ECRIT                                                     H. Schulzrinne
Internet-Draft                                               Columbia U.
Expires: November 20, 2006                                  May 19, 2006

               A Uniform Resource Name (URN) for Services

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   The content of many communication services depends on the context,
   such as the user's location.  We describe a 'service' URN that allows
   to register such context-dependent services that can be resolved in a
   distributed manner.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Registration Template  . . . . . . . . . . . . . . . . . . . .  5
   4.  Finding the Mapping Server . . . . . . . . . . . . . . . . . .  7
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
     5.1   New Service-Identifying Labels . . . . . . . . . . . . . .  8
     5.2   S-NAPTR Application Service Tag  . . . . . . . . . . . . .  8
     5.3   Sub-Services for the 'sos' Service . . . . . . . . . . . .  8
     5.4   Sub-Services for the 'counseling' Service  . . . . . . . .  9
   6.  International Considerations . . . . . . . . . . . . . . . . . 10
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1   Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2   Informative References . . . . . . . . . . . . . . . . . . 11
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12
   A.  Alternative Approaches Considered  . . . . . . . . . . . . . . 12
   B.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 13
       Intellectual Property and Copyright Statements . . . . . . . . 15

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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 911 in North
   America, 112 in Europe), community services and volunteer
   opportunities (211 in some regions of the United States), telephone
   directory and repair services (411 and 611 in the United States and
   Canada), government information services (311 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 accessing 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 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 directory providers that in turn manage how geographic
   locations are mapped to service providers.

   Availability of such service identifiers simplifies end system
   configuration.  For example, an IP phone could have a special set of
   short cuts or buttons that invoke emergency services, as it would not
   be practical to manually re-configure the device with local emergency
   contacts for each city or town a user visits with his or her mobile
   device.  Also, such identifiers make it possible to delegate routing
   decisions to third parties and to mark certain requests as having
   special characteristics while preventing these characteristics to be
   accidentally invoked on inappropriate requests.

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   This URN identifies services independent of a particular protocol to
   deliver the services.  It may appear in protocols that allow general
   URIs, such as the Session Initiation Protocol (SIP) [5] request URIs,
   web pages or mapping protocols.

   The service URN is generally not expected to be visible to humans.
   For example, it is expected that callers will still dial '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.)  Rather, protocol elements would carry
   the service URN described here, allowing universal identification.
   The translation of dial strings to service URNs is beyond the scope
   of this document; it is likely to depend on the location of the
   caller and may be many-to-one.  For example, a phone for a traveler
   could recognize the emergency dial string for both the traveler's
   home location and the traveler's visited location, translating both
   to the same universal service URN, urn:service:sos.

   Since service URNs are not routable, a 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.  LoST [21] is one
   resolution system for mapping service URNs to URLs based on
   geographic location.  It is anticipated that there will be several
   such systems, possibly with different systems for different services.

   Services are described by top-level service type, and may contain a
   hierarchy of sub-services further describing the service, as outlined
   in Section 3.  Mapping protocols SHOULD always provide a mapping just
   for the top-level service even if sub-services are in use.  This
   mapping for the top-level service MAY also be used if an entity is
   presented with an invalid sub-service and presenting an error
   condition to the user is inappropriate, e.g., during an emergency.

   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",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [2].

   Terminology specific to emergency services is defined in [23].

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3.  Registration Template

   Below, we include the registration template for the URN scheme
   according to RFC 3406 [15].
   Namespace ID: service
   Registration Information: Registration version: 1; registration date:

   Declared registrant of the namespace: TBD

   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 [1] and a subset of the labels
      allowed in [6].  Labels are case-insensitive and SHOULD be
      specified in all lower-case.  For any given service URN, service-
      identifiers can be removed right-to-left and the resulting URN is
      still valid, referring 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.

     "URN:service:" service
     service      = top-level *("." sub-service)
     let-dig      = ALPHA / DIGIT
     let-dig-hyp  = let-dig / '-'
     sub-service  = let-dig [ *let-dig-hyp let-dig ]
     top-level    = let-dig [ *25let-dig-hyp let-dig ]

   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 5).  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.

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   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., [13],
      [14], [18], [19]), types of telecommunications equipment [17],
      people or organizations [12]. tel URIs [16] 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).  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 5.

   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 5).

   Process for identifier resolution: 'service' identifiers are resolved
      by the mapping protocols, an instance of a Resolution Discovery
      System (RDS) as described in RFC 2276 [3].  Each top-level service
      can provide its own distinct set of mapping protocols.  Within
      each top-level service, all mapping protocols MUST return the same
      set of mappings.  Section 4 describes how the S-NAPTR mechanism is
      used to find an instance of a mapping 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 scheme.

   Validation mechanism: Validation determines whether a given string is
      currently a validly-assigned URN [15].  The S-NAPTR mechanism also
      allows to determine if a mapping protocol for a particular top-
      level service exists.  The mapping protocol itself would then
      answer the question whether the service identifier exists.  (The

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      issue of whether a particular combination of service and location
      yields a usable answer is beyond the scope of this specification.)

   Scope: The scope for this URN is public and global.

4.  Finding the Mapping Server

   When a network entity receives a service URN, it uses the S-NAPTR [6]
   mechanism to determine how to map the service URN, possibly using
   other information such as geographic location, to a routable URI.
   Each top-level service may define one or more such mapping protocols
   and mapping protocol servers may be operated by a range of providers.
   Thus, the network entity that needs to resolve the service URN
   queries an appropriate domain, typically its home or service provider
   domain, for NAPTR records and then selects records that match the
   service and the mapping protocols it supports.  The application
   service for this URN is registered in IANA Considerations (Section 5)
   of this document; the application protocols are registered in the
   appropriate protocol document.

   try until a working server has been found DNS name provided by DHCP
   (option X) reverse DNS lookup for all ICE derived addresses [20]
   application service provider

   The S-NAPTR entry MAY contain the "s" flag if the resolving client
   needs to perform an SRV resolution on the replacement string.

