No subject

Jim Shankland nanog at shankland.org
Tue Mar 27 22:32:34 UTC 2007


<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).  The
principal issue here is the relatively large bandwidth-delay
product, right?  So you need large TCP send buffers on the sending
side, a large (scaled) receive window on the receiver side, and
turn on selective acknowledgement (so that you don't have to
resend the whole send buffer if a packet gets dropped).

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.

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.

Jim Shankland



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