Network Working Group
Request for Comments: 5031
Category: Standards Track
H. Schulzrinne
Columbia U.
January 2008

A Uniform Resource Name (URN) for

Emergency and Other Well-Known Services

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.


The content of many communication services depends on the context, such as the user's location. We describe a 'service' URN that allows well-known context-dependent services that can be resolved in a distributed manner to be identified. Examples include emergency services, directory assistance, and call-before-you-dig hot lines.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Registration Template  . . . . . . . . . . . . . . . . . . . .  4
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  New Service-Identifying Labels . . . . . . . . . . . . . .  6
     4.2.  Sub-Services for the 'sos' Service . . . . . . . . . . . .  7
     4.3.  Sub-Services for the 'counseling' Service  . . . . . . . .  8
     4.4.  Initial IANA Registration  . . . . . . . . . . . . . . . .  9
   5.  Internationalization Considerations  . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Appendix A.  Alternative Approaches Considered . . . . . . . . . . 13
   Appendix B.  Acknowledgments . . . . . . . . . . . . . . . . . . . 14

1. Introduction

In existing telecommunications systems, there are many well-known communication and information services that are offered by loosely coordinated entities across a large geographic region, with well- known identifiers. Some of the services are operated by governments or regulated monopolies, others by competing commercial enterprises. Examples include emergency services (reached by dialing 9-1-1 in North America, 1-1-2 in Europe), community services and volunteer opportunities (2-1-1 in some regions of the United States), telephone directory and repair services (4-1-1 and 6-1-1 in the United States and Canada), government information services (3-1-1 in some cities in the United States), lawyer referral services (1-800-LAWYER), car roadside assistance (automobile clubs), and pizza delivery services. Unfortunately, almost all of them are limited in scope to a single country or possibly a group of countries, such as those belonging to the North American Numbering Plan or the European Union. The same identifiers are often used for other purposes outside that region, making access to such services difficult when users travel or use devices produced outside their home country.

These services are characterized by long-term stability of user- visible identifiers, decentralized administration of the underlying service, and a well-defined resolution or mapping mechanism. For example, there is no national coordination or call center for "9-1-1" in the United States; rather, various local government organizations cooperate to provide this service based on jurisdictions.

In this document, we propose a URN namespace that, together with resolution protocols beyond the scope of this document, allows us to define such global, well-known services, while distributing the actual implementation across a large number of service-providing entities. There are many ways to divide provision of such services, such as dividing responsibility by geographic region or by the service provider a user chooses. In addition, users can choose different mapping service providers that in turn manage how geographic locations are mapped to service providers.

Availability of such service identifiers allows end systems to convey information about the desired service to other network entities. For example, an IP phone could have a special set of short cuts, address book entries, or buttons that invoke emergency services. When such a service identifier is put into the outgoing Session Initiation Protocol (SIP) [RFC3261] message, it allows SIP proxies to unambiguously take actions, as it would not be practical to configure them with dial strings and emergency numbers used throughout the world. Hence, such service identifiers make it possible to delegate routing decisions to third parties and to mark certain requests as having special characteristics while preventing these characteristics from being accidentally invoked.

This URN identifies services independent of the particular protocol that is used to request or deliver the service. The URN may appear in protocols that allow general URIs, such as the SIP [RFC3261] request URIs, web pages, or mapping protocols.

The service URN is a protocol element and is generally not expected to be visible to humans. For example, it is expected that callers will still dial the emergency number '9-1-1' in the United States to reach emergency services. In some other cases, speed dial buttons might identify the service, as is common practice on hotel phones today. (Speed dial buttons for summoning emergency help are considered inappropriate by most emergency services professionals, at least for mobile devices, as they are too prone to being triggered accidentally.)

The translation of service dial strings or service numbers to service URNs in the end host is beyond the scope of this document. These translations likely depend on the location of the caller and may be many-to-one, i.e., several service numbers may map to one service URN. For example, a phone for a traveler could recognize the emergency service number for both the traveler's home location and the traveler's visited location, mapping both to the same universal service URN, urn:service:sos.

Since service URNs are not routable, a SIP proxy or user agent has to translate the service URN into a routable URI for a location- appropriate service provider, such as a SIP URL. A Location-to- Service Translation Protocol (LoST) [LOST] is expected to be used as a resolution system for mapping service URNs to URLs based on geographic location. In the future, there may be several such protocols, possibly different ones for different services.

Services are described by top-level service type, and may contain a hierarchy of sub-services that further describe the service, as outlined in Section 3.

We discuss alternative approaches for creating service identifiers, and why they are unsatisfactory, in Appendix A.

2. Terminology

In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 [RFC2119].

Terminology specific to emergency services is defined in [RFC5012].

