Network Working Group                                     H. Schulzrinne
Internet-Draft                                               Columbia U.
Expires: August 1, 2006                                 January 28, 2006

       Emergency Services URI for the Session Initiation Protocol

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   As part of an overall architecture for supporting emergency calling
   for the Session Initiation Protocol (SIP), this document defines
   universal emergency SIP URIs, sip:sos@domain and sips:sos@domain,
   that allows SIP user agents to contact the local emergency call
   center.  It also defines conventions that increase the high
   probability of reaching the appropriate emergency call center.  The
   document does not define any SIP protocol extensions.

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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.   Emergency URIs . . . . . . . . . . . . . . . . . . . . . . .   3
   4.   Request Handling . . . . . . . . . . . . . . . . . . . . . .   4
   5.   Alternative Approaches Considered  . . . . . . . . . . . . .   5
   6.   Media Feature Tag Registration: Service  . . . . . . . . . .   6
   7.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .   7
   8.   Security Considerations  . . . . . . . . . . . . . . . . . .   8
   9.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . .   8
   10.  References . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1   Normative References . . . . . . . . . . . . . . . . . .   8
     10.2   Informative References . . . . . . . . . . . . . . . . .   9
        Author's Address . . . . . . . . . . . . . . . . . . . . . .  10
        Intellectual Property and Copyright Statements . . . . . . .  11

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1.  Introduction

   Using the public switched telephone network (PSTN), emergency help
   can often be summoned at a designated, widely known number,
   regardless of where the telephone was purchased.  However, this
   number differs between localities, even though it is often the same
   for a country or continent-size region (such as many countries in the
   European Union or North America).  For end systems based on the
   Session Initiation Protocol (SIP) [RFC3261], it is desirable to have
   a universal identifier, independent of location, to simplify the user
   experience and to allow the device to perform appropriate processing.
   Here, we define a common user identifier, "sos", as the contact
   mechanism for emergency assistance.  This identifier is meant to be
   used in addition to any local emergency numbers.

   This document specifies only a small part of a comprehensive set of
   recommendations for operating emergency services.  Future documents
   will describe how a device that identifies a call as an emergency
   call can route it to the appropriate Public Safety Answering Point

   This document does not introduce any new SIP header fields, request
   methods, status codes, message bodies, or events.  User agents
   unaware of the recommendations in this draft can place emergency
   calls, but may not be able to provide the same user interface
   functionality.  The document suggests behavior for proxy servers, in
   particular outbound proxy servers.

   The solution described here is not as general as the alternative
   approach, service URNs [I-D.schulzrinne-sipping-service], but
   requires no changes to end systems or proxies.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
   [RFC2119] and indicate requirement levels for compliant

3.  Emergency URIs

   Having a single, global identifier for emergency services is highly
   desirable, as it allows end system and network devices to be built
   that recognize such services and can act appropriately.  Such actions
   may include restricting the functionality of the end system,
   providing special features, overriding user service constraints or
   routing session setup messages.

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   SIP user agents (UAs) that determine that a dialog or transaction
   relates to an emergency MUST use an an emergency SIP URI defined
   below as the Request-URI and "To" header field.

   It is RECOMMENDED that SIP-based [RFC3261] end systems and proxy
   servers support a uniform emergency call identifier, namely the SIP
   and SIPS URIs with the reserved user name "sos" within any domain,

   The reserved name is case-insensitive.

   The host part of the emergency URI SHOULD be the host portion of the
   address-of-record of the caller.  The "sips" form SHOULD be used to
   ensure integrity and confidentiality; the "sip" form MAY be used if a
   "sips" call fails with status code 416 (Unsupported URI Scheme).  All
   SIP requests with URIs of this form are assumed to be emergency

      The domain-of-record was chosen since a SIP user agent may not be
      able to determine the local domain it is visiting.  This also
      allows each user to test this facility, as the user can ensure
      that such services are operational in his home domain.  An
      outbound proxy in the visited domain can handle the call if it
      believes to be in a position to provide appropriate emergency

   In some cases, end users or, more likely, emergency service routing
   proxies may want to request specific emergency services.  We support
   this feature by leveraging the caller preferences [RFC3841] extension
   and define a new media feature tag, service, in Section 6.

   The SIP URI user name "sos" MUST NOT be assigned to any regular user.

