[e2e] Codel and Wireless

[Detlef Bosau]

Hari,

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
optimum.

Fine.

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.),
however:

It is never a problem to go wrong. But it is always  a problem to make
the same error more than once.


Detlef

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
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>>


-- 
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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