Subject: Re: [mobile-ip] Mobile IPv6 draft revision

From: "Charles E. Perkins" <[email-masked]>
Date: Fri, 07 Sep 2001 10:10:56 -0700

Hello Peter,

I'm resolving old issues one by one...

"Peter Grimstrup (TED)" wrote:
> 
> >From draft-14:
> 
> 8.5. Sending Binding Acknowledgements
> 
>                    and the packet MUST be sent using a Routing header
>                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>    in the same way as any other packet sent to a mobile node using a
>    care-of address (even if the binding was rejected), as described in
>    Section 8.9.
> 
> 8.9. Sending Packets to a Mobile Node
> 
>    Before sending any packet, the sending node SHOULD examine its
>    Binding Cache for an entry for the destination address to which the
>    packet is being sent.  If the sending node has a Binding Cache entry
>    for this address, the sending node SHOULD use a Routing header
> 
> I find the 2 sections in conflict. Further sec. 5.2 also says
> "The packet is sent using a routing header"

I agree that the wording is inconsistent.  I think that the original
idea was to to accomplish two results:
1. mandate use of the care-of address in Binding Acknowledgements
2. strongly recommend, but not absolutely mandate, the use of the
   care-of address by typical correspondent nodes whenever they have
   a valid Binding Cache entry with the mobile node's care-of address.

I'm not sure about all the reasons for result (2).  We could just
go ahead and mandate use of the care-of address, but I'm not so
confident that is the correct plan.  As it is, I will revise the
wording in sections 8.9 and 5.2 to eliminate, for instance, the
clause "in the same way as any other packet sent to a mobile node".
I think that will fix the immediate problem.

> Comments to De-registration (not touching the questions raised by
> Francis Dupont recently).
> 
> The deregistration may be sent from
>  a) the Home Address (any Lifetime)
>  b) the latest Care Of Address (Lifetime = 0)
>  c) another address (Lifetime = 0)
> 
> First the Binding Cache Entry (BCE) is deleted (see sec. 8.4 and 9.2)
> then the Binding Acknowledgement (BA) is sent.
> 
> As mentioned above, the BA must be sent with a routing header, but
> we have no BCE anymore.

Check.  I see the problem!

> In case a) the BA should be sent to the Home Address (in this case a
> routing header is not really needed, doesn't harm much either).
> In case b) and c) the BA could reasonably be sent back to the source
> address of the deregistration, which in case b) is the BCE just deleted.

Here I am not so sure.  Since the mobile node is considered to be
addressable at its home address, I think it should receive packets
that are addressed to its home address.  Use of the care-of address
is akin to multi-homing, and I don't think that should be considered
as a solution to this problem.

The problem is that:
- On the one hand, a Binding Update with a positive lifetime, and the
  'A' bit set, needs to have the Binding Cache updated before a Binding
  Acknowledgement sent by way of the _new_ care-of address.

- On the other hand, a Binding Update with a zero lifetime, and the
  'A' bit set, needs to have a Binding Acknowledgement send by way
  of a care-of address that arguably can no longer be used.

I think this is a basic inconsistency in the use of the Binding
Update, and I thought of a fairly trivial-to-implement solution:

   + Disallow Binding Updates with zero lifetimes in cases (b) and (c).

If the Binding Update, intended for deregistration, had a
lifetime of 1 second, then that would be enough time to use
the existing Binding Cache entry for delivering the Binding
Acknowledgement to the still non-local mobile node.

If you agree that this is a good solution, then the next step is
to figure out how to write the specification.  Help is always
appreciated!

Regards,
Charlie P.

---

From: Parijat <[email-masked]>
Date: Mon, 10 Sep 2001 16:31:59 -0400

> Acknowledgement to the still non-local mobile node.

That does not answer the basic question that a BA might be addressed to a CoA 
which is no longer valid.

I would suggest that cache deletion requests come in with zero lifetimes, 
(perhaps even in case (a)?), and the acknowledgements to _deletion_ requests 
be sent to the home address of the mobile node.

Of course, I am assuming that there is some need to keep a conceptual, 
specific difference between binding cache deletion requests and binding 
updates which merely have a very short lifetime.  It seems to be a neat thing 
to have a separate binding deletion message rather than one which is a 
degenerate case of a normal BU (okay, okay, zero lifetime BUs _are_ 
degenerate cases of normal BUs, but you know what I mean).

-- 
Parijat Mishra
R & D Engineer,
Centre for Wireless Communications, NUS
Tel: (65)8709353

---

From: "Peter Grimstrup (TED)" <[email-masked]>
Date: Mon, 10 Sep 2001 16:36:42 +0200

Comments inline

Peter

> which is no longer valid.

Well, if a MN send the deregistration from a Care-of Address it does
not want to use (any more) asking for an acknowledgement, it should
reasonably stay there for some minimum of time. Leaving before the BA
is received is OK though, the MN may wish to retransmit the deregistration
from its new location - but with the interpretation from Marc Hassons mail
(7 sep, 18:31) that will be OK as well.

> be sent to the home address of the mobile node.

But maybe the MN just wants to stay away in privacy for some time for some
reason - that happens, I guess. I think Marc's proposal will work.

> degenerate cases of normal BUs, but you know what I mean).
Actually a nice idea, but...