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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 draft-ietf-sipcore-location-conveyance

SIP Working Group                                         James M. Polk
Internet Draft                                            Cisco Systems
Expiration: Sept 6th, 2006                                  Brian Rosen
                                                                NeuStar




            Session Initiation Protocol Location Conveyance
               draft-ietf-sip-location-conveyance-02.txt
                             Mar 6th, 2006


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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document defines how the Session Initiation Protocol (SIP)
   conveys, or pushes, user location information from one SIP entity
   to another SIP entity.  SIP Location Conveyance is always end to
   end, but sometimes the embedded location information can be acted
   upon by SIP Servers to direct where the message goes, based on where
   the user agent client is.


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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .  2
       1.1 Conventions . . . . . . . . . . . . . . . . . . . . . . .  4
       1.2 Changes from Prior Versions . . . . . . . . . . . . . . .  4
   2.  Location In the Body or in a Header . . . . . . . . . . . . .  8
   3.  Requirements for SIP Location Conveyance  . . . . . . . . . .  9
   4.  Location Conveyance Using SIP . . . . . . . . . . . . . . . . 12
       4.1 New Option Tags and a Location Header Created . . . . . . 13
       4.2 424 (Bad Location Information) Response Code  . . . . . . 16
       4.3 Example PIDF-LO in Geo Format . . . . . . . . . . . . . . 16
       4.3 Example PIDF-LO in Civic Format . . . . . . . . . . . . . 17
   5.  SIP Element Behavior When Conveying Location  . . . . . . . . 18
       5.1 Location Conveyance Using the INVITE Method . . . . . . . 19
       5.2 Location Conveyance Using the MESSAGE Method  . . . . . . 21
       5.3 Location Conveyance Using the UPDATE Method . . . . . . . 22
       5.4 Location Conveyance Using the REGISTER Method . . . . . . 27
   6.  Special Considerations for Emergency Calls  . . . . . . . . . 29
   7.  Meeting RFC 3693 Requirements . . . . . . . . . . . . . . . . 29
   8.  Open issues . . . . . . . . . . . . . . . . . . . . . . . . . 30
   9.  Security Considerations . . . . . . . . . . . . . . . . . . . 30
   10. IANA Considerations   . . . . . . . . . . . . . . . . . . . . 31
       10.1 IANA Registration for the SIP Location Header  . . . . . 31
       10.2 IANA Registration of the Location Option Tags. . . . . . 31
       10.3 IANA Registration for Response Code 424  . . . . . . . . 31
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 31
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . . 31
       12.1 Normative References   . . . . . . . . . . . . . . . . . 31
       12.2 Informative References . . . . . . . . . . . . . . . . . 32
       Author Information  . . . . . . . . . . . . . . . . . . . . . 32
       Intellectual Property and Copyright Statements  . . . . . . . 33


1.  Introduction

   There are several situations in which it is desired or necessary for
   a Session Initiation Protocol (SIP) [RFC3261] user agent to convey,
   or push Location Information (LI) from one SIP entity to another.
   This document discusses the scenarios for such conveyance, and
   includes the requirements to be met when a SIP UAC wants or needs to
   convey its location to another SIP entity.  A concept of inheritance
   exists in which the conveyance of the location of a user agent means
   conveying the location of a user of that user agent.  This is not an
   absolute in SIP, but applies for the pushing of location using SIP.
   The privacy concerns of this topic are also discussed, and need to
   meet the requirements laid out in RFC 3693 [RFC3693].  This document
   does not discuss the pulling of location information from a user
   agent.  This is left for a future effort.

   Why would a SIP user agent (UA) push its location to another SIP UA?

   There are 3 reasonable scenarios why location can be, or needs to be


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   conveyed to a remote SIP element:

   1) to include location in a request message seeking the nearest
      instance of destination, where there could be more than one
      choice; (hey, here I am, I want to talk to the nearest instance
      of you? i.e. where's the nearest Pizza Hut relative to where I
      am).

   2) to push the user's location to a server that can deal with all
      the inquiries, leaving the UA to do other tasks; (Presence
      Server)

   3) to inform the user of another UA where the sending user is;
      (dude, he is where I am) or (I need help, here I am)

   Scenario #1 revolves around the idea of a user wanting to find the
   nearest instances of something else.  For example, where is the
   nearest  pizza parlor.  A chain of pizza parlors may be contacted
   through a single well known URI (sip:pizzaparlor.example.com).  This
   by itself does not solve enough to the sending UA.  The server at
   this well known URI needs to know where the nearest one is to the
   requester.  In SIP, this could be accomplished in the initial
   message by including the location of the UAC in the Request message.
   This allows the SIP message to be forwarded to the closest physical
   site by the pizzaparlor.com proxy server.  Additionally, the
   receiving site's UAS uses the UAC's location to determine the
   location your delivery.  A more immediate example may be: where's
   the nearest (car) garage repair shop, because the user of the UAC
   has a flat tire.

   Scenario #2 revolves around pushing the user's location information
   to an external server to deal with all location requests in the
   future.  This leaves a buffer layer between the user and the seeker
   of the user's location.  This server would typically handle all
   security checks and challenges of those seeking the user's location,
   as well as handling all the processing of the location target's
   profile rules entered into that server.  This external server
   c/would be a Presence server.  This scenario will not be addressed
   in this document because of the prevailing Presence solutions for
   conveying location information.

   Scenario #3 actually has a part A and a part B to it.  Both involve
   the UAC including its location in the request to the UAS within a
   SIP transaction.  Part A simply has the user, Alice, informing
   another user, Bob, where she is.  This could be for the loan purpose
   for this SIP message, or it could be part of another transaction -
   in which location were merely included, such as within a call set-
   up.

   Part B of this scenario has a user, Alice calling for help and
   including location to inform who she's calling where she is.  This
   is where the called party needs to come bring help to.  Within this


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   scenario, the UAC will need to know this is an emergency SIP request
   message, and to include the UAC's location in this message.

   While scenarios 1, 2 and 3A should use some form of SIP security,
   typically at the wishes of the user, scenario 3B may or may not
   involve SIP security measures.  This is because including any
   security measures may cause the SIP request to fail, and that is
   likely not a good result.  It is also conceivable that a first
   attempt with the user's security measures enabled is tried, and if
   there are any failures, the subsequent attempt or attempts do not
   involve security measures.  Most believe that completing the
   emergency call is more important than protecting the information in
   the SIP message.  Obviously this is up to local and jurisdictional
   policies, but is mentioned here as a hint of a rationale of a later
   section of this document.

   This document does not discuss how the UAC discovers or is
   configured with its location, however will specify how this spec
   meets the requirements for SIP qualifying as a "using protocol" as
   defined in [RFC3693], in section 7.

   Section 3 lists the requirements for SIP location conveyance.
   Section 4 defines how SIP conveys location.  Section 5 illustrates
   specifics about location conveyance in certain SIP request messages.
   Section 6 briefly discusses pertinent behaviors with respect to the
   unique nature of emergency calling.  Section 9 provides the security
   considerations and Section 10 IANA registers one new SIP header, two
   new option tags and one new 4XX Response codes.


1.1  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described
   in [RFC2119].


1.2  Changes from Prior Versions

   [NOTE TO RFC-EDITOR: If this document is to be published as an RFC,
   this section 1.2 is to be removed prior to that event.]

