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Versions: 00 01                                                         
ecrit                                                     H. Schulzrinne
Internet-Draft                                               Columbia U.
Expires: November 2, 2005                               R. Marshall, Ed.
                                                                May 2005

Requirements for Emergency Context Resolution with Internet Technologies

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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   This Internet-Draft will expire on November 2, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document enumerates requirements for 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.   High-Level Requirements  . . . . . . . . . . . . . . . . . .   9
   4.   Emergency Address  . . . . . . . . . . . . . . . . . . . . .  11
   5.   Identifying the Caller Location  . . . . . . . . . . . . . .  12
   6.   Identifying the Appropriate PSAP . . . . . . . . . . . . . .  13
   7.   Emergency Address Directory  . . . . . . . . . . . . . . . .  16
   8.   Supplemental Information . . . . . . . . . . . . . . . . . .  17
   9.   Security Considerations  . . . . . . . . . . . . . . . . . .  18
   10.  Contributors . . . . . . . . . . . . . . . . . . . . . . . .  19
   11.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  20
   12.  References . . . . . . . . . . . . . . . . . . . . . . . . .  21
     12.1   Normative References . . . . . . . . . . . . . . . . . .  21
     12.2   Informative References . . . . . . . . . . . . . . . . .  21
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  21
        Intellectual Property and Copyright Statements . . . . . . .  23

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1.  Introduction

   Users of telephone-like services expect to be able to call for
   emergency help, such as police, the fire department or an ambulance,
   regardless of where they are, what (if any) service provider they are
   using and what kind of device they are using.  Unfortunately, the
   mechanisms for emergency calls that have evolved in the public
   circuit-switched telephone network (PSTN) are not quite appropriate
   for evolving IP-based voice, text and real-time multimedia
   communications.  This document outlines the key requirements that end
   systems and network elements such as SIP proxies need to satisfy in
   order to provide emergency call services that offer at least the same
   functionality as existing PSTN services, with the goal of making
   emergency calling more robust, cheaper to implement and multimedia-

   In the future, users of other real-time and near real-time services
   may also expect to be able to summon emergency help.  For example,
   instant messaging (IM) users may want to use such services.  IM is
   particularly helpful for hearing-disabled users (RFC 3351 [4]) and in
   cases where bandwidth is scarce.

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

   This document identifies functional and security issues for
   determining the correct emergency identifier, for identifying the
   appropriate PSAP (emergency address) and for identifying the caller
   and its current location.

   Emergency calls need to be identified (Section 6).  Emergency
   identifiers are used by the emergency caller to declare a call to be
   an emergency call.  The device MUST recognize the emergency
   identifiers used and convert them to an emergency address to guide
   the call to a PSAP.  The emergency address MUST be a predefined
   "sip", "sips" or "tel" URI scheme.

   Emergency calls need to be routed to the appropriate PSAP (ref.
   Section 6).  Several terms are used for causing the call signaling to
   reach the geographically appropriate PSAP.  This has been referred to
   as call routing, (PSAP) lookup or location mapping, all capturing
   aspects of the problem.

   Emergency calls need to identify who placed the call (Section 7).  In
   most jurisdictions, callers do not have a choice as to whether they
   want to reveal their location or identity; such disclosure is

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   typically mandated by law.

   Emergency calls need to identify the location from which the call is
   initiated (Section 5).  The caller location needs to be identified
   for two purposes, namely to route the call to the appropriate PSAP
   and to display the caller location to the call taker to simplify
   dispatching emergency assistance to the correct location.

   Emergency calls may not be subject to access restrictions placed on
   non-emergency calls.  Also, some call features may interfere with
   emergency calls, particularly if triggered accidentally (Section 7).

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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] and
   indicate requirement levels for compliant implementations.

   Since a requirements document does not directly specify an implement
   able protocol, these compliance labels should be read as indicating
   requirements for the protocol or architecture, rather than an

   For lack of a better term, we will use the term "caller" or
   "emergency caller" to refer to the person placing an emergency call
   or sending an emergency IM.

