Re: [Bloat] Computer generated congestion control

[Jonathan Morton <chromatix99@gmail.com>]

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I think we've seen that before. The headline results are certainly
impressive. But there's a big caveat, which is in fact revealed by the
authors.

Remy works by tailoring the congestion control algorithm to the network
characteristics that it's been told about. If the actual network it's
running on matches those characteristics, then the results are good. The
more specific that information is, the better the results. But if the
network characteristics differ from the information given, the results are
bad - and the more specific the data was, the more likely a mismatch will
occur.

If we simply knew, a priori, what the delay bandwidth product was for a
given connection, we wouldn't need congestion control algorithms in the
first place, as we could simply maintain the congestion window at that
value. That need for a priori knowledge is a fundamental problem with
Remy's approach.

So while existing, hand written congestion control algorithms aren't
perfect, in practice they tend to do a competent job in difficult
circumstances, using the limited information available to them. If
anything, I'd like them to put some sane upper bound on the RTT - one
compatible with satellite links, but which would avoid flooding unmanaged
buffers to multi-minute delays. But when they get an unambiguous congestion
signal, they respond, and so they adapt to the myriad varieties of link
characteristics actually found on real networks.

- Jonathan Morton

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