4.  Request Handling

   Outbound proxy servers SHOULD check whether a tel URIs or a SIP URIs
   containing a dial string represents an emergency number within its
   geographic service area, but only if they can be reasonably certain
   that the call originated from within that area, e.g., if the call
   contained location information or the network is known to only be
   reachable from a restricted geographic area.  Typically, these
   service areas encompass whole countries since many countries now have
   nationwide emergency numbers.  Once they recognize an emergency
   number, they translate the Request-URI to an "sos" URI as described

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   The proxy MAY use any additional information contained in the call
   request to recognize additional numbers as emergency numbers.  Such
   information includes the Mobile Country Code and the Mobile Network
   Code for 3GPP devices or country information in location information
   available about the call.

5.  Alternative Approaches Considered

   The "sos" SIP URI reserved user name proposed here follows the
   convention of RFC 2142 [RFC2142] and the "postmaster" convention
   documented in RFC 2822 [RFC2822].  The approach has the advantage
   that only the home proxy for a user needs to understand the
   convention and that the mechanism is likely backwards-compatible with
   most SIP user agents, with the only requirement that they have to be
   able to generate alphanumeric URLs.  One drawback is that it may
   conflict with locally assigned addresses of the form "sos@domain".
   Also, if proxies not affiliated with the domain translate the URL,
   they violate the current SIP protocol conventions.

   There are a number of possible alternatives, each with their own set
   of advantages and problems:

   tel:NNN;context=+C This approach uses tel URIs [RFC3966].  Here, NNN
      is the national emergency number, where the country is identified
      by the context C. This approach is easy for user agents to
      implement, but hard for proxies and other SIP elements to
      recognize, as it would have to know about all number-context
      combinations in the world and track occasional changes.  In
      addition, many of these numbers are being used for other services.
      For example, the emergency number in Paraguay (00) is also used to
      call the international operator in the United States.  A number of
      countries, such as Italy, use 118 as an emergency number, but it
      also connects to directory assistance in Finland.
   tel:sos This solution avoids name conflicts, but is not a valid "tel"
      [RFC3966] URI.  It also only works if every outbound proxy knows
      how to route requests to a proxy that can reach emergency services
      since tel URIs.  The SIP URI proposed here only requires a user's
      home domain to be appropriately configured.
   urn:service:sos A related document [I-D.schulzrinne-sipping-service]
      defines a URN for identifying services, such as emergency calling.
      This solution fits most cleanly into the overall URI architecture,
      can support a variety of protocols beyond SIP and avoids
      dependencies on the home domain, but, like the tel URI solution
      above, also requires that every outbound proxy can resolve this
      URN and can route calls accordingly.  Alternatively, the end
      system has to be configured with a suitable URN-resolving proxy,
      e.g., in its home domain.

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   SIP URI user parameter: One could create a special URI, such as "aor-
      domain;user=sos".  This avoids the name conflict problem, but
      requires mechanism-aware user agents that are capable of emitting
      this special URI.  Also, the 'user' parameter is meant to describe
      the format of the user part of the SIP URI, which this usage does
      not do.  Adding other parameters still leaves unclear what, if
      any, conventions should be used for the user and domain part of
      the URL.  Neither solution is likely to be backward-compatible
      with existing clients.
   Special domain: A special domain, such as "" could be
      used to identify emergency calls.  This has similar properties as
      the "tel:sos" URI, except that it is indeed a valid URI.  To make
      this usable, the special domain would have to be operational and
      point to an appropriate emergency services proxy.  Having a
      single, if logical, emergency services proxy for the whole world
      seems to have undesirable scaling and administrative properties.

6.  Media Feature Tag Registration: Service

   Instead of defining additional, more specific, emergency services in
   the SIP URI, we propose the use of a new media feature tag [RFC3840],
   sip.service, that describe the desired emergency service.

   For example, a user agent could request to be routed to marine rescue
   by including the following header:

     Accept-Contact: *;sip.service="sos.marine"

   [Note: This mechanism fits with the Caller Preferences model, but
   reduces the backward-compatibility of the overall approach.]

   This specification defines an additional media feature tag, extending
   the SIP tree entries described in [RFC3840] and following the
   registration process in Section 12.1 of that document.  This section
   serves as the IANA registration for the service feature tags, which
   are made into the SIP media feature tag tree.

   This facility is not meant to encourage end users to select emergency
   services where a single PSAP for all such services exist.  Rather,
   these identifiers reflect current practice in jurisdictions that
   already have different numbers for the different emergency services.
   For example, in Germany, ambulance and fire use 112, while police
   uses 110.

