Internet Engineering Task Force                                R. Barnes
Internet-Draft                                               M. Lepinski
Intended status: Standards Track                        BBN Technologies
Expires: April 10, 2010                                  October 7, 2009

       Using Imprecise Location for Emergency Context Resolution

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   Emergency calling works best when precise location is available for
   emergency call routing.  However, there are situations in which a
   location provider is unable or unwilling to provide precise location,

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   yet still wishes to enable subscribers to make emergency calls.  This
   document describes the level of location accuracy that providers must
   provide to enable emergency call routing.  In addition, we descibe
   how emergency services and non-emergency services can be invoked by
   an endpoint that does not have access to its precise location.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Determining sufficient location precision  . . . . . . . . . .  4
     3.1.  Location filtering . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Constructing location filters  . . . . . . . . . . . . . .  7
       3.2.1.  Geodetic service boundaries  . . . . . . . . . . . . .  8
       3.2.2.  Civic service boundaries . . . . . . . . . . . . . . .  9
     3.3.  Maintaining location filters . . . . . . . . . . . . . . .  9
     3.4.  Applying location filters  . . . . . . . . . . . . . . . .  9
   4.  Requesting emergency and non-emergency services  . . . . . . . 10
     4.1.  Emergency calling  . . . . . . . . . . . . . . . . . . . . 10
     4.2.  Non-emergency services . . . . . . . . . . . . . . . . . . 11
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14

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1.  Introduction

   Information about the location of an emergency caller is a critical
   input to the process of emergency call establshment.  Endpoint
   location is used to determine which Public Safety Answering Point
   (PSAP) should be the destination of the call.  (The entire emergency
   calling process is described in detail in [1] and [2].)  This process
   is most likely to work properly when the endpoint is provided with
   the most accurate precise information available about its location.
   Using location information with maximal precision and accuracy
   minimizes the chance that a call will be mis-routed.  And when that
   location is provided to the endpoint, the endpoint is able to verify
   that the location is correct (to the extent of the endpoint's
   knowledge of its own location) prior to an emergency call, and is
   able to perform emergency call routing functions on its own,
   providing redundancy for network-provided functions.

   However, there may be situations in which it is not feasible for
   endpoints to be provided with maximally precise and accurate
   location.  These cases may arise when computing precise location is
   an expensive or time-consuming operation (e.g., in the case of
   wireless triangulation), and location is needed quickly (as is often
   the case in emergency situations).  Or they may arise because the
   policy of the location provider does not allow precise location to be
   provided to the endpoint (e.g. due to privacy considerations).  While
   it is undesirable to use imprecise location for emergency call
   routing, the possibility that precise location may not be available
   to the calling device must be accomodated in order to make emergency
   calling possible in the largest possible set of circumstances.

   This document is concerned imprecise location only in the context of
   routing emergency calls, i.e., for determining the correct PSAP to
   receive a given call (e.g., via a LoST query [3]).  (More generally,
   the provided location information will be needed to route the call to
   an entity that is authorized to request precise location, e.g., an
   Emergency Services Routing Proxy.)

   Location information may also be used in the emergency calling
   framework to direct the dispatch of emergency responders.  This usage
   is treated separately from call routing for purposes of this
   document, and this document does not place requirements on the
   location provided for dispatch (although it should obviously be as
   precise as possible).  The only provision for dispatch in this
   document is a recommendation that the location provider supply
   endpoints with a URI that can be used by a PSAP or other emergency
   authority to obtain a different location for use in dispatch,
   hopefully more precise than the one used for routing.

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   This document describes the use of imprecise location information in
   the emergency call routing system.  Section 3 describes how location
   providers can determine the precision necessary to support emergency
   call routing, and how they can use this information to optimize
   location delivery.  Section 4 describes how emergency calls are
   placed in such an environment, and how non-emergency services can be
   invoked when precise location is not available to the endpoint by

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [4].

   We consider in this document patterns of interaction as described in
   [1].  The two main parties of interest are endpoints and location
   providers.  Endpoints are hosts connected to the Internet that
   originate emergency calls in the emergency calling architecture,
   while location providers are entities that supply location
   information that is used for emergency calling.  In addition, we will
   discuss how these parties interact with the LoST mapping
   infrastructure [7], and with emergency and non-emergency location-
   based service providers.

