Subject: Re: Why is IPv6 a must?

From: "J. Noel Chiappa" <[email-masked]>
Date: Mon, 12 Nov 2001 14:27:58 -0500

> From: Robert Elz <[email-masked]>

    >> "no fully worked out example of separating location and identity"
    >> ... given the existence of an *IPv6* mechanism that does just that.

    > this is really not a very realistic claim.
    > Mobile IP doesn't separate location and identity really - the location
    > of the home agent is an important part of the identity of the mobile
    > node.

The home agent is not involved in the identification of the mobile node,
which is expressed solely by its "home address", in packets sent to it (see
the discussion of sending packets to a mobile host, Section 8.9. of the IPv6
Mobility spec, "Sending Packets to a Mobile Node" - this is the version 12
draft I'm looking it, it may have moved since).

You have an point that the home agent is tied in, but it's only because it is
being used as a rendezvous mechanism (i.e. once a node has "gone mobile", how
do you get in touch with it) - but that's quite a different beast. (The home
agent is also used as an error recovery mechanism, for cases where the
correspondent node loses track of the current location of the mobile node,
but the mechanism works the same in either case.)

In other words, what is being overloaded onto the home address in this case
is nothing to do with the location of node (the other function we were
discussing of the address), but rather a toehold for the rendezvous mechanism.

(It's clear that rendezvous/error-recovery is indeed a separate function,
since some other scheme for separating location and identity could use some
entirely different mechanism for rendezvous and error. Such a mechanism
might, for instance, make it possible to move the rendezvous point to a
location other than the node's "home" location.)

So there are really three functions being served by the IPv6 address - host
identification, rendezvous toehold, and host location. Once a node has gone
mobile, the latter function is indeed fully split off into the care-of
address.

    > So, while the location and identity of the mobile node itself may
    > have been unlinked, the two concepts haven't been

I'm a bit puzzled as to how you can agree that "the location and identity of
the mobile node ... [has] been unlinked", but still argue that "the two
concepts haven't been". This is some sort of very fine hair-shaving indeed.

	Noel

---

From: Robert Elz <[email-masked]>
Date: Tue, 13 Nov 2001 17:59:59 +0700

Date:        Mon, 12 Nov 2001 14:27:58 -0500
    From:        "J. Noel Chiappa" <[email-masked]>
    Message-ID:  <[email-masked]>

  | I'm a bit puzzled as to how you can agree that "the location and identity of
  | the mobile node ... [has] been unlinked", but still argue that "the two
  | concepts haven't been". This is some sort of very fine hair-shaving indeed.

I meant that if you consider a mobile node in isolation, it appears as if
its location and identity are unrelated.

However, to come up with an architecture which has separate concepts of
identity and location, there can't be any direct relationship between them
at all (other than mapping through some kind of database).

That isn't what mobile IP offers - the identity of a mobile node is
tightly coupled to the location of its home node (or if you like, the home
location of the mobile node).  The two things just aren't separable.

Mobile IP neither is, nor pretends to be, any kind of demonstration of
a model with separate ID and location spaces.

kre

---

From: Keith Moore <[email-masked]>
Date: Tue, 13 Nov 2001 13:41:06 -0500

> location of the mobile node).  The two things just aren't separable.

actually, I think this is wrong.

if you map through some kind of a database then you immediately have the
question of how to find that database.  and if you have to derive the
location of the database from the address of the host which you are trying
to reach, you're adding (yet) another level of indirection that has to be
maintained, is an opportunity for failure, etc.  not to mention one which 
doesn't much, if any, value.  (other than address portability across 
mapping database providers)

mobile IP doesn't require that there be any tight coupling of the 
"identity" of the mobile node to the location of that database, 
it just encodes the location of that mapping database in the home address
of the mobile node.

Keith