ECRIT                                                     H. Schulzrinne
Internet-Draft                                               Columbia U.
Expires: December 11, 2006                              R. Marshall, Ed.
                                                            June 9, 2006

      Requirements for Emergency Context  Resolution with Internet

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document enumerates requirements for the context resolution of
   emergency calls placed by the public using voice-over-IP (VoIP) and
   general Internet multimedia systems, where Internet protocols are
   used end-to-end.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Basic Actors . . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.  High-Level Requirements  . . . . . . . . . . . . . . . . . . . 12
   5.  Identifying the Caller's Location  . . . . . . . . . . . . . . 15
   6.  Emergency Service Identifier . . . . . . . . . . . . . . . . . 18
   7.  Mapping Protocol . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
   10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 27
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 28
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     12.1.  Normative References  . . . . . . . . . . . . . . . . . . 29
     12.2.  Informative References  . . . . . . . . . . . . . . . . . 29
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
   Intellectual Property and Copyright Statements . . . . . . . . . . 31

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1.  Introduction

   Users of both voice-centric (telephone-like) and non voice type
   services (e.g., text communication for hearing disabled users (RFC
   3351 [2]) have an expectation to be able to initiate a request for
   help in case of an emergency.

   Unfortunately, the existing mechanisms to support emergency calls
   that have evolved within the public circuit-switched telephone
   network (PSTN) are not appropriate to handle evolving IP-based voice,
   text and real-time multimedia communications.  This document outlines
   the key requirements that IP-based end systems and network elements,
   such as SIP proxies, need to satisfy in order to provide emergency
   call services, which at a minimum, offer the same functionality as
   existing PSTN services, with the additional overall goal of making
   emergency calling more robust, less costly to implement, and

   This document only focuses on end-to-end IP-based calls, i.e., where
   the emergency call originates from an IP end system and terminates
   into an IP-capable PSAP, conveyed entirely over an IP network.

   Outlined within this document are various functional issues which
   relate to placing an IP-based emergency call, including a description
   of baseline requirements (Section 4), identification of the emergency
   caller's location (Section 5), use of an service identifier to
   declare a call to be an emergency call (Section 6), and finally, the
   mapping function required to route the call to the appropriate PSAP
   (Section 7).

   The primary intent of the mapping protocol is to produce a PSAP URI
   (from a preferred set of URIs, e.g., SIP:URI, SIPS:URI) based on both
   location information [6] and a service identifier in order to
   facilitate the IP end-to-end completion of an emergency call.  Aside
   from obtaining a PSAP URI, the mapping protocol is useful for
   obtaining other information as well.  There may be a case, for
   example, where an appropriate dial string is not known, only
   location.  The mapping protocol can then return a geographically
   appropriate dial string based on the input.

   Since some PSAPs may not immediately support IP, or because some end
   devices (UAs) may not initially support emergency service URNs, it
   may be necessary to also support emergency service identifiers that
   utilize less preferred URI schemes, such as a tel URI in order to
   complete an emergency call via the PSTN.

   Identification of the caller, while not incompatible with the
   requirements for messaging outlined within this document, is

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   considered to be outside the scope of this document.

   Location is required for two separate purposes, first, to support the
   routing of the emergency call to the appropriate PSAP and second, to
   display the caller's location to the call taker for help in
   dispatching emergency assistance to the appropriate location.

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2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [1],
   with the qualification that unless otherwise stated these words apply
   to the design of the mapping protocol, not its implementation or

   Basic emergency service: Basic Emergency Service allows a user to
      reach a PSAP serving its current location, but the PSAP may not be
      able to determine the identity or geographic location of the
      caller, except by having the call taker ask the caller.

   Enhanced emergency service: Enhanced emergency services add the
      ability to identify the caller's identity or location to basic
      emergency services.  (Sometimes, only the caller location may be
      known, e.g., when a call is placed from a public access point that
      is not owned by an individual.)

   Internet Attachment Provider (IAP): An organization that provides
      physical and layer 2 network connectivity to its customers or
      users, e.g., through digital subscriber lines, cable TV plants,
      Ethernet, leased lines or radio frequencies.  Examples of such
      organizations include telecommunication carriers, municipal
      utilities, larger enterprises with their own network
      infrastructure, and government organizations such as the military.

   Internet Service Provider (ISP): An organization that provides IP
      network-layer services to its customers or users.  This entity may
      or may not provide the physical-layer and layer-2 connectivity,
      such as fiber or Ethernet, i.e., it may or may not be the role of
      an IAP.

