[e2e] Why do we need TCP flow control (rwnd)?
David P. Reed
dpreed at reed.com
Thu Jul 3 07:32:18 PDT 2008
I think all mathematical + tools are useful... I hope I have not given
an impression to the contrary.
What is not useful is "cookbook thinking" that is often characterized by
using such models without understanding their limitations, or worse, by
people who assume they are "correct" in any sense other than being
self-consistent mathematical systems.
In the late nineteenth century, the bulk of practicing physicists (and
essentially ALL EE's) acted as if the world were linear (in the sense
that all physical laws were linear systems of differential equations).
Such models were very useful, and when used in their domain of
applicability, made much progress - essentially curve-fitting linear
models piecewise to reality.
This is the modern version of "epicycles".
As a simple thought experiment: we assume in radio or optical
communications that the world is characterized by Gaussian white
noise. But if you look outside your window on a sunny day, do you see
a Gaussian optical field? I would argue that to see the world as a
Gaussian optical field you have to be wearing a blindfold. I hope you
Scientists who stand blindfolded in the world are making the most
fundamental scientific error. Presuming their models are more correct
than the world of experience and experiment.
Choosing to see the world of a network as a Poisson process is wearing a
blindfold of the same sort.
guol at cs.bu.edu wrote:
>> Good rant.
>> And I coudln't agree more.
>> Only one problem remains.... And that's a very honest question:
>> If we agree upon some facts:
>> - Poisson processes and Markov prossesses are of little use in
>> networking research,
> I don't think this has been agreed upon. I believe the concensus is that
> Markovian model under-estimates variability existing in the current
> Internet/human behavior. But these processes are definitely very valuable
> in that they give ways to estimate, at least qualitatively, the effect of
> variability to networking systems.
>> - Analysis is not really helpful (and frankly spoken, I hardly believe
>> those analytical TCP models, which are around),
> Again I think they are very helpful to get insights
>> - Simulation can prove anything and nothing,
> Simulation is for validating results or for discovering problems, not for
> proving something.
>> - Observation is not reproducible and not systematic,
>> so, if we agree upon the fact, that research on networking is basically
>> impossible, _how_ can we accomplish research on networking then?
>> It's of course allowed - and it's always scientifically correct to do so
>> - to put in question anything we have.
>> But if we only see blind alleys, how do we find a way out?
>> Detlef Bosau Mail: detlef.bosau at web.de
>> Galileistrasse 30 Web: http://www.detlef-bosau.de
>> 70565 Stuttgart Skype: detlef.bosau
>> Mobile: +49 172 681 9937
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