Subject: RE: on NAT-PT

From: Margaret Wasserman <[email-masked]>
Date: Tue, 21 Jan 2003 17:05:52 -0500

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