North American Network Operators Group

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Re: MTU of the Internet?

  • From: Marc Slemko
  • Date: Thu Feb 05 13:01:24 1998

On Thu, 5 Feb 1998, Jeff Stehman wrote:

> 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.

Erm... I think you are getting a few things a bit confused here.

First, I _really_ doubt the web server set the DF bit.  It is almost
certainly the OS, probably trying to do path MTU discovery.

If there is a smaller MTU in the middle, and someone is filtering ICMP
can't fragment errors then yes, you will have trouble with PTMU discovery.
The fix is to not blindly filter ICMP.  Increasing the client MTU won't
fix this.

If the client lowers their MTU, there is no problem because then they
advertise an appropriate MSS and no stack should try sending packets that
it knows will need to be fragmented given the client MSS.