ECRIT                                                     H. Schulzrinne
Internet-Draft                                               Columbia U.
Expires: November 3, 2006                               R. Marshall, Ed.
                                                             May 2, 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 Identifier . . . . . . . . . . . . . . . . . . . . . 18
   7.  Mapping Protocol . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   9.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 26
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 27
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     11.1.  Normative References  . . . . . . . . . . . . . . . . . . 28
     11.2.  Informative References  . . . . . . . . . . . . . . . . . 28
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
   Intellectual Property and Copyright Statements . . . . . . . . . . 30

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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 [8]) 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.

   This document outlines the 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 emergency 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).

   Ideally, the mapping protocol would yield a URI from a preferred set
   of URIs (e.g., SIP:URI, SIPS:URI) which would allow an emergency call
   to be completed using IP end-to-end.  Despite this goal, some PSAPs
   may not immediately have IP based connectivity, and therefore it is
   imperative that the URI scheme not be fixed, in order to ensure
   support for a less preferred set of URIs such as, for example, a TEL
   URI which may be used to complete a call via the PSTN.

   Identification of the caller, while not incompatible with the
   requirements for messaging outlined within this document, is
   considered to be outside the scope of the ECRIT charter.

   Location is required for two separate purposes, first, to route the
   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.

   As used in this document, validation of location does not require

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   that we ascertain as to whether or not the location actually exists.
   For example, validation might only check that the house number in a
   civic address falls within the assigned range, not whether a
   building, known by a specific building number, exists at that
   location.  However, such higher precision validation is desirable.

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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

   Codes: "caller" or "emergency caller" refers to the person placing an
   emergency call or sending an emergency instant message (IM).

   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.

   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.

   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.

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

   Emergency address: The URI (e.g., SIP:URI, SIPS:URI, XMPP:URI, IM:
      URI, etc.) which represents the address of the PSAP useful for the
      completion of an emergency call.

   Emergency call routing support: 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.

   Emergency caller: The user or user device entity which sends his/her
      location to another entity in the network.

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   Emergency identifier: An identifier that marks a call as an emergency

   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
      URL 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

   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.)

   Geographic location: A reference to a locatable point 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.

   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

   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.

   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.

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   Location-dependent emergency dial string: Location-dependent
      emergency dial strings should be thought of as the digit sequence
      that is dialed in order to reach emergency services.  There are
      two dial strings, namely either a "home emergency dial string", or
      a "visited emergency dial string", and is something separate from
      an emergency identifier, since each represents specific emergency
      dial string key sequences which are recognized within a local
      geographic area or jurisdiction.

   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.  Location validation ensures that a location is
      able to be referenced for mapping, but makes no assumption about
      the association between the caller and the caller's location.

   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.

   PSAP (Public Safety Answering Point): 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.

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   PSAP URI: PSAP URI is a general term, used to refer to the output of
      the mapping protocol, and represents either the actual PSAP IP
      address, or the IP address of some other intermediary, e.g., an
      ESRP, which points to the actual PSAP.

   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.

   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/
      Voice Service Provider" or "ASP/VSP".

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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), PSAPs as endpoints for emergency calls, mapping
   services or other infrastructure elements that assist during the call

   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, or obtained via a third party method.  Even if
   location information is known to the network it might be made
   available to the end host via DHCP (RFC 3825 [2]) or some other
   mechanism.  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 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

   (5) Location Information is used by emergency call routing 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

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   UE-based emergency call routing), the emergency call routing support
   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 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 handle emergency calls.

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

      Motivation: The design 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: In 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 4103 [10]),

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      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 [9]).

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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 as a geographic location.  How
   does the location (or location reference) become associated with the
   call?  In general, we can distinguish three modes of operation of how
   a location is associated with an emergency call:

   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 (RFC 3825 [2]) and
      I-D.ietf-geopriv-dhcp-civil [7]) or utilizing the Link Layer
      Discovery Protocol (LLDP) [see IEEE8021AB].

   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 service 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.

   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 not remove location
      Information so that the PSAP can still receive the caller

   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.

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

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   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), prior
      to initiating an emergency call.

      Motivation: Location validation provides an opportunity to help
      assure ahead of time, whether or not 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 provisioning systems will accept
      both 2D and 3D data.  When a 3D request is presented to an area
      only defined by 2D data, the mapping result would be the same as
      if the height/altitude dimension was omitted in the request.

   Lo10.  Location validation indicator: The mapping protocol MAY
      support a mechanism which indicates whether a civic location does
      or does not fall within an existing range of addresses listed
      within a referenced address database.

      Motivation: It is helpful to get an indication of whether the
      validation process worked or not.

   Lo11.  Matched element indication: The mapping protocol MAY support a
      mechanism which returns an indication of specific data elements
      which were matched as a result of a validation query.

