[Tmrg] Traffic Generators (Harpoon and Tmix)

[Lachlan Andrew]

Greetings Sally and all,

On 09/12/2007, Sally Floyd <sallyfloyd at mac.com> wrote:
> On Dec 6, 2007, at 8:23 PM, Lachlan Andrew wrote:
> > 1. We agreed at the meeting that the load would be "open loop".  That
> > allows us to specify the offered load in a protocol-independent way.
> > If the traffic is entirely closed-loop then the load depends on the
> > protocols, making comparisons difficult.  (Being open-loop does not
> > preclude modelling the think-time between arrivals within a session.)
>
> Closed-loop models are just as protocol-independent as open-loop models,
> I would say.
> The overall transfer time depends on the protocol used in either case.

The transfer times depends on the protocol in both cases.  However,
the *total* amount of cross traffic depends on the protocol in one
case but not in the other.

With an open-loop model, it is meaningful to talk about "10% cross
traffic", because we specify how much data arrives in what long period
of time.  With a purely closed-loop model, inefficient algorithms will
receive less traffic, because flows arrive less often.  There is AFAIK
no way to specify that a closed-loop model gives x% cross traffic.


> Even if  I was only looking at metrics about the behavior of long-lived
> flows,  I would prefer for the "background traffic" to have user think times
> within TCP connections.  This is more realistic, and increases the burstiness
> of the aggregate traffic in a way that affects all of the competing
> traffic.

There is certainly a good case for making the background traffic as
realistic as possible, all else being equal.  If  Tmix  does all the
hard work and comes complete with representative traces, I'd be happy
for us to specify that cross traffic be non-greedy.

I still don't know what metrics would be meaningful if we're measuring
non-greedy traffic.  This will affect what we do for the cases where
all traffic comes from the traffic generators (such as the  throughput
vs delay  scenarios).


> One of the good areas for future work is for researchers to  say
> "by the way, these results are quite sensitive to parameter X", or
> "these results are not at all sensitive to parameter Y".  It is
> unavoidable,
> I think, that we will have to learn these things as we go along.

Agreed.

> There are two ways to go:
> (1) models where the total load requested in a user session is
> independent of the level of congestion: and
> (2) models where the total load requested in a user session is
> explicitly dependent on the level of congestion.
>
> I assume that the world is like (2).  As far as I know, more traffic
> generators are based on model (1).   We could make an arbitrary
> attempt at model (2), or we could use model (1) and explicitly ask
> researchers to give us model (2) for traffic generation for the future.
> Either one sounds ok to me.

I'd vote for using (1) and explicitly suggesting that it be modified.
It would be a shame to "standardize" an arbitrary model.

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