[e2e] RES: Why Buffering?

Detlef Bosau detlef.bosau at web.de
Tue Jun 23 09:02:39 PDT 2009

Jon Crowcroft wrote:
> the naive argument for bw*delay buffer
> was that AIMD involves rate halving 

It's not the rate being halved but the congestion window.
And I'm always curious to see that some people expect some easy 
relationship between rate and sending window / congestion window. Maybe, 
there is one. Than it's certainly not a trivial one.

> so by corollary, just before the rate halved
> it was twice the bottleneck's capacity

Yes. That's the common rationale. However, Alexandre is correct here: 
Some router in between has no idea of a path's capacity - at least as 
this cannot be easily described by some "latency bandwidth product" in 
some networks.

So, the idea of having the "latency bandwidth product" of a path doubled 
as buffer capacity in a router is a nice one for theoretical papers. But 
I'm not quite sure what this should mean for reality: Consider a router 
in Rotterdam which sees both, traffic from Wladiwostok to London, 
Hamburg to New York and from Den Haag to Santiago to Chile.

Which is "the" latency bandwidth product which should be doubled?

Surely not a different one for any possible flow.

> if there's lots of flows
> and the individual flows are all even reasonably 
> unsynchronised in their AIMD phase
> (very good chance due to random local perturbation
> (law of large number argument says..)
> then you are right (well, nick mckeown's right:)
> if you use some smart queue management and ECN
> then you're right
> if most flows stop before reaching their operating point
> your right 
> (well actually, there's a way to be wrong
> in a quite ghastly way if  you have many sy cnhronised flows
> in "slow start", heading thru the same bottleneck...
> but it ought to be quite a rare event  - like
> black swans and mrket meltdowns:-)

So, after a long list of scenarios where Alexandre is right: Is there a 
scenario, where he is wrong? ;-)

Detlef Bosau		Galileistraße 30	70565 Stuttgart
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