Subject: RE: [mobile-ip] Question about the I Bit in the Revocation Acknow ledgement Messag e

From: "Steven M. Glass" <[email-masked]>
Date: Fri, 12 Apr 2002 17:52:33 -0400 (EDT)

> Thanks for your reply.

    Not a problem!  (I hope you don't mind, but I've forwarded a copy of this
to the list mainly for archival reasons, but also to answer the more general
audience).

> I have been thinking about Registration Revocation for sometimes, now. 
> I like your approach, BUT I have been asking myself the following question:
> 
> Why we do not think of something more secure and End-to-End between the HA
> and the MN.

    Ahmad,

   Actually, there is (but it's not what you expect).  Since there's a
constraint in the base protocol that a home agent can't generate a
'registration request' that is NOT in response to a 'registration reply' it
means that the MN will not be in a state to receive any registration messages. 
I don't want this to change, since it means a significant change in MN
architecture.  As I said, there is more secure end2end solution, and that is
what happens after the revocation.  If the FA notifies the MN its binding has
been lost, or certainly when the binding would normally expire, the MN will
[likely] attempt to reregister.  If the Home domain revoked the registration,
the home agent will respond that it is denying the registration with some
error code, e.g. administratively prohibited, and that IS secure end2end with
the HA-MN authenticator.  The other thing to consider is if the FA revoked the
registration, it may reply with an error [on the attempt to re-register], or
it may simply not repond if that's the policy in the foriegn domain.

    The key thing here is the relationship between home and foreign policies,
and we shouldn't design a revocation mechanism that shifts the balance between
the two.  As we get deeper in detail, local policies can be enforced to
'squeeze' the correct thing out of the other domain.  E.g. if the MN wants to
get revocation notifications directly, it should attempt to colocate, and will
get revocation messages with the HA-MN authenticator.  If, however, the
foreign domain wants get these messages, it can set the 'R' bit to suggest the
MN register through it (though enforcing it is a topological consideration
beyond the scope of mobile ip, but simply preventing the MN from getting a
topologically correct IPsrc address for the foreign subnet is sufficient).

                              Cheers,
                                  Steve