While it is an edge case in our configuration it is technically
possible for a (6Lo) router not to maintain an address resolution state
machine. This fix allows for that with the `gnrc_ndp` module.
Otherwise, it may happen that `::` or a global address is chosen by
the IPv6 header fill function. Both types of addresses are
[not valid for RAs](https://tools.ietf.org/html/rfc4861#section-4.2)
Since gcc-7 `Wimplicit-fallthrough` is activated by using `-Wextra`.
This leads to the following problem when compiling `gnrc_networking`:
```
RIOT/sys/net/gnrc/network_layer/ndp/internal/gnrc_ndp_internal.c: In function ‘gnrc_ndp_internal_set_state’:
RIOT/sys/net/gnrc/network_layer/ndp/internal/gnrc_ndp_internal.c:106:15: error: this statement may fall through [-Werror=implicit-fallthrough=]
t = ipv6_iface->reach_time;
~~^~~~~~~~~~~~~~~~~~~~~~~~
RIOT/sys/net/gnrc/network_layer/ndp/internal/gnrc_ndp_internal.c:109:9: note: here
case GNRC_IPV6_NC_STATE_DELAY:
^~~~
```
The fall-through in this code is intentional. There are several ways to
warn the comiler about such intentional fall-throughs, which include
e.g. attributed empty statements (`__attribute__ ((fallthrough));`).
I don't like tis approach however. The best way would probably be to
remove this fall-through from the code. However, to keep the diff
minimal, and since ndp will change in the future, I went for warning
the compiler using comments.
The compiler checks comments for several *fall through* regexs to
decide whether a fallthrough was intentional or not.
You can read more about this gcc option in [1]. A note about
fallthrough comment regexs is at the bottom of this article.
[1] https://developers.redhat.com/blog/2017/03/10/wimplicit-fallthrough-in-gcc-7/
RFC4861 says: "If a Neighbor Cache entry is created for the router
[while processing route advertisements], its reachability state MUST be
set to STALE as specified in Section 7.3.3."
The forms of the Source/Target Link-layer Address option for 6LoWPAN are defined
in RFC 4944 Section 8:
https://tools.ietf.org/html/rfc4944#section-8
The address is 16 bit if length is 1, 64 bit if length is 2.
