- 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 is the first fundamental advance in the state of the art of network Active Queue Management in many, many years.
It is pronounced "coddle", because it handles network streams in a gentle way.
With fq_codel it is 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.
We don't say several orders of magnitude lightly. We have the benchmarks to back it up. Some more recent benchmarks on cable systems are here: http://burntchrome.blogspot.com/2014/05/fixing-bufferbloat-on-comcasts-blast.html - and there are multiple papers on the subject now of widely varying quality.
Papers and Publications¶
The most up to date descriptions of codel and fq_codel are now the subject of IETF internet drafts.
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.
We are working on updating the codel and fq_codel ns3 simulations in a github repository - several outstanding bugs have been fixed already and we hope to be baked by the end of august, 2014 as part of the Google Summer of Code.
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 The code as published in the original paper is obsolete.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 irc.freenode.net in the #bufferbloat chat room.
Please go to the codel mailing list if you have questions.
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 10 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 "debloat" 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.
The CeroWrt research router project was started, specifically, to test new AQM technologies, and has been tracking codel closely.
A pretty solid build is now available
The fq_codel code has already migrated into the OpenWrt mainline (upon which Cerowrt is based), so the research has 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 host (rather than router) based queue management, we recomend sch_fq instead of fq_codel as of linux 3.12.
It can be enabled by default via a single sysctl option in /etc/sysctl.conf
net.core.default_qdisc = fq # for hosts, fq_codel for routers. Note that in a virtualized environment the host is a router, the guest VMs are hosts and we don't have a lot of data regarding either qdiscs' performance at 10gigE speeds.
Code for other operating systems¶
A port of Codel exists for BSD and is available in pfsense and elsewhere.
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.
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.
It is clear that without chaning the target or interval variables, CoDel is not appropriate for AQM of traffic 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.