   Access Infrastructure Provider (AIP): An organization that provides
      physical network connectivity to its customers or users, e.g.
      through digital subscriber lines, cable TV plants, Ethernet,
      leased lines or radio frequencies.  This entity may or may not
      also provide IP routing, IP addresses, or other Internet protocol
      services.  Examples of such organizations include
      telecommunication carriers, municipal utilities, larger
      enterprises with their own network infrastructure, and government
      organizations such as the military.

      [Ed.  AIP vs. IAP vs. ? not yet clear as to general agreement on a
      single term.]

   address: A description of a location of a person, organization, or
      building, most often consisting of numerical and text elements
      such as street number, street name, and city arranged in a
      particular format.

   administrative domain: An area or group of services falling with in a
      specific category or jurisdictional boundary.

   Application (Voice) Service Provider (ASP, VSP): The organization
      that provides voice or other application-layer services, such as
      call routing, a SIP URI or PSTN termination.  This organization
      can be a private individual, an enterprise, a government or a
      service provider.  We avoid the term voice service provider as
      emergency calls are likely to use other media, including text and
      video, in the future.  For a particular user, the ASP may not be
      the same organization as the AIP or ISP.

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   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).

   domain authentication and validation entity: A node that has
      authority within a given domain to authenticate and validate user
      location information.

   Emergency Control Center (ECC): Facilities used by emergency
      organizations to accept and handle emergency calls.  A PSAP
      (below) forwards emergency calls to the emergency control center,
      which dispatches police, fire, rescue and other emergency
      services.  An ECC serves a limited geographic area.  A PSAP and
      ECC can be combined into one facility (ETSI SR 002 180
      definition).  We assume that the ECC is reachable by IP-based
      protocols, such as SIP for call signaling and RTP for media.

   emergency address: The  sip:uri, sips:uri, or tel:uri which
      represents the network address of the PSAP useful for the
      completion of a VoIP emergency call.

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

   emergency identifier: The numerical and/or text identifier which is
      supplied by a user or a user device, which identifies the call as
      an emergency call and is translated into an emergency address for
      call routing and completion.

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

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   ESRP (Emergency Services Routing Proxy): An ESRP is a call routing
      entity that invokes the location-to-URL mapping, which in turn may
      return either the URL for another ESRP or the PSAP.  (In a SIP
      system, the ESRP would typically be a SIP proxy, but could also be
      a Back-to-back user agent (B2BUA).

   geocoding: The process of finding the location of a street address on
      a map.  The location can be an x,y coordinate or a feature such as
      a street segment, postal delivery location, or building.  In GIS,
      geocoding requires a reference dataset that contains address
      attributes for the geographic features in the area of interest.

   geographic coordinates: A representation (measurement) of a location
      on the earth's surface expressed in degrees of latitude and

   geographic coordinate system: A reference system that uses latitude
      and longitude to define the locations of points on the surface of
      a sphere or spheroid.

   geographic transformation: A method of converting data between two
      geographic coordinate systems (datums).

   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 WGS-84 datum)

   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.

   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 address.  In the geocoding process,
      the location is defined with an x,y coordinate value according to
      the distance north or south of the equator and east or west of the
      prime meridian.

   Location Key (LK): A key identifier used to query a location server
      in order to retrieve a specific end user or end user device

   location validation: A caller location is considered valid if the
      civic or geographic location is recognizable within an acceptable
      location reference systems (e.g.  USPS, WGS84, etc.), and can be
      mapped to one or more PSAPs.  Location validation ensures that a
      location is reference able, but makes no assumption about the

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      association between the caller and the caller's location.

   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.

   x,y coordinates: A pair of values that represents the distance from
      an origin (0,0) along two axes, a horizontal axis (x) representing
      east-west, and a vertical axis (y) representing north-south.  On a
      map, x,y coordinates are used to represent features at the
      location they are found on the earth's spherical surface.

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3.  High-Level Requirements

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

   R1.  Application Service Provider:  The existence of a Application
      Service Provider (ASP) MUST NOT be assumed.

      Motivation: The caller may not have a voice service provider,
      i.e., a corporate entity that provides voice services as a
      business.  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.