   Application Service Provider (ASP): The organization or entity that
      provides application-layer services, which may include voice (see
      "Voice Service Provider").  This entity can be a private
      individual, an enterprise, a government, or a service provider.
      An ASP is more general than a Voice Service Provider, since
      emergency calls may use other media beyond voice, including text
      and video.  For a particular user, the ASP may or may not be the
      same organization as his IAP or ISP.

   Voice Service Provider (VSP): A specific type of Application Service
      Provider which provides voice related services based on IP, such
      as call routing, a SIP URI, or PSTN termination.  In this
      document, unless noted otherwise, any reference to "Voice Service
      Provider" or "VSP" may be used interchangeably with "Application/

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      Voice Service Provider" or "ASP/VSP".

   Emergency Service Routing Proxy (ESRP): An ESRP is an emergency call
      routing support entity that invokes the location-to-PSAP URI
      mapping, to return either the URI for the appropriate PSAP, or the
      URI for another ESRP.  (In a SIP system, the ESRP would typically
      be a SIP proxy, but may also be a Back-to-back user agent

   Emergency Call Routing Support (ECRS): An intermediary function which
      assists in the routing of an emergency call via IP.  An ESRP is an
      example of an emergency call routing support entity.

   Public Safety Answering Point (PSAP): Physical location where
      emergency calls are received under the responsibility of a public
      authority.  (This terminology is used by both ETSI, in ETSI SR 002
      180, and NENA.)  In the United Kingdom, PSAPs are called Operator
      Assistance Centres, in New Zealand, Communications Centres.
      Within this document, it is assumed, unless stated otherwise, that
      PSAP is that which supports the receipt of emergency calls over
      IP.  It is also assumed that the PSAP is reachable by IP-based
      protocols, such as SIP for call signaling and RTP for media.

   Location: A geographic identification assigned to a region or feature
      based on a specific coordinate system, or by other precise
      information such as a street number and name.  It can be either a
      civic or geographic location.

   Civic location: A described location based on some defined grid, such
      as a jurisdictional, postal, metropolitan, or rural reference
      system, (e.g., street address).

   Geographic location: A reference to a point which is able to be
      located as described by a set of defined coordinates within a
      geographic coordinate system, (e.g., lat/lon within the WGS-84
      datum).  For example, (2-D) geographic location is defined as an
      x,y coordinate value pair according to the distance North or South
      of the equator and East or West of the prime meridian.

   Location validation: A caller location is considered valid if the
      civic or geographic location is recognizable within an acceptable
      location reference system (e.g., USPS, WGS-84, etc.), and can be
      mapped to one or more PSAPs.  While it is desirable to determine
      that a location exists, validation may not ensure that such a
      location exists, but rather may only ensure that the location
      falls within some range of known values.  Location validation
      ensures that a location is able to be referenced for mapping, but
      makes no assumption about the association between the caller and

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      the caller's location.

   (Location-dependent) emergency dial string: A location-dependent
      emergency dial string should be thought of as the digit sequence
      that is dialed in order to reach emergency services.  There are
      two dial strings described within this document, namely a "home
      emergency dial string", and a "visited emergency dial string".

   Home emergency dial string: A home emergency dial string represents a
      (e.g., dialed) sequence of digits, that is used to initiate an
      emergency call within a geographically correct location of a
      caller if it is considered to be a user's "home" location or

   Visited emergency dial string: A visited emergency dial string
      represents a sequence of digits that is used to initiate an
      emergency call within a geographically correct location of the
      caller if outside the caller's "home" location or vicinity.

   Service identifier: A general identifier that has applicability to
      both emergency and non-emergency contexts (specifically referred
      to within this document as "emergency service identifier").

   Service URN: An implementation of a service identifier, which has
      applicability to both emergency and non-emergency contexts (e.g.,
      urn:service:sos, urn:service:info, etc.)  Within this document,
      service URN is specifically referred to as 'emergency service URN'

   Emergency service identifier (ESI): A specific service identifier
      that is used to request a PSAP URI in order to initiate an
      emergency call, and may be used to mark any call as an emergency
      call.  An ESI is a more general term than 'emergency service URN',
      since it could also refer to an alternate identifier, such as a
      tel URI (Section 6).

   Emergency service URN: An emergency-context specific service URN that
      is an implementation of an emergency service identifier (e.g.,
      urn:service:sos).  Is often referred to as, and is equivalent with
      'sos service URN'.

   PSAP URI: The URI (e.g., SIP:URI, SIPS:URI, XMPP:URI, etc.) at which
      the PSAP may be contacted with an emergency call.  This contact
      could be done directly, or via an intermediary, (e.g., ESRP).

