[Codel] why RED is not considered as a solution to bufferbloat.

Dave Taht dave.taht at gmail.com
Tue Feb 24 13:15:25 EST 2015

and I meant to mention my other big kvetch...

Nearly no paper tests an asymmetric network, where upload speeds are a
fraction of the download speeds. This leads to acks dominating
upstream traffic, and A) a big upload is starved, or B) that upload
acks starves the download by a providers desparate attempts to
optimize - the combination of any traffic accros sthat link gets
subject to crazy ideas like ack prioritization and other scenarios
that break other forms of traffic.

As this is actually how the edge of the internet is deployed - DSL has
ratios of 6 and even 20 to 1, cable modems are in the range of 6 and
10 to 1, the only technology that is symmetric is fiber - it totally
boggles my mind that this is not also the standard benchmark test of
an aqm or fq algorithm.

The lack of an asymmetric network test should also be a fundamental
bar to publication of any new paper on the AQM or FQ subjects.

So we fixed that, in creating ns3 tests as well. It turned out to be
hard to do, we had a ton of bugs to sort out in the code (now mostly
done except for getting fq_codel itself mainlined) and I have no idea
if ns2 has the same problems or not, but it certainly explains why so
little in the liturature actually sees the real problems.

And lastly - on another subject entirely - no aqm we know of yet is
correctly structured to deal with the taxi-stand topology half duplex
nature of wifi and wireless, and no sim I have yet tried looks
anything like measured reality.

we're working on it.

On Tue, Feb 24, 2015 at 10:00 AM, Dave Taht <dave.taht at gmail.com> wrote:
> On Tue, Feb 24, 2015 at 8:32 AM, Jonathan Morton <chromatix99 at gmail.com> wrote:
>> Most of us on this list believe that to be true, in many cases after
>> performing experiments ourselves, or at least looking through data generated
>> by others' experiments.
>> However, if as I suspect you are investigating various AQM algorithms as
>> part of your education, you should probably examine the data yourselves and
>> come to your own conclusions. You may even get extra credit for being able
>> to describe the difference between AQM and Fair Queuing, and how they can be
>> combined (as in fq_codel) to give the benefits of both types in one go. But
>> for that, you ARE going to need to read some boring papers like "RED in a
>> different light".
> It is not particularly easy to keep up with the onslought of AQM
> literature since the bufferbloat effort started, but a review of stuff
> since 2011, or even as late at 2013 via google scholar should be
> illuminating. Many papers use RED as a reference, but nearly all of
> them miss the major points in the original bufferbloat experiments.
> Those experiment, long ago documented on Jim´s earliest writings on
> the subject, available in video form, in various papers etc. One
> example:
> https://gettys.wordpress.com/2012/02/01/bufferbloat-demonstration-videos/
> where jim performed an upload and a ping, at the same time, on a
> network optimized for downloads and and obviously not tested for
> uploads. The later rrul test suite (in netperf-wrapper, open source,
> anybody can use it, and I really wish they did) was designed to
> exercise both directions of the link with tcp data, and do a latency
> measurement, at the same time.
> Either experiment is consistently not replicated by experimenters *to
> date*, it frustrates me, and the only thing that gives hope is the
> slow progress in science of resolving the problems in this experiment:
> http://en.wikipedia.org/wiki/Oil_drop_experiment#Millikan.27s_experiment_as_an_example_of_psychological_effects_in_scientific_methodology
> But I am not willing to wait 70 years to get it all sorted out!
> The core observation I have is :
> Drop tail queues and AQMs do not do well in the face of cross traffic,
> (a mixture of small ack packets and larger data packets at saturating
> loads). This is apparently one of those problems that most aqm-ers
> (but not van and kathie!) wish to sweep under the rug, as if having a
> car that can steer on a downhill run only, was acceptable and safe to
> society at large.
> I made for-damn-sure that there was a rrul-like test for that scenario
> in the ns3 code now being mainlined, in the hope that new
> experimenters and designers of new algorithms would rigorously test
> for circumstances with cross traffic. I think I should also have got
> around to doing one in ns2.
> Moving on, codel was co-designed by the RED guy - van jacobson - and
> if you don´t  believe him when he explains how RED was flawed, please
> stay away from my networks.
> http://www.pollere.net/Pdfdocs/QrantJul06.pdf
> There is no information in average queue length.
> The whole story about red in a different light, is sad and amusing at
> the same time, when someone finds he has made a mistake, and tries to
> retract it, and cant get published, and the pile on and noise level
> since by folk since writing RED related papers, even since we managed
> to get the correctly contradictory information on RED out there, is
> annoying.
> If *all* future aqm oriented papers made sure to address the
> cross-traffic problem - that  mixture of big and small packets under
> saturating loads - with their latest-aqm-idea-de-jure as part of their
> *core criteria for worthiness* - it would be a better world.
> While I certainly believe that you can make an AQM that works better
> with cross traffic - and actually have some revisions for codel that
> do so -
> You cannot predict the traffic load or traffic types going in either
> direction in most environments and thus you *must* handle it at peak
> load in both directions, well, in order to have a deployable solution.
> For all the aqm-related papers of DASH and web traffic down, there are
> nearly none that test torrent/scp/dropbox or videoconferencing traffic
> up, at the same time. I would like to be in a world where I could
> refuse to read any paper that did not address the cross traffic
> problem, AND such papers were summarily rejected before publication.
> So, thus, codel is a AQM that combines well with FQ, unlike all other
> AQMs published to date - and nearly nobody that uses it, uses it
> without also doing FQ.
> As it is, fq_codel is deploying rapidly across the edge where it was
> designed to go, and I see nearly no implementations of RED in the
> field from extensive talks with operators and firmware makers around
> the world. *None*, in my last poll at the New Zealand network
> operators group meeting.
> Some form of fair queuing, on the other hand, was deployed by over 1/3
> the network operators there. (Convincing advocates of FQ that AQM is
> also needed, has often been a frustrating exercise as well!)
> Lastly, if you have trouble reading english, there is always google translate.
>> - Jonathan Morton
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> --
> Dave Täht
> Let's make wifi fast, less jittery and reliable again!
> https://plus.google.com/u/0/107942175615993706558/posts/TVX3o84jjmb

Dave Täht
Let's make wifi fast, less jittery and reliable again!


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