[Tmrg] Traffic Generators (Harpoon and Tmix)

[Lachlan Andrew]

On 04/12/2007, Constantine Dovrolis <dovrolis at cc.gatech.edu> wrote:
> folks, my apologies for jumping into the discussion

Not at all.  Thanks for your input!

> 1. I want to loudly agree with Sally that we should be
> considering non-greedy TCP flows with heavy-tailed size
> distribution, and
> 2. we should be asking whether these non-greedy TCP flows
> are generated by an open-loop flow arrival process or by a
> closed-loop process that takes user thinking times (and
> perhaps limited patience) into account.

Just to clarify, in point 2, are you suggesting that there are
idle/think times both within and between flows?

I agree entirely that these are all important effects, which should be
included in "version 2" of the test suite.  I have several reason for
supporting the simpler models for the initial "version 1".  I'd be
interested in your thoughts on each.  My strongest concerns are points
2(ii) and 4.

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.)

2. We need to ask what cost/benefit we get from the more complex models.
(i) For some of our tests, this traffic is "cross traffic" which we're
not measuring.  In these tests, the results of Hohn, Veitch, and Abry
(e.g., "The impact of the flow arrival process in Internet traffic")
suggest that structure in the flow arrival process doesn't greatly
affect the packet level traffic.
(ii) For cases where we're going to measure the performance of
non-greedy flows, we need to define metrics for their performance
which reflect the non-greediness.  I don't think such measures are
obvious.  We can't use connection completion times, average rates, ...

3. These tests are not intended to be exhaustive.  As I said before
the meeting, I'd rather the meeting result in one or two
clearly-defined tests than a complete first draft of a test suite
where none of the tests is specified well enough to allow comparisons.

4.  I'm afraid of models with too many parameters which have to be
estimated.  I was under the impression that many studies have found
distributions of *connection* sizes, but many fewer (if any) have
studied the sizes of "bursts" within a connection.  Will it matter if
we get the sizes wrong?

Another point related to parameter estimation is that I'm worried by
the approach we agreed on of assuming that the file-size distribution
is independent of the load, so that the load is simply proportional to
the session arrival rate.  It seems likely to me that higher load
occurs when there is a brief influx of longer connections (say some
BitTorrent users start up), rather than a brief rise in the session
arrival rate.  Could this have as big an impact as the choice of
whether new "bursts" start their own connections or not?

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