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   Mapping: The process of resolving a location to one or more PSAP URIs
      which directly identify a PSAP, or point to an intermediary which
      knows about a PSAP and that is designated as responsible to serve
      that location.

   Mapping client: A mapping client interacts with the mapping server to
      learn one or more PSAP URIs for a given location.

   Mapping protocol: A protocol used to convey the mapping request and

   Mapping server: The mapping server holds information about the
      location-to-PSAP URI mapping.

   Mapping service: A network service which uses a distributed mapping
      protocol to perform a mapping between a location and a PSAP, or
      intermediary which knows about the PSAP, and is used to assist in
      routing an emergency call.

   (Emergency) caller: The term "caller" or "emergency caller" refer to
      the person placing an emergency call or sending an emergency
      instant message (IM).

   Call taker: A call taker is an agent at the PSAP that accepts calls
      and may dispatch emergency help.  Sometimes the functions of call
      taking and dispatching are handled by different groups of people,
      but these divisions of labor are not generally visible to the
      outside and thus do not concern us here.

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3.  Basic Actors

   In order to support emergency services covering a large physical
   area, various infrastructure elements are necessary, including:
   Internet Attachment Providers (IAPs), Application/Voice Service
   Providers (ASP/VSPs), Emergency Call Routing Support (ECRS)
   providers, mapping service providers, and PSAPs.

   This section outlines which entities will be considered in the
   routing scenarios discussed.

      Information     +-----------------+
          |(1)        |Internet         |   +-----------+
          v           |Attachment       |   |           |
     +-----------+    |Provider         |   | Mapping   |
     |           |    | (3)             |   | Service   |
     | Emergency |<---+-----------------+-->|           |
     | Caller    |    | (2)             |   +-----------+
     |           |<---+-------+         |          ^
     +-----------+    |  +----|---------+------+   |
          ^           |  |   Location   |      |   |
          |           |  |   Information<-+    |   |
          |           +--+--------------+ |(5) |   | (6)
          |              |                |    |   |
          |              |    +-----------v+   |   |
          |   (4)        |    |Emergency   |   |   |
          +--------------+--->|Call Routing|<--+---+
          |              |    |Support     |   |
          |              |    +------------+   |
          |              |          ^          |
          |              |      (7) |          |  +----+--+
          |    (8)       |          +------------>|       |
          +--------------+----------------------->| PSAP  |
                         |                     |  |       |
                         |Application/         |  +----+--+
                         |Voice                |
                         |Service              |
                         |Provider             |

   Figure 1: Framework for emergency call routing

   Figure 1 shows the interaction between the entities involved in the
   call.  There are a number of different deployment choices, as can be
   easily seen from the figure.

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   o How is location information provided to the end host?  It might
   either be known to the end host itself via manual configuration,
   provided via GPS, made available via DHCP (RFC 3825 [4]) or some
   other mechanisms.  Alternatively, location information is used as
   part of call routing and inserted by intermediaries.

   o Is the Internet Attachment Provider also the Application/Voice
   Service Provider?  In the Internet today these roles are typically
   provided by different entities.  As a consequence, the Application/
   Voice Service Provider is typically not able to learn the physical
   location of the emergency caller.

   The overlapping squares in the figure indicate that some functions
   can be collapsed into a single entity.  As an example, the
   Application/Voice Service Provider might be the same entity as the
   Internet Attachment Provider.  There is, however, no requirement that
   this must be the case.  Additionally, we consider that end systems
   might act as their own ASP/VSP, e.g., either for enterprises or for
   residential users.

   Various potential interactions between the entities depicted in
   Figure 1, are described in the following:

   (1) Location information might be available to the end host itself.

   (2) Location information might, however, also be obtained from the
   Internet Attachment Provider (e.g., using DHCP or application layer
   signaling protocols).

   (3) The emergency caller might need to consult a mapping service to
   determine the PSAP (or other relevant information) that is
   appropriate for the physical location of the emergency caller,
   possibly considering other attributes such as appropriate language
   support by the emergency call taker.

   (4) The emergency caller might get assistance for emergency call
   routing by infrastructure elements that are emergency call routing
   support entities, (e.g., an Emergency Service Routing Proxy (ESRP),
   in SIP).

   (5) Location information is used by emergency call routing support
   entities for subsequent mapping requests.

   (6) Emergency call routing support entities might need to consult a
   mapping service to determine where to route the emergency call.