   For convenience, we say that location information (either in LoST
   queries or in service boundaries) is provided "in geodetic form" if
   it is provided in the "geodetic-2d" location profile, and "in civic
   form" if it is provided in the "civic" profile.

3.  Determining sufficient location precision

   A location provider wishing to provide location information usable
   for emergency call routing requires a mechanism for determining when
   a description of location (e.g., a polygon) is precise enough to be
   used for emergency call routing.  This mechanism might be used to
   decide when to terminate a positioning mechanism that converges over
   time, or to choose a polygon larger than the known location of the
   endpoint (in order to obscure the known location of the endpoint),
   while preserving the utility of the location for emergency call

   There are two base requirements for a location to be usable for
   emergency call routing:

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   1.  The location SHOULD be sufficiently precise that a LoST request
       with the location and any service URN will return a unique URI
       mapping value.  This may not be possible in all cases, e.g.,
       because of overlapping service boundaries (leading to areas that
       do not have a unique mapping) or positioning limitations (leading
       to insufficient precision).

   2.  When the location of the endpoint is known by the provider to
       greater precision than is being provided, the provided location
       MUST return the same mappings from LoST (for all service URNs) as
       the known location.

   3.  When the location of the endpoint is known by the provider to
       greater precision than is being provided, the provided location
       MUST contain the precise location (as a geographic subset).

   In this section, we describe how to use a "location filter" to
   determine whether a given location is usable for emergency call
   routing, and how to construct and maintain such a filter.

3.1.  Location filtering

   With each service-to-URI mapping, a LoST query provides a service
   boundary that represents the set of locations in which that mapping
   is valid.  A consequence of this is that given a set of service
   boundaries for difference services (say, one mapping
   "" to "" and one mapping
   "urn:service:sos.police" to ""), the
   intersection of those service boundaries is the region in which two
   mappings are valid ("" maps to
   "" and "urn:service:sos.police" maps to
   "").  Outside that area, one or more of the
   mappings is invalid.  Said differently, any region contained in an
   intersection uniquely determines mappings for the services used in
   the intersection, and any two locations within the same intersection
   are equivalent for the purpose of LoST mapping (i.e., emergency call

   A location filter is thus a set of regions (optionally, each region
   may be assigned a list of LoST mappings), as illustrated in Figure 1.
   Each region is the intersection of the service boundaries for all
   services available within the region, and the lists represent the
   mappings that are valid within that region.  A filter is used to
   determine whether a location is useable for emergency call routing in
   the following way:

   1.  The location SHOULD be contained in exactly one of the regions in
       the filter.  This guarantees that LoST mappings are unique.

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   2.  When the precise location of the endpoint is known, the provided
       location MUST be contained in the same region(s) of the filter as
       the known location.  This guarantees that LoST queries with the
       provided location return the same results as those done with the
       known location.

   3.  When the precise location of the endpoint is known, the provided
       location MUST contain the precise location (as a geographic

   When the regions are bound to lists of URN-URI mappings, the
   resulting filter can also be used as a cache for LoST mappings; the
   LoST mappings for a location are the mappings bound to the region(s)
   containing it.

                  Service boundaries for individual services


                     +-------+                +-------+
                     | A     |                | C     |
                     |       +---+            |   +---+---+
                     |       |   |            |   |       |
                     +---+---+   |            +---+       |
                         |     B |                |     D |
                         +-------+                +-------+

                           |                        |
                           |                        |

                               | A,C   |
                               |   +---+
                               |   | +---+
                               +---+ |A,D| +---+
                                     +---+ |   |
                                       +---+   |
                                       |   B,D |

                        Resulting Location Filter Regions

           Figure 1: Generating a filter from service boundaries

   When the location of the endpoint is known to more precision than the

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   location provided to the endpoint, although any location meeting the
   two criteria above is equivalent to the known location for purposes
   of LoST, the provided location MUST contain the known location in
   order to avoid errors if the location is used for other purposes in
   the course of an emergency (e.g., if the location is provided to
   first responders for dispatch).  This guarantee also allows the
   endpoint to do some course verification that the provided location is
   correct (in order to prevent very gross errors in routing).  Thus,
   any location that (1) contains the known location and (2) is
   contained in the same filter region as the known location is
   allowable.  Locations that also are contained in only one filter
   region are preferred.  Adding randomness to the provided locations
   may have privacy benefits in some cases, as discussed in the security
   considerations below.

