[e2e] RTO Estimation... was "Agility..."

Detlef Bosau detlef.bosau at web.de
Mon Aug 1 11:07:46 PDT 2005

Sireen Habib Malik wrote:

> Second point: It is clear that present RTO estimation will work in the
> frame of assumptions under which it is supposed to work. Like Detlef
> says, "nobody will complain that a car does not run if it is out of
> gas", so nobody should complain if RTO estimator does not work when
> traffic parameters do not fall inside the space of the relevant assumptions.

And we should well consider the consequences, if it´s true that RTO
actually don´t work, as Christian suggested. For the particular case of
mobile wireless networks, we would have to reconsider the whole work on
"spurious timeouts", because what´s called "spurious timeout" is perhaps
not the problem, but a symptom. Unduly often spurious timeouts are
nothing else 
than a too high probability for unwanted retransmissions, to remain in
the words chosen e.g. by Edge.

> If that is true then one way to resolve this issue is to adjust/shape
> traffic in such a way that RTO should work (I think this is what Detlef

Exactly. In my post, I focussed solely on the routers. However, any
change in a router´s behaviour directly influences the traffic switched
by this.
Even more difficult: Influencing the traffic on a router will perhaps
not only affect this router, but the behaviour of other routers as well.
So, I´m not quite sure whether we are allowed to consider the router
queues as being decoupled.

> is saying), or make a "general purpose" RTO estimator that
> reduces/relaxes the set of assumptions - ideally, it should work if IID
> assumption holds, or not.
> I think the work in the second direction is more general, and conducive
> to practical environments. How difficult or easy it is, I don't know! A

The assumptions made by Edge are extremely general. E.g., for RTT and
VAR he assumes hardly more than the pure existence. "Weakly stationary"
means that all observation variables must share the same E and V,
moreover the correlation of the latest observation variable to some
arbitrary other one in a given sample does not depend on the sample

Bearing in mind, that we look for E and V, these assumptions appear
rather weak to me.

I´ve looked around for other estimators. In fact, there may be
estimators which yield better values for forecasting, however under much
stronger assumptions, e.g. the forecasted process must be stationary and
obey a normal distribution.

> good way is to first find out, if there is any work already done in this
> direction?

In addition I would be interested in work on the convergence speed of
EWMA filters. I´m not quite sure, whether I can access the work on EWMA
filters quoted by Edge, these are older textbooks by Cox (1965) or Box

I think, a signal theory perspective would be helpful here. An EWMA
filter is basically nothing else as a grade 1 lowpass IIR filter.
So it may be helpful to consider step response and impulse response
functions of this one, particularly the step impulse because this will
the behaviour in case of sudden steps in the latency. However, if we
know the impulse response, we can describe the general behaviour on
arbitrary signals here.

One difficulty here is that the EWMA filters impulse response is that
one of a time discrete system and hence its consequences on a real time
(continuous time!) system  depend on the sampling freuquency, i.e. on
the acknowledgement rate. This becomes extremely important in mobile
networks where path characteristics may change due to physical or
enviorenmental circumstances which are more or less beyound our
influence and where e.g. a filtering of changes, which is of course a
time discrete filtering, must be adapted to the flows "sampling
frequency", i.e. the ACK rate.


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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