3. Registration Template

Below, we include the registration template for the URN scheme according to RFC 3406 [RFC3406].

   Namespace ID:  service

Registration Information:

      Registration version:  1
      Registration date:  2006-04-02

Declared registrant of the namespace:

      Registering organization:  IETF
      Designated contact:  Henning Schulzrinne
      Designated contact email:
   Declaration of syntactic structure:  The URN consists of a
      hierarchical service identifier, with a sequence of labels
      separated by periods.  The left-most label is the most significant
      one and is called 'top-level service', while names to the right
      are called 'sub-services'.  The set of allowable characters is the
      same as that for domain names [RFC1123] and a subset of the labels
      allowed in [RFC3958].  Labels are case-insensitive, but MUST be
      specified in all lower-case.  For any given service URN, service-
      identifiers can be removed right-to-left; the resulting URN is
      still valid, referring to a more generic service.  In other words,
      if a service 'x.y.z' exists, the URNs 'x' and 'x.y' are also valid
      service URNs.  The ABNF [RFC4234] is shown below.
     service-URN  = "URN:service:" service
     service      = top-level *("." sub-service)
     top-level    = let-dig [ *25let-dig-hyp let-dig ]
     sub-service  = let-dig [ *let-dig-hyp let-dig ]
     let-dig-hyp  = let-dig / "-"
     let-dig      = ALPHA / DIGIT
     ALPHA        = %x41-5A / %x61-7A   ; A-Z / a-z
     DIGIT        = %x30-39 ; 0-9
   Relevant ancillary documentation:  None
   Community considerations:  The service URN is believed to be relevant
      to a large cross-section of Internet users, including both
      technical and non-technical users, on a variety of devices, but
      particularly for mobile and nomadic users.  The service URN will
      allow Internet users needing services to identify the service by
      kind, without having to determine manually who provides the
      particular service in the user's current context, e.g., at the
      user's current location.  For example, travelers will be able to
      use their mobile devices to request emergency services without
      having to know the emergency dial string of the visited country.
      The assignment of identifiers is described in the IANA
      Considerations (Section 4).  The service URN does not prescribe a
      particular resolution mechanism, but it is assumed that a number
      of different entities could operate and offer such mechanisms.
   Namespace considerations:  There do not appear to be other URN
      namespaces that serve the same need of uniquely identifying
      widely-available communication and information services.  Unlike
      most other currently registered URN namespaces, the service URN
      does not identify documents and protocol objects (e.g., [RFC3044],
      [RFC3187], [RFC4179], and [RFC4195]), types of telecommunications
      equipment [RFC4152], people, or organizations [RFC3043].  tel URIs
      [RFC3966] identify telephone numbers, but numbers commonly
      identifying services (such as 911 or 112) are specific to a
      particular region or country.
   Identifier uniqueness considerations:  A service URN identifies a
      logical service, specified in the service registration (see IANA
      Considerations (Section 4)).  Resolution of the URN, if
      successful, will return a particular instance of the service, and
      this instance may be different even for two users making the same
      request in the same place at the same time; the logical service
      identified by the URN, however, is persistent and unique.  Service
      URNs MUST be unique for each unique service; this is guaranteed
      through the registration of each service within this namespace,
      described in Section 4.
   Identifier persistence considerations:  The 'service' URN for the
      same service is expected to be persistent, although there
      naturally cannot be a guarantee that a particular service will
      continue to be available globally or at all times.
   Process of identifier assignment:  The process of identifier
      assignment is described in the IANA Considerations (Section 4).
   Process for identifier resolution:  There is no single global
      resolution service for 'service' URNs.  However, each top-level
      service can provide a set of mapping protocols to be used with
      'service' URNs of that service.
   Rules for lexical equivalence:  'service' identifiers are compared
      according to case-insensitive string equality.
   Conformance with URN syntax:  The BNF in the 'Declaration of
      syntactic structure' above constrains the syntax for this URN
   Validation mechanism:  Validation determines whether a given string
      is currently a validly-assigned URN [RFC3406].  Due to the
      distributed nature of the mapping mechanism, and since not all
      services are available everywhere and not all mapping servers may
      be configured with all current service registrations, validation
      in this sense is not possible.  Also, the discovery mechanism for
      the mapping mechanism may not be configured with all current top-
      level services.
   Scope:  The scope for this URN is public and global.

4. IANA Considerations

This section registers a new URN scheme with the registration template provided in Section 3.

Below, Section 4.1 details how to register new service-identifying labels. Descriptions of sub-services for the first two services to be registered, sos and counseling, are given in Section 4.2 and Section 4.3, respectively. Finally, Section 4.4 contains the initial registration table.