3.2.  Constructing location filters

   For simplicity, we assume that the entity performing filtering will
   only be using the filter to test locations contained within a
   particular geographic "coverage area".  (In principle, this coverage
   area could be the entire world, but assuming a more limited coverage
   area allows for a filter to be built more quickly) Given a coverage
   area and the ability to act as a LoST client, a location service
   provider can autonomously compute a location filter using the
   following algorithm:

   First, the server must obtain mappings and service boundaries for all
   services and for all points within the coverage area.  For each
   emergency service URN, the server goes through the following process
   to build a service map: First, the server queries LoST for the
   complete coverage map for the desired service.  This can be done with
   a LoST <findService> query of the following form:

           <?xml version="1.0" encoding="UTF-8"?>
               <!-- Coverage Area -->
               <!-- Service URN -->

   If LoST returns a set of mappings whose service boundaries cover the

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   coverage area (i.e., if LoST is configured to return all possible
   matches for the queried location), then the process termintes here.
   The coverage map for this service is the set of returned service

   If service boundaries in the LoST response to the above query do not
   cover the location provider's coverage area, then the location server
   must perform further queries.  The location included in each query is
   the difference between the coverage area and the current coverage
   map, that is, the coverage area with all currently-known service
   boundaries removed.  The server repeats this process (query, then
   remove service boundaries from the query location, then query again)
   until either (1) the coverage area is covered by the collected
   service boundaries or (2) LoST returns a <notFound> error.

   After the location server has performed this procedure for each
   service, it will have a set of LoST mappings for each service, for
   every point in its coverage region where that service is offered.

   The regions in the location filter are computed separately for
   service boundaries provided in civic form and in geodetic form.  If
   all service boundaries are provided in one form (e.g., if all
   boundaries are provided in geodetic form, even if some are also
   provided in civic form), the server MAY perform the algorithm for
   that form.  If both algorithms are being performed, and some mappings
   provide both civic and geodetic service boundaries, the server MUST
   input those mappings to both the civic and geodetic computations.

3.2.1.  Geodetic service boundaries

   The regions in the location filter are computed from these mappings
   by iterating over URI tuples: For each service URN, let uris(urn) be
   the set of PSAP URIs for that service URN (collected from the
   mappings).  The set of URI tuples is then the cartesian product of
   these sets; if the set of servuce URNs is {urn1,...,urnN}, then the
   set of URI tuples is uris(urn1) x ... x uris(urnN).  The server
   computes the regions in the filter by iterating through the set of
   URI tuples, either by constructing the set of URI tuples and directly
   iterating, or by using nested iteration through all the sets

   For each URI tuple, the server MUST compute the intersection of the
   service boundaries for the URIs in the tuple.  This becomes an entry
   in the location filter: The stored region is the intersection of the
   service boundaries, and the corresponding mapping table is the list
   of (URN, URI) pairs, where the URIs are the URIs from the tuple and
   the URNs are the services used to obtain them from LoST.  (Empty
   filter regions, corresponding to URIs in a tuple with disjoint

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   service boundaries, can of course be discarded.)

3.2.2.  Civic service boundaries

   As in the case of geodetic location, regions of a civic address
   filter are computed based on URI-tuples.  For tuples where all
   mappings have the same service boundary, that service boundary MUST
   be used as the filter region for that type.  For all other cases
   (i.e., tuples with different civic locations), the regions of the
   filter must be computed as the intersections of the locations
   according to an algorithm that is determined by local addressing

   Note that the resulting filter regions SHOULD still cover the
   location server's coverage area, i.e., there should be a filter
   region that contains every civic address within the coverage area.
   In particular, the server SHOULD NOT use a specific address to
   represent a filter region: Such an address would not include many
   points in the service region (i.e., it would not meet the third rules
   from both lists of rules above).  If the server chooses to return a
   civic address that does not, then it MUST set the 'method' element of
   the PIDF-LO it returns to value 'area-representative' registered in
   Section 7.

