Subject: RE: on NAT-PT

From: Margaret Wasserman <[email-masked]>
Date: Tue, 21 Jan 2003 08:38:07 -0500

Hi Juha,

Thanks for the references.

>- IMS is an extensive concept; when saying that IMS is "exclusively IPv6", 
>it is a valid statement for all elements that are connected to the IMS / 
>considered to be a part of the IMS. Both signalling and user data traffic
>belong to the IMS. E.g. the connections between two IMS UEs belong to the 
>IMS. Connections can only be IPv6, not IPv4 and not IPv4 & IPv6 
>simultaneously.

I don't understand why the IMS being "exclusively IPv6" would prevent two
IMS-capable UEs from opening IPv4 PDP contexts and connecting to each other
for non-IMS services.  I'm not even sure how you could prevent this, as
an IMS-capable UE has no real way of knowing whether the remote service
that it is trying to access is/isn't hosted on another IMS-capable UE.

It seems very likely that, regardless of the capabilities of the handset,
some folks will want to use IPv4-only devices behind the handset (IPv4-only
palm pilots, laptops, etc.) for some time.  Even after IPv6 is available on
most laptops, palm pilots, etc. they may still need/want to run IPv4-only
applications.  I can't understand how it would be in the best interests of
3GPP, IPv6 or anyone else to prevent two people who buy very high-end
IMS-capable phones from making an IPv4 connection between their two phones
to run an IPv4-only application...

>- The user has to activate an IPv6 PDP context to access IMS and 
>registrate to the P-CSCF (i.e. registrate to the IMS SIP server) using an 
>IPv6 address.

Right, this is clear.  The IMS-capable UE will have to use IPv6 to access
SIP and other IMS services.  IMS connections to regular Internet SIP services
(which may be IPv4-only) will be done through a proxy.  So, the IPv6 <-> IPv4
conversion can also be handled by that proxy.  Are there other services that
are part of IMS that need to be proxied?

>- E.g. tunneling traffic (IPv4 in IPv6) through the IMS and IPv4 
>connection from the IMS UE to the IPv4 correspondent node is not a 
>feasible solution for IMS scenario 1.
>
>However, the (3GPP Release 5) UE can use IPv4 (type of PDP contexts) to 
>access other services than IMS, e.g. web browsing, MMS, e-mail...

When you say that tunneling (IPv4 in IPv6) is not feasible, is that just a
dogmatic position (based on the dogma that IMS is IPv6-only), or is there
a technical reason why you couldn't tunnel IPv4 over the IPv6-only parts of
the IMS infrastructure?

As far as handsets go, I think it is fine that they have to use IPv6 to
access IMS services, as those services are IPv6-only.  As long as they can
use IPv4 to access non-IMS services, I don't see where the need for NAT-PT
comes in.

I've just printed out the latest 3GPP scenario/analysis documents, and I'll
read them again...  But, I'm still not convinced that NAT-PT is really
needed in this situation, and I'd like to see a clearer technical analysis
of why less damaging alternatives (dual-stack, possibly combined with IPv4
in IPv6 tunnels) would not work.

Margaret

>
>
>The latest version of the 23.221 specification can be found here:
>ftp://ftp.3gpp.org/specs/latest/Rel-5/23_series/23221-570.zip
>
>Best Regards,
>                 -Juha W.-
>
>-----Original Message-----
>From: ext Margaret Wasserman [mailto:[email-masked]]
>Sent: 20 January, 2003 18:00
>To: BELOEIL Luc FTRD/DMI/CAE
>Cc: Erik Nordmark; [email-masked]; [email-masked]
>Subject: RE: on NAT-PT
>
>
>At 04:51 PM 1/20/2003 +0100, BELOEIL Luc FTRD/DMI/CAE wrote:
> >Hi all,
> >
> > >
> > > The proxy alternative may make sense for the 3GPP IMS case, as this
> > > is a special case where only certain services are required to be
> > > IPv6-only.  IMS-capable handsets may still be capable of opening
> > > IPv4 PDP contexts for communication with non-IMS IPv4 nodes and
> > > services.
> > >
> >I've proposed that solution for months but it seems that current 3GPP
> >specifications do not allow such a solution.
>
>My understanding is that the IMS subsystem is specified to be IPv6-only,
>which will require that some services are only available on IPv6 (i.e.
>the 3GPP version of SIP).  This may also result in some parts of the
>3GPP infrastructure (routers and equivalent) being IPv6-only.
>
>While a handset can be IMS-capable, I don't think that an IMS-capable
>handset is required to be IPv6-only.  An IMS handset could still be
>capable of establishing IPv4 PDP contexts, and using IPv4 to access
>the Internet and/or non-IMS 3GPP networks and services...
>
>Could someone correct me if I'm wrong, and please cite relevant parts
>of the 3GPP standards?
>
>Margaret