   (7) For infrastructure-based emergency call routing (in contrast to
   UE-based emergency call routing), the emergency call routing support

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   entity needs to forward the call to the PSAP.

   (8) The emergency caller (UE) may interact directly with the PSAP
   (e.g., UE invokes mapping, and initiates a connection), without
   relying on any intermediary emergency call routing support entities.

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4.  High-Level Requirements

   Below, we summarize high-level architectural requirements that guide
   some of the component requirements detailed later in the document.

   Re1.  Application/Voice service provider existence: The initiation of
      an IP-based emergency call SHOULD NOT assume the existence of an
      Application/Voice Service Provider (ASP/VSP).

      Motivation: The caller may not have an application/voice service
      provider.  For example, a residence may have its own DNS domain
      and run its own SIP proxy server for that domain.  On a larger
      scale, a university might provide voice services to its students
      and staff, but might not be a telecommunication provider.

   Re2.  International applicability: Regional, political and
      organizational aspects MUST be considered during the design of
      protocols and protocol extensions which support IP-based emergency

      Motivation: It must be possible for a device or software developed
      or purchased in one country to place emergency calls in another
      country.  System components should not be biased towards a
      particular set of emergency numbers or languages.  Also, different
      countries have evolved different ways of organizing emergency
      services, e.g., either centralizing them or having smaller
      regional subdivisions such as United States counties or
      municipalities which handle emergency calls.

   Re3.  Distributed administration: Deployment of IP-based emergency
      services MUST NOT depend on a sole central administration

      Motivation: The design of the mapping protocol must make it
      possible to deploy and administer emergency calling features on a
      regional or national basis without requiring coordination with
      other regions or nations.  The system cannot assume, for example,
      that there is a single global entity issuing certificates for
      PSAPs, ASP/VSPs, IAPs or other participants.

   Re4.  Multi-mode communication: IP-based emergency calls MUST support
      multiple communication modes, including, for example, audio, video
      and text.

      Motivation: Within the PSTN, voice and text telephony (often
      called TTY or text-phone in North America) are the only commonly
      supported media.  Emergency calling must support a variety of
      media.  Such media should include voice, conversational text (RFC

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      4103 [5]), instant messaging and video.

   Re5.  Alternate mapping sources: The mapping protocol MUST implement
      a mechanism that allows for the retrieval of mapping information
      from different sources.

      Motivation: This provides the possibility of having available
      alternative sources of mapping information when the normal source
      is unavailable or unreachable.

   Re6.  Currency indication: The mapping protocol SHOULD support an
      indicator describing how current the information provided by the
      mapping source is.

      Motivation: This is especially useful when an alternate mapping is
      requested, and alternative sources of mapping data may not have
      been created or updated with the same set of information or within
      the same timeframe.  Differences in currency between mapping data
      contained within mapping sources should be minimized.

   Re7.  Mapping result usability: The mapping protocol MUST return one
      or more URIs that are usable within a standard signaling protocol
      (i.e., without special emergency extensions).

      Motivation: For example, a SIP specific URI which is returned by
      the mapping protocol needs to be usable by any SIP capable phone
      within a SIP initiated emergency call.  This is in contrast to a
      "special purpose" URI, which may not be recognizable by a legacy
      SIP device.

   Re8.  PSAP URI accessibility: The mapping protocol MUST support
      interaction between the client and server where no enrollment to a
      mapping service exists or is required.

      Motivation: The mapping server may well be operated by a service
      provider, but access to the server offering the mapping must not
      require use of a specific ISP or ASP/VSP.

   Re9.  Common data structures and formats: The mapping protocol SHOULD
      support common data structures and formats from the mapping

      Motivation: Location databases should not need to be transformed
      or modified in any unusual or unreasonable way in order for the
      mapping protocol to use the data.  For example, a database which
      contains civic addresses used by location servers may be used for
      multiple purposes and applications beyond emergency service
      location-to-PSAP URI mapping.

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   Re10.  Anonymous mapping: The mapping protocol MUST NOT require the
      true identity of the target for which the location information is

      Motivation: Ideally, no identity information is provided via the
      mapping protocol.  Where identity information is provided, it may
      be in the form of an unlinked pseudonym (RFC 3693 [3]).