   This is a list of the changes that have been made from the SIP WG
   version -01 to this version -02:

   - streamlined the doc by removing text (ultimately removing 42 pages
     of text).

   - Limited the scope of this document to SIP conveyance, meaning only
     how SIP can push location information.



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   - reduced emergency calling text to just a few paragraphs now that
     the ECRIT WG is taking most of that topic on.

   - greatly reduced the number of requirements in this version.

   - changed the requirements groups from "UA-to-UA", "UA-to-Proxy",
     etc to "UAC Reqs", "UAS-Reqs" and "Proxy-Reqs" to focus on what is
     being asked of each SIP element.

   - Removed the full SIP message examples.

   - completed the ABNF for the Location header, including a cid-url to
     point at a message body part to help in parsing for location.

   - Deleted the call for a new 425 (Retry Location) response code, as
     it appears this can easily be used to spoof a UA into providing
     where it is inadvertently, even if the intent is legitimate by the
     UAC.

   This is a list of the changes that have been made from the SIP WG
   version -00 to this version -01:

   - cleaned up a lot of loose ends in the text

   - created a new Location header to convey many means (location is in
     the body - even if not viewable, which location format is present,
     which format is requested in a query, how to request more than one
     location format in a query, whether the UAC understands location
     at all, if the UA knows its location, how to push location from
     one UA to through a second to a third UA, etc).

   - added the ability to convey location by-reference, but only under
     certain conditions.

   - Added support for the OPTIONS Request to query a server for the
     UAC's location, through the use of the new Location header.

   - moved both new Response code sections forward in the document for
     their meaning to be clearer, earlier for necessary discussion.

   - Changed the message flows to only have the pertinent message
     headers shown for brevity.

   - Added text to the SUB/NOT section showing how and why the location
     of a UA can be refreshed or updated with an interval, or by a
     trigger.

   This is a list of the changes that have been made from the SIPPING
   WG version -02 to this SIP WG item document version -00:

   - Changed which WG this document is in from SIPPING to SIP due to
     the extension of the protocol with new Response codes (424 and


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     425) for when there is an error involving the LO message body.

   - Moved most of the well formed SIP messages out of the main body of
     this document and into separate appendixes.  This should clean up
     the document from a readability point of view, yet still provide
     the intended decode examples to readers of this document who wish
     that level of detail per flow.  The first few flows still have the
     decoded SIP messages (unencrypted and encrypted).

   - Removed some flow examples which no longer made sense

   - Changed all references of "ERC" (Emergency Response Center) to
     "PSAP" (Public Safety Answering Point) as a result of discussion
     within the new ECRIT WG to define a single term

   This is a list of the changes that have been made from the sipping-
   01 working group version of this effort to the sipping-02 version:

   - added requirements for 2 new 4XX error responses (Bad Location
     Information) and (Retry Location Body)

   - added "Bad Location Information" as section 8.6

   - added "Retry Location Body " as section 9.3

   - added support for session mode to cover packet sizes larger than
     the single packet limit of 1300 bytes in the message body

   - added requirement for a SIP entity to SUBSCRIBE to another for
     location information

   - added SUBSCRIBE and NOTIFY as section 8.5

   - added requirement to have user turn off any tracking created by
     subscription

   - removed doubt about which method to use for updating location
     after a INVITE is sent (update)

   - cleaned up which method is to be used if there is no dialog
     existing (message)

   - removed use of reINVITE to convey location

   - clarified that UAs include <provided-by> element of PIDF-LO when
     placing an emergency call (to inform PSAP who supplied Location
     information)

   - updated list of open issues

   - added to IANA Considerations section for the two new 4XX level
     error responses requested in the last meeting


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   This is a list of the changes that have been made from the sipping-
   00 working group version of this ID to the sipping-01 version:

   - Added the offered solution in detail (with message flows,
     appropriate SIP Methods for location conveyance, and

   - Synchronized the requirements here with those from the Geopriv
     Working Group's (attempting to eliminate overlap)

   - Took on the task of making this effort the SIP "using protocol"
     specification from Geopriv's POV

   - Refined the Open Issues section to reflect the progress we've made
     here, and to indicate what we have discovered needs addressing,
     but has not been to date.

   This is a list of the changes that have been made from the -01
   individual submission version to the sipping-00 version of this ID:

   - Brian Rosen was brought on as a co-author

   - Requirements that a location header were negatively received in
     the previous version of this document.  AD and chair advice was to
     move all location information into a message body (and stay away
     from headers)

   - Added a section of "emergency call" specific requirements

   - Added an Open Issues section to mention what hasn't been resolved
     yet in this effort

   This is a list of the changes that have been made from the
   individual submission version -00 to the -01 version

   - Added the IPR Statement section

   - Adjusted a few requirements based on suggestions from the
     Minneapolis meeting

   - Added requirements that the UAC is to include from where it
     learned its location in any transmission of its LI

   - Distinguished the facts (known to date) that certain jurisdictions
     relieve persons of their right to privacy when they call an PSAP,
     while other jurisdictions maintain a person's right to privacy,
     while still others maintain a person's right to privacy - but only
     if they ask that their service be set up that way.

   - Made the decision that TLS is the security mechanism for location
     conveyance in emergency communications (vs. S/MIME, which is still
     the mechanism for UA-to-UA non-emergency location conveyance


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     cases).

   - Added the Open Issue of whether a Proxy can insert location
     information into an emergency SIP INVITE message, and some of the
     open questions surrounding the implications of that action

   - added a few names to the acknowledgements section


2.  Location In the Body or in a Header

   In determining where "location" is placed in a SIP message,
   consideration is taken as to where the trust model is based on the
   architecture involved.

   If the user agent has the location stored within it, and this user
   agent wants to inform another user agent where it is, it seems
   reasonable to have this accomplished by placing the location
   information (coordinate or civic) in an S/MIME registered and
   encoded message body, and sending it as part of a SIP request or
   response.  No routing of the request based on the location
   information is required in this case; therefore no SIP Proxies
   between these two UAs need to view the location information
   contained in the SIP messages.  The UAC should know messages will be
   routed based on location when creating a message.  This is location
   by-value.

   SIP currently does not permit SIP intermediaries to modify
   or delete a message body [RFC3261].  There is, however, no
   restriction on intermediaries viewing message bodies.  S/MIME
   protected message bodies, implemented on bodies for end-to-end
   communications only (i.e. between user agents), would render the
   location object opaque to a proxy server from any viewing of the
   message body.  This problem is similar to that raised in Session
   Policy [ID-Sess-Pol], where an intermediary may need information in
   a body, such as IP address of media streams or codec choices to
   route a call properly.  Requirements in [ID-Sess-Pol] are applicable
   to routing based on location, and are incorporated in these
   requirements by reference.

   The location format is defined in [RFC4119] as a "Presence
   Information Data Format - Location Object", or PIDF-LO.  The amount
   of information that is necessary to appropriately transmit location
   information in a format that is understandable is larger than a SIP
   header could realistically include.  However, there must be a means
   for both a UAC to include a reference point to where location can be
   retrieved from a remote server, and in some cases, a SIP server
   wants or needs to add location to a SIP message as it is processed
   by that server.  This must be in a compact form in a SIP header.  A
   URI satisfies this description.  This is location-by-reference.

