Subject: Re: PPPoE vs ETHERMTU
To: der Mouse <mouse@Rodents.Montreal.QC.CA>
From: Martin Husemann <>
List: tech-net
Date: 07/01/2001 08:38:45

This was one of the first obstacles I hit when doing my in-kernel PPPoE
stuff. For quite some time I searched for bugs in my driver...

I have exactly the same problem, but in the opposite direction: if I turn
on PMTU discovery on my mail machine, I can't pop3 sometimes. When I turn
it off, it works.

> I'm considering adding a sysctl to put a cap on advertised TCP MSS
> values regardless of interface MTUs; I can't see any way at present to
> make it advertise a value less than that appropriate for the outgoing
> interface's MTU.

But this won't help connections from other machines behind the router
(I have a few Wintel machines for my wife + kids running there). It will
help machines running NetBSD where you turn this option on.

BTW: the rp-pppoe userland PPP stuff has an option to fiddle with the 
advertised MSS even for connections only routed through this machine.
While this is something no router should do (look beyound IP header),
it may be ok for a NAT box - so an option for this in IPF may be ok.