugs in HTB fixed prior to Linux 3.11.

While fq_codel and codel are “no knobs” qdiscs, several other knobs can be tweaked for the lowest latency results. An example script for doing that is called “” which is available from the deBloat repository on github. It tunes up BQL, turns off various forms off tcp offloads, and offers both a fq_codel and codel + qfq model to play with. Turning off BQL is not really needed since linux 3.8 and later, the autotuning works well. TCP small queues has evolved to
where it does the right thing with TCP offloads as of linux 3.12.

So most of the debloat script is no longer needed.

All codel and fq_codel development was pushed into the linux mainline as of linux 3.6 and you should look there for the most up to date code.

iproute2 added support for fq_codel in the 3.6 release and configuration and statistics should be available by default in most Linux systems shipped today.

CeroWrt Version

The CeroWrt research router project was started, specifically, to test new AQM technologies. It was completed in 2014, and most of the innovations landed upstream in OpenWrt, Linux and BSD.

A pretty solid build is still available

The fq_codel code has already migrated into the OpenWrt mainline (upon which Cerowrt is based), so the research paid off! - there is more to come…

Binary code and kernels for Linux based operating systems:

All modern linux distros now ship with fq_codel.

For servers with tcp-only workloads, particularly at 10GigE speeds, for queue management, we recomend sch_fq instead of fq_codel.

Either qdisc can be enabled by default via a single sysctl option in /etc/sysctl.conf:

net.core.default_qdisc = fq\_codel - best general purpose qdisc

net.core.default_qdisc = fq - for fat servers, fq_codel for routers.

Note that in a virtualized environment the underlying server IS a router, the guest VMs are hosts and we don’t have a lot of data regarding either qdiscs’ performance at 10gigE speeds on vms - and early data on fq shows fq_codel’s lowered quantums of benefit at 1GigE and below. We certainly expect fq to continue to improve on hosts and servers… and we expect fq_codel to improve also.

Code for other operating systems

A port of Codel exists for BSD and is available in pfsense and elsewhere.

Known issues

At very low bandwidths (e.g. .5Mbps) on ADSL, we’re having to play with the target; Kathie did not have to in her simulations. This is due to inevitable buffering
in htb or in the device driver. We have a version under development that does bandwidth limiting without buffering an extra packet, called cake. It’s looking good so far.

People have tried to run CoDel in very big routers, with hundreds of simultaneous flows, a situation not simulated in advance. There, it isn’t controlling the queue the way it should: whether this is a problem with the algorithm, or the implementation, is not yet understood. fq_codel does better in this case.

It is clear that without lowering the target and interval variables, CoDel is not appropriate for AQM of traffic solely inside a data center; it does not react in a timely enough fashion. Whether the modifications of the ideas in CoDel will solve this problem is not yet known. Again, this is an area which CoDel was not designed to solve or it simulated in before publication.

To edit this page, submit a pull request to the Github repository.
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Comcast Research Innovation Fund
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Bufferbloat Related Projects

OpenWrt Project
Congestion Control Blog
Flent Network Test Suite
The Cake shaper
CeroWrt (where it all started)

Network Performance Related Resources

Jim Gettys' Blog - The chairman of the Fjord
Toke's Blog - Karlstad University's work on bloat
Voip Users Conference - Weekly Videoconference mostly about voip
Candelatech - A wifi testing company that "gets it".