   Location-by-Reference allows a UA to place its location on a remote


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   node, to be retrieved by who has this URI.  This allows the server
   to use its processing power to handle all policy rule operations the
   user wants performed per request, and all security challenges done
   as well.

   [RFC3693] prefers S/MIME for security of Location Information, and
   indeed S/MIME is preferable in SIP [RFC3261] for protecting a
   message body.  Accordingly, these requirements specify location be
   carried in a body when it is known to/stored in a user agent.

   It is the use of S/MIME however, that limits message routing based
   on the location of the UAC, scenario 3B from above.  Therefore, it
   seems appropriate to require that, where routing is dependent on
   location, protection of the location information object be
   accomplished by other mechanisms visible to SIP proxies: here TLS
   ("sips:" from [RFC3261]).  The UAC will need to know the difference
   in the call's intent as to which security mechanism to engage for
   location conveyance.

   It is conceivable that an initial attempt to communicate with
   location included may fail due to the security measures used.
   Subsequent requests ought to use less security.  For example, if an
   initial request used S/MIME and failed.  A subsequent request could
   downgrade the security measures used to that of TLS.  This is a
   matter for local and jurisdictional policy, and is merely a hint at
   implementation possibilities.


3.  Requirements for SIP Location Conveyance

   The following subsections address the requirements placed on the
   user agent client, the user agent server, as well as SIP proxies
   when conveying location.

3.1 Requirements for a UAC Conveying Location

   The following are the requirements for location conveyance by a user
   agent client.  There is a motivational statement below each
   requirements that is not obvious in intent.

   UAC-1  The SIP INVITE Method [RFC3261] MUST support Location
          Conveyance.

   UAC-2  The SIP MESSAGE method [RFC3428] MUST support Location
          Conveyance.

   UAC-3  SIP Requests within a dialog SHOULD support Location
          Conveyance.

   UAC-4  Other SIP Requests MAY support Location Conveyance.

   UAC-5  There MUST be one, mandatory to implement means of


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          transmitting location confidentially.

   Motivation:  interoperability

   UAC-6  It MUST be possible for a UAC to update location conveyed
          prior to dialog establishment.

   Motivation: in case a UAC has moved prior to the establishment of a
          dialog between UAs, the UAC must be able to send new location
          information.

   UAC-7  The privacy and security rules established within [RFC3693]
          that would categorize SIP as a 'using protocol' MUST be met.
          See Section 7 for analysis.

   UAC-8  The PIDF-LO [RFC4119] is a mandatory to implement format for
          location conveyance within SIP, whether included by-value or
          by-reference.

   Motivation:  interoperability

   UAC-9  A UAC MUST be capable of transmitting a SIP request without
          protecting the PIDF-LO message body.  It is RECOMMENDED this
          not be the default configuration of any UA.  This requirement
          is orthogonal to the use of TLS or IPSec hop-by-hop between
          SIP elements.

   Motivation:  If a SIP request is part of an emergency call,
          therefore includes the UAC's location, the UAC may understand
          through local policy or configuration that a proxy server
          will need to learn the UAC's location to route the message
          correctly.  Using S/MIME on the PIDF-LO defeats this
          capability in proxies.

   UAC-10 A UAC MUST allow its user to be able to disable providing
          location within any SIP request message.  It is RECOMMENDED
          this not be the default configuration of any UA.

   Motivation:  local laws may give this right to all users within a
          jurisdiction, even when the request is initiating an
          emergency call.


3.2 Requirements for a UAS Receiving Location

   The following are the requirements for location conveyance by a user
   agent server:

   UAS-1  SIP Responses MUST support Location Conveyance.

   UAS-2  There MUST be one, mandatory to implement means of
          transmitting location confidentially.


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   Motivation:  interoperability

   UAS-3  The PIDF-LO [RFC4119] is a mandatory to implement format for
          location conveyance within SIP, whether included by-value or
          by-reference.

   Motivation:  interoperability

   UAS-4  There MUST be a unique 4XX error response code informing
          the UAC it did not provide applicable location information.

   UAS-5  SIP UAs MUST be prepared to receive location without privacy
          mechanisms enabled.  It is RECOMMENDED this not be the
          default configuration of any UA, however, this is possible
          based on local laws.

   Motivation:  Because a SIP request can fail in transit for security
          reasons, UACs are allowed to transmit, or retransmit requests
          including location without any security mechanisms utilized,
          even when this SIP transaction is an emergency call.  UAs
          must be prepared to receive the messages without confidential
          location.

   UAS-6  There MUST be a unique 4XX error response code informing the
          UAC it did not provide applicable location information.


3.3 Requirements for SIP Proxies and Intermediaries

   The following are the requirements for location conveyance by a SIP
   proxies and intermediaries:

   Proxy-1  Proxy servers MUST NOT modify or remove a location
            message body part, and SHOULD NOT modify or remove a
            location header or location header value.

   Motivation:  [RFC3261] forbids the removal of a message body part,
            and the proxy may not have all the relevant information as
            to why location was included in this message (meaning it
            might need to be there), and should not remove this
            critical piece of information.

   Proxy-2  Proxy servers MUST be capable of adding a Location header
            during processing of SIP requests.

   Motivation:  If the proxy determines a message needs to have the
            location of the UAC in the message, and knows the UAC's
            location by-reference, it must be able to add this header
            and URI to the message during processing.  This MUST NOT
            violate requirement Proxy-3 below.



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   Proxy-3  If a Proxy server detects "location" already exists within
            a SIP message, it MUST NOT add another location header or
            location body to the message.

   Motivation:  This may lead to confusion, and should be left for the
            UAC to do on purpose.

   Proxy-4  There MUST be a unique 4XX error response code informing
            the UAC it did not provide applicable location information.


4.  Location Conveyance Using SIP

   RFC 4119 defines the PIDF-LO location object to be inside a RFC 3693
   defined "using protocol" message from one entity to another entity.
   For SIP location conveyance, using the PIDF-LO body satisfies the
   entire format and message-handling requirements as stated in the
   baseline Geopriv Requirements [RFC3693].

   Although a PIDF-LO is to be used to indicate location of a UA, the
   actual PIDF-LO does not need to be contained in the message itself,
   it can be as a by-reference URI in a SIP header or message body
   part, pointing to the PIDF-LO of that UA on a remote node.

   Section 26 of [RFC3261] defines the security functionality SIPS for
   transporting SIP messages with either TLS or IPSec, and S/MIME for
   encrypting message bodies from SIP intermediaries that would
   otherwise have access to reading the clear-text bodies.  SIP
   endpoints MUST implement S/MIME to encrypt the PIDF-LO message body
   (part) end-to-end.  The SIPS-URI from [RFC3261] SHOULD be used for
   message protection (message integrity and confidentiality) and MUST
   be used when S/MIME is not used (when not violating the requirements
   for emergency messaging detailed in section 3 of this document).
   The entities sending and receiving location MUST obey the privacy
   and security rules in the PIDF-LO to be compliant with this
   specification.

   Self-signed certificates SHOULD NOT be used for protecting PIDF-LO,
   as the sender does not have a secure identity of the recipient.

