[e2e] link between Kelly's control and TCP's AIMD

Ted Faber faber at ISI.EDU
Fri Feb 18 10:33:43 PST 2005

On Fri, Feb 18, 2005 at 08:02:35AM +0000, Jon Crowcroft wrote:
> so the idea of packet conservation and ack clocking was completely
> there in Van's original 88 paper. the continuous control theoretic
> model  was also pretty much there - and is partly from the early DEC
> work but i would guess that if you look at Davies work and some of the
> early french packet switched net work, there was some analogy with
> fluids already in the back of peoples minds.

I said "at least."  I'm not asserting that they were the first, just
that the ideas and analysis have been around for quite some time.  I
wouldn't claim to be definitive without digging back around in my
congestion library.
> I guess the interesting thing is how much work is based on something
> which for a long time was pretty poor approx (the percentage of flows
> that make it into a steady state being so low for ages, that the
> notion that each new packet is only sent in because an ack came out
> was kind of fantasy land) - when Van did this mid to late 80s, the net
> was full of long usenet and ftp transfers - by early 90s, it was full
> of tiny intsy web transfers - nowadays, i guess we may be nearly back
> in the regime where  there's a reasonable percentage of longish (p2p)
> tcp transfers although the ratio of access link speed to core is so
> huge, that from the core perspective, most tcp flows are pretty tiny
> things - its only really at the inter-pop traffic that the aggregate
> flow might look like something fluid approximations work for...but
> then it aint a TCP AIMD process, its an aggregate of a set of flows
> starting and stopping which is quite a different thing...

Those darn transients and corner cases sure make thing interesting when
the're neither so transient nor so uncommon, that's for sure.  If you're
arguing that there's still a lot to do in congestion control, I
certainly agree. :-)  (And I share your surprise that such a gross
approximation still yielded significant improvements.)

> as steven hand just said to me - reasoning by analogy is like trying
> to fix a car with a leaky screwdriver

And yet without some analogies to guide us, people don't do very well in
exploring the unknown.

Ted Faber
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