Subject: [Geopriv] Location Conveyance: Scenarios

From: "Tschofenig, Hannes" <[email-masked]>
Date: Tue, 18 Jul 2006 16:22:54 +0200

Hi all, 

I have compiled a few message flows that would make it easier to discuss
the different scenarios. I have listed some challenges along with each
of the scenarios. 

1) Location by End Host / Per Value/ in Body

    UA Alice                     SIP Proxy   UAS-A   UAS-B   UAS-C

       |  SIP Message                |          |       |       |
       |---------------------------->|          |       |       |
       |  Location (value/body)      |                  |
       |                             |----------------->|
       |                             |                  |
       |                SIP Message                     |
       |<-----------------------------------------------|
       |                                                |

Problems: 
* Likelyhood of the UA using S/MIME; nice on paper but does it reflect
reality?
* UA Alice needs to know that (a) intermediaries do not need to process
the PIDF-LO 
(b) recipient has a certificate
* Unclear what happens if there are multiple recipients. 

2) Location by End Host / Per Reference / in Body

   UA Alice                     SIP Proxy   UAS-A   UAS-B   UAS-C

      |  SIP Message                |          |       |       |
      |---------------------------->|          |       |       |
      |  Location (ref./body)       |                  |
      |                             |----------------->|
      |                             |                  |
      |              Dereference                       |
      |<==============================================>|
      |                                                |
      |                                                |
      |                SIP Message                     |
      |<-----------------------------------------------|
      |                                                |

Problems: 
* Can be protected using S/MIME (see issue above)
* If not protected then the considerations of who is allowed to
dereference to a PIDF-LO are applicable. There is certainly a difference
between a reference pointing to a PIDF-LO at the UA itself and a
reference obtained using one of the discussed Geopriv L7 protocols. 
It might be difficult for the end host to make the PIDF-LO available. It
might be difficult for him to have a server-side certificate but it
might be easier to control access to it. 

3) Location by SIP Proxy / Per Reference / in Header

   UA Alice                     SIP Proxy   UAS-A   UAS-B   UAS-C

      |  SIP Message                |          |       |       |
      |---------------------------->|          |       |       |
      |                             | SIP Message      |
      |                             |----------------->|
      |                             | Location         |
      |                             | (ref./header)    |
      |                             |                  |
      |                             |                  |
      |                             |   Dereference    |
      |                             |<================>|
      |                             |                  |
      |                SIP Message                     |
      |<-----------------------------------------------|
      |                                                |
      |                                                |
      |                                                |

Problems:

* What should the SIP proxy but into the PIDF-LO when it is created
(policies, identifiers, etc.)?
* Authorization aspects: see above (Why cannot just everyone that sees
the reference in the header passing by dereference it?)
* Signing of the reference theoretically possible. SIP Proxy, however,
needs to know the intended recipient. 

4) Location by SIP Proxy / Per Value / in Header

  UA Alice                     SIP Proxy   UAS-A   UAS-B   UAS-C

     |  SIP Message                |          |       |       |
     |---------------------------->|          |       |       |
     |                             | SIP Message      |
     |                             |----------------->|
     |                             | Location         |
     |                             | (value/header)   |
     |                             |                  |
     |                SIP Message                     |
     |<-----------------------------------------------|
     |                                                |
     |                                                |
     |                                                |

Problems:

* What should the SIP proxy but into the PIDF-LO when it is created
(policies, identifiers, etc.)?
* Data URI approach possible but with increased message size. 
* Signing of the PIDF-LO possible but approach similar. SIP Proxy,
however, needs to know the intended recipient.

5) Location by SBC / Per Value / in Body

  UA Alice                     SIP Proxy   UAS-A   UAS-B   UAS-C
                                 (SBC)
     |  SIP Message                |          |       |       |
     |---------------------------->|          |       |       |
     |  Location (value/body)      | SIP Message      |
     |                             |----------------->|
     |                             | Location         |
     |                             | (value/body)     |
     |                             |                  |
     |                SIP Message                     |
     |<-----------------------------------------------|
     |                                                |
     |                                                |
     |                                                |

Problems:

* If there is no solution for the cases available where the SIP proxy
can add a PIDF-LO and a reference is not acceptable (because of the
latency) it is likely that boxes will just do it. 
* Does not play "nicely" with SIP. 
* Problems with E2E security

6) Location by End Host / Per Referene / in Header

  UA Alice                     SIP Proxy   UAS-A   UAS-B   UAS-C

     |  SIP Message                |          |       |       |
     |---------------------------->|          |       |       |
     |  Location (ref./header)     |                  |
     |                             |----------------->|
     |                             |                  |
     |              Dereference                       |
     |<==============================================>|
     |                                                |
     |                                                |
     |                SIP Message                     |
     |<-----------------------------------------------|
     |                                                |

Problems:

* Not sure about S/MIME usage
* Simlar considerations as in (2)

Finally, I think it would be useful to differentiate between location
based services in general and emergency services since the requirements
are, in most cases, different. 

Ciao
Hannes

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---

From: Henning Schulzrinne <[email-masked]>
Date: Tue, 18 Jul 2006 10:57:28 -0400

Thanks, Hannes; these are helpful to focus the discussion.

I think, however, that the issue of security (encryption or integrity) 
and conveyance are essentially orthogonal. In other words, you can apply 
the same techniques, with similar problems, in all three conveyance 
mechanisms (by-value, by-attachment, by-reference). The procedural 
details differ (such as XML-DSIG and XML encryption vs. S/MIME vs. TLS), 
but the strength and the certificate problems are similar, albeit not 
identical. (For example, it is probably somewhat harder to deal with 
multi-recipient cases in S/MIME than in other mechanisms.)

It might help streamline the discussion if we can agree on this general 
statement or show that it is mistaken.

Henning