[e2e] Why do we need TCP flow control (rwnd)?
huitema at windows.microsoft.com
Wed Jul 2 10:27:57 PDT 2008
> I was meaning to address the statement that it is bad to model
> congestion collapse by people continuing to pound a congested link
> with Poisson traffic. My point was that, for a core/edge link,
> individuals each only need to pound once, and we can still have a
> (batch) Poisson process during the congestion interval. An individual
> leaving the system because of congestion doesn't stop the Poisson-ish
> arrivals of new individuals (flows, bursts, ...) coming.
Actually, the arrival of individuals in the system does not appear to match the Poisson hypothesis. Part of the heavy tail behavior seems to be due precisely to correlations in the arrival process. A given individual is more likely to become active if he or she was recently active; multiple individuals are more likely to become active if other individuals are already active. This "piling up" effect contributes to the heavy tail effect, or to power law distributions.
The problem with Poisson modeling is not that it overestimate congestion. On the contrary, the Poisson hypothesis tends to underestimate the variability of the aggregate, which leads to underestimating the risk of congestion. Basically, the Poisson hypothesis says that arrivals are independent, and thus that aggregates will be smooth. The "self correlation" hypothesis says that more arrivals are likely if many arrivals are already observed, and thus that peaks of traffic are not likely to be smooth.
-- Christian Huitema
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