ipv6 next-hop link-local

[S.P.Zeidler]

Thus wrote Mark Smith (nanog at 85d5b20a518b8f6864949bd940457dc124746ddc.nosense.org):

> For link local unicast or multicast traffic, the ifindex of the
> interface to use is supplied in the sin6_scope_id field of the
> sockaddr_in6 structure, passed to connect() or bind().

.. and if it's not specified it will have a strong tendency (depending on
the struct memory having being cleared or not) to be 0 and thus loopback,
which is both a safe fallback and a bofhly punishment for the sin of
omission :-P

> I do agree though, the ambiguity of link locals can be an issue, and
> that is the trade off with using them. So the benefits of link locals
> are (a) they're always there, regardless of whether global or ULA
> addresses are also available or being deprecated, (b) they have a well
> known prefix distinct from other types of addresses, and (c) they're not
> reachable from off-link destinations. The drawback is their ambiguity on
> multi-homed hosts or routers.

And c) (non-reachability from off-link) too. It Depends (tm).

Intuition says to use routable addresses for routing unless I would pick
unnumbered, but I'll need to let that percolate a bit for the whys and
their applicability.

> Fundamentally, for non-link local addresses, the subnet number is used
> to distinguish the outbound/inbound interface when the prefix is the
> same. I've thought it could be useful to have link locals with
> properties similar to ULAs -
[...]

This is either not stateless, or you have trouble after all your systems
have been off (UPS outage, anyone?), and I fail to see an advantage
over ULA.

regards,
	spz
-- 
spz at serpens.de (S.P.Zeidler)