Rule 2 of the source address algorithm outlined in [RFC6724] states the
possible source addresses must also be compared among each other:
> Rule 2: Prefer appropriate scope.
> If Scope(SA) < Scope(SB): If Scope(SA) < Scope(D), then prefer SB and
> otherwise prefer SA. Similarly, if Scope(SB) < Scope(SA): If
> Scope(SB) < Scope(D), then prefer SA and otherwise prefer SB.
Our current implementation doesn't do that. It just checks if the scope
of a possible source is lesser than the scope of the destination
(which involves the second "If" in the rule).
This fix grants points according to the scope of an address. If the
scope matches, they get the highest points, ensuring that the selected
source will always be reachable from the destination.
[RFC6724]: https://tools.ietf.org/html/rfc6724
46fd632e91