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RE: on NAT-PT

Hi Karim,
There are other services that can be run over an IMS PDP Context which would
require translation. If you open up an IPv6 PDP Context (IMS) you can only
talk IPv6. I think that's now clear to all. However you may want to run
other (non-SIP) v6 app.s on that same PDP Context without establishing another
PDP Context. You're allowed to do that. It's better to use the same PDP Context
because it takes longer to establish a 2nd PDP Context and it uses up more
network resources than necessary. Say that the user opens app.x which is not
widely used so that it doesn't make sense for the operator to deploy an app. proxy.
This won't be the mainstream case which is a good thing, but it is still to
be covered. In this case you have an IPv6 IMS PDP Context on which you run
IMS IPv6 SIP app.s and "other v6" app.s. For the "other v6" app.s which are
mainstream (e.g. http) you can use an app. proxy to talk to v4. However for the
remaining v6 app.s you would need a NAT-PT to talk to v4.
Why would you run IPv6 apps to talk to IPv4?  It would make much more
sense to open an IPv4 PDP context to talk to a remote IPv4 host than to
communicate over IPv6 with either proxies or translation.
It's only for a limited
amount of traffic, so it's not like the plan is to have millions of users sending
all their data through the NAT-PT. The bottom line is that IMS is IPv6-only but it
does not exclude other v6 applications. An IMS IPv6 PDP Context can carry IPv6 SIP
and IPv6 non-SIP traffic but no IPv4 traffic. Since you cannot send IPv4 traffic,
some of the IPv6 non-SIP traffic needs to go through a NAT-PT to talk to v4.
Hope it makes more sense although it's not easy to explain.
No IPv6 PDP context can carry IPv4 traffic -- IMS PDP contexts are not special
in this regard.  To send IPv4 traffic, you need to open an IPv4 PDP context.
If I'm running e.g. web browsing over an IPv6 IMS PDP Context
(which has a good reason as mentioned above) my understanding is that you shouldn't
use IPv4. The reason is the good old "IMS is IPv6-only" story. So there would be
a need for some form of translation/app. proxying (dual-stack http proxy in this case).
Other services which are not mainstream like http would require a translator
such as a NAT-PT. You can also use a separate IPv4 PDP Context to access non-IMS
services such as web browsing, but if the user already has an active IMS IPv6
PDP Context we can't and shouldn't prevent this other scenario which has its
advantages as mentioned above.
I don't see any advantages that would outweigh the serious disadvantages
of using NAT-PT...  Why do you consider it such a problem to open an IPv4
PDP context to communicate to IPv4-only nodes and services?

Margaret