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.