ECRIT                                                     H. Schulzrinne
Internet-Draft                                               Columbia U.
Expires: August 6, 2006                                 R. Marshall, Ed.
                                                        February 2, 2006

Requirements for Emergency Context Resolution with Internet Technologies

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   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.  Basic Actors . . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.  High-Level Requirements  . . . . . . . . . . . . . . . . . . . 12
   5.  Identifying the Caller Location  . . . . . . . . . . . . . . . 14
   6.  Emergency Identifier . . . . . . . . . . . . . . . . . . . . . 16
   7.  Mapping Protocol . . . . . . . . . . . . . . . . . . . . . . . 19
   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 messaging for hearing disabled users, (RFC 3351
   [7]) 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 multimedia-capable.

   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 outlines the various functional issues which relate to
   making 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).

   Identification of the caller, while not incompatible with the
   requirements for messaging outlined within this document, is not
   currently considered within the scope of the ECRIT charter, and is
   therefore, left for a future draft to describe.

   Note: 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 correct location.

   Ideally, the mapping protocol would yield a URI from a preferred set
   of URIs (e.g. sips:uri; sip:uri), which would allow an emergency call
   to be completed using IP end-to-end (possibly via the Internet).
   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 a TEL URI which may be used to complete a call over the

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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 a protocol to
   implement, 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.

   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 defined as something more general than a Voice Service
      Provider, since emergency calls are sometimes likely to use other
      media, including text and video.  Note: For a particular user, the
      ASP may or may not be the same organization as the IAP and/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 scheme (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 caller: The user or user device entity which sends his/her
      location to another entity in the network.

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   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,
      useful for call routing and completion of the emergency call.

   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.)

   ESRP (Emergency Service 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).

   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)

   Home emergency dial-string: A home emergency dial-string (ref.
      Location-dependent emergency identifier) represents a sequence of
      digits that is used to initiate an emergency call within a
      geographic vicinity considered to be a user's "home" location or

   Internet Attachment Provider (IAP): 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.  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.

   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.  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.

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   Location-dependent emergency identifier: Location-dependent emergency
      identifiers, also referred to as "emergency dial-strings" within
      this document, 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 a
      universal emergency identifier, since each represents specific
      emergency identifiers 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 systems (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: Process of resolving a location to a URI (or multiple URIs).

   Mapping client: A Mapping Client interacts with the Mapping Server to
      learn one or multiple 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 URI mappings.

   Mapping service: A network service which uses a distributed mapping
      protocol to provide information about the 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 some other intermediary's IP address, which points to
      the actual PSAP.

   Universal identifier: An emergency identifier which is recognized by
      any compatible endpoint, from any geographic location as useful
      for initiating an emergency request.  A general approach to using
      universal identifiers is outlined in the service URN draft
      (I-D.schulzrinne-sipping-service [5]).

   Visited emergency dial-string: A visited emergency dial-string (ref.
      Location-dependent emergency identifier) represents a sequence of
      digits that is used to initiate an emergency call within a
      geographic vicinity other than a user's "home" location or

   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.

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3.  Basic Actors

   In order to support emergency services covering a large physical area
   various infrastructure elements are necessary: Internet Attachment
   Providers, Application/Voice Service Providers, PSAPs as endpoints
   for emergency calls, mapping services or other infrastructure
   elements that assist in during the call routing and potentially many
   other entities.

   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<-+    |   |
          |           +--+--------------+ |(8) |   | (5)
          |              |    +-----------v+   |   |
          |   (4)        |    |Emergency   |   |   |
          +--------------+--->|Call Routing|<--+---+
          |              |    |Support     |   |
          |              |    +------------+   |
          |              |       ^             |
          |              |   (6) |        +----+--+
          |    (7)       |       +------->|       |
          +--------------+--------------->| PSAP  |
                         |                |       |
                         |Application/    +----+--+
                         |Voice                |
                         |Service              |
                         |Provider             |

   Figure 1: Framework

   Figure 1 shows the interaction between the entities involved in the
   call.  There are a number of different deployment choices, as it can
   be easily seen from the figure.  The following deployment choices
   need to be highlighted:

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   o How is location information provided to the end host?  It might
   either be known to the end host itself (due to manual configuration
   or provided via GPS) or available via a third party.  Even if
   location information is known to the network it might be made
   available to the end host.  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.

   Please note that the overlapping squares aim to indicate that certain
   functionality 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 and they might also operate the
   PSAP.  There is, however, no requirement that this must be the case.
   Additionally it is worth pointing out that end systems might be its
   own VSP, e.g., for enterprises or residential users.

