[e2e] Historical question: Link layer flow control / silent discard

Noel Chiappa jnc at mercury.lcs.mit.edu
Sat May 25 08:41:15 PDT 2013

    > From: Bob Braden <braden at ISI.EDU>

    > The following sentence from that document says it all;
    >    "All of our algorithms oscillate, some worse than others."

That just means we didn't have a good enough control theorist working with
us! :-)

(My memory of control theory is very dim now, but I seem to recall that _any_
system with feedback will oscillate unless damped properly... and usually
even when damped, there is _some_ oscillation, it's just contolled to the
point that it's not harmful, but instead is helpful, as 'probing'.)

But seriously, I do think we probably still don't really understand if SQ
would work or not. (Although the ECN mechanism explores a somewhat similar
space, it depends on the packet getting to the ultimate destination, and then
back, successfully, to get the congestion signal back to the originating

    > Sending a packet when there is an overload must be a losing strategy!

But the SQ travels over a path which is not (yet) congested...

    > From: John Day <jeanjour at comcast.net>

    > I define flow control as feedback co-located with the resource being
    > controlled and congestion control as feedback not co-located with the
    > resource

That's an interesting - and perhaps useful - distinction, but I fear we are
trying to put too many meanings onto too few terms.

I still maintain that because of the grubby low-level engineering
distinctions (different causes, different mechanisms, etc) it's useful to
distinguish between end-end control at the transport layer, and indications
of congestion on the path, at the internetwork layer.

I guess I was committing the same sin as you, in re-purposing existing terms
(flow and congestion control) to the cases I was interested in distinguishing.

Maybe instead we need a whole new taxonomy/terminology for this area?


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