Re: Internet speed report...

[Simon Leinen <simon () limmat switch ch>]

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.