[CentOS] Sendmail and pmtu discovery

Ralph Angenendt ra+centos at br-online.de
Tue Oct 14 18:56:25 UTC 2008


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?

Fragmentation is bad. That's why you do PMTUD - to see which is the lowest
MTU in the path. You then set your packet sizes accordingly and set the DF
bit. If the lowest MTU in the path changes to an even lower one you get an
error and can continue with smaller packet sizes.

If you disallow PMTUD - well, you're asking for trouble >:)

<http://www.znep.com/~marcs/mtu/> has a rather good discussion about that.

Ralph
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