Subject: Re: PPPoE vs ETHERMTU
To: None <>
From: der Mouse <mouse@Rodents.Montreal.QC.CA>
List: tech-net
Date: 07/01/2001 02:58:10
> 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...

Fun fun fun....

> 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

That's a non-issue for me; machines behind the router tunnel
everything, and the tunnel endpoints limit packet sizes so they don't
end up trying to generate oversize Ethernet packets.  (They also
generate need-frag ICMPs correctly, which may or may not be happening
here - it's entirely possible from what I've seen that the DSL provider
is generating oversize Ethernet packets and counting on their provided
hardware and (Wintel) software to unpack them "correctly"...and the
SPARC le isn't - and for some reason the same Ethernet, in the Alpha as
ne0, isn't either, a driver issue perhaps.)

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