   The first entry in the following example indicates that 'sos' service
   URNs should be mapped to URIs using the LoST [21] protocol server at, a DNS A record.  The second entry is for an
   imaginary top-level service 'pizza', using the equally imagined
   'Pizza Location Protocol', offered by the
   server, which should be queried for the appropriate DNS SRV record.
   Note that these NAPTR records are maintained by, i.e., does not actually provide the mapping service itself.
   ;      order pref flags service      regexp
   IN NAPTR 50   50  "a"  "SURN.sos:LoST"     ""
   ;  replacement

   IN NAPTR 10 50 "s" ""  ""

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5.  IANA Considerations

5.1  New Service-Identifying Labels

   New service-identifying labels and sub-services are to be managed by
   IANA, according to the processes outlined in [4].  The policy for
   top-level service names is 'IETF Consensus'.  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.

   To allow use within the constraints of S-NAPTR [6], all top-level
   service names MUST NOT exceed 27 characters.

5.2  S-NAPTR Application Service Tag

   Since each top-level service could use one or more different
   resolution protocols, we need to indicate the top-level service in
   the S-NAPTR application service tag.  To indicate the URN-to-service
   mapping service, all such services start with the string "SURN." (for
   "service URN"), followed by the top-level service identifier.  Note
   that application service tags are case-insensitive and rendered here
   in mixed case purely for readability.

   This document effectively creates a sub-registry of labels under
   SURN, but the contents of that registry are exactly the same as those
   defined in Section 5.1 and thus no separate IANA action is needed.

   This document registers the label "SURN.sos" as the S-NAPTR
   application service tag according to [6] for emergency services and
   defines the intended usage, interoperability considerations and
   security considerations (Section 7).

5.3  Sub-Services for the 'sos' Service

   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 should be of general public interest
   and have a similar emergency nature.  The expert review should take
   into account whether these emergency services 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 or social

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   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 below.
   urn:service:sos.ambulance This service identifier reaches an
      ambulance service that provides emergency medical assistance and
   urn:service:sos.animal-control Animal control is defined as control
      of dogs, cats, and domesticated or undomesticated animals. 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.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.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.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.
   urn:service:sos.suicide The 'suicide' service refers to the suicide
      prevention hotline.

5.4  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 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

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   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.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.

6.  International Considerations

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

7.  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 [22].

8.  References

8.1  Normative References

   [1]  Braden, R., "Requirements for Internet Hosts - Application and
        Support", STD 3, RFC 1123, October 1989.

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

   [3]  Sollins, K., "Architectural Principles of Uniform Resource Name
        Resolution", RFC 2276, January 1998.

   [4]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA

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        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

   [5]  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.

   [6]  Daigle, L. and A. Newton, "Domain-Based Application Service
        Location Using SRV RRs and the Dynamic Delegation Discovery
        Service (DDDS)", RFC 3958, January 2005.

8.2  Informative References

   [7]   Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
         March 1997.

         FUNCTIONS", RFC 2142, May 1997.

   [9]   Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
         specifying the location of services (DNS SRV)", RFC 2782,
         February 2000.

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

   [11]  Mealling, M. and R. Daniel, "The Naming Authority Pointer
         (NAPTR) DNS Resource Record", RFC 2915, September 2000.

   [12]  Mealling, M., "The Network Solutions Personal Internet Name
         (PIN): A URN Namespace for People and Organizations", RFC 3043,
         January 2001.

   [13]  Rozenfeld, S., "Using The ISSN (International Serial Standard
         Number) as URN (Uniform Resource Names) within an ISSN-URN
         Namespace", RFC 3044, January 2001.

   [14]  Hakala, J. and H. Walravens, "Using International Standard Book
         Numbers as Uniform Resource Names", RFC 3187, October 2001.

   [15]  Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
         "Uniform Resource Names (URN) Namespace Definition Mechanisms",
         BCP 66, RFC 3406, October 2002.

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

   [17]  Tesink, K. and R. Fox, "A Uniform Resource Name (URN) Namespace
         for the Common Language Equipment Identifier (CLEI) Code",
         RFC 4152, August 2005.

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   [18]  Kang, S., "Using Universal Content Identifier (UCI) as Uniform
         Resource Names (URN)", RFC 4179, October 2005.

   [19]  Kameyama, W., "A Uniform Resource Name (URN) Namespace for the
         TV-Anytime Forum", RFC 4195, October 2005.

   [20]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Methodology for Network  Address Translator (NAT) Traversal for
         Offer/Answer Protocols", draft-ietf-mmusic-ice-08 (work in
         progress), March 2006.

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

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

   [23]  Schulzrinne, H. and R. Marshall, "Requirements for Emergency
         Context Resolution with Internet Technologies",
         draft-ietf-ecrit-requirements-09 (work in progress), May 2006.

Author's Address

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

   Phone: +1 212 939 7004

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 [16].  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

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      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 Finland.

   tel:sos This solution avoids name conflicts, but is not a valid "tel"
      [16] 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.

   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 [8] and the "postmaster" convention
      documented in RFC 2822 [10].  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

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   benefited from the comments of Leslie Daigle, Keith Drage, Benja
   Fallenstein, Paul Kyzivat, Andrew Newton, Brian Rosen, Jonathan
   Rosenberg, Martin Thomson and Hannes Tschofenig.

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