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

Detlef Bosau detlef.bosau at web.de
Tue Jul 1 09:29:12 PDT 2008

David P. Reed wrote:
> Michael Scharf wrote:
>> BTW: I wonder how all those TCP simulators out there address this
>> problem.
> Trivial arrival models:  e.g. the wondrous Poisson model, with all its 
> huge flaws of assuming that the sender will keep banging away despite 
> getting no app-layer results.

Actually, this introcudes a second flow control mechanism.

The first one, Michael is talking about, is TCP flow control which is 
achieved by propagating the rwnd.
The second one, which you just mentioned, is on the application layer:


watiing for the SMTP server, waiting, waiting, waiting.

"RCPT TO: someone.who.loves at me"



Actually, receivers can (and do) control senders that way.

And even worse: Human beings exist. So, when I'm chatting with somebody, 
the anwer at my peer's site may hang - not because of network problems 
or because of wrong application models but simply because I'm going for 
a mug of coffee instead of typing an answer.

> This is awful: most predictions of "congestion collapse" (though not 
> all) were created by mindlessly presuming Poisson arrival.  No human 
> that I know of *ever* generates a Poisson distribution.   We use them 
> on student problem sets because they are analytically easy, not 
> because they are realistic in any way.   Then a whole generation of 
> theory students grow up thinking that network traffic is best thought 
> of as Poisson.  And they dominate the conversation, blasting out any 
> more thoughtful approaches by generating minimum publishable units and 
> reviewing each others' papers as if they were problem sets to be graded.

Detlef Bosau                          Mail:  detlef.bosau at web.de
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