      Motivation: Given a query using "123 Main St. Anytown"
      (represented, as A1, A2, A3, A5 in this example) it may be helpful
      to receive an indication that the validation process matched only
      elements A2, A3, A5 (but not A1).

   Lo12.  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 Identifier

   Id1.  Emergency identifier support: The mapping protocol MUST support
      one or more emergency identifiers for delivery back to mapping
      clients to be used for call setup purposes.

      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 "urn:service:sos".

   Id2.  Emergency identifier resolution: Where multiple emergency
      identifiers exist, the mapping protocol MUST be able to
      differentiate between identifiers 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.  Emergency identifier marking: The mapping protocol MUST include
      an emergency identifier with the signaling, if one does not exist,
      for the purpose of 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.

   Id4.  Prevention of fraud: If a call is identified as an emergency
      call, the mapping protocol MUST support that call being
      successfully routed to a PSAP.

      Motivation: This prevents use of the emergency call indication to
      gain access to call features or authentication override for non-
      emergency purposes.

   Id5.  Extensible emergency 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 identifier is locally

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   Id6.  Discovery of emergency dial string: The mapping protocol MUST
      support a mechanism to discover an existing location-dependent
      emergency dial string, (e.g., "9-1-1", "1-1-2"), which are
      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.

   Id7.  Home emergency dial string translation: The mapping protocol
      MUST support end device translation (e.g.  SIP UA) of a home
      emergency dial string into an emergency 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.

   Id8.  Emergency dial string replacement: The mapping protocol SHOULD
      support replacement of the original dial string with a reserved
      emergency 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 identifier or set of identifiers is preferred.

   Id9.  Emergency identifier not recognized: The mapping protocol MUST
      support calls which are initiated as emergency calls even if the
      specific emergency service requested is not recognized, based on
      the emergency identifier used.

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

   Id10.  Discovery of visited emergency dial strings: The mapping
      protocol MUST support a mechanism to allow the end device to learn
      visited emergency dial strings.

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      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 invoking a mapping service.  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 [6].

   For caller-based resolution, the caller's user agent invokes a
   mapping service 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, a call signaling server, such as a SIP
   (outbound) proxy or redirect server invokes the mapping service.

   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.  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 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

   Ma2.  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.

   Ma3.  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.

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

   Ma4.  Multiple response URIs: The mapping protocol MUST support the
      possible inclusion of multiple URIs in a mapping response.

      Motivation: Multiple URIs may be available from the mapping

   Ma5.  URI alternate contact: In addition to returning a primary
      contact, the mapping protocol MUST support the return of a URI or
      contact method explicitly marked as an alternate contact.

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

   Ma6.  URL properties: The mapping protocol MUST support the ability
      to provide ancillary information about a contact or URI that
      allows the mapping client to determine relevant properties of the

      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 URLs 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.

   Ma7.  Traceable resolution: The mapping protocol SHOULD support the
      ability of the mapping client to be able to determine the entity
      or entities which 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.

   Ma8.  URI for error reporting: The mapping protocol MUST support the
      return of a URI 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 URI which points to a resource which can explain or
      potentially help resolve the error.

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   Ma9.  Resilience against failure: The mapping protocol MUST support a
      mechanism which enables fail over to different (replica) mapping
      server in order to obtain a successful mapping.

      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.

   Ma10.  Incrementally deployable: The mapping protocol MUST be
      designed in such a way that supports the incremental deployment of
      mapping services.

      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.

   Ma11.  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.

      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

   Ma12.  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

      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.

   Ma13.  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 (RFC 4119 [3]).

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   Ma14.  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.

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

   Ma15.  Baseline query protocol: A mandatory-to-implement protocol
      MUST be specified.

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

   Ma16.  Multiple PSAP URIs: The mapping protocol MUST support a method
      to receive multiple PSAP URIs which cover the same geographic

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

   Ma17.  Single URI per contact protocol: Though the mapping protocol
      supports the return of multiple URIs, it SHOULD return only one
      URI per contact protocol, so that clients are not required to
      select among different targets for the same contact protocol.

      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.

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8.  Security Considerations

   Security considerations are discussed in the ECRIT security document
   I-D.ietf-ecrit-security-threats [4] .

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9.  Contributors

   The information contained in this document is a result of a joint
   effort based on individual contributions by those involved in the
   ECRIT WG.  The contributors 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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10.  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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11.  References

11.1.  Normative References

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

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

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

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

   [5]  Schulzrinne, H., "A Uniform Resource Name (URN) for Services",
        draft-ietf-ecrit-service-urn-02 (work in progress), April 2006.

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

   [7]  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.

11.2.  Informative References

   [8]   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.

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

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

   [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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