[PATCH] Babel: add RFC9229 (v4 via v6) support

[Juliusz Chroboczek]

> btw, there is one question that i noticed. If an Update is ignored for
> semantic reasons (e.g. update with valid metric, but missing next hop or
> router id), should it update last prefix with P-flag?

Such a packet would be incorrect.  What to do in presence of an incorrect
packet is left to the implementation.  Babeld will log an error and stop
parsing the packet at this point and discard all remaining TLVs.  Other
solutions would include dropping the whole packet, dropping the whole
adjacency, or even adding the neighbour to a blacklist of buggy neighbours.

> RFC says that for sub-TLV errors but not for other errors:

A packet with an unknown sub-TLV is correct, it's just that the
implementation doesn't know how to interpret this particular sub-TLV.  The
behaviour is well-defined in this case (drop the sub-TLV if the mandatory
bit is unset, drop the TLV otherwise).  This is unlike the case described
above, which indicates a buggy neighbour.

-- Juliusz