[RFCv2] Babel: add v4viav6 support

[Toke Høiland-Jørgensen]

Andreas Rammhold <andreas at rammhold.de> writes:

> This implements [draft-ietf-babel-v4viav6] an IPv4 via IPv6 extension
> to the Babel routing protocol that allows annoncing routes to an IPv4
> prefix with an IPv6 next-hop, which makes it possible for IPv4 traffic
> to flow through interfaces that have not been assigned an IPv4 address.
>
> The implementation is compatible with the current Babeld version (the
> relevant changes can be seen in the [babeld PR]). I've verified this
> with a few VMs in the following setup:
>
> Bird <- v4 only -> Bird <- v6 only -> Babeld <- v4 only -> Babeld
>
> Each routing daemon was running on their own VM and had L2 connectivity
> to only its direct neighbors. At the nodes at the edges v4 networks have
> been announced and full end-to-end communication was possible over the
> mixed AF network. The v6 only link between Babel and Bird (at the
> "center" of the above setup) did transport the v4 packets via the v6
> link-local next hop addresses just as expected.
>
> Thanks to Toke Høiland-Jørgensen for early review on this work.
>
> [draft-ietf-babel-v4viav6]: https://datatracker.ietf.org/doc/draft-ietf-babel-v4viav6/
> [babeld PR]: https://github.com/jech/babeld/pull/56

Thank you for the respin! Code LGTM; only comment is that it would be
nice if the documentation was changed to refer to the proper RFC number
instead of draft-ieft-babel-v4viav6; but that can be fixed up when
applying (after the number gets assigned hopefully soonish).

Also, one small nit regarding the comment you left in:

> +  /*
> +   * When receiving requests, AEs 1 (IPv4) and 4 (v4-via-v6) MUST be
> +   * treated in the same manner: the receiver processes the request as
> +   * described in Section 3.8 of [RFC6126bis].  If an Update is sent, then
> +   * it MAY be sent with AE 1 or 4, as described in Section 2.1 above,
> +   * irrespective of which AE was used in the request.
> +   */

This is a bit long, especially when repeated in two places; also the ref
to RFC6126bis is outdated. But maybe just shorten it to:

/* RFCXXXX section 2.3: When receiving requests, AEs 1 (IPv4) and 4
 * (v4-via-v6) MUST be treated in the same manner.
 */

With those comments, please consider this:

Acked-by: Toke Høiland-Jørgensen <toke at toke.dk>

-Toke