Internet speed report...
Simon Leinen
simon at limmat.switch.ch
Mon Sep 6 19:54:21 UTC 2004
Mikael Abrahamsson writes:
> On Mon, 6 Sep 2004, Simon Leinen wrote:
>> Rather than over-dimensioning the backbone for two or three users
>> (the "Petabyte crowd"), I'd prefer making them happy with a special
>> TCP.
> Tune your max window size so it won't be able to use more than say
> 60% of the total bandwidth, that way (if the packets are paced
> evenly) you won't ever overload the 10GE link with 30% background
> "noise".
Hm, three problems:
1.) Ideally the Petabyte folks would magically get *all* of the
currently "unused bandwidth" - I don't want to limit them to 60%.
(Caveat: Unused bandwidth of a path is very hard to quantify.)
2.) When we upgrade the backbone to 100GE or whatever, I don't want to
have to tell those people they can increase their windows now.
3.) TCP as commonly implemented does NOT pace packets evenly.
If the high-speed TCP
1.) notices the onset of congestion even when it's just a *small*
increase in queue length, or maybe a tiny bit of packet drop/ECN
(someone please convince Cisco to implement ECN on the OSR :-),
2.) adapts quickly to load changes, and
3.) paces its packets nicely as you describe,
then things should be good. Maybe modern TCPs such as FAST or BIC do
all this, I don't know. I'm pretty sure FAST helps by avoiding to
fill up the buffers.
As I said, it would be great if it were possible to build fast
networks with modest buffers, and use end-to-end (TCP) improvements to
fill the "needs" of the Petabyte/Internet2 Land Speed Record crowd.
--
Simon.
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