North American Network Operators Group
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Re: MTU of the Internet?

  • From: Phil Howard
  • Date: Thu Feb 05 13:31:16 1998 
Jeff Stehman writes...

> On Wed, Feb 04, 1998 at 01:18:56PM -0500, Dennis Simpson wrote:
> > 
> > We recommend that clients who care about interactive response use small
> > MTUs, and clients who care about download speed use higher MTUs.
> 
> There's an extremely annoying potential gotcha in having clients set
> lower MTUs.  At least one release of Netscape's web server set the
> Don't Fragment bit.  In the few cases we've seen, if there was not a 
> 1500 MTU pipe between server and client, the server could be reached,
> but no HTML would be downloaded.  Usually it's easier to work around
> the problem on the client end than convince the server admins they
> might want change things on their end.

If the client sets the MTU lower, that becomes the connection MTU and
MTU discovery doesn't take place.  The problem only occurs if there is
a router in between (not the client) that has a lower MTU than what the
connection is using, and hence the fragmentation (or discarding if DF
is on) occurs.

There's probably a whole thread due on whether DF should be set or not.
But there really are links that are smaller than 1500, such as SLIP at
1006.

IMHO setting DF should not be allowed where the MTU is greater than 576,
or whatever number today constitutes the "minimum reasonable requirement"
which I would say isn't larger than 1006.  Maybe in a few years we can
kiss SLIP bye-bye and make sure everything is 1500.

Netscape's web server is broken.  And in more ways than just the DF thing.

But then that begs the question, why 1500?  Why not 4000?  Or 32000?

-- 
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