   R2.  International:  The protocols and protocol extensions developed
      MUST support regional, political and organizational differences.

      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.

   R3.  Distributed Administration:  Deployment of emergency services
      MUST NOT depend on a sole central administration authority.

      Motivation: Once common standards are established, it must be
      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, ASPs, AIPs or other participants.

   R4.  Multiple Modes:  Multiple communication modes, including
      Multimedia data and services MUST be supported.

      Motivation: Emergency calling must support a variety of media, not
      just voice and TDD (telecommunication device for the deaf) beyond
      the capabilities of  current limitations.  Such additional media
      should include conversational text, instant messaging and video.
      In addition, it should be possible to convey telemetry data, such
      as data from automobile crash sensors.

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   R5.  Minimum Connectivity:  An emergency call should succeed as long
      as there is a working network path between the caller and the
      PSAP.  In particular, reliance during call set-up and calls on
      entities and network paths that are located elsewhere should be

      Example: A caller in New York who needs to contact a PSAP in the
      same city shouldn't have to get information from some entity in
      Texas to make that call, as the call would then fail if the New
      York to Texas path is unavailable.  (To avoid this, the caller
      could, for example, have cached mapping information, use a local
      server that has the necessary information, or use other mechanisms
      to avoid such off-path dependencies.)

      [Ed.  No resolution yet agreed to for the above requirement.]

   R6.  Incremental Deployment The output of the ECRIT mapping protocol
      will be one or more URIs that can be used as the target of an
      emergency communication.  These must be usable by an appropriately
      capable device even if that device has no knowledge of the mapping
      protocol.  As an example, if the mapping protocol returns a SIP
      URI any SIP-capable phone should be able to use it as a target of
      the call; no special extension to SIP should be required.

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4.  Emergency Address

   A1.  Universal: Each device and all network elements MUST recognize
      one or more universal (global) emergency identifiers, regardless
      of the location of the device, the service provider used (if any)
      or other factors.  Examples of these might include: 911, 112, and

      Motivation:  SIP and other call signaling protocols are not
      specific to one country or service provider and devices are likely
      to be used across national or service provider boundaries.  Since
      services such as disabling mandatory authentication for emergency
      calls requires the cooperation of outbound proxies, the outbound
      proxy has to be able to recognize the emergency address and be
      assured that it will be routed as an emergency call.  Thus, a
      simple declaration on a random URI that it is an emergency call
      will likely lead to fraud and possibly attacks on the network
      infrastructure.  A universal address also makes it possible to
      create user interface elements that are correctly configured
      without user intervention.  UA features could be made to work
      without such an identifier, but the user interface would then have
      to provide an unambiguous way to declare a particular call an
      emergency call.

   A3.  Recognizable: Emergency calls MUST be recognizable by user
      agents, proxies and other network elements.  To prevent fraud, an
      address identified as an emergency number for call features or
      authentication override MUST also cause routing to a PSAP.

   A4.  Minimal configuration: Any local emergency identifiers SHOULD be
      configured automatically, without user intervention.

      Motivation:  A new UA "unofficially imported" into an organization
      from elsewhere should have the same emergency capabilities as one
      officially installed.

   A6.  Backwards-compatible: Existing devices that predate the
      specification of emergency call-related protocols and conventions
      MUST be able reach a PSAP.

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5.  Identifying the Caller Location

   This section supplements the requirements outlined in RFC 3693 [5].
   Thus, the requirements enumerated there are not repeated here.  In
   general, we can distinguish three modes of operation:

   UA-inserted: The caller's user agent inserts the location
      information, derived from sources such as GPS, DHCP or link-layer
      announcements (LLDP).

   UA-referenced: The caller's user agent provides a reference, via a
      permanent or temporary identifier, to the location which is stored
      by a location service somewhere else and then retrieved by the

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

   L6.  Validation of civic location: It MUST be possible to validate an
      address prior to its use in an actual emergency call.

      Motivation:  Location validation refers to a process to determine
      whether or not a given civic location is valid or not.  A location
      is said to be valid if it can be mapped exactly to a unique
      emergency address for a PSAP, known to the emergency services
      directory/mapping database.