3.3.  Maintaining location filters

   As the LoST mappings that underlie the filter change, the filter will
   need to be updated.  The entity maintaining the filter MUST obtain a
   new mapping for a region when an existing mapping expires.  The
   service boundary from the new mapping is compared to the service
   boundary from the old mapping: If they are the same, then the filter
   need not be updated.  If they differ, then regions in the filter that
   intersect either the old service boundary or the new service boundary
   will need to be recomputed.  Note that since this operation only
   requires the server to determine if two service boundaries are
   identical, the server need only store a hash of the old boundary (to
   which it can compare a hash of the new boundary).

3.4.  Applying location filters

   After constructing a location filter, a location server can use it to
   optimize how it delivers location.  When the location server is using
   a positioning algorithm that grows more accurate with time, the
   filter tells it how long to run the algorithm.  Namely, the algorithm
   can be terminated when the estimated location is within one of the
   regions in the filter.

   When the location provider knows the precise location of the caller,

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   a location filter can also be used as a "location cache".  That is,
   the location provider can simply look up which of the filter regions
   contains the caller's precise location and return that region as the
   caller's location (or some subset that contains the precise

   This allows an additional optimization in some cases: If the location
   server knows that the caller's precise location will be within the
   same region for a period of time, it can instruct the client not to
   re-query in that time.  For instance, if the server is delivering
   location over HELD, then it can use the HTTP cache-control headers
   (e.g., Expires).  However, the location server MUST NOT instruct the
   client to wait for longer than the current filter is valid; the
   expiry time of the location MUST be before the earliest expiry of a
   LoST mapping used in the filter.

4.  Requesting emergency and non-emergency services

   When a location provider wishes to deliver endpoints location
   information that is below its maximum available precision while still
   supporting emergency calling, it MUST provide to the endpoint both a
   location (by value) that is sufficient for emergency call routing
   (see above) and a location reference (i.e., a URI) that can
   subsequently be used by authorized parties to obtain more precise
   information about the location of the endpoint.  The endpoint then
   can then use both the location value and the location reference to
   request location-based services (LBS) as described below.

4.1.  Emergency calling

   The procedure for placing an emergency call is indentical to that
   described in [1].  In particular, the endpoint requirements in
   Sections 8 and 9 of [2] still apply to an endpoint that receives
   imprecise location.

   In addition, an endpoint that receives location both by value and by
   reference from its location provider MUST include both the location
   value and the location reference in the SIP INVITE message that
   initiates an emergency call, as specified in [5].  When the endpoint
   supports LoST, it SHOULD use the location value to obtain a PSAP URI
   for LoST queries (as opposed to attempting to dereference the
   location reference).  Note that the caller would also have to add the
   "used-for-routing" parameter to the geolocation header that points to
   the location value as inserted into the INVITE message.  Note that
   this process crucially relies on the location value having sufficient
   precision for routing emergency calls (see Section 3 for techniques
   to ensure the location value is suitable for emergency call routing).

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   When a PSAP receives a SIP INVITE that contains both a location value
   and a location reference, if the value is too imprecise for use in
   dispatch then the PSAP SHOULD dereference the LbyR to obtain more
   precise information.  In turn, the location provided by the location
   provider MUST allow access by all PSAPs whose service boundaries
   overlap with the region served by the location provider.  This means
   that either the provider must supply a reference that can be
   dereferenced by any party, or else the provider must establish
   explicit authentication and authorization relationships with all
   PSAPs in its service area.

4.2.  Non-emergency services

   Non-emergency LBSs will generally require more precise information
   than is required for emergency call routing.  Therefore, when
   requesting a non-emergency LBS, the endpoint SHOULD include the
   location reference provided by its location provider, and MAY
   additionally provide the location value.  If the provided location
   value is not sufficiently precise to deliver the requested service,
   then the LBS provider should then dereference the location value to
   request location information of sufficient precision from the
   location provider.  If the dereference fails, then the request for
   service may fail as well.

   Note that when the location reference provided by the location
   provider is access-controled, this dereference may require a pre-
   existing authentication and authorization agreement between the LBS
   provider and the location provider.  In such a case, the endpoint may
   not know whether a given non-emergency service is authorized to
   obtain the endpoint's precise location using the location reference.
   The endpoint is always capable of requesting services without knowing
   whether they are authorized; in this way, the endpoint can discover
   authorized services by trial and error.  In order to simplify this
   process, a location provider may supply the endpoint with references
   to authorized service providers, although there is currently no
   standard protocol for this transaction.