   More than one location representation or format MAY be included in
   the same message body part, but all MUST point at the same position
   on the earth (altitude not withstanding), as this would confuse the
   recipient by pointing at more than one position within the same
   PIDF-LO.  There MAY be a case in which part of one location format
   and part of another exist in the same message body part.  These all
   still MUST point at the same position on the earth, yet are
   incomplete within their own format. For example, there maybe be a
   latitude and longitude in coordinate format and a civic altitude
   value to complete a 3-dimenttional position of a thing (i.e. which
   floor of a building the UA is on in a building at a particular
   lat/long coordinate pair).


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   There MAY be several PIDF-LOs in separate message body parts in the
   same message, and each MAY point at different positions on the earth
   (altitude not withstanding).  If the message length exceeds the
   maximum message length of a single packet (1300 bytes), TCP MUST to
   be used for proper message fragmentation and reassembly.

   Several push-based SIP Request Methods are capable (and applicable)
   of carrying location, including:

      INVITE,
      REGISTER,
      UPDATE, and
      MESSAGE,

   While the authors do not yet see a reason to have location conveyed
   in the ACK, PRACK, BYE, REFER and CANCEL Methods, we do not see a
   reason to prevent carrying a PIDF-LO within these Method Requests as
   long as the SIP message meets the requirements stated within this
   document.  Discussing Location in the PUBLISH Request Method will be
   for another document.

   SIP Methods such as SUBSCRIBE and NOTIFY are considered a pull-based
   location retrieval mechanism, and are therefore not part of this
   document.

   A 200 OK to a SIP Request MAY carry the UAS's PIDF-LO back to the
   UAC that provided its location in the original request, but this is
   not something that can be required due to the timing of the request
   to 200 OK messages, with potential local/user policy requiring the
   called user to get involved in determining if the caller is someone
   they wish to give their location to (and at what precision).


4.1  New Option Tags and a Location Header Created

   This document creates and IANA registers two new option tags,
   "location" or "unknown-location".  User agent clients who support
   this specification will indicate that support by including either of
   these option-tags in a Supported header.

   This document also creates and IANA registers a new Location header.
   The Location header, if present, will have one of three header
   values defined by this document:

   o  a Location-by-reference URI

   o  a Content ID indicating where location is within the message body

   o  a location based option tag

   A location-by-reference URI is a pointer to a record on a remote


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   node containing the PIDF-LO of a UA.

   If the PIDF-LO of a UA is contained in a SIP message, a Location
   header will be present in the message with a content-ID (cid-url)
   [RFC2392] indicating where in the message body location is for this
   UA.  This is to aid a node in not having to parse the whole message
   body or body parts looking for this body type.

   The Unknown-Location option tag in a Location header indicates a UA
   understands the concept of location conveyance, but does not have
   its location to provide.  This can save error messages from being
   generated looking for an answer the UA does not have to give.  It
   can also allow a processing entity the immediate knowledge it needs
   to act as if the UA will not learn location on its own, and perhaps
   call on another process to address the location needs for that
   message.

   The purpose of the Location option-tag is to indicate support for
   this document in the Requires, Supported and Unsupported headers.
   It gives a UAS the proper means to indicate it does not support the
   concept of location in an Unsupported header in a response message
   that might otherwise not be clear that the lack of support for
   location is the problem with the request message.

   The new "Location" header has the following BNF syntax:

   Location           =  "Location" HCOLON Location-value *(COMMA
                         Location-value)
   location-value     =  (addr-spec / option-tag / token)
   addr-spec          =  cid-url / absoluteURI
   option-tag         =  string
   token              =  token / quoted-string
   cid-url            =  "cid" ":" content-id /
   absoluteURI        =  SIP or SIPS-URI
   content-id         =  url-addr-spec
   url-addr-spec      =  addr-spec ; URL encoding of RFC 822 addr-spec

   The Content-ID (cid) is defined in [RFC2392] to locate message body
   parts.

   The absoluteURI is the SIP or SIPS URI of the location-by-reference,
   which points at a PIDF-LO of a UA in a record on a remote node.

   The following table extends the values in Table 2/3 of RFC3261
   [RFC3261].

      Header field             where proxy INV ACK CAN BYE REG OPT PRA
      ----------------------------------------------------------------
      Location                 Rr    ar     o   -   -   o   o   o   -





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      Header field             where proxy SUB NOT UPD MSG REF INF PUB
      ----------------------------------------------------------------
      Location                 Rr    ar     -   -   o   o   o   o   -

   The Location header MAY be added, or read if present in a Request
   message listed above.  A proxy MAY add the location header in
   transit if one is not present.  [RFC3261] states message bodies
   cannot be added by proxies.  A proxy MAY read the location header in
   transit if present.

   It is RECOMMENDED that only one Location header be in the same
   message, but this is not mandatory.  That said, there MUST NOT be
   more than one cid-url pointing to a location message body (part) in
   a SIP message, regardless of how many Location headers there are in
   that message.  There MUST NOT be more than one location by-reference
   URI in any SIP message, regardless of how many Location headers
   there are in a message.

   Here is an example INVITE that includes the proper Location and
   Supported headers (without the PIDF-LO message body part):

   INVITE sip:bob@biloxi.example.com SIP/2.0
   Via: SIP/2.0/TCP pc33.atlanta.example.com
     ;branch=z9hG4bK74bf9
   Max-Forwards: 70
   To: Bob <sip:bob@biloxi.example.com>
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   Call-ID: 3848276298220188511@atlanta.example.com
   Location: cid:alice123@atlanta.example.com
   Supported: location
   Accept: application/sdp, application/pidf+xml
   CSeq: 31862 INVITE
   Contact: <sip:alice@atlanta.example.com>
   Content-Type: multipart/mixed; boundary=boundary1
   Content-Length: ...

   --boundary1

   Content-Type: application/sdp

   ...SDP here

   --boundary1

   Content-Type: application/pidf+xml
   Content-ID: alice123@atlanta.example.com

   ...PIDF-LO with geo-location coordinates here

   --boundary1--




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   The location header from the above INVITE:

      Location: cid:alice123@atlanta.example.com

   indicates the Content-ID location [RFC2392] within the multipart
   message body of were location information is.

   If the Location header were this instead:

      Location: <server5@atlanta.example.com/alice123>

   this would indicate location by-reference was included in this
   message.  It is expected that any node wanting to know where user
   alice123 is would fetch the PIDF-LO from the server5 URI.


4.2 424 (Bad Location Information) Response Code

   In the case that a UAS or SIP intermediary detects an error
   in a Request message specific to the location information supplied
   by-value or by-reference, a new 4XX level error is created here to
   indicate this is the problem with the request message.  This
   document creates the new error code:

      424 (Bad Location Information)

   The 424 (Bad Location Information) Response code is a rejection of
   the location contents, whether by-value or by-reference of the
   original SIP Request.  The server function of the recipient (UAS or
   intermediary) has deemed this location by-reference or location by-
   value to be bad.  No further action by the UAC is required.  The UAC
   can use whatever means it knows to verify/refresh its location
   information before attempting a new request.  There is no cross-
   transaction awareness expected by either the UAS or SIP intermediary
   as a result of this error message.

   This new error code will be IANA registered in Section 10.


