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

[Juliusz Chroboczek]

> Problem is when the underlay L3 network is composed of more tunnels and not
> 1500 MTU ethernet links, then at each hop the path MTU could be reduced by
> the tunnel overhead again and again and again (across the entire
> path). Hence no predictable MTU I can deploy across all my interfaces
> exists. QED :)

I'm still not following.  Every tunnel has an MTU, right?  And every
tunnel is able to carry packets up to its MTU?  If that's not the case,
then there's no way your network can work, since IP does not support
variable MTU links.

> Enable a config option for "minimum path MTU" on each babel node. Nodes
> then pad all hello packets to this size and set appropriate sockopts to
> stop the kernel from doing PMTUdisc behind our backs (on IPv6) and setting
> DF=1 (on IPv4).

We can only control fragmentation in the overlay.  Can you explain what
the tunnelling protocol will do, and whether it will prevent fragmantation
in the underlay?

-- Juliusz