[Tmrg] Round-table PFLDnet submission

[Sally Floyd]

Cesar -

> I think the general scenario could explore a good sub-space of
> parameters that fit in a 3-day simulation. I believe that's the right
> thing to do.

My assumption was that the plan was to come up with a basic set of
tests that could be run in simulations over two to three days, and
that could be run by an experimenter in a testbed with a reasonable
amount of effort, with the basic set of tests including not only
the general scenario (Part A of the draft paper) but also
delay/throughput tradeoffs (Part B), convergence times (Part C),
transients (Part D), impact on TCP traffic (Part E), intra-protocol
fairness (Part F), and multiple bottlenecks (Part G).  Many of the
more delailed scenarios, such as the exploration of transients,
might be able to be run is a fairly moderate amount of simulation
time;  e.g., exploring a very small subset of the parameter space
for the exploration of transients might be just fine.

My suggestion for the "general scenario" would be that we choose
the types of congested links to be explored (some characterized
purely by bandwidth, with others characterized more broadly as
"congested satellite link" or "congested data center link", as I
said in my earlier email), and that for each congested link, with
the dumbbell topology, we explore that subset of the parameter space
that seems somewhat realistic, *and* that seems most likely to cause
problems or to illustrate a new set of tradeoffs for proposed
congestion control mechanisms.  In particular, for each type of
congested link, I would suggest that we explore a range of levels
of congestion (as that is a fundamental parameter to explore for
congestion control mechanisms).

For *some* of the types of congested links in the general scenario,
we might want to vary the range of RTTs, or the queue management
parameters (buffer size, packets or bytes, AQM or Drop-Tail), or
the parameter for the heavy-tailed distribution for the transfer
sizes, or something else, but given the desire for a core set of
simulations/experiments that can be run in a reasonable amount of
time, I think we have to leave the task of exploring much of the space
as an open research task for someone else, and in this test suite
explicitly concentrate on those scenarios that have been shown to
be problematic in the past for someone (e.g., for HSTCP, or for
delay-based congestion control protocols, or for very aggressive
protocols, or for very timid protocols, etc.), or that have been
shown to differentiate between different congestion control mechanisms.
And as new scenarios are uncovered by researchers that are somewhat
realistic and that illustrate a new set of strengths or weaknesses
of particular congestion control mechanisms, they can be added to
the core set of scenarios.

In particular, my own experience is that two or three days of
simulation time is not really all that much, and that we are going
to have to be rather draconian in fashioning a core set of tests
that can all be run by a researcher in a few days of simulations.
The basic set of tests to be run in test-beds might be able to be
much larger that the set of tests to be run in simulators (particular
the subset of tests that explore a high-bandwidth congested link).
For the "general scenario" in Section A, it might be necessary
pretty early on to propose different subsets of tests for simulations
and for testbeds, and to try to prune the proposed tests for simulations
down to the essential subset.

Take care,
- Sally
http://www.icir.org/floyd/