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

[Daniel Gröber]

Hi Juliusz,

On Wed, Jul 19, 2023 at 11:25:52PM +0200, Juliusz Chroboczek wrote:
> Could you please test the new branch "probe-mtu"?  It's now using the
> IPV6_DONTFRAG cmsg in sendmsg, so it's enough to say
> 
>     default probe-mtu true
> 
> (No global options, only per-interface options.)

I can observe (some) hellos using the padding depending on the option
setting. Problem is when I force the interface MTU to 1280 instead of the
initial 1420 the padded hellos get dropped and don't reach the other side
as you'd expect, but the regular sized hellos still make it through and so
the neighbourship relationship stays up.

Here's an idea: what if we pad the IHU response instead of all hellos? That
might have slightly less control overhead when RTT isn't enabled as you
don't need to respond to every hello then? I'm not sure how babeld
schedules IHU sending exactly.

--Daniel