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5.  Identifying the Caller's Location

   Location can either be provided directly, or by reference, and
   represents either a civic location, or a geographic location.  An
   important question is how and when to attach location information to
   the VoIP emergency signaling.  In general, we can distinguish three
   modes of operation of how a location is associated with an emergency

   UA-inserted: The caller's user agent inserts the location information
      into the call signaling message.  The location information is
      derived from sources such as GPS, DHCP (see [4] for geographic
      location information and [10]) for civic location information or
      utilizing the Link Layer Discovery Protocol (LLDP) [see

   UA-referenced: The caller's user agent provides a pointer (i.e., a
      location reference), via a permanent or temporary identifier, to
      the location which is stored by a location server somewhere else
      and then retrieved by the PSAP, ESRP, or other authorized service

   Proxy-inserted: A proxy along the call path inserts the location or
      location reference.

   Lo1.  Reference datum: The mapping protocol MUST support the WGS-84
      coordinate reference system and MAY support other coordinate
      reference systems.

      Motivation: Though many different datums exist around the world,
      the WGS-84 datum is recommended here since it is designed to
      describe the whole earth, rather than a single continent, etc.

   Lo2.  Location object/info preservation: The mapping protocol MUST
      retain any location information which is provided to it, even
      after mapping is performed.

      Motivation: The ESRP and the PSAP use the same location
      information object, but for a different purpose.  Therefore, it is
      imperative that the mapping protocol does not remove the location
      information from the messaging, so that it can be provided to the

   Lo3.  Location delivery by-value: The mapping protocol MUST support
      the delivery of location information using a by-value method,
      though it MAY also support de-referencing a URL that references a
      location object.

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      Motivation: The mapping protocol is not required to support the
      ability to de-reference specific location references.

   Lo4.  Alternate community names: The mapping protocol MUST support
      both the jurisdictional community name and the postal community
      name fields within the PIDF-LO data.

      Motivation: A mapping query must be accepted with either or both
      community name fields, and provide appropriate responses.  If a
      mapping query is made with only one field present, and if the
      database contains both jurisdictional and postal, the mapping
      protocol response should return both.

   Lo5.  Validation of civic location: The mapping protocol MUST support
      location validation for civic location (street addresses).

      Motivation: Location validation provides an opportunity to help
      assure ahead of time, whether or not a successful mapping to the
      appropriate PSAP will likely occur when it is required.
      Validation may also help to avoid delays during emergency call
      setup due to invalid locations.

   Lo6.  Validation resolution: The mapping protocol MUST support the
      ability to provide ancillary information about the resolution of
      location data used to retrieve a PSAP URI.

      Motivation: The mapping server may not use all the data elements
      in the provided location information to determine a match, or may
      be able to find a match based on all of the information except for
      some specific data elements.  The uniqueness of this information
      set may be used to differentiate among emergency jurisdictions.
      Precision or resolution in the context of this requirement might
      mean, for example, explicit identification of the data elements
      that were used successfully in the mapping.

   Lo7.  Indication of non-existent location: The mapping protocol MUST
      support a mechanism to indicate and resolve any associated issues
      attributed to a location or a part of a location that is known to
      not exist, despite the receipt of a successful mapping response.

      Motivation: The emergency authority for a given jurisdiction may
      provide a means to resolve addressing problems, e.g., a URI for a
      web service that can be used to report problems with an address.

   Lo8.  Limits to validation: Successful validation of a civic location
      MUST NOT be required to place an emergency call.

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      Motivation: In some cases, a civic location may not be considered
      valid.  This fact should not result in the call being dropped or
      rejected by any entity along the call setup signaling path to the

   Lo9. 3D sensitive mapping: The mapping protocol MUST implement
      support for both 2D and 3D location information, and may accept
      either a 2D or 3D mapping request as input.

      Motivation: It is expected that end devices or location servers
      will provide either 2D or 3D data.  When a 3D request is presented
      within an area only defined by 2D data within the mapping server,
      the mapping result would be the same as if the height/altitude
      dimension was omitted in the request.

   Lo10.  Database type indicator: The mapping protocol MAY support a
      mechanism which provides an indication describing a specific
      "type" of location database used.

      Motivation: It is useful to know the source of the data stored in
      the database used for location validation.  This is applicable for
      either civic or geographic location matching (e.g., USPS, MSAG,
      GDT, etc.).

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6.  Emergency Service Identifier

   The term, service identifier, is a general term that incorporates all
   service URNs [8], but which may also refer to other identifiers which
   are not service URNs, for example, a tel URI.  In protocol exchanges,
   any request to invoke an emergency service along with the specific
   type of emergency service desired, such as fire department or police,
   is indicated by the service URN.

   Since this document addresses only emergency service context specific
   requirements for mapping, the terms service identifier and service
   URN, which have a more general applicability than that of only
   emergency services, are replaced by the terms "emergency service
   identifier" (ESI) and "emergency service URN", respectively,
   throughout this document.  The term "sos service URN" is used
   interchangeably with "emergency service URN".

