Subject: [mobile-ip] MIPv6 question re L bit

From: "Greg O'Shea" <[email-masked]>
Date: Wed, 11 Dec 2002 17:07:53 -0000

The Link-Local Address Compatibility (L) bit is set when the
         home address reported by the mobile node has the same interface
         identifier (IID) as the mobile node's link-local address.

What I fail to follow is that a multi-homed mobile node may have
*several* interfaces, as in more than one, each capable of connecting to
the home net with a different EUI64 LL addr on each.

Presumably then the MN could need to register one HoA for each interface
whose LLA it wants protecting; but then when Ifa is attached to the home
net and Ifb isn't then the HoA for Ifb would nevertheless be "at home"
and so the MN would deregister the Ifb HoA and the HA would stop
defending the LLA for Ifb and someone else could grab the LLA for Ifb
before Ifb attaches to the home net. Or did I miss something?

Apparently L=1 is **mandatory** if S=0, so presumably someone must care
deeply about this [11.7.1]

---

From: "Ed Remmell" <[email-masked]>
Date: Wed, 11 Dec 2002 09:43:32 -0700

MIPv6 question re L bitGreg -

Basically, the MN must pick what home address(es) it wants to support MIPv6
with. Once those are identified, the next question is were any of those home
addresses formed/auto-configured using a globally unique interface ID? Note
that registration with L=1 is not allowed for random interface IDs generated
per RFC-3041.

For each of the home addresses which were formed using a globally unique
interface ID, regardless of what interface, if the MN could form a
link-local scope address from that interface ID for operation on the home
network (i.e. if/when that interface is active on the home network), then
the MN should set L=1 when registering that home address. The MN can set
either S=0 or S=1 in this case.

The reason why L=1 is mandatory when S=0 is mainly (I think) so that the HA
can optimize DAD in this case, for the set of MN home addresses formed from
the specified interface ID, specifically the HA only needs to perform DAD on
the link-local scope address formed from this interface ID. Or, you can look
at it another way: if another node on the home network attempts to
auto-configure and use an address with that same interface ID, it will first
perform DAD on the link-local scope address formed from that interface ID -
so, the HA needs to defend that link-local scope address, so that the other
node will fail DAD in this case.

In your example below, even when the MN is at home I assume that all
interfaces are not simultaneously attached and active on the home network,
i.e. only a single of the multiple interfaces will be opened/used on the
home network at a given time. So, in this case even when the MN is home the
scenario you describe when the MN is not currently using lfb some other node
on the home network could attempt to use addresses (i.e. perform DAD) formed
from lfb's interface ID, and end up configuring the addresses that the MN
would normally auto-configure were lfb attached to the home network.

IMHO, if you have 2 nodes on the same home network that are using the same
globally unique interface ID, it is a network administration problem, not in
the scope of the protocol specification. But, we do have DAD to detect this
situation. So, MIPv6 is designed so that the HA proxys as much as possible
for the MN while the MN is away from home, including defending the MN's
link-local scope address formed from the interface ID when the MN registers
that interface ID with L=1.
Thanks.
- Ed Remmell
Elmic Systems, USA
http://www.elmic.com

  -----Original Message-----
  From: [email-masked]
[mailto:[email-masked]]On Behalf Of Greg O'Shea
  Sent: Wednesday, December 11, 2002 10:08 AM
  To: Mobile IPv6 mailling list
  Subject: [mobile-ip] MIPv6 question re L bit

  What I fail to follow is that a multi-homed mobile node may have *several*
interfaces, as in more than one, each capable of connecting to the home net
with a different EUI64 LL addr on each.

  Presumably then the MN could need to register one HoA for each interface
whose LLA it wants protecting; but then when Ifa is attached to the home net
and Ifb isn't then the HoA for Ifb would nevertheless be "at home" and so
the MN would deregister the Ifb HoA and the HA would stop defending the LLA
for Ifb and someone else could grab the LLA for Ifb before Ifb attaches to
the home net. Or did I miss something?