North American Network Operators Group

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  • From: Perry Lorier
  • Date: Tue Mar 27 20:55:56 2007

Jim Shankland wrote:
<[email protected]> 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:

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.