[e2e] Reacting to corruption based loss
weddy at grc.nasa.gov
Tue Jun 28 07:53:30 PDT 2005
On Tue, Jun 28, 2005 at 02:12:40AM +0200, Detlef Bosau wrote:
> I see the point. But one question remains (admittedly, I did not yet
> read the paper, therefore I apologize if you have given the answer
There is much better information in the paper than this email provides,
but I'll try to answer anyways :).
> How do you achieve _fairness_, when beta may vary?
The paper splits this into two questions, so that it makes more sense:
1) Are a bunch of competing CETEN flows "fair" to each other?
2) Are CETEN flows "friendly" to competing "legacy" TCP flows?
Define fair to mean "equal sharing of resources", and define friendly (in
a way that's a bit different from what TFRC uses) to mean "doesn't reduce
the throughput of a normal competing TCP flow any more than another normal
TCP flow would." In other words, by fairness, we mean to say that the
enhanced TCP only gains performance improvements from utilizing unused
link capacity, not by stealing from competing flows.
The answers given to these questions in the paper are:
1) Yes, in fact, at high error rates, CETEN flows are more fair to each
other than normal TCP flows are. Under CETEN, each flow has it's own
floating point value for beta that's computed from observations of its
own TCP behavior and some hints on error rates observed by routers; so
it's safe to say that few flows have the same beta, although for most
long-lived flows the beta values should be fairly closely grouped. The
paper has experimental (simulation-based) evidence that acceptable
fairness can be acheived even if beta isn't totally uniformly
2) The paper's answer to this comes from several simulations over
varying levels of bottleneck saturation where the total number of flows
is kept constant, but the ratio of CETEN to stock TCP flows is varied.
We see that in normal cases, at high numbers of CETEN flows, the
throughput that the stock flows' throughput is not significantly lower
than it is with no CETEN flows. So, CETEN is friendly to stock TCP.
However, we also show that it is possible to construct edge cases where
the CETEN flows starve out normal TCP flows.
Wesley M. Eddy
Verizon FNS / NASA GRC
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