[e2e] Why do we need TCP flow control (rwnd)?
huitema at windows.microsoft.com
Fri Jul 11 08:54:21 PDT 2008
> The reason folks thought that data networks would be Poisson was that
> 1. Phone networks were demonstrably Poisson
> 2. They had nothing else in the arsenal (that is, if you were going
> to do anything non-trivial, you lacked an analytic alternative)
The point about nothing else is certainly valid. If you have to perform your computation using log tables and an actual spreadsheet, then tackling anything besides Poisson will be very challenging. However, it has been known for some time that Poisson was at best a first degree approximation, even for phone traffic.
Consider two widely documented cases, time variations in traffic and flash crowd, both of which have been part of traffic engineering for a long time. The average traffic load is known to vary as a function of the time of day, day of the week, month of the year, not to mention Mother's day. There is no way you can model that as a single "independent arrival" process. Flash crowds happen when many people decide to pick up their phone at the same time, e.g. to call in a TV show, or to report about a particular incident. Again, there is no way to model flash crowds as independent arrivals. Even the duration of phone calls was probably not well modeled by time independent departure processes, e.g. if you take into account the behavior of teenagers in the 70's.
I suspect that any process that results from human behavior is very likely to exhibit much more variability than predicted by a Poisson model. That's certainly true for phone traffic as well.
-- Christian Huitema
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