4.3 Example PIDF-LO in Geo Format

   This subsection will show a sample of what just the PIDF-LO can look
   like, as defined in [RFC4119].  Having this here will first offer a
   look at a location by-value message body, and secondly, give readers
   an appreciation for how large a location message body is so that
   this document does not have to show a PIDF-LO in every message flow
   example.  This section shows a coordinate position based PIDF-LO.
   Section 4.4 shows this same position in a civic address format.
   Full example message flows will be left for another document.

   Whether this PIDF-LO message body is S/MIME encrypted in the SIP
   message or not, the PIDF-LO stays exactly the same.  There is no


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   change to its format, text or characteristics.  Whether TLS or IPSec
   is used to encrypt this overall SIP message or not, the PIDF-LO
   stays exactly the same.  There is no change to its format, text or
   characteristics.  The examples in section 4.3 (Geo format) taken
   from [RFC3825] and 4.4 (Civic format) taken from [ID-CIVIC] are for
   the exact same position on the Earth.  The differences between the
   two formats is within the <gp:location-info> are of the examples.
   Other than this portion, of each PIDF-LO, the rest the same for both
   location formats.

   <?xml version="1.0" encoding="UTF-8"?>
       <presence xmlns="urn:ietf:params:xml:ns:pidf"
          xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
          xmlns:gml="urn:opengis:specification:gml:schema-
                    xsd:feature:v3.0"
          entity="pres:alice@atlanta.example.com">
        <tuple id="sg89ae">
         <timestamp>2006-03-20T14:00:00Z</timestamp>
         <status>
          <gp:geopriv>
            <gp:location-info>
              <gml:location>
                <gml:Point gml:id="point96" srsName="epsg:4326">
                  <gml:coordinates>33.001111N
                                   96.68142W</gml:coordinates>
                </gml:Point>
               </gml:location>
            </gp:location-info>
            <method>dhcp</method>
            <provided-by><nena>www.cisco.com</nena></provided-by/>
            <gp:usage-rules>
              <gp:retransmission-allowed>no</gp:retransmission-allowed>
              <gp:retention-expiry>2006-03-24T18:00:00Z</gp:retention-
                            expiry>
             </gp:usage-rules>
          </gp:geopriv>
         </status>
        </tuple>
       </presence>


4.4 Example PIDF-LO in Civic Format

   This subsection will show a sample of what just the PIDF-LO can look
   like, as defined in [RFC4119].  Having this here will first offer a
   look at a location by-value message body, and secondly, give readers
   an appreciation for how large a location message body is so that
   this document does not have to show a PIDF-LO in every message flow
   example.  This section shows a civic address based PIDF-LO.  Section
   4.3 shows this same position in a coordinate format.  Full example
   message flows will be left for another document.



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   Whether this PIDF-LO message body is S/MIME encrypted in the SIP
   message or not, the PIDF-LO stays exactly the same.  There is no
   change to its format, text or characteristics.  Whether TLS or IPSec
   is used to encrypt this overall SIP message or not, the PIDF-LO
   stays exactly the same.  There is no change to its format, text or
   characteristics.  The examples in section 4.3 (Geo format) taken
   from [RFC3825] and 4.4 (Civic format) taken from [ID-CIVIC] are for
   the exact same position on the Earth.  The differences between the
   two formats is within the <gp:location-info> are of the examples.
   Other than this portion, of each PIDF-LO, the rest the same for both
   location formats.

   <?xml version="1.0" encoding="UTF-8"?>
      <presence xmlns="urn:ietf:params:xml:ns:pidf"
          xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
          xmlns:gml="urn:opengis:specification:gml:schema-
                     xsd:feature:v3.0"
          entity="pres:alice@atlanta.example.com">
        <tuple id="sg89ae">
         <timestamp>2006-03-20T14:00:00Z</timestamp>
         <status>
          <gp:geopriv>
            <gp:location-info>
              <cl:civilAddress>
                <cl:country>US</cl:country>
                <cl:A1>Texas</cl:A1>
                <cl:A3>Colleyville</cl:A3>
                <cl:HNO>3913</cl:HNO>
                <cl:A6>Treemont</cl:A6>
                <cl:STS>Circle</cl:STS>
                <cl:PC>76034</cl:PC>
                <cl:LMK>Polk Place</cl:LMK>
                <cl:FLR>1</cl:FLR>
              <cl:civilAddress>
            </gp:location-info>
            <method>dhcp</method>
            <provided-by><nena>www.cisco.com</nena></provided-by/>
            <gp:usage-rules>
              <gp:retransmission-allowed>no</gp:retransmission-allowed>
              <gp:retention-expiry>2006-03-24T18:00:00Z</gp:retention-
                            expiry>
            </gp:usage-rules>
          </gp:geopriv>
         </status>
        </tuple>
       </presence>


5.  SIP Element Behavior When Conveying Location

   The SIP Request Methods that MUST convey location are the INVITE,
   REGISTER, UPDATE and MESSAGE Methods.  It is not forbidden by this


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   document to convey location with any other SIP method.  However, no
   other methods are detailed here.

   The message flows in this document will be example messages
   containing only the key headers to convey the point being made that
   do not include all the requisite SIP headers.  All well formed SIP
   message flows are to be in a separate document for brevity here.

5.1 Location Conveyance Using the INVITE Method

   Below is a common SIP session set-up sequence between two user
   agents.  In this example, Alice will provide Bob with her location
   in the INVITE message.

   UA Alice                                  UA Bob

      |               [M1] INVITE               |
      |---------------------------------------->|
      |               [M2] 200 OK               |
      |<----------------------------------------|
      |               [M3] ACK                  |
      |---------------------------------------->|
      |                    RTP                  |
      |<=======================================>|
      |                                         |

   Figure 1. Location Conveyance in INVITE Requests

   User agent Alice invites user agent Bob to a session [M1 of Figure
   1].

   INVITE sips:bob@biloxi.example.com SIP/2.0
   To: Bob <sips:bob@biloxi.example.com>
   From: Alice <sips:alice@atlanta.example.com>;tag=1928301774
   Supported: Location
   Location: cid:alice123@atlanta.example.com

   If the message were S/MIME encrypted, this would be the Content-type
   header:

   Content-Type: application/pkcs7-mime;
      smime-type=enveloped-data; name=smime.p7m

   If this INVITE were not S/MIME encrypted, this would be the
   Content-Type header:

   Content-Type: multipart/mixed; boundary=boundary1

   The obvious reason this for a multipart/mixed Content-Type is that
   this is an INVITE message and there is an SDP message body part
   included.  This is not mandatory, but highly likely.  The cid-url in
   the Location header points a parsing entity that can view the


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   message body to where the PIDF-LO is in the message.

     Within the non-S/MIME message body is this:

   --boundary1

   Content-Type: application/sdp
   v=0
   ...

   --boundary1

   Content-type: application/pidf+xml
   PIDF-LO

   --boundary1--

   In the INVITE, Alice's UAC included the Supported header with the
   location option tag, and the Location header with the cid:url
   pointing at the by-value PIDF-LO.  These two headers MAY be hidden
   in the S/MIME encrypted message body next to the topmost
   Content-Type header to hide the fact that this message is carrying
   location in transit.  Bob's UAS, the destination UA of Alice's
   message, will read these headers when deciphering the overall
   message body.

