[e2e] Codel and Wireless

[Daniel Havey]

On Wednesday, December 4, 2013 4:42 AM, Detlef Bosau <detlef.bosau at web.de> wrote:

Am 03.12.2013 22:11, schrieb Daniel Havey:
> CoDel parameters are interval and target.  

Particularly "interval" is a yet to be discovered fundamental physical
constant, the Nobel price and the Turing award for which are still pending.

>> Haha!  And the answer to life, the universe and everything is 42!  Anyways I read a really nice paper about a really cool parameterless algorithm (CoDel) and I find that it has 2 parameters.  Huh?!  Then I read on and find out that they are not parameters, they are magic numbers!  Now I really want to know!  I am ready to believe that 100ms and 5ms are pretty darn good numbers.  But the paper didn't explain why.  I still want to know why.  Clearly the interval value affects response time.  What else?  

And finally the pragmatic question.  Just like Emmanuel asked.  Do these numbers work over a slow laggy satellite link.  I think the parameters work well over a wide range of scenarios just like it says in the paper, but, if you put that thing on my colleague's connection it will kill you.

The connection is a 128 Kbps with burst rates up to 256 Kbps satellite and we have measured 10 seconds of ping time under load.  Clearly 100ms and 5ms will perform miserably on this link since >5ms standing queue time is normal performance.  The interval value is suspect here too.  One hundred milliseconds?  Give me a break!  We have 500 ms just in flight time to outer space and back.

Now that I am done with my rant, let me just say that I like CoDel and I think fair queuing is cool.  Heck yes I want a queue for every flow!  Who would say no to that?  But I will (and did) recommend to my African friend not to use those numbers.  That is my disclaimer since I am not as flameproof as you!  ;^)

...Daniel



(Less pathetically spoken: It is a constant discovered by three or four
experiments and now, it is "the" fundamental constant which is valid in
the whole universe. When I did so in papers, these papers were rejected,
without exception. What are we pursuing? Science or gambling?)
> If the queue drains before the interval then there shouldn't be any drops. 

Yes, Daniel. I know the code.

And when I claim this is nonsense, then I'm first ready to take flames
and send, you can be assured that I read the code and the comments and
papers VERY MUCH more than once.

And even in this very moment, I have the pseudocode visible on the other
screen. And actually, the "sojourn time" is a qeue sojourn time and it
is calculated from time stamps, where the "clock" is read when the
packet is enqueued and dequeued.

With respect to queueing systems: CoDel measures queueing time, CoDel
doesn't meaure sojourn time. (:= Queueing Time + Service Time as you
remember from lectures.)


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