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<div class="moz-cite-prefix">Am 04.06.2014 10:59, schrieb Jon
Crowcroft:<br>
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<div dir="ltr">I dont think there's anything wrong here, </div>
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"wrong" wouldn't be an adequate word. I think, we have different
goals here.<br>
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<div dir="ltr">but maybe a note on buffer bloat is in order:-
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<div>alongside the feedback/AIMD and ack clocking mechanisms for
tcp, there was a long discussion on right sizing buffers in
the net - since AIMD naively applied led to the sawtooth rate
behaviour in TCP, a back of envelope calculation</div>
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^^^^^^^^^^^^^^^^^^^^^very appropriate for a physicist :-)<br>
Even Einstein did so :-)<br>
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<div> led to the notion that the bottleneck had to have a buffer
to cope with the peak, which at worst case would be
bandwidth*delay product </div>
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and exactly this might be a problem. What is the "delay" then? The
more buffer space you introduce in the path, the greater the delay,
and hence the product, will be....<br>
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<div>worth of packets (basically 3/2 times the mean rate) so
that when 1 more packet was sent at that rate, one loss would
be incurred triggering the MD part of AIMD once every ln(W)
worth of RTTs...[al this is academic in reality for lots of
reasons, including the various other triggers like dupacks and
the fact that this case is a corner one - since usually there
are lots of flows multiplexed at the bottleneck(s) and
multiple bottlenecks, so the appropriate buffer siz could be
way smaller - and of course, any one running a virtual queue
and rate estimater (i.e. AQM a la codel etc) and especially
ding ECN rather than loss baed feedback can avoid all this
rediculsous provisioning of packet memory all ov er the net</div>
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My concern is that I doubt, that this calculation should be done for
the "whole path end to end". And of course, you will perhaps
provide sufficient buffer that the links will work at full load.
Hence, the delay will vary from propagation and serialization delays
only (empty queues) and ./. + queueing delays. Extremely rough-<br>
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<div>but alas, the rule of thumb for a corner case became dogma
for a lot of router vendors for way too long to get it
disestablished....</div>
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what doesn't forbid to ask questions ;-)<br>
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<div>and many of the bottlenecks today are near the edge, and
more common than not, probably in the interface between
cellular data and backhaul, where, as you say, thee radio link
may not exhibit any kind of stationary capacity at all etc etc</div>
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When I got the insight of non stationary radio links fourteen years
ago, I certainly would have less grey hairs than today ;-)<br>
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