5.  Acknowledgements

   This document generalizes the concept of "rough location" that was
   originally discussed in the context of the location hiding problem.
   This concept was put forward by Henning Schulzrinne and Andy Newton,
   among many others, in a long-running ECRIT discussion.

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6.  Security Considerations

   The use of rough location to support emergency calling enables a
   location provider to provide low-precision location with low
   assurance (e.g., of requestor identity) and high-precision location
   with higher assurance.  The fact that lower-precision location has
   lower value -- to location providers and LBS providers as a
   commercial asset, and to targets as private information -- this
   trade-off allows a location provider to avoid the cost of protecting
   location with high-assurance access controls when this location has
   low value.

   However, in order to support emergency services, this expense cannot
   be avoided entirely.  Because PSAPs require high-precision location
   for emergency response planning, a location provider that normally
   provides rough location MUST provide a location URI that a PSAP can
   use to obtain high-precision location.  This constraint means that
   the provided URI MUST have either no access control at all or a
   policy that allows access by appropriate PSAPs (and other emergency
   response systems, e.g., ESRPs).  That is, if such a location URI is
   access controlled, then the location provider MUST be able to
   authenticate requests from PSAPs.

   One reason for a location server to provide location information
   below its maximum precision is to protect the privacy of the target.
   Some location provisioning protocols do not enable the location
   provider to obtain strong assurance of the identity of the location
   recipient; in particular, the location provider may be unable to
   verify that the recipient is the target of the location being
   provided.  Therefore, there is a risk that a sufisticated attacker
   might be able to spoof the identifier (e.g.  IP address) used by the
   location provider to identify the target, and obtain the target's
   location in this way.  One way to mitigate this risk is to provide
   only imprecise location information to the end-point (without
   authentication), and to provide precise information only to trusted
   entities that can authenticate themselves to the location provider.
   Additionally, in some deployment scenarios, location providers have
   concerns about the comprimise of endpoint devices.  Providing only
   imprecise location to the endpoint, prevents malware on a comprised
   device from obtaining the precise location of the target.

   As described in Section 3.1 above, the location provider choosing to
   provide a less precise location than a known location has a
   significant amount of choice in deciding which location to provide:
   Any location that contains the known location and is in the same
   filter region will do.  When the provider is reducing precision for
   privacy purposes, there is a signficant benefit to choosing a random
   location meeting these criteria.  If a watcher is interested in

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   whether or not the endpoint is moving, an imprecise location may
   still reveal that fact if it is constant when the endpoint is at
   rest.  If the provided location is randomized each time it is
   provided, then the watcher is unable to obtain even this level of

7.  IANA Considerations

   This document requests that IANA register a new PIDF-LO 'method'
   token in the registry defined by RFC 4119 [6]

   area-representative:  Location chosen as a representative of a region
      in which the target is located; may not be the target's location

8.  References

8.1.  Normative References

   [1]  Rosen, B., Schulzrinne, H., Polk, J., and A. Newton, "Framework
        for Emergency Calling using Internet Multimedia",
        draft-ietf-ecrit-framework-10 (work in progress), July 2009.

   [2]  Rosen, B. and J. Polk, "Best Current Practice for Communications
        Services in support of Emergency  Calling",
        draft-ietf-ecrit-phonebcp-13 (work in progress), July 2009.

   [3]  Hardie, T., Newton, A., Schulzrinne, H., and H. Tschofenig,
        "LoST: A Location-to-Service Translation Protocol", RFC 5222,
        August 2008.

   [4]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [5]  Polk, J. and B. Rosen, "Location Conveyance for the Session
        Initiation Protocol", draft-ietf-sip-location-conveyance-13
        (work in progress), March 2009.

   [6]  Peterson, J., "A Presence-based GEOPRIV Location Object Format",
        RFC 4119, December 2005.

8.2.  Informative References

   [7]  Schulzrinne, H., "Location-to-URL Mapping Architecture and
        Framework", draft-ietf-ecrit-mapping-arch-04 (work in progress),
        March 2009.

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Authors' Addresses

   Richard Barnes
   BBN Technologies
   9861 Broken Land Pkwy, Suite 400
   Columbia, MD  21046

   Phone: +1 410 290 6169

   Matt Lepinski
   BBN Technologies
   10 Moulton St
   Cambridge, MA  02138

   Phone: +1 617 873 5939

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