   L10.  Preferred datum: The preferred geographic coordinate system for
      emergency calls SHALL be WGS-84.

   L28.  Location Provided: If location is provided to the routing
      proxy, it MUST be provided to the PSAP.

      Motivation:  Transmission of the current location of the
      contacting device to the PSAP.

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6.  Identifying the Appropriate PSAP

   From the previous section, we take the requirement of a single (or
   small number of) emergency addresses which are independent of the
   caller's location.  However, since for reasons of robustness,
   jurisdiction and local knowledge, PSAPs only serve a limited
   geographic region, having the call reach the correct PSAP is crucial.
   While a PSAP may be able to transfer an errant call, any such
   transfer is likely to add tens of seconds to call setup latency and
   is prone to errors.  (In the United States, there are about 6,100

   There appears to be two basic architectures for translating an
   emergency identifier into the correct PSAP emergency address.  We
   refer to these as caller-based and mediated.  In caller-based
   resolution, the caller's user agent consults a directory and
   determines the correct PSAP based on its location.  We assume that
   the user agent can determine its own location, either by knowing it
   locally or asking some third party for it.  A UA could conceivably
   store a complete list of all PSAPs across the world, but that would
   require frequent synchronization with a master database as PSAPs
   merge or jurisdictional boundaries change.

   For mediated resolution, a call signaling server, such as a SIP
   (outbound) proxy or redirect server performs this function.  Note
   that the latter case includes the architecture where the call is
   effectively routed to a copy of the database, rather than having some
   non-SIP protocol query the database.  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

   The resolution may take place well before the actual emergency call
   is placed, or at the time of the call.

   The problem is harder than for traditional web or email services.
   There, the originator knows which entity it wants to reach,
   identified by the email address or HTTP URL.  However, the emergency
   caller only dialed an emergency identifier.  Depending on the
   location, any of several ten thousand PSAPs around the world could be
   valid.  In addition, the caller probably does not care which specific
   PSAP answers the call, but rather that it be an accredited PSAP, e.g.
   one run by the local government authorities.  (Many PSAPs are run by
   private entities.  For example, universities and corporations with
   large campuses often have their own emergency response centers.)

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   I1.  Correct PSAP: Calls Must be routed to the PSAP responsible for
      this particular geographic area.

      Motivation:  In particular, the location determination should 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.

   I3.  Multi-stage resolution: A mapping server for a large geographic
      area SHOULD be able to refer clients to mapping servers
      responsible for subsets of the geographic area.

      Motivation: In some cases, an initial mapping may provide a single
      URL for a large geographic area.  The ESRP identified by that URL
      then re-invokes the mapping protocol on a different database to
      obtain another URL for an ESRP or PSAP covering a smaller area.

   I4.  Return multiple PSAPs: The mapping protocol MUST be able to
      return multiple URLs for different PSAPs that cover the same area.

      The mapping protocol MUST provide additional information that
      allows the querying entity 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.  The choice would typically be made by an ESRP based on
      local policy, not by a human user.

   I7.  Traceable resolution: The entity requesting mapping SHOULD be
      able to definitively and securely determine the entity or entities
      who provided the emergency address resolution information.

   I8.  Resilience against server failure: A client MUST be able to fail
      over to another replica of the mapping server, so that a failure
      of a server does not endanger the ability to perform the mapping.

   I10.  Incrementally deployable: The mapping function MUST be capable
      of being deployed incrementally.  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 boundary

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   I13.  Existing infrastructure support: It SHOULD be possible for the
      mapping function to provide information that allows the requesting
      entity to determine if ecrit compatible emergency call support is
      available in the jurisdiction where the location is proferred for
      mapping.  Where ecrit compatible emergency calling is NOT
      available, the mapping function MAY yield information which could
      be used to route emergency calls using existing, country specific
      methods.  For example, a tel URI may be provided for a PSTN routed
      call, or a routing code which has meaning only within a country
      specific routing mechanism.

   I25.  Mapping can be requested from anywhere: The mapping protocol
      MUST be able to provide the mapping regardless of where the
      querier is located, either geographically or by network location.