   Id1.  Emergency service identifier support: The mapping protocol MUST
      be able to return one or multiple emergency service identifiers in
      response to a query.

      Motivation: Since there is a need for any device or network
      element to recognize an emergency call throughout the call setup,
      there is also a need to have the mapping protocol provide support
      for such an identifier.  This is regardless of the device location
      or the ASP/VSP used.  An example of this kind of identifier might
      be the emergency service URN, 'urn:service:sos'.

   Id2.  Emergency service identifier resolution: Where multiple
      emergency service identifiers exist, the mapping protocol MUST be
      able to differentiate between ESIs based on the specific type of
      emergency help requested.

      Motivation: Some jurisdictions may have multiple types of
      emergency services available, (e.g., fire, police, ambulance), in
      which case, it is important that any one could be selected

   Id3.  Extensible emergency service identifiers: The mapping protocol
      MUST support an extensible list of emergency identifiers, though
      it is not required to provide mapping for every possible service.

      Motivation: The use of an emergency service identifier is locally

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   Id4.  Discovery of emergency dial string: There MUST be support for a
      mechanism to discover an existing location-dependent emergency
      dial string, (e.g., "9-1-1", "1-1-2"), contextually appropriate
      for the location of the caller.

      Motivation: Users are trained to dial the appropriate emergency
      dial string to reach emergency services.  There needs to be a way
      to figure out what the dial string is within the local environment
      of the caller.

   Id5.  Home emergency dial string translation: There MUST be support
      for end device translation (e.g., SIP UA) of a home emergency dial
      string into an emergency service identifier.

      Motivation: The UA would most likely be pre-provisioned with the
      appropriate information in order to make such a translation.  The
      mapping protocol would be able to support either type for those
      clients which may not support dial string translation.

   Id6.  Emergency dial string replacement: There SHOULD be support for
      replacement of the original dial string with a reserved emergency
      service identifier for each signaling protocol used for an
      emergency call.  This replacement of the original dial string
      should be based on local conventions, regulations, or preference
      (e.g., as in the case of an enterprise).

      Motivation: Any signaling protocol requires the use of some
      identifier to indicate the called party, and the user terminal may
      lack the capability to determine the actual emergency address
      (PSAP URI).  The use of local conventions may be required as a
      transition mechanism.  Note: Such use complicates international
      movement of the user terminal.  Evolution to a standardized
      emergency service identifier or set of identifiers is preferred.

   Id7.  Emergency service identifier marking: There MUST be support for
      an emergency service identifier to be used for marking the call as
      an emergency call.

      Motivation: Marking ensures proper handling as an emergency call
      by downstream elements that may not recognize, for example, a
      local variant of a logical emergency address, etc.  This marking
      mechanism is assumed to be different than a QoS marking mechanism.

   Id8.  Emergency service identifier not recognized: There MUST be
      support for calls which are initiated as emergency calls even if
      the specific emergency service requested is not recognized, based
      on the emergency service identifier used.

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      Motivation: In order to have a robust system that supports
      incremental service deployment while still maintaining a fallback

   Id9.  Discovery of visited emergency dial strings: There MUST be
      support for a mechanism to allow the end device to learn visited
      emergency dial strings.

      Motivation: Scenarios exist where a user dials a visited emergency
      dial string that is different from the home emergency dial string:
      If a user (i.e., UA operator) visits a foreign country, observes a
      fire truck with 999 on the side, the expectation is one of being
      able to dial that same number to summon a fire truck.  Another use
      case cited is where a tourist collapses, and a "good Samaritan"
      uses the tourist's cell phone to enter a home emergency dial
      string appropriate for that foreign country.

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7.  Mapping Protocol

   There are two basic approaches to invoke the mapping protocol.  We
   refer to these as caller-based and mediated.  In each case, the
   mapping client initiates a request to a mapping server via a mapping
   protocol.  A proposed mapping protocol is outlined in the document
   I-D.hardie-ecrit-lost [9].

   For caller-based resolution, the caller's user agent invokes the
   mapping protocol to determine the appropriate PSAP based on the
   location provided.  The resolution may take place well before the
   actual emergency call is placed, or at the time of the call.

   For mediated resolution, an emergency call routing support entity,
   such as a SIP (outbound) proxy or redirect server invokes the mapping

   Since servers may be used as outbound proxy servers by clients that
   are not in the same geographic area as the proxy server, any proxy
   server has to be able to translate any caller location to the
   appropriate PSAP.  (A traveler may, for example, accidentally or
   intentionally configure its home proxy server as its outbound proxy
   server, even while far away from home.)

