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

[Juliusz Chroboczek]

>> IP does not support variable MTU links.
> 
> Excuse me, but that's plain false. IP was designed in an environment where
> (non-ethernet) networks with various MTU standards were commonplace

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.

Now, I agree that it is possible to simulate a variable-MTU link, as
describes in RFC 4459 Section 3.2, and it will mostly work.  But that's
not what IP was designed for, and I don't know whether it's possible to
make it reliable.

> There is a way: My routing protocol just has to stop picking links that are
> obviously going to cause a problem.

Could you please describe the problem in detail?  Because I'm probably
missing something.

If Wireguard implements RFC 4459 Section 3.2, then pushing a too large
packet over the tunnel, then Wireguard should synthesise an ICMP "packet
too large", which will cause the sender to retry with a smaller packet.
Is that not the case?

I'm not opposed to your probing idea, but I'd really prefer to fully
understand the problem first.

-- Juliusz