I've been watching all the discussion of different TCP flavours with a
certain amount of disquiet; this is not because I think working on
improvements to TCP are bad; in fact, it is clear for wireless we could
do with improvements in algorithms. I'm not trying to discourage work
on this topic.
My disquiet is otherwise; it is:
0) the buffers can be filled by any traffic, not necessarily your
own (in fact, often that of others), so improving your behaviour, while
admirable, doesn't mean you or others sharing any piece of your won't
suffer.
1) the bloated buffers are already all over, and updating hosts is
often a very slow process.
2) suffering from this bloat is due to the lack of signalling
congestion to congestion avoiding protocols.
OK, what does this mean? it means not that we should abandon improving
TCP; but that doing so won't fundamentally eliminate bufferbloat
suffering. It won't get us to a fundamentally different place, but only
to marginally better places in terms of bufferbloating.
The fundamental question, therefore, is how we start marking traffic
during periods when the buffers fill (either by packet drop or by ECN),
to provide the missing feedback in congestion avoiding protocol's servo
system. No matter what flavour of protocol involved, they will then back
off.
Back last summer, to my surprise, when I asked Van Jacobson about my
traces, he said all the required proof was already present in my traces,
since modern Linux (and I presume other) operating systems had time
stamps in them (the TCP timestamps option).
Here's the off the wall idea. The buffers we observe are often many
times (orders of magnitude) larger than any rational RTT.
So the question I have is whether there is some technique whereby
monitoring the timestamps that may already be present in the traffic
(and knowing what "sane" RTT's are) that we can start marking traffic in
time prevent the worst effects of bloating buffers?
- Jim
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