4.1. New Service-Identifying Labels

Services and sub-services are identified by labels managed by IANA, according to the processes outlined in [RFC2434] in a new registry called "Service URN Labels". Thus, creating a new service requires IANA action. The policy for adding top-level service labels is

'Standards Action'. (This document defines the top-level services 'sos' and 'counseling'.) The policy for assigning labels to sub- services may differ for each top-level service designation and MUST be defined by the document describing the top-level service.

Entries in the registration table have the following format:

   Service  Reference  Description
   foo      RFCxyz     Brief description of the 'foo' top-level service  RFCabc     Description of the '' service

To allow use within the constraints of S-NAPTR [RFC3958], all top- level service names MUST NOT exceed 27 characters.

4.2. Sub-Services for the 'sos' Service

This section defines the first service registration within the IANA registry defined in Section 4.1, using the top-level service label 'sos'.

The 'sos' service type describes emergency services requiring an immediate response, typically offered by various branches of the government or other public institutions. Additional sub-services can be added after expert review and must be of general public interest and have a similar emergency nature. The expert is designated by the ECRIT working group, its successor, or, in their absence, the IESG. The expert review should only approve emergency services that are offered widely and in different countries, with approximately the same caller expectation in terms of services rendered. The 'sos' service is not meant to invoke general government, public information, counseling, or social services.

   urn:service:sos  The generic 'sos' service reaches a public safety
      answering point (PSAP), which in turn dispatches aid appropriate
      to the emergency.  It encompasses all of the services listed
   urn:service:sos.ambulance  This service identifier reaches an
      ambulance service that provides emergency medical assistance and
   urn:service:sos.animal-control  Animal control typically enforces
      laws and ordinances pertaining to animal control and management,
      investigates cases of animal abuse, educates the community in
      responsible pet ownership and wildlife care, and provides for the
      housing and care of homeless animals, among other animal-related
      services.  The 'fire' service identifier summons the fire
      service, also known as the fire brigade or fire department.
   urn:service:sos.gas  The 'gas' service allows the reporting of
      natural gas (and other flammable gas) leaks or other natural gas
   urn:service:sos.marine  The 'marine' service refers to maritime
      search and rescue services such as those offered by the coast
      guard, lifeboat, or surf lifesavers.
   urn:service:sos.mountain  The 'mountain' service refers to mountain
      rescue services (i.e., search and rescue activities that occur in
      a mountainous environment), although the term is sometimes also
      used to apply to search and rescue in other wilderness
   urn:service:sos.physician  The 'physician' emergency service connects
      the caller to a physician referral service.
   urn:service:sos.poison  The 'poison' service refers to special
      information centers set up to inform citizens about how to respond
      to potential poisoning.  These poison control centers maintain a
      database of poisons and appropriate emergency treatment.
   urn:service:sos.police  The 'police' service refers to the police
      department or other law enforcement authorities.

4.3. Sub-Services for the 'counseling' Service

The 'counseling' service type describes services where callers can receive advice and support, often anonymous, but not requiring an emergency response. (Naturally, such services may transfer callers to an emergency service or summon such services if the situation warrants.) Additional sub-services can be added after expert review and should be of general public interest. The expert is chosen in the same manner as described for the 'sos' service. The expert review should take into account whether these services are offered widely and in different countries, with approximately the same caller expectation in terms of services rendered.

   urn:service:counseling  The generic 'counseling' service reaches a
      call center that transfers the caller based on his or her specific
   urn:service:counseling.children  The 'children' service refers to
      counseling and support services that are specifically tailored to
      the needs of children.  Such services may, for example, provide
      advice to run-aways or victims of child abuse.
   urn:service:counseling.mental-health  The 'mental-health' service
      refers to the "diagnostic, treatment, and preventive care that
      helps improve how persons with mental illness feel both physically
      and emotionally as well as how they interact with other persons".
      (U.S. Department of Health and Human Services)
   urn:service:counseling.suicide  The 'suicide' service refers to the
      suicide prevention hotline.

4.4. Initial IANA Registration

The following table contains the initial IANA registration for emergency and counseling services.

   Service                   Reference  Description
   counseling                RFC 5031   Counseling services
   counseling.children       RFC 5031   Counseling for children
   counseling.mental-health  RFC 5031   Mental health counseling
   counseling.suicide        RFC 5031   Suicide prevention hotline
   sos                       RFC 5031   Emergency services
   sos.ambulance             RFC 5031   Ambulance service
   sos.animal-control        RFC 5031   Animal control                  RFC 5031   Fire service
   sos.gas                   RFC 5031   Gas leaks and gas emergencies
   sos.marine                RFC 5031   Maritime search and rescue
   sos.mountain              RFC 5031   Mountain rescue
   sos.physician             RFC 5031   Physician referral service
   sos.poison                RFC 5031   Poison control center
   sos.police                RFC 5031   Police, law enforcement

5. Internationalization Considerations

The service labels are protocol elements [RFC3536] and are not normally seen by users. Thus, the character set for these elements is restricted, as described in Section 3.

