North American Network Operators Group

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Re: MTU of the Internet?

  • From: Marc Slemko
  • Date: Wed Feb 04 14:19:43 1998

On Tue, 3 Feb 1998, Peter Ford wrote:

> Several people have noted to the Microsoft Support and Product groups that
> they want the Windows 95 PPP MTU to be set to 576 (down from 1500).  this
> change is in Windows 98.

Yuck.  Why bother implementing PTMU discovery if you are going to use a
MTU that larger than almost every MTU around?

> The reason for this change cited by many customers is that many ISPs have
> 576 MTUs set "inside" their networks and packets get fragmented.

I really don't buy that.  Many or most backbone links have MTU >1500, and
MTUs <1500 outside of low-speed dialup connections aren't that common.
They are there, yes.  But not that common.

My understanding of why a lower MTU is demonstratable better under Win95
is because the Win95 TCP stack is broken, and it is a good workaround.
Most of the people raving about it are saying they are getting 2-4 times
speed increases from changing their MTU from 1500 to 576.  Something is
very wrong there.  I thought I had heard details about exactly what is
broken in the Win95 TCP stack that causes this problem, but can't recall
them at the moment.  It could have no basis in reality and just be a

There are all sorts of people spouting all sorts of lies around Windows
newsgroups about why small MTUs are good; I think novice users are simply
getting drawn in by supposed experts.  

I guess systems receiving data from servers with broken retransmission
timers (eg. how Solaris used to be) could be helped by lowering the MTU
which would result in faster ACKs so bogus retransmissions won't happen
all the time, but the fix for this really isn't to lower the MTU.

You also get the obvious improvements in interactive performance, and you
start getting data more quickly.

I would suggest that you would be well advised to find a handy user or
four where this effect is easily observable, and find out what is really
going on.