   Below, we describe various interactions between the entities shown in
   Figure 1 are described:

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

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

   o (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 (and considering other attributes such as a
   certain language support by the Emergency Call Takers).

   o (4) The Emergency Caller might get assistance for emergency call
   routing by infrastructure elements (referred as Emergency Call
   Routing Support entities).  In case of SIP these entities are

   o (5) Individual Emergency Call Routing Support entities might need
   to consult a mapping service to determine where to route the
   emergency call.

   o (6) The Emergency Call Routing Support entities need to finally
   forward the call, if infrastructure based emergency call routing is

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   o (7) The emergency caller might interact directly with the PSAP
   without any Emergency Call Routing Support entities.

   o (8) Location Information is used by emergency call routing entities
   to determine appropriate PSAP mapping.

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

      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: 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.

   Re3.  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, IAPs or other participants.

   Re4.  Multiple Modes: Multiple communication modes, such as audio,
      video and text messaging MUST be supported.

      Motivation: In PSTN, voice and text telephony (often called TTY or
      textphone 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 [9]), instant
      messaging and video.

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   Re5.  Alternate Mapping Sources: The mapping protocol SHOULD allow
      for alternative redundant sources of mapping information, possibly
      of different degrees of currency.

      Motivation: This provides the possibility of having available
      alternative sources of mapping information when the normal source
      is unavailable or unreachable, without specifying the means by
      which the alternative source is created or updated.

   Re6.  Incremental Deployment: The ECRIT mapping protocol MUST return
      URIs that are usable by a standard signaling protocol (i.e.,
      without special emergency extensions) unless an error is returned.

      Motivation: The format of the output returned by the mapping
      protocol is in a standard format for communication protocol.  For
      example, it should return something SIP specific (e.g.  URI), that
      any SIP capable phone would be able to use if used in a SIP
      context.  Special purpose URIs would not be understood by "legacy"
      SIP devices since they do not have knowledge about the mapping
      protocol, and therefore are not to be used.

   Re7.  Ubiquitous Triggering: It MUST be possible to invoke the
      mapping protocol at any time, from any location, by any client
      which supports the mapping protocol.

      Motivation: While end devices are the typical initiators of
      mapping service requests, it is also expected that other mapping
      clients, such as relays, 3rd party devices, PSAPs, etc. may also
      trigger a mapping request.

   Re8.  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.

   Re9X. No Modification of Location Databases: The mapping protocol
      SHOULD NOT require that data within location databases be
      transformed or modified in any unusual or unreasonable way in
      order for the mapping protocol to use the data.

      Motivation: Databases which contain civic addresses (used within
      location information servers), may be used for multiple purposes
      and applications, (in addition to being used for emergency service
      mapping only).

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5.  Identifying the Caller 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, derived from sources such as GPS, DHCP (RFC 3825 [2])
      and I-D.ietf-geopriv-dhcp-civil [6]) or utilizing the Link Layer
      Discovery Protocol (LLDP) [see IEEE8021AB].

   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.

   Lo1.  Validation of Civic Location: It MUST be possible to validate a
      civic location prior to its use in an actual emergency call.

      Motivation: Location validation provides an opportunity to help
      assure ahead of time, whether 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.

   Lo1X. Validation Resolution: The mapping protocol MUST support the
      return of additional information which can be used to determine
      the precision or resolution of the data elements used to determine
      a PSAP URI, for example.

   Lo1XX.  Indication of non-existent location: The protocol MUST
      support a mechanism to indicate that a location or a part of a
      location is known to not exist, even if a valid location-to-PSAP
      uri mapping can be provided.

   Lo2.  Limits to Validation: Validation of a civic location MUST NOT
      be required to enable any feature that is part of the emergency
      call process.

      Motivation: In some cases, (based on a variety of factors), 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 signaling path to the PSAP.

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   Lo3.  Reference Datum: The mapping server MUST understand WGS-84
      coordinate reference system and may understand other reference

   Lo4.  Location Provided: An Emergency Services Routing Proxy (ESRP)
      MUST NOT remove location information after performing location
      based routing.

      Motivation: The ESRP and the PSAP use the same location
      information object but for a different purpose.  Therefore, the
      PSAP still requires the receipt of information which represents
      the end device's location.

   Lo5. 3D Sensitive Mapping: The mapping protocol MUST accept either a
      2D or 3D mapping request, and return an appropriate result, based
      on which type of input is used.

      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 on the request."

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6.  Emergency Identifier

   Id1.  Universal Identifier Setup: One or more universal emergency
      identifiers MUST be recognized by any device or network element
      for call setup purposes

      Motivation: There must be some way for any device or element to
      recognize an emergency call throughout the call setup.  This is
      regardless of the device location, the application (voice) service
      provider used (if any at all), or of any other factor.  Examples
      of these might include: 911, 112, and sos.*.

