[RP-PPPoE] (pppoe manual page) MTU/MSS confusion

[Jens Stimpfle]

Hi David,

On Thu, Jul 03, 2014 at 02:10:09PM -0400, dfs at roaringpenguin.com wrote:
> > I think there's some confusion of MTU and MSS in the pppoe(8) manual
> > page. Find below a patch for the current (3.11) version.
>
> No, there's no confusion.  The original wording is correct.

If the wording is correct, what puzzles me is why does it say the
firewalled machines should have an MTU of 1412? I don't see why they
should have an MTU lower than 1492. And 1412 is actually the recommended
MSS value for the -m option. (Which might be not low enough because TCP
and IP headers can both be up to 60 bytes. So I think it's only safe
below 1372).

> TCP picks the MSS itself; you can't control it.  All you can do is
> set the MTU of the interfaces on the machines *behind* the firewall so
> that Linux picks a suitably-low MSS.

Well so you can control it no? That's how I tried to understand the
manpage but it seems I interpreted too much.

> The notes about MTU/MSS are probably obsolete, actually.  Almost
> everyone
> nowadays uses the MSS clamping feature, either in pppoe itself using
> the Linux iptables "--clamp-mss-to-pmtu" or "--set-mss" option.

This is what I do as well, and it's why I proposed adding a reference to
the -m option at the two locations.

Jens