   - If Bob's UA wants to join the call, his UA responses with a 200 OK
     [M2].  Bob can include his location in the 200 OK response, but
     this shouldn't be expected to due to user timing.

   A 424 (Bad Location Information) Response with a Unsupported header
   (option tag of 'location') is the proper response if Bob's UA cannot
   display this information, but does understand the concept of
   location.

   [Alternative M2 of Figure 2]
   SIP/2.0 424 Bad Location Information
   To: Bob <sips:bob@biloxi.example.com>
   From: Alice <sips:alice@atlanta.example.com>;tag=1928301774
   Unsupported: location

   - If Bob's UA accepts with a 200 OK message, Alice's UA replies with
     an ACK and the session is set up.

   - If Bob's UA does not accept the INVITE for reasons other than
     location included, a 488 (Not Acceptable Here) may be the
     response.

   Figure 1 does not include any Proxies because in it assumed they
   would not affect the session set-up with respect to whether or not
   Alice's location is in a message body part, and Proxies do not react
   to S/MIME encrypted bodies, making their inclusion more or less moot


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   and asking for more complex message flows than necessary here.


5.2 Location Conveyance Using the MESSAGE Method

   Alice can choose to merely want to communicate her location to Bob
   point-to-point, without starting a (voice) conversation, the MESSAGE
   Method MAY be used here.

   To comply with privacy concerns raised in [RFC3693] and [RFC4119], a
   MESSAGE Method Request would be built according to [RFC3428] that
   includes a location message body.  S/MIME encryption SHOULD be used
   on the message body (part), as outlined in [RFC3261].  Figure 2 here
   shows a simplistic MESSAGE method message flow.


   UA Alice                                  UA Bob

      |               MESSAGE [M1]              |
      |---------------------------------------->|
      |                200 OK [M2]              |
      |<----------------------------------------|
      |                                         |

   Figure 1. Location Conveyance in MESSAGE Requests

   Below is a sample, non-well-formed MESSAGE Method message from Alice
   to Bob conveying her geo location:

   [M1 of Figure 2]
   MESSAGE sips:bob@biloxi.example.com SIP/2.0
   To: Bob
   From: Alice
   Supported: location
   Location: cid:alice123@atlanta.example.com

   If the message were S/MIME encrypted, this would be the Content-type
   header:

   Content-Type: application/pkcs7-mime;
      smime-type=enveloped-data; name=smime.p7m

   If this MESSAGE request were not S/MIME encrypted, this would be the
   Content-Type header:

   Content-Type: multipart/mixed; boundary=boundary1

   --boundary1

   Content-Type: text/plain
   Here's my location, Bob?



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   --broundary1

   Content-Type: application/pidf+xml
   Content-Disposition: render
   [Alice's PIDF-LO goes here]

   --broundary1--

   The Content-type of M1 here is "multipart/mixed" to have a text
   message incorporated into the message.  Within the PIDF-LO message
   body, there is a Content-Disposition of "render" to display this
   location information to Bob when his UA receives it.  The cautions
   about whether or not Bob actually reads this message are outlined in
   [RFC3428].

   The 200 OK to M1 of Figure 2 is a simple 200 OK.

   A 424 (Bad Location Information) Response with a Unsupported header
   (option tag of 'location') is the proper response if Bob's UA cannot
   display this information, but does understand the concept of
   location.

   [Alternative M2 of Figure 2]
   SIP/2.0 424 Bad Location Information
   To: Bob
   From: Alice
   Unsupported: location

   If Bob is declining the M2 MESSAGE Request message, a 488 (Not
   Acceptable Here) is the appropriate response.  A Supported header
   with a location option tag indicates location was not the reason
   this message was declined.

   [Alternative M2 of Figure 2]
   SIP/2.0 488 Not Acceptable Here
   To: Bob
   From: Alice
   Supported: location


5.3 Location Conveyance Using the UPDATE Method

   The UPDATE Method [RFC3311] is to be used any time location
   information is to be updated between UAs setting up a dialog or
   after the dialog has been established, no matter how long that
   dialog has been operational.  reINVITE is inappropriate here, and
   the MESSAGE Method is for non-dialog location conveyance between UAs
   only.  The same security properties used in the INVITE MUST be
   applied in the UPDATE message.

   There are 3 conditions UPDATE is to be used to convey location
   between UAs:


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   1) During dialog establishment, but before the final 200 OK (see
      section 5.3.1)

   2) After dialog establishment, but no prior location information has
      been convey (see section 5.3.2), and

   3) After dialog establishment, when a UA has determined it has moved
      (see section 5.3.3)


5.3.1 UPDATE Updates Location During Session Establishment

   Figure 3a. shows the first example of what the UPDATE Method is
   used: during dialog establishment when Alice updates Bob with her
   location information [M3].  This might be different location
   information than was in message [M1] of Figure 3a. or it could be
   the first time Alice conveys location to Bob during the dialog
   set-up.

   UA Alice                                  UA Bob

      |                INVITE [M1]              |
      |---------------------------------------->|
      |                UPDATE [M2]              |
      |---------------------------------------->|
      |            200 OK (UPDATE) [M3]         |
      |<----------------------------------------|
      |            200 OK (INVITE) [M4]         |
      |<----------------------------------------|
      |              ACK (UPDATE) [M5]          |
      |---------------------------------------->|
      |                   RTP                   |
      |<=======================================>|
      |                                         |

      Figure 3a. Updating Location During Dialog Establishment

   [M2 of Figure 3a]
   UPDATE sips:bob@biloxi.example.com SIP/2.0
   To: Bob
   From: Alice
   Supported: location
   Location: cid:alice123@atlanta.example.com
   Content-Type: multipart/mixed; boundary=boundary1

   --boundary1

   Content-Type: application/sdp
   v=
   ...



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   --broundary1

   Content-Type: application/pidf+xml
   [Alice's PIDF-LO goes here]

   --broundary1--

   The above example has Alice also changing something within her
   original SDP, but this is not necessary for this update of location
   information.

   - If Bob agrees with this INVITE and the UPDATE, there his UA
     transits 200 OKs for each [M4] and [M5] in Figure 3a.

   - Alice, upon receiving the 200 OKs, sends an ACK to establish the
     dialog with her modified location.

   Bob's UA should send a 424 (Bad Location Information) Response with
   a Unsupported header (stating 'location') if his UA does not
   understand the concept of location conveyance; meaning to the INVITE
   in [M1].  Therefore, a 424 SHOULD NOT be sent to the UPDATE of
   location information if the PIDF-LO is well formed and has valid
   (not validated!) location fields.  If Bob's UA sends a 424 to this
   UPDATE without an Unsupported header containing a location option
   tag, Alice's UA MUST interpret that to mean the location in the
   PIDF-LO was poorly generated.  Perhaps it was missing a field.
   Perhaps a field was incomplete.

   If Bob is declining the M2 UPDATE Request message, a 488 (Not
   Acceptable Here) is the appropriate response.  A Supported header
   with a location option tag indicates location was not the reason
   this message was declined.