   Id2.  Universal Identifier Resolution: Where multiple emergency
      service types exist, it MUST be possible to treat each emergency
      identifier separately, based on the specific type of emergency
      help requested.

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

   Id3.  Emergency Marking: Any device in the signaling path that
      recognizes by some means that the signaling is associated with an
      emergency call MUST add the emergency indication called for in Id1
      to the signaling before forwarding it.  This marking mechanism
      must be different than QoS marking.

      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.

   Id4.  Emergency Identifier-based Marking: User agents, proxies, and
      other network elements that process signaling associated with
      emergency calls SHOULD be configured to recognize a reasonable
      selection of logical emergency identifiers as a means to initiate
      emergency marking.

      Motivation: Since user devices roam, emergency identifiers may
      vary from region to region.  It is therefore important that a
      network entity be able to perform mapping and/or call routing
      within the context of its own point of origin rather than relying
      on non-local logical emergency identifiers as the only basis for
      emergency marking of calls.

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   Id5.  Prevention of Fraud: A call identified as an emergency call or
      marked as such in accordance with the above requirements for
      marking MUST be 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.

   Id6.  Extensibility of emergency service types: The list of emergency
      service types MUST be extensible, and it is not necessary to
      provide mapping for every possible service type.

      Motivation: The use of a service type is locally determined.

   Id6X. Discovery of emergency dial-string: The mapping protocol MUST
      support a mechanism to discover existing location-dependent
      emergency identifiers, known as emergency dial-strings, (e.g.
      9-1-1, 1-1-2), 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.

   Id6XX.  XXX: The SIP UA SHOULD translate home emergency dial-strings
      to universal emergency identifiers.  The UA is most likely pre-
      provisioned with this information to be able to make such a
      translation.  A mechanism to provide the user's home emergency
      dial-strings MUST be available.

   Id7.  Emergency Identifier Replacement: For each signaling protocol
      that can be used in an emergency call, reserved identifiers SHOULD
      be allowed to replace the original emergency identifier, 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, and evolution to a standardized
      universal emergency identifier or set of identifiers is preferred.

   Id8.  Universal Identifier Recognition: Universal identifier(s), MUST
      be universally recognizable by any network element which supports
      the ECRIT protocol."

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   Id9.  Universal Identifier Unrecognized: A call MUST be recognized as
      emergency call even if the specific emergency service requested is
      not recognized."

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

   Id10X. Translation of emergency dial-strings: The SIP UA SHOULD
      translate both home and visited emergency dial-strings into a
      universal emergency identifier.

   Id11X. Detection of visited emergency dial-strings: The mapping
      protocol MUST support 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 of a UA visits a foreign country, observes a fire truck
      with 999 on the side, the expectation is to be 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 dial a local emergency number.

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7.  Mapping Protocol

   Given the requirement from the previous section, that of a single (or
   small number of) emergency identifier(s) which are independent of the
   caller's location, and since PSAPs only serve a limited geographic
   region, and for reasons of jurisdictional and local knowledge, having
   the call reach the appropriate PSAP based on a mapping protocol, is

   There are two basic architectures described for translating an
   emergency identifier into the appropriate PSAP emergency address.  We
   refer to these as caller-based and mediated.

   For caller-based resolution, the caller's user agent consults 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 performs this function (a request
   for mapping) by invoking the mapping protocol.

   Note that this case relies on an 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 home.)

   The problem at hand is more difficult to resolve than that for
   traditional web or email services.  In this case, the emergency
   caller only dialed an emergency identifier, and depending on the
   location, any one of several thousand PSAPs around the world could be
   appropriate PSAP.  In addition, there may be a finer resolution of
   routing (which the caller isn't aware of), which results in a
   particular "accredited" PSAP (i.e. one run by local authorities)
   answering to call.  (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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   Ma1.  Appropriate PSAP: Calls MUST be routed to the PSAP responsible
      for this particular geographic area.  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.

      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.

   Ma2.  Mapping redirection: The mapping protocol MUST support
      redirection functionality, since in some cases, an initial mapping
      may provide a single URL for a large geographic area.  Redirection
      is needed to then re-invokes the mapping protocol on a different
      database to obtain another URL for a more resolute ESRP or PSAP,
      which covers a smaller area.

      Motivation: The more local the mapping output is, the more
      favorable (in most cases) the likely outcome will be for the
      emergency caller.

   Ma3.  Minimal additional delay: The execution of the mapping protocol
      SHOULD minimize the amount of additional delay to 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.

