Re: [CentOS] Sendmail and pmtu discovery

On Tue, October 14, 2008 12:31, Kai Schaetzl wrote:

Ralph Angenendt wrote on Tue, 14 Oct 2008 17:24:08 +0200:

...

If you don't know the smallest MTU on the path to the mail server, you might not be able to send packets over that path, especially if DF is set.

But if it's not set? Shouldn't most devices have it not set?

Yes, most devices should fragment if necessary (DF not set).

Most devices should also pass/accept ICMP messages relating to their connections. Deliberately configuring them not to is asking for trouble; those messages are part of the protocol for a reason.

(Fragmentation introduces more work and effectively many more lost packets in most setups, so the flow will be jumpy and less efficient even if it mostly works.)