SIPPING                                                   H. Schulzrinne
Internet-Draft                                               Columbia U.
Expires: August 19, 2006                               February 15, 2006

               A Uniform Resource Name (URN) for Services

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   Copyright (C) The Internet Society (2006).


   The content of many communication services depend 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.  Registration Template  . . . . . . . . . . . . . . . . . . . .  4
   3.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     5.1   Normative References . . . . . . . . . . . . . . . . . . .  6
     5.2   Informative References . . . . . . . . . . . . . . . . . .  7
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  8
   A.  Alternative Approaches Considered  . . . . . . . . . . . . . .  8
   B.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
       Intellectual Property and Copyright Statements . . . . . . . . 10

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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 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 911; 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 to
   define such global, well-known services, while distributing the
   actual implementation across a large number of service-providing
   entities.  While there are many ways to divide provision of such
   services, we focus on geography as a common way to delineate service
   regions.  In addition, users can choose different directory providers
   that in turn manage how geographic locations are mapped to service

   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 allow to delegate routing decisions
   to third parties and mark certain requests as having special
   characteristics while preventing these characteristics to be
   accidentally invoked on inappropriate requests.

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

   Existing technologies address the mapping of service identifiers to a
   service for a particular DNS domain (DNS SRV [9], DNS NAPTR [11]) or
   a local area network (SLP [8]).

   The tel URI [16] allows to express service codes such as 911 by
   adding a context parameter, but does not address the problem of
   global validity.

   LUMP [20] is a prototype resolution system for mapping URNs to URLs
   based on geographic location.  However, it is anticipated that there
   will be several such systems.

2.  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 IRIs, that used for domain names [1] except that
      there is no restriction on the first character being a letter;
      labels are case-insensitive and SHOULD be specified in all lower-
      case.  Any string of service labels can be used to request
      services that are either more generic or more specific.  In other
      words, if a service 'x.y.z' exists, the URNs 'x' and 'x.y' are
      also valid service URNs.

     "URN:service:" top-level-service *("." service-identifier)
     top-level-service = ALPHA / DIGIT / "-" /
     service-identifier = ALPHA / DIGIT / "-" /

   Relevant ancillary documentation: None
   Community considerations: The service URN is believe 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

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      kind, without having to determine manually who provides the
      particular service in the user's current context, e.g., at his
      current location.  For example, a traveler will be able to use his
      mobile device to request emergency services without having to know
      the local emergency number.  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.  The ECRIT working group is currently
      discussing several approaches, including solutions based on DNS,
      IRIS and a web-services protocol.  Software prototypes for some of
      these are currently already available and are believed to be
      readily developed.
   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.
   Identifier persistence considerations: The 'service' URN for the same
      service is expected to be persisent, 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: Details of the service assignment
      depend on the service and national regulations.  In general, it is
      assumed that providers of services can register through a service
      mapping mechanism for a particular service in a particular
      geographic area.  The provision of some services may be restricted
      by local or national regulations.  (As a hypothetical example,
      providing emergency services may be restricted to government-
      authorized entities, which may limit the region where each entity
      can advertise its services.)  The rules for each service are
      described in a service-specific document.
   Process for identifier resolution: 'service' identifiers are resolved
      by the TBD mapping protocol, an instance of a Resolution Discovery
      System (RDS) as described in RFC 2276 [3].  (In theory, there
      could be several such mapping protocols in concurrent use, as long
      as there are reasonable guarantees that all services are available

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      in all mapping protocols.)
   Rules for Lexical Equivalence: 'service' identifiers are compared
      according to domain name comparison rules.  The use of homographic
      identifiers is NOT RECOMMENDED.
   Conformance with URN Syntax: There are no special considerations.
   Validation mechanism: The RDS mechanism is also used to validate the
      existence of a resource.  As noted, by its design, the
      availability of a resource may depend on where service is desired
      and there may not be service available in all or most locations.
      (For example, roadside assistance service is unlikely to be
      available on about 70% of the earth's surface.)
   Scope: The scope for this URN is public and global.

3.  Example

   For discussion and illustration purposes only, we include an example
   of a particular service.  We choose emergency services as an example,
   with the top-level service identifier 'sos'.  A possible list of
   identifiers might include:


4.  IANA Considerations

   New service-identifying tokens and sub-registrations are to be
   managed by IANA, according to the processes outlined in [4].  The
   policy for top-level service names is TBD, but could be
   'specification required', 'IETF Consensus' or 'Standards Action'.
   The policy for assigning names to sub-services may differ for each
   top-level service designation and MUST be defined by the document
   describing the top-level service.

5.  References

5.1  Normative References

   [1]  Mockapetris, P., "Domain names - concepts and facilities",
        STD 13, RFC 1034, November 1987.

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   [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
        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]  Duerst, M. and M. Suignard, "Internationalized Resource
        Identifiers (IRIs)", RFC 3987, January 2005.

5.2  Informative References

         FUNCTIONS", RFC 2142, May 1997.

   [8]   Guttman, E., Perkins, C., Veizades, J., and M. Day, "Service
         Location Protocol, Version 2", RFC 2608, June 1999.

   [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.

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   [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.

   [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]  Schulzrinne, H., "Location-to-URL Mapping Protocol (LUMP)",
         draft-schulzrinne-ecrit-lump-01 (work in progress),
         October 2005.

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 "sos" SIP URI reserved user name proposed here follows the
   convention of RFC 2142 [7] and the "postmaster" convention documented
   in RFC 2822 [10].  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:

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   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
      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"
      [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,  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
   benefitted from the comments of Benja Fallenstein and Leslie Daigle.

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