PMTU Discovery support

[Bruce M. Simpson]

Kevin Lahey wrote:
>
> The boxes were running FreeBSD-6.1, but I can't really vouch for the
> particular kernel configuration.  It could well be that the problem is
> with the loose nut behind the wheel, rather than with FreeBSD. :-)
>   

I believe PMTU measurements may only be relied upon for active TCP 
connections, but it's been a while since I read this code.

It would be useful if non-TCP drivers such as gre(4) could be extended 
to perform PMTU discovery and auto-tune their MTU based on this, as 
manually setting the MTU is a bit random and can result in horrible 
fragmentation when going across the big-I Internet.

I imagine doing this would require changes to the icmp input path and a 
bit of abstraction.

Regards,
BMS