Location Conveyance: Scenarios

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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