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

   I31: In response to a mapping request, a server will normally provide
      a URI or set of URIs for contacting the appropriate PSAP.  The
      protocol must also be to return a URI or contact method explicitly
      marked as an alternate contact.  When this is used will be
      described in an operational document.

   I39: It SHOULD be possible to have updates of location (which may
      occur when measuring devices provider early, but imprecise "first
      fix" location) which can change routing of calls.

   I40.  The mapping protocol MUST be extensible to allow for the
      inclusion of new location fields.

      Motivation: This is needed, for example, to accommodate future
      extensions to location information that might be included in the

   I41.  Split responsibility: The mapping protocol MUST allow that
      within a single level of the civic address hierarchy, multiple
      mapping servers handle subsets of the data elements.

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

   I42.  The mapping function MUST be able to be invoked at any time,
      including while an emergency call is in process.

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7.  Emergency Address Directory

   D1.  PSAP Identification:  The mapping information MUST be available
      without having to enroll with a service provider.

      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 VSP.

   D5.  Call setup latency: The directory lookup SHOULD minimize any
      added delay to the call setup.

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

   D7.  Referral: The querier MUST be able to contact any server and be
      referred to another server that is more qualified to answer the

      Motivation:  This requirement alleviates the potential for
      misconfigurations to cause calls to fail, particularly for caller-
      based queries.

   D9.  Baseline query protocol: A mandatory-to-implement protocol MUST
      be specified.

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

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8.  Supplemental Information

   SD1 The format both of the query and of the result returned by the
      protocol must be extensible to accommodate new types of

      Motivation: In addition to information sent with the call,
      additional information may be available, supplemental to the call,
      which is retrieved from internal or external databases using a key
      to the information included with the call.  This key may also
      include information to identify/address the database.

   SD2 Additional information MAY be available to the call taker based
      on the location of the caller.

   SD3 Additional information MAY be available to the call taker based
      on the owner of the structure.

   SD4 Additional information MAY be available to the call taker based
      on the tenant of the structure.

   SD5 Where a vehicle is involved, additional information MAY be

   SD6 Additional information MAY be available based on the Address of
      Record (AoR) of the caller.  In this context, AoR equates to the

   SD7 Consideration SHOULD be given to permitting users to have domain
      independent mechanisms to supply information related to the
      caller, for example, another datum related to user.

   SD8.  Additional Data:  Transfer of additional data SHOULD be

      Motivation:  Capabilities to contact PSAP by automatic means and
      for the transfer of additional information (alarm equipment, cars,
      buses, trucks with dangerous loads, ...)

   SD9 Mechanism MUST be provided to automatically generate and provide
      misroute and location error reports.

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9.  Security Considerations

   Note: Security Considerations are referenced in the ECRIT security
   document [3].

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10.  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          nabbott@telcordia.com

   Hideki Arai            arai859@oki.com

   Martin Dawson          mdawson@nortelnetworks.com

   Motoharu Kawanishi     kawanishi381@oki.com

   Brian Rosen            br@brianrosen.net

   Richard Stastny        Richard.Stastny@oefeg.at

   Martin Thomson         marthom@nortelnetworks.com

   James Winterbottom     winterb@nortelnetworks.com

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11.  Acknowledgments

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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.

   [2]  Polk, J., "Requirements for Session Initiation Protocol Location
        Conveyance", draft-ietf-sipping-location-requirements-02 (work
        in progress), October 2004.

   [3]  Tschofenig, H., "Security Threats and Requirements for Emergency
        Calling", draft-tschofenig-ecrit-security-threats-00 (work in
        progress), May 2005.

12.2  Informative References

   [4]  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.

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

   [6]  National Emergency Number Assocation, "NENA technical
        information document on the interface between the E9-1-1 service
        providers network and the Internet protocol (IP) PSAP",
        NENA NENA-08-501, February 2003.

Authors' Addresses

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

   Phone: +1 212 939 7004
   Email: hgs+ecrit@cs.columbia.edu
   URI:   http://www.cs.columbia.edu

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

   Phone: +1 206 792 2424
   Email: rmarshall@telecomsys.com
   URI:   http://www.telecomsys.com

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