No subject

Wed Mar 28 00:53:46 UTC 2007

Jim Shankland wrote:
> <michael.dillon at bt.com> writes:
> 
>> Use GigE cards on the servers with a jumbo MTU and only buy IP network
>> access from a service provider who supports jumbo MTUs end-to-end
>> through their network.
> 
> I'm not sure that I see how jumbo frames help (very much).  

Jumbograms don't change your top speed, but they do mean you acclerate 
through slow start more quickly.  If there is non-congestion based 
packet loss on a link you can end up with slow start being stopped 
early, and waiting for linear increase which can mean it will take hours 
to reach steady state instead of minutes.  Jumbograms reduces this by a 
factor of 6 which of course helps (60 minutes -> 10 minutes...).

<snip other good advice>

> At 45 Mb/s and 120 ms RTT, you need to be able to have ca. 700 KBytes
> of data "in flight"; round up and call it a megabyte.

I have written a calculator to help people explore these issues:
http://wand.net.nz/~perry/max_download.php

It also includes TFRC to show how non-congestion-related packet loss 
impacts your performance too (got a dodgy wireless hop there somewhere? 
Well expect everything to be glacially slow...)

> Having said that, I too have tried to configure Windows to use
> a large send buffer, and failed.  (In my case, it was Windows
> machines at a remote location sending to Linux machines.)
> I'm not a Windows person; maybe I didn't try hard enough.  In
> the event, I threw up my hands and installed a Linux proxy server
> at the remote site, appropriately configured, and went home happy.

I've never really been a windows guy either and I've never had a windows 
machine in a position that it needed to be tuned.  Of course most of the 
tuning is just upping the rwin.  Apparently Vista has a larger default 
rwin, and an optional "Compound TCP" congestion control system designed 
for use over high bandwidth delay WAN links if upgrading windows is an 
option.