6. Security Considerations

As an identifier, the service URN does not appear to raise any particular security issues. The services described by the URN are meant to be well-known, even if the particular service instance is access-controlled, so privacy considerations do not apply to the URN.

There are likely no specific privacy issues when including a service URN on a web page, for example. On the other hand, ferrying the URN in a signaling protocol can give attackers information on the kind of service desired by the caller. For example, this makes it easier for the attacker to automatically find all calls for emergency services or directory assistance. Appropriate, protocol-specific security mechanisms need to be implemented for protocols carrying service URNs. The mapping protocol needs to address a number of threats, as detailed in [RFC5069]. That document also discusses the security considerations related to the use of the service URN for emergency services.

7. References

7.1. Normative References

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.
   [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.
   [RFC3958]  Daigle, L. and A. Newton, "Domain-Based Application
              Service Location Using SRV RRs and the Dynamic Delegation
              Discovery Service (DDDS)", RFC 3958, January 2005.
   [RFC4234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

7.2. Informative References

   [LOST]     Hardie, T., "LoST: A Location-to-Service Translation
              Protocol", Work in Progress, March 2007.
              FUNCTIONS", RFC 2142, May 1997.
   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.
   [RFC3043]  Mealling, M., "The Network Solutions Personal Internet
              Name (PIN): A URN Namespace for People and Organizations",
              RFC 3043, January 2001.
   [RFC3044]  Rozenfeld, S., "Using The ISSN (International Serial
              Standard Number) as URN (Uniform Resource Names) within an
              ISSN-URN Namespace", RFC 3044, January 2001.
   [RFC3187]  Hakala, J. and H. Walravens, "Using International Standard
              Book Numbers as Uniform Resource Names", RFC 3187,
              October 2001.
   [RFC3406]  Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
              "Uniform Resource Names (URN) Namespace Definition
              Mechanisms", BCP 66, RFC 3406, October 2002.
   [RFC3536]  Hoffman, P., "Terminology Used in Internationalization in
              the IETF", RFC 3536, May 2003.
   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, December 2004.
   [RFC4152]  Tesink, K. and R. Fox, "A Uniform Resource Name (URN)
              Namespace for the Common Language Equipment Identifier
              (CLEI) Code", RFC 4152, August 2005.
   [RFC4179]  Kang, S., "Using Universal Content Identifier (UCI) as
              Uniform Resource Names (URN)", RFC 4179, October 2005.
   [RFC4195]  Kameyama, W., "A Uniform Resource Name (URN) Namespace for
              the TV-Anytime Forum", RFC 4195, October 2005.
   [RFC5012]  Schulzrinne, H. and R. Marshall, Ed., "Requirements for
              Emergency Context Resolution with Internet Technologies",
              RFC 5012, January 2008.
   [RFC5069]  Taylor, T., Ed., Tschofenig, H., Schulzrinne, H., and M.
              Shanmugam, "Security Threats and Requirements for
              Emergency Call Marking and Mapping", RFC 5069,
              January 2008.

Appendix A. Alternative Approaches Considered

The discussions of ways to identify emergency calls has yielded a number of proposals. Since these are occasionally brought up during discussions, we briefly summarize why this document chose not to pursue these solutions.

   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.  As another
      example, a number of countries, such as Italy, use 118 as an
      emergency number, but it also connects to directory assistance in
   tel:sos  This solution avoids name conflicts, but requires extending
      the "tel" URI "tel" [RFC3966].  It also only works if every
      outbound proxy knows how to route requests to a proxy that can
      reach emergency services since tel URIs do not identify the
      destination server.
   sip:sos@domain  Earlier work had defined a special user identifier,
      sos, within the caller's home domain in a SIP URI, for example,  Such a user identifier follows the
      convention of RFC 2142 [RFC2142] and the "postmaster" convention
      documented in RFC 2822 [RFC2822].  This approach had the advantage
      that dial plans in existing user agents could probably be
      converted to generate such a URI and that only the home proxy for
      the domain has to understand the user naming convention.  However,
      it overloads the user part of the URI with specific semantics
      rather than being opaque, makes routing by the outbound proxy a
      special case that does not conform to normal SIP request-URI
      handling rules and is SIP-specific.  The mechanism also does not
      extend readily to other services.
   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.

Appendix B. Acknowledgments

This document is based on discussions with Jonathan Rosenberg and benefited from the comments of Leslie Daigle, Keith Drage, Benja Fallenstein, Paul Kyzivat, Andrew Newton, Brian Rosen, Jonathan Rosenberg, Martin Thomson, and Hannes Tschofenig.

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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