[73849] in North American Network Operators' Group
Re: Internet speed report...
daemon@ATHENA.MIT.EDU (Simon Leinen)
Mon Sep 6 15:55:02 2004
To: Mikael Abrahamsson <swmike@swm.pp.se>
Cc: nanog@merit.edu
From: Simon Leinen <simon@limmat.switch.ch>
In-Reply-To: <Pine.LNX.4.44.0409061636320.12238-100000@uplift.swm.pp.se> (Mikael
Abrahamsson's message of "Mon, 6 Sep 2004 16:37:41 +0200 (CEST)")
Date: Mon, 06 Sep 2004 21:54:21 +0200
Errors-To: owner-nanog-outgoing@merit.edu
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.