   Ma4.  Referral: The mapping client MUST be able to contact any server
      and be referred to another server that is more qualified to answer
      the query.

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

   Ma5.  Multiple Response URIs: The mapping protocol response MUST
      allow the return of multiple URIs.

      Motivation: In response to a mapping request, a server will
      normally provide a URI or set of URIs for contacting the
      appropriate PSAP.

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   Ma6.  URI - Alternate Contact: The mapping protocol MUST be able to
      return a URI or contact method explicitly marked as an alternate

      Motivation: In response to a mapping request, if an expected URI
      is unable to be returned, then mapping server may return an
      alternate URI.  When and how this would be used will be described
      in an operational document.

   Ma7.  Multiple PSAP URI's: The mapping protocol MUST be able to
      return multiple URIs for different PSAPs that cover the same area.

   Ma8.  URL properties: The mapping protocol must provide additional
      information that allows the querying entity to determine relevant
      properties of the URL.

      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 trigger methods.

   Ma9.  Traceable resolution: The entity requesting mapping SHOULD be
      able to determine the entity or entities which provided the
      emergency address resolution information.

      Motivation: To provide operational traceability in case of errors.

   Ma9X. URI for error reporting: The mapping protocol MUST have a
      mechanism to return a urii that can be used to report a suspected
      or known error within the mapping database.

   Ma10.  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

   Ma11.  Incrementally deployable: The mapping function MUST be capable
      of being deployed incrementally.

      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 boundary information.

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   Ma12X. URI for error resolution: The mapping protocol MUST have a
      mechanism to return a reference URI which can be used to report a
      suspected error in the mapping database.

   Ma13.  Mapping requested from anywhere: The mapping protocol MUST be
      able to provide the mapping regardless of where the mapping client
      is located, either geographically or by network location.

      Motivation: The mapping client, (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.

   Ma14.  Location Updates: It SHOULD be possible to have updates of

      Motivation: Updated location information may have an impact on
      PSAP routing.  In some cases it may be possible to redirect that
      call to a more appropriate PSAP (some device measurement
      techniques provide quick (i.e. early), but imprecise "first fix"

   Ma15.  Extensible Protocol: 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
      PIDF-LO (RFC 4119 [3]).

   Ma16.  Split responsibility: The mapping protocol MUST allow that
      within a single level of the civic location 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.

   Ma17.  Pervasive Mapping: The mapping function MUST be able to be
      invoked at any time, including while an emergency call is in

   Ma18.  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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   Ma19.  Single URI Scheme: The mapping protocol MAY return multiple
      URIs, though it SHOULD return only one URI per scheme, 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.

   Ma20X. Separation of Identity from mapping: The mapping function MUST
      NOT require the true identity of the target for which the location
      information is attributed.  Ideally, no identity information is
      provided to the mapping function.  Where identity information is
      provided, it may be in the form of an unlinked pseudonym as
      defined in RFC 3963.

   Ma21X. Location delivery by-value: The mapping protocol MUST support
      the delivery of location information by-value and MAY support de-
      referencing of location references.  Location by-reference is not
      one of the evaluation criteria.  The mapping protocol is not
      required to support the ability to de-reference location

   Ma22X. Alternate community names: The mapping protocol MUST support
      both the jurisdiction community name and the postal community name
      fields in the PIDF-LO.

      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, given that the
      database has both fields populated, the mapping protocol response
      should return both available fields.

   Ma23X. Support for alias locations: The mapping protocol MUST support
      one or more aliases for a specific location entry.

      Motivation: It should be possible to relate one entry to another
      and be able to determine which is the "primary" entry and which is
      the alias.  The result of aliasing is always that mapping from the
      primary or any of the aliases is the same.

   Ma24X. Pre-call mapping for fallback: The mapping protocol MUST
      support LCMS queries prior to making an emergency call.

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      Motivation: Used as a fallback mechanism only, if a LCMS 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 an LCMS query at any time prior to an emergency

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8.  Security Considerations

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

   We would like to thank Michael Hammer, Ted Hardie, Marc Linsner,
   Andrew Newton, James Polk, Tom Taylor, and Hannes Tschofenig for
   their 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]  Schulzrinne, H., "Security Threats and Requirements for
        Emergency Calling", draft-taylor-ecrit-security-threats-01 (work
        in progress), December 2005.

   [5]  Schulzrinne, H., "A Uniform Resource Name (URN) for Services",
        draft-schulzrinne-sipping-service-01 (work in progress),
        October 2005.

   [6]  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

   [7]   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.

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

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

   [10]  Wijk, A., "Framework of requirements for real-time text
         conversation using SIP", draft-ietf-sipping-toip-03 (work in
         progress), September 2005.

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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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