   [Alternative M3 of Figure 3a]
   SIP/2.0 488 Not Acceptable Here
   To: Bob
   From: Alice
   Supported: location


5.3.2 UPDATE Updates Location After Session Establishment

   Figure 3b. shows the second example of what the UPDATE Method is
   used for: if a dialog exists between Alice and Bob without location
   having been conveyed previously in either direction, and one of the
   UAs wants to convey location to the other.  For example, if Alice
   invites Bob to a dialog, but does not include her location in that
   dialog establishment.  Anytime during that dialog that Alice's UA
   decides to convey location, she uses the UPDATE Method, not the
   INVITE Method (in a reINVITE), to update the location parameters of
   that dialog.



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   Once a dialog has been established, a UAC MUST NOT use the INVITE
   Method as a reINVITE to convey location within a dialog.  The UPDATE
   Method MUST be used.

   Consider the following example message flow in Figure 3b.:

   UA Alice                                  UA Bob

      |                INVITE [M1]              |
      |---------------------------------------->|
      |            200 OK (INVITE) [M2]         |
      |<----------------------------------------|
      |                  ACK [M3]               |
      |<----------------------------------------|
      |                   RTP                   |
      |<=======================================>|
      |                UPDATE [M4]              |
      |---------------------------------------->|
      |            200 OK (UPDATE) [M5]         |
      |<----------------------------------------|
      |                                         |

   Figure 3b. Updating Location After Dialog Establishment

   For whatever reason, Alice decides to send Bob her location for the
   first time.  [M4] is an example of the UPDATE message used to
   accomplish this.

   [M4 of Figure 3b]
   UPDATE sips:bob@biloxi.example.com SIP/2.0
   To: Bob
   From: Alice
   Supported: location
   Location: cid:alice123@atlanta.example.com
   Content-Type: application/pidf+xml

   [Alice's PIDF-LO goes here]

   A 424 (Bad Location Information) Response with a Unsupported header
   (stating Location) is the proper response if Bob's UA does not
   understand the concept of location.  In this case, the dialog MUST
   remain unaffected by this rejection message.  Here is a rough idea
   of this 424:

   [Alternative M5 of Figure 3b]
   SIP/2.0 424 Bad Location Information
   To: Bob
   From: Alice
   Unsupported: location

   If Bob is declining the M4 UPDATE Request message, a 488 (Not
   Acceptable Here) is the appropriate response.  A Supported header


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   with a location option tag indicates location was not the reason
   this message was declined.

   [Alternative M5 of figure 3b]
   SIP/2.0 488 Not Acceptable Here
   To: Bob
   From: Alice
   Supported: location


5.3.3 UPDATE Updates Location After a UA Moves in a Dialog

   Figure 3c. shows the first example of what the UPDATE Method is
   used: if one UA that already conveyed location to the other UA, and
   has moved since the dialog was originally sent up.  How a UA
   determines it has moved is out of scope for this document.

   However that "movement" trigger occurred, M4 of Figure 3c. is the
   result: an UPDATE Method Request indicating new location by Alice,
   to keep Bob current with Alice's position.

   UA Alice                                  UA Bob

      |                INVITE [M1]              |
      |---------------------------------------->|
      |            200 OK (INVITE) [M2]         |
      |<----------------------------------------|
      |                  ACK [M3]               |
      |<----------------------------------------|
      |                   RTP                   |
      |<=======================================>|
   **Alice's UA determines it has moved, and needs to update Bob**

      |                UPDATE [M4]              |
      |---------------------------------------->|
      |            200 OK (UPDATE) [M5]         |
      |<----------------------------------------|
      |                                         |

      Figure 3c. Updating Location During Dialog After Movement

   This message flow assumes Alice conveyed location in [M1], and that
   Bob's UA supports location conveyance by not rejecting the INVITE
   request.

   Message M4 of Figure 3c. shows the UPDATE of Alice's location
   information to Bob.  That message may look like this (non-well-
   formed SIP message):

   [M4 of Figure 3c]
   UPDATE sips:bob@biloxi.example.com SIP/2.0
   To: Bob


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   From: Alice
   Supported: location
   Location: cid:alice123@atlanta.example.com
   Content-Type: application/pidf+xml

   [Alice's PIDF-LO goes here]

   There currently is not an indication within the PIDF-LO for Alice to
   tell Bob this PIDF-LO is new, replacement location information from
   a previous message (here in the M1 INVITE message).

   Because of the 200 OK to the INVITE containing location, Alice knows
   Bob's UA understands location conveyance.  Therefore, if Bob's UA
   sends a 424 to this UPDATE, it MUST NOT contain an Unsupported
   header containing a location option tag.

   If Alice does receive a 424 (with the Unsupported header with a
   location option tag), Alice's UA MUST interpret that to mean the
   location in the PIDF-LO was poorly generated.  Perhaps it was
   missing a field.  Perhaps a field was incomplete.

   If Bob is declining the M4 UPDATE Request message, a 488 (Not
   Acceptable Here) is the appropriate response.  A Supported header
   with a location option tag indicates location was not the reason
   this message was declined.

   [Alternative M5 of figure 3c]
   SIP/2.0 488 Not Acceptable Here
   To: Bob
   From: Alice
   Supported: location


5.4 Location Conveyance Using the REGISTER Method

   Alice can choose to merely want to communicate her location to Bob
   point-to-point, without starting a (voice) conversation, the
   REGISTER Method MAY be used here.

   To comply with privacy concerns raised in [RFC3693] and [RFC4119], a
   REGISTER Method Request MUST S/MIME encrypt the PIDF-LO, as outlined
   in [RFC3261].  A UAC SHOULD use a SIPS-URI, as outlined in
   [RFC3261].  Figure 4 here shows a  simplistic REGISTER method
   message flow.

   UA Alice                                 Registrar

      |               REGISTER [M1]             |
      |---------------------------------------->|
      |                200 OK [M2]              |
      |<----------------------------------------|
      |                                         |


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   Figure 4. Location Conveyance in REGISTER Requests

   Below is a sample, non-well-formed REGISTER Method message from
   Alice to Bob conveying her geo location:

   [M1 of Figure 2]
   REGISTER sips:registrar1@biloxi.example.com SIP/2.0
   To: Alice <sips:alice@atlanta.example.com>;
   From: Alice <sips:alice@atlanta.example.com>;tag=1928301774
   Supported: location
   Location: cid:alice123@atlanta.example.com
   Expires: 21600

   If the message were S/MIME encrypted, this would be the Content-type
   header:

   Content-Type: application/pkcs7-mime;
      smime-type=enveloped-data; name=smime.p7m

   If this REGISTER request were not S/MIME encrypted, this would be
   the Content-Type header:

   Content-Type: application/pidf+xml

   provided there were no other registration event message bodies.

   The 200 OK to M1 of Figure 2 is a simple 200 OK.

   A 424 (Bad Location Information) Response with a Unsupported header
   (option tag of 'location') is the proper response if the Registrar
   server does not understand location conveyance.

   [Alternative M2 of Figure 2]
   SIP/2.0 424 Bad Location Information
   To: Alice
   From: Alice
   Unsupported: location

   If the Registrar Server is declining the original [M1] REGISTER
   Request, a 488 (Not Acceptable Here) is the appropriate response.  A
   Supported header with a location option tag indicates location was
   not the reason this message was declined.

