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

[David Schinazi]

Hi Juliusz,

While you're absolutely right that this MUST NOT happen, in practice it
does. A rare scenario is when routes change deep in a network causing the
e2e PMTU to change without the link MTU on the endpoints observing any
change. This phenomenon happens much more commonly on tunnels when the
tunnel takes a new path (e.g., moving IKEv2/IPsec to a different underlying
interface via RFC 4555) - in that scenario the endpoint experiencing the
migration (e.g. the cell phone) knows that something changed but the e2e
peer does not. In IPv4 this can be (poorly) solved by in-network
fragmentation, but that's not allowed in v6.

If Babel were to magically know the MTU of its interfaces (including
tunnels), it would make sense to consider that information as part of route
metrics. The remaining question is where to perform the PMTUD, it feels
like the responsibility of the tunnel but could also be reused across
different tunnel types.

[ ... an hour passes by with this email half written ... ]

Oh, and in the meantime Juliusz just went ahead and implemented probe-mtu.
Nicely done, sir! Looking at the PR it validates that the kernel-provided
MTU gets through the network. I wonder if that breaks popular tunnel
implementations today, as I suspect many don't set that correctly.

David

On Tue, Jul 18, 2023 at 1:42 PM Juliusz Chroboczek <jch at irif.fr> wrote:

> >> RFC 2460: "link MTU - the maximum transmission unit, i.e., maximum
> packet
> >>            size in octets, that can be conveyed over a link."
>
> > I read this as "link MTU" being the maximum packet size that you could
> ever
> > hope to be able send but the link technology could very well not allow
> the
> > maximum at times.
>
> Daniel, the specs are perfectly clear: there is no licence given to nodes
> to systematically drop packets smaller than MTU.  In fact, such links
> break TCP, as you've discovered.
>
> > I'm still not sold on your argument, but it hardly matters. Tunnels on
> top
> > of the internet exist so we kind of just have to deal with it.
>
> Nobody is denying that.  Please see RFC 4459, which describes how to make
> them work reasonably well.
>
> -- Juliusz
>
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