IPv6 and PPPoE with MSSFIX

Luiz Angelo Daros de Luca luizluca at gmail.com
Wed Aug 23 19:01:53 UTC 2023

> Hi,

Hi Daniel,

> On Wed, Aug 23, 2023 at 04:58:40PM +0200, Marek Küthe wrote:
> > PPPoE adds 8 bytes of overhead so that an MTU of 1432 can be used. I
> > also have to do this at home with my DSL line for example.
> > The MTU should be set on each side (on both peers) for this to work.
> Oh, I just realized I used the 1432 MTU in my earlier reply based on
> Marek's math but since Luiz's underlay network is IPv6 this is not actually
> correct. MTU=1440 is only correct on top of IPv4, for IPv6 the "optimal"
> MTU is 1420 so with PPPoE involved that's MTU=1412.
>   1500 Ethernet payload
>    -40 IPv6 header
>     -8 UDP header
>    -32 Wg header
>     -8 PPPoE
> ===================
>   1412 wg tunnel MTU

In my case, the PPPoE interface got MTU=1480. They might be stacking
something else on top of it or PPPoE might have optional fields. I
read somewhere that PPPoE might use either 8 or 20 bytes, but I'm not
an expert on PPPoE. If I don't control both sides, I would use 1400 by

> --Daniel

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