[e2e] Codel and Wireless

Jon Crowcroft Jon.Crowcroft at cl.cam.ac.uk
Fri Dec 6 04:49:37 PST 2013

i think the variations in wireless properties
over the length of their traes for their experiment
probably covers most tof hte cases of variation you might 
see in future scenarios....there just isn't that much 
design space for the channel to vary over - they aren't trying to
evolve/deisgn a channel condition predictor - just a reactive system
that does betterer than others....

i think the Reverend Bayes has some things to observe in this space
about a priori and a posteriori, although i might be talking out of my
posterior when I say that

In missive <52A1AED7.7040302 at web.de>, Detlef Bosau typed:

 >>yes, you use traces obtained from somewhere. And when I remember TCP ex
 >>machina, which was mentioned by Keith,
 >>you run into the same fallacy again and again.
 >>In TCP ex machina you calculated ex post an optimal CC algorithm - that
 >>means you optimized a target function in some variables and found an
 >>And completely useless for the next concrete run of the protocols where
 >>the scenario my be completely different from the "obtained one".
 >>The same now.
 >>We can discuss the TCP RTO, which is basically a confidence interval
 >>which is expected to cover, say, 95% of observered RTT samples.
 >>Now you observe 10 Millions of TCP samples and calculate the RTO from
 >>these. And guess what is most likely to be found (by the Strong Law of
 >>Large Numbers).
 >>Of course, 95% of the observed RTT will stay in the "predicted"
 >>interval, because you have drawn "random numbers" by completely
 >>independent and identically distributed random variables obeying exactly
 >>the one distribution which is defined by your observations.
 >>And of course, the same formulae applied to _REAL_ observations will NOT
 >>hold. Because the  _REAL_ observations are neither independent, nor
 >>identically distributed.
 >>This is always what is sometimes called "ratio ex post", sometimes
 >>"conclusion in the wrong direction", however this is mathematically wrong.
 >>And as you know from elementary physics (applied to Rayleigh Fading):
 >>You will NEVER be able to reproduce a mobile wireless scenario.
 >>Never. Under no circumstances. So it absolutely does not matter, whether
 >>you model doesn't match reality, or the "real world trace" - where the
 >>mobile phone was placed 5 centimetres away from its current location
 >>doesn't match the (current) reality.
 >>The error is always the same and you will, with "best QoS guarantees",
 >>fall into exactly the same pitfall and the same fallacy again and again.
 >>And although I don't remember the paper: I've read even the "trace
 >>story" in the context of adaptive multimedia flows over wireless networks
 >>years ago, the story is as old as wrong.
 >>Excuse my being upset. It's just a result of 14 years of frustration :-(
 >>Particularly as I well understand the problems. I made many of these
 >>mistakes myself and several times in my life (I'm aged 50 years, btw.),
 >>It is never a problem to go wrong. But it is always  a problem to make
 >>the same error more than once.
 >>Am 05.12.2013 17:30, schrieb Hari Balakrishnan:
 >>> Detlef,
 >>> May I suggest that you please look at the paper? The simulation experiments don't use a wireless link model but are obtained over packet delivery traces collected over multiple different cellular networks (in both directions).
 >>> Hari
 >>> On Dec 5, 2013, at 8:47 AM, Detlef Bosau wrote:
 >>>> Keith,
 >>>> as long as we don't have compelling wireless link models in the ns-2, I
 >>>> don't think simulations in this area are useful.
 >>>> Particularly, quantitative results from those simulations are extremely
 >>>> questionable.
 >>>> -- 
 >>>> ------------------------------------------------------------------
 >>>> Detlef Bosau
 >>>> Galileistraße 30   
 >>>> 70565 Stuttgart                            Tel.:   +49 711 5208031
 >>>>                                           mobile: +49 172 6819937
 >>>>                                           skype:     detlef.bosau
 >>>>                                           ICQ:          566129673
 >>>> detlef.bosau at web.de                     http://www.detlef-bosau.de
 >>Detlef Bosau
 >>Galileistraße 30   
 >>70565 Stuttgart                            Tel.:   +49 711 5208031
 >>                                           mobile: +49 172 6819937
 >>                                           skype:     detlef.bosau
 >>                                           ICQ:          566129673
 >>detlef.bosau at web.de                     http://www.detlef-bosau.de



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