   Ma1.  Baseline query protocol: A mandatory-to-implement protocol MUST
      be specified.

      Motivation: An over-abundance of similarly-capable choices appears
      undesirable for interoperability.

   Ma2.  Extensible protocol: The mapping protocol MUST be designed to
      support the extensibility of location data elements, both for new
      and existing fields.

      Motivation: This is needed, for example, to accommodate future
      extensions to location information that might be included in the
      PIDF-LO ([6]).

   Ma3.  Incrementally deployable: The mapping protocol MUST be designed
      in such a way that supports the incremental deployment of mapping

      Motivation: It must not be necessary, for example, to have a
      global street level database before deploying the system.  It is
      acceptable to have some misrouting of calls when the database does
      not (yet) contain accurate PSAP service area information.

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   Ma4.  Any time mapping: The mapping protocol MUST support the ability
      of the mapping function to be invoked at any time, including while
      an emergency call is in process and before an emergency call is

      Motivation: Used as a fallback mechanism only, if a mapping query
      fails at emergency call time, it may be advantageous to have prior
      knowledge of the PSAP URI.  This prior knowledge would be obtained
      by performing a mapping query at any time prior to an emergency

   Ma5.  Anywhere mapping: The mapping protocol MUST support the ability
      to provide mapping information in response to an individual query
      from any (earthly) location, regardless of where the mapping
      client is located, either geographically or by network location.

      Motivation: The mapping client, such as an ESRP, may not
      necessarily be anywhere close to the caller or the appropriate
      PSAP, but must still be able to obtain mapping information.

   Ma6.  Appropriate PSAP: The mapping protocol MUST support the routing
      of an emergency call to the PSAP responsible for a particular
      geographic area.

      Motivation: Routing to the wrong PSAP will result in delays in
      handling emergencies as calls are redirected, and therefore will
      also result in inefficient use of PSAP resources at the initial
      point of contact.  It is important that the location determination
      mechanism not be fooled by the location of IP telephony gateways
      or dial-in lines into a corporate LAN (and dispatch emergency help
      to the gateway or campus, rather than the caller), multi-site LANs
      and similar arrangements.

   Ma7.  Multiple PSAP URIs: The mapping protocol MUST support a method
      to return multiple PSAP URIs which cover the same geographic area.

      Motivation: Two different mapping servers may cover the same
      geographic area, and therefore have the same set of coverage

   Ma8.  Single primary URI per contact protocol: Though the mapping
      protocol supports multiple URIs being returned, it SHOULD return
      only one primary URI per contact protocol used, so that clients
      are not required to select among different targets for the same
      contact protocol.

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      Motivation: There may be two or more URIs returned when multiple
      contact protocols are available (e.g., SIP and SMS).  The client
      may select among multiple contact protocols based on its
      capabilities, preference settings, or availability.

   Ma9.  URI alternate contact: In addition to returning a primary
      contact, the mapping protocol MUST support the return of a PSAP
      URI or contact method explicitly marked as an alternate contact
      for use when a fallback contact is needed.

      Motivation: In response to a mapping request, the mapping server
      will also return an alternate URI.  Implementation details to be
      described within an operational document.

   Ma10.  Non-preferred URI schemes: The mapping protocol MAY support
      the return of a less preferred URI scheme, (e.g., TEL URI).

      Motivation: In order to provide incremental support to non-IP
      PSAPs it may be necessary to be able to complete an emergency call
      via the PSTN.

   Ma11.  URI properties: The mapping protocol MUST support the ability
      to provide ancillary information about a contact that allows the
      mapping client to determine relevant properties of the PSAP URI.

      Motivation: In some cases, the same geographic area is served by
      several PSAPs, for example, a corporate campus might be served by
      both a corporate security department and the municipal PSAP.  The
      mapping protocol should then return URIs for both, with
      information allowing the querying entity to choose one or the
      other.  This determination could be made by either an ESRP, based
      on local policy, or by direct user choice, in the case of caller-
      based methods.

   Ma12.  Mapping referral: The mapping protocol MUST support a
      mechanism for the mapping client to contact any mapping server and
      be referred to another mapping server that is more qualified to
      answer the query.

      Motivation: To help avoid the case of relying on incorrect
      configuration data which may cause calls to fail, particularly for
      caller-based mapping queries.

