[Tmrg] Mix of RTTs

[Lachlan Andrew]

Greetings Sally,

Thanks for your reply.

On 21/02/2008, Sally Floyd <sallyfloyd at mac.com> wrote:
>
> Actually, the *real world* contains users whose behavior is a function
> of congestion and download times experienced so far.  And in the real
> world (with current TCP), users over connections with longer RTTs
> have much slower download times that users over connections with
> shorter RTTs.  And therefore will download less.
>
> But since our simulations and experiments don't yet have user
> behavior sensitive to past congestion and to past download times,
> this doesn't happen in our simulations and experiments...

True.  However, we can easily model "users with long RTTs choose to
download less" in a way which doesn't need their behaviour to reflect
actual experience.  We can just choose the load at each RTT.

> > Yes, the long flows will have more unfilled demand.  However, if the
> > simulation has been run long enough, the unfilled demand of the
> > remaining flows will be a small fraction of the total data.
>
> Yep, if the average load is less than 100%.  If the average load is
> greater than 100%, then the unfilled demand increases and
> increases, the longer we run the simulation, with a lot
> of the unfilled demand from the longer-RTT flows.

True.  In the "better models" paper, were the RTT comparison tests run
at over 100% load?  I would have thought that comparing the RTT
distribution at a load which lets all the traffic through would be the
natural setting.

> For me, the reason to take measurements over the second half
> of the experiment is to avoid the odd and atypical period in the
> beginning of the simulation when all flows are slow-starting at
> the same time.

Yes, that is certainly the biggest artefact to avoid.

> But personally, I am perfectly happy to run
> simulations for finite, specified time periods when the average
> load is greater than 100%, and there is no equilibrium.
> (In fact, I think it is probably quite necessary, if one wants scenarios
> with higher levels of congestion over the lifetime of the simulation.)

OK.  I'll get back to you on this when/if I try some simulations which
start in equilibrium...

> Yep, I am happy with the  current text.

Great.

Cheers,
Lachlan

-- 
Lachlan Andrew  Dept of Computer Science, Caltech
1200 E California Blvd, Mail Code 256-80, Pasadena CA 91125, USA
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http://netlab.caltech.edu/~lachlan