[Babel-users] MTU based routing for tunnel based babel networks?

[Juliusz Chroboczek]

>> Sorry, I wasn't clear.  IP requires every link to have a well-defined
>> MTU: all the nodes connected to a link must agree on the link's MTU.

> I don't think that can be true either. PMTU can vary and paths can be
> asymmetric so two nodes could very well see different MTUs across the
> internet. There's just not many ASen that run with less than 1500 MTU :)

I'm not speaking about PMTU.  I'm speaking about link MTU.

> Do you have a referece for this "MTU well-definedness" criteria, I don't
> think I ever heard of this.

RFC 2460: "link MTU - the maximum transmission unit, i.e., maximum packet
           size in octets, that can be conveyed over a link."

RFC 4861: "All nodes on a link must use the same MTU (or Maximum Receive
           Unit) in order for multicast to work properly."

> Wireguard drops packets when they exceed the underlay network's MTU.
> this happens no PTB ICMP errors are generated by wireguard inside the
> tunnel,

If true, that's very surprising, and looks to me like a bug in Wireguard.

But yeah, I'll add an option to probe for MTU on each Hello.

-- Juliusz