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

[Juliusz Chroboczek]

> While you're absolutely right that this MUST NOT happen, in practice it does.

I think we're in at least partial agreement.  The point I'm making is that
this configuration is not something that's supported by IP, and that VPN
implementations that cause MTU blackholes are quite simply buggy.

  (There's an argument to be made that IPv6 should support variable MTU
  links.  Good luck pushing this idea at the IETF, which, of late, appers
  to be mostly interested in breaking the e2e principle and proxying
  everything at the application layer.  Sorry for the rant.)

Of course, in practice misconfiguration happens, and so it's a good thing
to be able to be able to automatically detect misconfiguration and discard
the link.  It would be even better to be able to notify the network
administrator of the issue, but that would be a little more work than I'm
willing to do right now.

(For example, we could send Hellos in a small packets, in order to
discover neighbours, and then send a small number of Ack Requests padded
to MTU to every discovered neighbour.  If a neighbour never answers the
Ack Request, then it's fairly strong evidence that there's something wrong.)

-- Juliusz