Where to control [was Re: [e2e] TCP in outer space]

Steven Low slow at caltech.edu
Sat Apr 14 15:49:06 PDT 2001


While I agree that routers should do more to help contorl congestion,
I believe they should *not* determine source rate, as done
in ABR Explicit Rate.   Rather, a router should figure out the right
measure of congestion and feed back this information to sources that
use this router.   A source, knowing the congestion information
on its *path*, is in a better position to determine its rate.   A
potential bonus of this approach is that
sources with different valuation of bandwidth can adjust
their rates differently even when the congestion on their paths is
"identical".   For example, they can choose utility functions that
are tailored to their applications, and use that to adjust their
rates.   In summary, a router only knows how congested it is,
and this is what it should inform the source about.   This is
necessary and sufficient for a source to figure out its rate.

All these can be made precise, and it can be shown that
this approach can be made optimal, stable, and robust as network
scales up *arbitrarily* in delay, capacity and load.


> The point is that router/switch code can do far more these days than ever
> imagined when the decision to offload performance and capacity decisions from
> 'gateways' (routers) was made years ago.  The corollary is that this is not a
> surprising reality.  So, for example, rather than simply using the hardware
> RED capability now available to drop packets, use it to generate a more
> intelligent control statement to the sender.  Source Quench and its original
> purposes have been discussed, but consider that intelligent folks might even
> go further -- let a little of this processor-cycle wealth be directed at the
> network-layer without tricking the assumed transport, which is not the source
> of all the traffic.  This is, of course being discussed.

Steven Low, Assoc Prof of CS & EE 
slow at caltech.edu			netlab.caltech.edu
Tel: (626) 395-6767			Caltech MC256-80
Fax: (626) 792-4257			Pasadena CA 91125

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