[e2e] Why do we need TCP flow control (rwnd)?

John Day day at std.com
Fri Jul 4 18:23:40 PDT 2008

At 16:56 -0700 2008/07/01, Fred Baker wrote:
>On Jul 1, 2008, at 3:52 PM, slblake at petri-meat.com wrote:
>>Quoting John Day <day at std.com>:
>>>No kidding. There are some textbook authors who have helped with this
>>>too.  I was shocked to see Stallings say in one of his books, something
>>>to the effect of 'In the early 90s we discovered network traffic wasn't
>>>Poisson.'  (!)  We had known *that* since the 70s!!! I remember prior
>>>to 1976 scouring the literature looking for work that was closer to
>>>real, i.e. didn't assume Poisson, or would give us the tools to get
>>>closer to real.
>>The following paper may be of interest to those following this thread:
>>T. Karagiannis, M. Molle, M. Faloutsos, and A. Broido,
>>"A Nonstationary Poisson View of Internet Traffic",
>>IEEE Infocom 2004.
>As the paper notes, the Poisson model tends to be a limiting case - 
>its results will be similar to but more conservative than one would 
>expect in reality. I use the equations too, because they are simple, 
>but with that caveat very explicitly in place.
>The thing that I find a little hard to grasp is why folks might have 
>thought the network was Poisson in the first place.


>I think the thing that makes Gaussian and Poisson models attractive 
>is the relative simplicity of their math. With Gaussian models we 
>can discuss standard deviations, and with poisson models we can pull 
>formulae out of textbooks. But in both cases, they are more 
>conservative than we observe, and have predictive only to the extent 
>that they are used as conceptual limits.

And herein lies the rub.  If I had to guess, I would say that pre-76, 
we didn't think it was Poisson but it was the only math we could do 
and we couldn't simulate very large networks then. There were also 
those just infatuated with queuing theory and insisted it was the 
only way to attack the problem.  As the field grew, some researchers 
forgot the "we assume Poisson although we know it isn't" proviso or 
assumed everyone knew it and others started to believe it.

Take care,

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