ipv6 next-hop link-local

Mark Smith nanog at 85d5b20a518b8f6864949bd940457dc124746ddc.nosense.org
Mon Feb 21 11:48:40 CET 2011

On Mon, 21 Feb 2011 11:02:28 +0100
"S.P.Zeidler" <spz at serpens.de> wrote:

> 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?),

Just need to define a tie breaking algorithm to select the new seed to
then choose the new subnet id. Highest or lowest IID would probably do
after DAD has been completed. I think most of the "packet functionality"
exists in RAs - PIOs for SLAAC can be announced by an RA without the
device issuing an RA being considered a default router, by setting the
router lifetime in the RA to zero.

> and I fail to see an advantage
> over ULA.

ULAs would go close, and I've thought about using them in this
manner, however I think there would be a few advantages in these
"unique link locals" using a different prefix to the ULA one. Existing
link-locals have all the necessary properties, except the uniqueness
of subnet identifier.

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

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