Why the temptation for dial users to crank back rwin/mtu?

Forrest W. Christian forrestc at iMach.com
Sat Feb 6 18:11:31 UTC 1999

On Sat, 6 Feb 1999 mcmanus at appliedtheory.com wrote:

> apparently there's some performance value in this (at least to the
> immediate user) because they keep doing it in droves. It's not obvious
> to me why the heck this would be. (warning: I am a protocol guy, but
> I'm not a dialup guy at all.. and even less of a windows guy)

This has been generally beat to death on nanog in the past.  If you
weren't around back then, dig around in the archive.   I remember one of
the subjects being "PC Bozoworld strikes again" or something like that.

The short recap is that for some unknown reason the Microsoft TCP/IP stack
is broken in some bizzare way that setting down the MTU on a good chunk of
the machines out there will result in a dramatic speed increase.   Why
this occurs, I'm not sure anyone really knows.   It would be really
interesting to see a study of what the MS stack is doing and why it's

> MTU - at least this makes a little bit of sense.. If they're doing
> HTTP/1.0 stuff with parallel connections then a smaller MTU is going
> to make that parallelization latency much more effective and perceived
> performance will go up some.. it doesn't impact full document

Just for my information, does the MTU setting affect <received> packets
in some way?   My understanding was that a machine wouldn't send packets
over the MTU size, but could recieve anything up to whatever the TCP/IP
stack writer included in the stack.  Guess I'll have to go dig out the

- Forrest W. Christian (forrestc at imach.com) 
iMach, Ltd., P.O. Box 5749, Helena, MT 59604      http://www.imach.com
Solutions for your high-tech problems.                  (406)-442-6648

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