   Ma13.  Split responsibility: The mapping protocol MUST support the
      division of data subset handling between multiple mapping servers
      within a single level of a civic location hierarchy.

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      Motivation: For example, two mapping servers for the same city or
      county may handle different streets within that city or county.

   Ma14.  URL for error reporting: The mapping protocol MUST support the
      ability to return a URL that can be used to report a suspected or
      known error within the mapping database.

      Motivation: If an error is returned, for example, there needs to
      be a URL which points to a resource which can explain or
      potentially help resolve the error.

   Ma15.  Resiliance to failure: The mapping protocol MUST support a
      mechanism which enables fail over to different (replica) mapping
      server in order to obtain and return a successful mapping to the
      mapping client.

      Motivation: It is important that the failure of a single mapping
      server does not preclude the mapping client's ability to receive
      mapping from a different mapping server.

   Ma16.  Traceable resolution: The mapping protocol SHOULD support the
      ability of the mapping client to be able to determine the entity
      or entities that provided the emergency address resolution

      Motivation: It is important for public safety reasons, that there
      is a method to provide operational traceability in case of errors.

   Ma17.  Minimal additional delay: Mapping protocol execution SHOULD
      minimize the amount of delay within the overall call-setup time.

      Motivation: Since outbound proxies will likely be asked to resolve
      the same geographic coordinates repeatedly, a suitable time-
      limited caching mechanism should be supported.

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8.  Security Considerations

   Threats and security requirements are discussed in a separate
   document, see I-D.ietf-ecrit-security-threats [7] .

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9.  IANA Considerations

   This document does not require actions by the IANA.

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10.  Contributors

   The information contained in this document is a result of a several
   original joint contributions of text, which was then discussed and
   refined by those and many others within the working group.  These
   contributors to the early text include, Nadine Abbott, Hideki Arai,
   Martin Dawson, Motoharu Kawanishi, Brian Rosen, Richard Stastny,
   Martin Thomson, James Winterbottom.

   The contributors can be reached at:

   Nadine Abbott

   Hideki Arai  

   Martin Dawson

   Motoharu Kawanishi

   Brian Rosen  

   Richard Stastny

   Martin Thomson

   James Winterbottom

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11.  Acknowledgments

   In addition to thanking those listed above, we would like to also
   thank Guy Caron, Barry Dingle, Keith Drage, Tim Dunn, Patrik
   Faeltstroem, Clive D.W. Feather, Raymond Forbes, Randall Gellens,
   Michael Haberler, Michael Hammer, Ted Hardie, Gunnar Hellstrom,
   Cullen Jennings, Marc Linsner, Rohan Mahy, Patti McCalmont, Don
   Mitchell, John Morris, Andrew Newton, Steve Norreys, Jon Peterson,
   James Polk, Benny Rodrig, John Rosenberg, Jonathan Rosenberg, John
   Schnizlein, Shida Schubert, James Seng, Byron Smith, Tom Taylor,
   Barbara Stark, Hannes Tschofenig, and Nate Wilcox, for their
   invaluable input.

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12.  References

12.1.  Normative References

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

12.2.  Informative References

   [2]   Charlton, N., Gasson, M., Gybels, G., Spanner, M., and A. van
         Wijk, "User Requirements for the Session Initiation Protocol
         (SIP) in Support of Deaf, Hard of Hearing and Speech-impaired
         Individuals", RFC 3351, August 2002.

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

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

   [5]   Hellstrom, G. and P. Jones, "RTP Payload for Text
         Conversation", RFC 4103, June 2005.

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

   [7]   Taylor, T., "Security Threats and Requirements for Emergency
         Call Marking and Mapping", draft-ietf-ecrit-security-threats-01
         (work in progress), April 2006.

   [8]   Schulzrinne, H., "A Uniform Resource Name (URN) for Services",
         draft-ietf-ecrit-service-urn-03 (work in progress), May 2006.

   [9]   Hardie, T., "LoST: A Location-to-Service Translation Protocol",
         draft-hardie-ecrit-lost-00 (work in progress), March 2006.

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

   [11]  Wijk, A., "Framework for real-time text over IP using SIP",
         draft-ietf-sipping-toip-04 (work in progress), March 2006.

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Authors' Addresses

   Henning Schulzrinne
   Columbia University
   Department of Computer Science
   450 Computer Science Building
   New York, NY  10027

   Phone: +1 212 939 7004

   Roger Marshall (editor)
   TeleCommunication Systems
   2401 Elliott Avenue
   2nd Floor
   Seattle, WA  98121

   Phone: +1 206 792 2424

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