[e2e] Codel and Wireless

Andrew Mcgregor andrewmcgr at google.com
Thu Dec 5 15:22:33 PST 2013

On 6 December 2013 02:49, Detlef Bosau <detlef.bosau at web.de> wrote:

> Am 04.12.2013 23:52, schrieb Andrew Mcgregor:
> > CoDel has a definition of 'sojourn time', which is the way Van and Kathie
> > defined it.  You're using a different definition, and therefore we are
> > talking at cross purposes.
> >
> > The CoDel definition is equivalent to 'queue residence time'.  I don't
> know
> > what you mean by 'sojourn time',
> Then you should have a look at an arbitrary text book on queueing theory.

I have, and I still don't know what you mean.  But 'queue residence time'
has us on the same page now, so let's just ignore sojourn as a term.

> Basically, the queueing residence time, you're talking about, and I had
> a closer look at the CoDel code yeserterday so I know at least what
> CoDel is talking about here, is simply not feasible to assess a links
> load situation, particularly in mobile networks.

Of course it isn't a measure of load, but that is not what CoDel is trying
to do with it, still less fq_codel.  What these algorithms are doing is
effectively saying, we know that excess queueing delay (due to TCP windowed
flow control) is a problem in and of itself, so we shall measure and
control it directly.

> I succumbed to this fallacy myself - and it took me years to admit my
> error.
> I've seen even PhD theses based on this nonsense, however in science we
> should some day make a difference between truth and fallacy. And using
> queueing times as a means for network load assessment is a fallacy.

I agree entirely, it doesn't work for that purpose.  But that's not what
the AQM is aiming for.

> Perhaps, I'm completely isolated with my position - I can't help it.
> I have no academic affiliation and no academic contacts here in Germany,
> so perhaps I'm about to completely ruin my reputation here, however to
> my understanding, science deals with truth and error. And CoDel as a
> means of queue management in wireless networks is an error.

I don't think so, not empirically, so long as we agree that it's not about
load but about direct management of queueing delay and nothing more.
 However, I expect there's better ways out there, and the purpose of the
AQM working group is, at least partly, to find out what they are.

There's also little theoretical support for CoDel, still less fq_codel, so
fq_codel has the empirical status of 'works really well in the (many) tests
we have done', but not the theoretical support of 'and we know exactly why
it works and what its limitations are analytically'.  I hope we can find
some algorithm with both, but I'm not holding my breath.

By the way, fq_codel is not a combination of SFQ and CoDel, but something
else.  The code deserves really close study to understand what is really
going on there, because there is a very interesting contribution in terms
of heuristically measuring whether flows are building queue in the qdisc or
not.  Empirically, this works really well on user access links, including
LTE, by my own direct observation having run it on my home network's LTE
link for many months now.

Andrew McGregor | SRE | andrewmcgr at google.com | +61 4 8143 7128

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