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

[David Schinazi]

On Wed, Jul 26, 2023 at 5:18 AM Juliusz Chroboczek <jch at irif.fr> wrote:

> > 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.
>

Agreed.

  (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.)
>

(As a proxy enthusiast, I have thoughts :P. In my view, the e2e principle
as we knew it broke when people started deploying TCP "accelerators".
We brought back transport-layer e2e with QUIC thanks to e2e encryption.
So in my view, QUIC is e2e but TCP, UDP, and IP are not. In that world,
CONNECT-UDP allows you to maintain e2e because it allows QUIC.
Sorry for the rant reply, but I couldn't resist)

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.


Definitely. Thanks for implementing and deploying that by the way.


> 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.
>

babeld automatically emailing sysadmins sounds like a fun time :-)

(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.)
>

(You could even perform dichotomy there to measure the exact MTU and update
the OS link MTU based on that, but I agree that's not necessarily babeld's
job.)

David
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