[e2e] congestion collapse definition

Noel Chiappa jnc at mercury.lcs.mit.edu
Tue Sep 8 20:24:22 PDT 2009

    > From: rick jones <perfgeek at mac.com>

    > It was my understanding that congestion collapse was defined based on
    > the effective throughput (goodput?) dropping to epsilon while the link
    > utilization was 100%.

Well, I think that general concept may be a good thing to have in there (the
link getting little real work done, even though its utilization is high), but
two caveats.

First, I do think the definition of CC needs to say something about what's
_causing_ that situation (because it's probably possible to have other causes
of epsilon goodput with 100% utilization) - and for CC, it is 'excess offered
load' that is the cause.

Second, I'm not sure it has to be as low as epsilon; I'd say that a link
which which is 100% used, but only 20% goodput, is in in something very close
to congestive collapse. OK, so maybe 'total congestive collapse' is epsilon
goodput, but then what is, say, only 20% goodput? Maybe we should just throw
a generic term at it, rather than a specific level; perhaps something like
say 'little or no productive throughput with high utilization'.

    > I would assert, without any additional substantiation, that unless the
    > effective throughput goes to epsilon, it isn't "congestion collapse"
    > and one would indeed need another term.

Now that I think about it, David's scenario is quite interesting, because
it's not clear that any packets are being uselessly retransmitted or
discarded. In other words, in David's case, goodput may actually be quite
high (as in, all the packets will eventually be accepted and processed at
their destination); it's just that the packets are _so_ delayed that the
human (or automaton) users have given up.

So once again we circle back to the definition of 'the offered load is so
high, above and beyond the capacity of the network, that the network can't
provide a useful level of service'. But is that 'congestive collapse', or
just plain old 'way overloaded'?

So maybe we need to have some definition in terms of queue lengths and/or
drop rates: 'when because of excess offered load, either i) queue lengths get
ridiculously long, or ii) drop rates get very high, the network is in
congestive collapse'.


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