[Iccrg] Re: [Tmrg] convergence time

[Lachlan Andrew]

Greetings Andrea,

On 30/10/2007, Andrea Baiocchi <andrea.baiocchi at uniroma1.it> wrote:
>
> At 13:00 -0700 27-10-2007, Lachlan Andrew wrote:
> >How should we measure the responsiveness of a TCP algorithm?
>
> can it be sensible to look at responsiveness from a point of view
> closer to the user, by measuring the time required to deliver a given
> amount of data Bklg as a function of the value of Bklg? This is a
> curve not a single value, but useful indication could be extracted
> from there (i.e. asymptotic growth rate).

That is a very useful metric for the "efficiency" or "aggressiveness"
of the algorithm.  It is essentially the "flow completion time" metric
that Nandita and Nick have been promoting.

When I spoke of "responsiveness", I meant "response to changes in
network conditions", which is something not captured by the metric you
mention.  Whatever we call them, we need to measure both effects.

> My proposal above suffers from slow start dependence. It could be
> "generalized" (albeit also complicated) by considering the amount of
> time required to deliver one INCREMENT of Bklg, say DeltaB, after an
> amount Bklg0 has already been delivered. As an example, given Bklg0,
> time required to deliver further data DeltaB=alpha*Bklg0, with
> alpha=0.1. Parameters to choose for this measurement are in general
> Bklg0 and alpha. This last measurement could also be normalized to
> the overall time required to deliver Bklg0 amount of data.

That metric is equivalent to the average rate over some interval.  Can
you think of a way to average out the effects of AIMD in that
measurement?  It would either need  alpha  to be quite large, or to be
specially tuned to the time between loss events.  Otherwise, for given
parameters  alpha  and  Bklg0,  an algorithm could alternate between
doing well and doing poorly as the RTT increases, as the interval
moves from (just before a loss) to (just after a loss).

> Thank you for you attention.

Thank you for your input!

Cheers,
Lachlan

-- 
Lachlan Andrew  Dept of Computer Science, Caltech
1200 E California Blvd, Mail Code 256-80, Pasadena CA 91125, USA
Ph: +1 (626) 395-8820    Fax: +1 (626) 568-3603
http://netlab.caltech.edu/~lachlan