[e2e] end2end-interest Digest, Vol 17, Issue 26

Detlef Bosau detlef.bosau at web.de
Thu Aug 4 13:19:25 PDT 2005

S. Keshav wrote:
> Detlef,
>     In general, what you are asking for is difficult. Consider the following
> scenario. Suppose a router forecasts that the queueing delays at a
> particular interface are small at time t and expects this forecast to hold
> until t+200ms. Now, suddenly, a burst of packets from multiple input ports
> destined to that interface arrive at time t+epsilon. This builds up the
> queue, increasing delays. You have two choices:
> 1. violate the forecast
>    or
> 2. drop packets in order to meet the forecast.
> Neither one is a good alternative. If you violate the forecast, then what
> use is it? If you drop packets to meet the forecast, that's a waste, because
> adequate buffers exist. I do not think that dropping packets in order to
> make RTO computations sane is a good tradeoff.

Perhaps, we talk a litte bit cross purposes here. What I´m trying to 
understand is the estimation of mean and variation of RTT in TCP flows.
I don´t want to give any guarantees.

So the purpose of a forecast is only to estimate latencies for the near 
future. If there is a traffic burst, then the forecast may be violated. 
So what? It´s an _estimate_. Moreover, it´s an estimate for a _mean_. An 
actual latency may well be greater or less.

Basically, there are two objectives:

1. provide an RTO estimator with _less_ assumptions than e.g. Edge´s 
2. alleviate the settling behaviour and the consequences of the 
sometimes quite rough sampling done by the usual RTT observation.

Perhaps, this could be helpful, I don´t know yet.

Detlef Bosau
Galileistrasse 30
70565 Stuttgart
Mail: detlef.bosau at web.de
Web: http://www.detlef-bosau.de
Mobile: +49 172 681 9937

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