   [Alternative M2 of Figure 2]
   SIP/2.0 488 Not Acceptable Here
   To: Alice
   From: Alice
   Supported: location





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6.  Special Considerations for Emergency Calls

   Emergency calling, such as 911, 112 and 999 calling today,
   necessitates a UAC to understand the type of call it is about to
   generate with an INVITE message to a PSAP.  First of all, the
   purpose of calling for emergency help is to get someone to respond
   to the UAC's location, therefore, location MUST be included in the
   INVITE, if known by the UAC.

   The emergency services community strongly prefers that message
   routing occur in the network with the freshest available Public
   Safety Answering Point (PSAP) information.  Message routing, in this
   context, means choosing which SIP(S)-URI to place in the Request-URI
   field of the status line.

   If a UAC knows it is generating an emergency request towards a PSAP,
   there MAY be unique message handling characteristics that diminish
   the level of confidentiality of the location information within the
   SIP message(s).  This is because emergency call routing requires
   proxies to know the location of the message originating UAC in order
   to make a decision on where to route the message.  This is because
   emergency calls are directed to the PSAP local to the caller's
   location.  A proxy performing this function requires that proxy to
   learn the location of the UAC during message processing.

   How a message is routed based on the location of the UAC, and if and
   by how much the level of confidentiality of location information is
   diminished when calling for emergency help are both out of scope of
   this document.

   Hop-by-hop confidentiality mechanisms, as defined in [RFC3261] MUST
   be attempted initially by a UAC that includes location.  Local
   configuration MAY allow a subsequent retry, after a security related
   failure, to be without hop-by-hop confidentiality.  SIP elements
   MUST obey the rules set forth in [RFC3261] regarding maintaining
   hop-by-hop confidentiality when a message using a SIPS-URI.

   While many jurisdictions force a user to reveal their location
   during an emergency call set-up, there is a small, but real, number
   of jurisdictions that allow a user to configure their calling device
   to disable providing location, even during emergency calling.  This
   capability MUST be configurable, but is not RECOMMENDED as the
   default configuration of any UA.  Local policies will dictate this
   ability.


7.  Meeting RFC3693 Requirements

   Section 7.2 of [RFC3693] details the requirements of a "using
   protocol".  They are:

   Req. 4.  The using protocol has to obey the privacy and security


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      instructions coded in the Location Object and in the
      corresponding Rules regarding the transmission and storage of the
      LO.

   This document requires, in Section 3, that SIP entities sending or
   receiving location MUST obey such instructions.

   Req. 5.  The using protocol will typically facilitate that the keys
      associated with the credentials are transported to the respective
      parties, that is, key establishment is the responsibility of the
      using protocol.

   [RFC3261] and the documents it references define the key establish
   mechanisms.

   Req. 6.  (Single Message Transfer)  In particular, for tracking of
      small target devices, the design should allow a single
      message/packet transmission of location as a complete
      transaction.

   This document specifies that the LO be contained in the body of a
   single message, which may be fragmented via TCP, but is still not a
   streaming delivery.


8. Open issues

   This is a list of open issues that have not yet been addressed to
   conclusion:

   none

9.  Security Considerations

   Conveyance of physical location of a UAC is problematic for many
   reasons.  This document calls for that conveyance to normally be
   accomplished through secure message body means (like S/MIME or TLS).
   In cases where a session set-up is routed based on the location of
   the UAC initiating the session or SIP MESSAGE, securing the location
   with an end-to-end mechanism such as S/MIME is problematic, due to
   the probability of a proxy from requiring the ability to read that
   information to route the message appropriately.  This means the use
   of S/MIME may not be possible.  This leaves location information of
   the caller available in each proxy through to the PSAP.  This may
   not be a perfect solution, but may be a pill we need to swallow to
   enable this functionality.

   A bad implementation of SIP location conveyance would have a UAC
   send location in cleartext, without hop-by-hop confidentiality, or
   have any SIP element along the path towards the PSAP alter the
   transport of any message carrying location to be without hop-by-hop
   confidentiality between elements.  The latter would be in clear


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   violation of RFC3261 rules surrounding the use of a SIPS-URI.


10.  IANA Considerations

   This section defines one new SIP header, two new option tags, and
   one new 4XX error response code within the sip-parameters section of
   IANA.  [NOTE: RFC XXXX denotes this document].


10.1 IANA Registration for the SIP Location Header

   The Location header is created by this document, with its definition
   and rules in Section 4 of this document.

10.2 IANA Registration for Two New SIP Option Tags

   Two new SIP option tags are created by this document, "Location" and
   "Unknown-location", with the definitions and rules for each in
   Section 4 of this document.

10.3 IANA Registration for Response Code 4XX

   Reference: RFC-XXXX (i.e. this document)
   Response code: 424
   Default reason phrase: Bad Location Information


11.  Acknowledgements

   To Dave Oran for helping to shape this idea. To Jon Peterson and
   Dean Willis on guidance of the effort. To Henning Schulzrinne,
   Jonathan Rosenberg, Dick Knight, Mike Hammer and Keith Drage for
   constructive feedback.

   To Paul Kyzivat for inspiring some of the recent text addressing
   lingering issues the authors could not resolve.

   To Jon Peterson for his guidance in this effort.


12. References

12.1 References - Normative

 [RFC3261] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
           Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP:
           Session Initiation Protocol", RFC 3261, May 2002.

 [RFC3693] J. Cuellar, J. Morris, D. Mulligan, J. Peterson. J. Polk,
           "Geopriv Requirements", RFC 3693, February 2004



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 [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
           Requirement Levels", RFC 2119, March 1997

 [RFC4119] J. Peterson, "draft-ietf-geopriv-pidf-lo-03", Internet
           Draft, Sept 2004, work in progress

 [RFC3428] B. Campbell, Ed., J. Rosenberg, H. Schulzrinne, C. Huitema,
           D. Gurle, "Session Initiation Protocol (SIP) Extension for
           Instant Messaging" , RFC 3428, December 2002

 [RFC2392] E. Levinson, " Content-ID and Message-ID Uniform Resource
           Locators", RFC 2393, August 1998

 [RFC3311] J. Rosenberg, "The Session Initiation Protocol (SIP) UPDATE
           Method", RFC 3311, October 2002


12.2 References - Informative

 [ID-Sess-Pol] J. Rosenberg, "Requirements for Session Policy for the
           Session Initiation Protocol”, draft-ietf-sipping-session-
           policy-req-02", Internet Draft, July, 2004, "work in
           progress"

 [RFC3825] J. Polk, J. Schnizlein, M. Linsner, "Dynamic Host
           Configuration Protocol Option for Coordinate-based Location
           Configuration Information", RFC 3825, July 2004

 [ID-CIVIC] H. Schulzrinne, " Dynamic Host Configuration Protocol
           (DHCPv4 and DHCPv6) Option for Civic Addresses Configuration
           Information ", draft-ietf-geopriv-dhcp-civil-09, "work in
           progress", January 2006

   Author Information

   James M. Polk
   Cisco Systems
   3913 Treemont Circle                              33.00111N
   Colleyville, Texas  76034                         96.68142W

   Phone: +1-817-271-3552
   Email: jmpolk@cisco.com


   Brian Rosen
   470 Conrad Dr.                                    40.70497N
   Mars, PA  16046                                   80.01252W
   US

   Phone: +1 724 382 1051
   Email: br@brianrosen.net



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