[Tsvwg] RTT fairness

[Frank Kelly <[e-mail]>]

Mark, Sally, Bob

A quick comment prompted by a paragraph in
"Comments on the Usefulness of Simple Best-Effort Traffic"
http://www.icir.org/floyd/papers/draft-floyd-tsvwg-besteffort-00b.txt

The paragraph reads
"RTT-fairness: What is the desired relationship between flow
bandwidth and round-trip times, for best-effort traffic?  As shown in
Section 3.3 of [FJ92], it would be straightforward to modify TCP's
congestion control so that flows with similar packet drop rates
but different round-trip times would receive roughly the same
throughput."

I don't think it would be that straightforward!
There would be an impact on the stability or speed of convergence 
of the control loop: the modification in [FJ92] would, I think, 
lead to instability on routes where the ratio RTT/rate is large, 
or convergence that is slower than necessary on routes
where the ratio is small. More detail on why I think this is in
Section 5.1 of 
"Fairness and stability of end-to-end congestion control"
http://www.statslab.cam.ac.uk/~frank/PAPERS/fse2ecc.html

Frank