Re: [RFC PATCH 0/4] Introduce per-peer MTU setting

["Toke Høiland-Jørgensen" <toke@toke.dk>]

leon@is.currently.online writes:

> From: Leon Schuermann <leon@is.currently.online>
>
> This patch series is an attempt to integrate a per-peer MTU setting
> into WireGuard. With matching changes to the wireguard-tools,
> individual MTU values can be set and retrieved for each registered
> peer.
>
> While Linux supports setting an MTU metric for specific FIB route
> entries [which I've only found out after implementing this :)], and
> thus allows to lower the MTU for individual peers, this appears to
> disable regular path MTU discovery (PMTUD) entirely on the
> route. While regular PMTUD does not work over the tunnel link, it
> should still be usable on the rest of the route.

I'm not sure I understand the use case? Either PMTUD works through the
tunnel and you can just let that do its job, or it doesn't and you have
to do out-of-band discovery anyway in which case you can just use the
FIB route MTU? Or what do you mean by "usable on the rest of the route"?

> Furthermore, with the goal of eventually introducing an in-band
> per-peer PMTUD mechanism, keeping an internal per-peer MTU value does
> not require modifying the FIB and thus potentially interfere with
> userspace.

What "in-band per-peer PMTUD mechanism"? And why does it need this?

-Toke