      We expect that users will rarely invoke specific emergency
      services directly.  Rather, they might be generated by outbound
      proxy servers translating dial strings or be generated when
      pressing icon-bearing speed dial buttons.

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      Using feature tags has the advantage that they are not affected by
      entities that translate URIs, e.g., to route emergency calls to a
      specific PSAP.

   The service types for this feature tag are case-insensitive.
   Additional service types can be registered with IANA (Section
   Section 7).

   Media feature tag name: sip.service
   ASN.1 Identifier: New assignment by IANA.
   Summary of the media feature indicated by this tag: Each feature tag
      indicates the type of communication service requested.
   Values appropriate for use with this feature tag: Token with an
      equality relationship.  Initial values include a number of
      emergency services:

      sos: general emergency service fire brigade
      sos.marine: marine guard
      sos.mountain: mountain rescue
      sos.police: police (law enforcement)
      sos.rescue: ambulance, emergency medical service
      sos.test: testing, not a real emergency call
   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a user agent providing a particular
      type of communication service.
   Examples of typical use: Allowing an emergency service proxy to
      select the desired emergency service, such as police or ambulance.
   Related standards or documents: RFC3840.
   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 11.1 of RFC3840.

7.  IANA Considerations

   Subaddresses of the "sos" address are registered with IANA This
   specification establishes the "sos" subaddres sub-registry under

   Subaddresses are registered by the IANA when they are published in
   standards track RFCs.  The IANA Considerations section of the RFC
   must include the following information, which appears in the IANA
   registry along with the RFC number of the publication.

   o  Name of the subaddress.  The name MAY be of any length, but SHOULD
      be no more than twenty characters long.  The name MUST consist of
      NVT alphanumeric characters only and is case-insensitive.

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   o  Descriptive text that describes the emergency service.

8.  Security Considerations

   The SIP specification [RFC3261] details security considerations that
   apply to emergency calls as well.  Security for emergency calls has
   conflicting goals, namely to make it as easy and reliable as possible
   to reach emergency services, while discouraging and possibly tracing
   prank calls.  It appears unlikely that classical authentication
   mechanisms can be required by emergency call centers, but SIP proxy
   servers may be able to add identifying information.

   Given the sensitive nature of many emergency calls, it is highly
   desirable to use the "sips" URI to ensure transport-level
   confidentiality and integrity.  However, this may cause the call to
   fail in some environments.

   Allowing the user agent to clearly and unambiguously identify
   emergency calls makes it possible for the user agent to make
   appropriate policy decisions.  For example, a user agent policy may
   reveal a different amount of information to the callee when making an
   emergency call.  Local laws may affect what information network
   servers or service providers may be allowed or be required to release
   to emergency call centers.  They may also base their decision on the
   user-declared destination of the call.

   Recognizing only "sos" in the user's home domain, i.e., the domain of
   the user's AOR, prevents spoofing where a link points to a fake
   emergency calling number and leads the user to, for example, include
   location information in the request.

   Additional security considerations related to call routing,
   destination authentication and other issues are detailed in
   [I-D.ietf-ecrit-requirements] and [I-D.taylor-ecrit-security-

9.  Acknowledgements

   Andrew Allen, Keith Drage, Cullen Jennings, Mike Pierce, James Polk,
   Brian Rosen, John Schnizlein and Hannes Tschofenig contributed
   helpful comments.

10.  References

10.1  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3361]  Schulzrinne, H., "Dynamic Host Configuration Protocol
              (DHCP-for-IPv4) Option for Session Initiation Protocol
              (SIP) Servers", RFC 3361, August 2002.

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840, August 2004.

   [RFC3841]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
              Preferences for the Session Initiation Protocol (SIP)",
              RFC 3841, August 2004.

10.2  Informative References

              Schulzrinne, H. and R. Marshall, "Requirements for
              Emergency Context Resolution with Internet Technologies",
              draft-ietf-ecrit-requirements-02 (work in progress),
              January 2006.

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

              Schulzrinne, H., "Security Threats and Requirements for
              Emergency Calling", draft-taylor-ecrit-security-threats-01
              (work in progress), December 2005.

              FUNCTIONS", RFC 2142, May 1997.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, December 2004.

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Author's Address

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

   Phone: +1 212 939 7004

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