Subject: Re: [MOBILE-IP] Cellular IP for Private Network Access

From: Yingchun Xu <[email-masked]>
Date: Thu, 6 Jan 2000 09:27:26 -0600

Phil,
Sprint network does not have Cellular IP. In Cellular IP draft, no PPP is used.
Between GW and BS, Mobile Host IP addresses are used for packet routing.
Cellular IP nodes maintain Route Cache.

The problem occurs when two Mobile Hosts accessing different private networks
get assigned the same IP address at WAN level. In this case, Cellular IP node
will have problem for IP packet routing based on Mobile Host IP address.

--Yingchun.

Phil Karn <[email-masked]> on 01/05/2000 05:38:02 PM

Please respond to Phil Karn <[email-masked]>

Sent by:  Phil Karn <[email-masked]>

To:   [email-masked]
cc:    (Yingchun Xu/MW/US/3Com)
Subject:  Re: [MOBILE-IP] Cellular IP for Private Network Access

>conflicted Mobile Node IP addresses may be assigned which will cause the
failure
> of IP packet forwarding.

I don't know what this means. I do know from personal observation how
Sprint PCS's wireless web service is set up, though. PPP gives you an
address on network 10, and routes you through a bidirectional NAT in
Kansas City. I.e., the NAT assigns you a routable address and
translates between that and your network 10 address without any port
translation.

This forces you to run Mobile IP with tunneling in both directions if
you want to give your mobile computer a predictable routable address.

Is this what you had in mind?

Phil

---

From: Phil Karn <[email-masked]>
Date: Fri, 7 Jan 2000 01:28:00 -0800

>will have problem for IP packet routing based on Mobile Host IP address.

How can/why should two mobile hosts get assigned the same IP address?
We have no trouble avoiding that in wired networks, why should
wireless be any different?

Or are you referring to NON-global IP addresses (eg., on network 10)
belonging to distinct private networks that multiple mobile users on
the same cellular FA may wish to use at the same time?

If so, this is a totally artificial problem created by the cellular
carrier's pointless insistence on implementing its own MIP FA.  If you
strip all vestiges of MIP out of the carrier, put a co-located FA in
the mobile hosts, and have the carrier simply provide a unique,
possibly temporary, globally routable IP address to each mobile user
for the duration of his session, the problem disappears. Each mobile
user's FA peers with his own HA, and each FA/HA association can
encapsulate packets with non-global IP addresses belonging to his home
network without any conflict with anyone else doing the same.

Once again, we see that co-locating the FA at the mobile host is the
*only* MIP configuration that makes any technical or operational
sense.  But I suppose I'll be told (and not for the first time) that
it doesn't make "marketing" sense from the carrier's perspective.

Over-the-air encapsulation overhead is a red herring, for several
reasons. First, there are more efficient ways to encapsulate than
sending two complete IP headers. Second, even if you do send two full
IP headers, VJ header compression can be easily extended to work on
them. Third and most important, encapsulated MIP traffic will be a
tiny (if not zero) fraction of most mobile packet data users' traffic
for a very simple reason: the vast majority of mobile users run
clients, not servers. Those clients can initiate their transactions
with whatever temporary IP address is assigned by the carrier. Because
most network transactions are apt to be very short compared to the
rate at which addresses change due to inter-cellular-system mobility,
an address change during a transaction will be a rare event, and if it
happens it can easily be repeated. I.e., MIP solves a problem they
just don't have, and it imposes some substantial costs in the process
(e.g., routing that can be wildly suboptimum).

Virtually every dialup ISP already uses PPP dynamic address
assignment. I don't know of any that provide MIP FAs. Why is cellular
any different?

Phil

---

From: "Charles E. Perkins" <[email-masked]>
Date: Fri, 7 Jan 2000 12:57:33 -0800

Hello Phil,

I second most of your comments on pipes (spectrum) and end-to-end
design.

But I finally have to respond to the following.

> carrier's pointless insistence on implementing its own MIP FA.

Whether or not a foreign agent is pointless should be decided in
each particular instance.  You may say that it's pointless for
ISPs to charge more for using multiple IP addresses, or that many
other existing practices are pointless.  I think it is important
to allow for the _possibility_ that a mobile node may not wish to
operate with a co-located care-of address.  I also think it's fine
if it wishes to do so.

>                                                              If you
> for the duration of his session, the problem disappears.

That depends on what you mean the problem is.  There are a lot
of reasons why an IP address should survive the duration of a
single "session", and those reasons may well become even more
important over time.

>                                                        Each mobile

I agree that this is a great model for mobile nodes that can support it.

> it doesn't make "marketing" sense from the carrier's perspective.

I also don't agree that it's the "only" MIP configuration that
makes sense, so I don't fit in your first person plurality,
althought I certainly agree that it does make sense.  Somehow,
I don't feel as if I am being blinded by my marketing background.

>                                                        Because

From my understanding, you are saying that things only seem to be
a little bit broken now.  I compare your solution to the user-oriented
retransmission algorithm (UORA):
        If it doesn't work the first time, hit the mouse button again.
I'd rather build better systems than that.  Furthermore, it's not
so easy for embedded systems, which are increasingly likely to be
mobile.
And it would happen more and more often, as cell sizes shrink to offer
more bandwidth.

Lastly, I _agree_ with you that a mobile node should be able to use
its temporary IP address for many such applications where retries
are O.K, and transaction time short.

While I'm spinning on this topic, I would like to point out that
most of the recent problems that have been raised are involved with
melding network authorization with Mobile IP.  Putting in network
authorization with Mobile IP might be easier than with any other
infrastructure capable of rapid mobility, but it still isn't simple.
I don't think that building up layer-2 mobility systems to replace
the more natural mobility handling at layer-3 is the best way to
make progress towards our future always-on, seamless mobility.

Regards,
Charlie P.