Re: Smallest Transit MTU

[Tony Rall <trall () almaden ibm com>]

On Wednesday, 2004-12-29 at 17:04 EST, Joe Abley <jabley () isc org> wrote:
> On 29 Dec 2004, at 16:33, Tony Rall wrote:
> > But that only affects tcp traffic - it does nothing to help other
> > protocols.
>
> Are there any common examples of the DF bit being set on non-TCP
> packets?

Common?  It depends on what you're doing.  To some people, the only common 
application is 80/tcp. 

Remember that the DF bit is in the IP header - it can be on in any 
protocol.  I know that AIX and my old RH Linux (at least) defaults to 
PMTUD enabled for tcp and udp.  You can even see it in dns lookups.

> > The better solution is to ensure that PMTUD works correctly for your
> > network, and get on the case of any correspondent or provider for
> > which it doesn't.
>
> Making sure that pMTUd works in your own network doesn't solve the
> problem. You need to make sure it works properly in every other network
> with which you ever might want to exchange a 1500-byte packet.

I thought that's what I just said.

Tony Rall