Network Working Group                                 URI Interest Group
Internet-Draft                                                  W3C/IETF
Expires: March 25, 2002                               September 24, 2001


        URIs, URLs, and URNs: Clarifications and Recommendations
       Report from the joint W3C/IETF URI Planning Interest Group
                      draft-mealling-uri-ig-01.txt

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   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   This Internet-Draft will expire on March 25, 2002.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This paper addresses and attempts to clarify two issues pertaining to
   URIs, and presents recommendations.  Section 1 addresses how URI
   space is partitioned and the relationship between URIs, URLs, and
   URNs.  Section 2 describes how URI schemes and URN namespace ids are
   registered.  Section 3 mentions additional unresolved issues not
   considered by this paper and section 4 presents recommendations.
   Questions concerning this document should be directed to the
   uri@w3.org mailing list.







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Table of Contents

   1.      URI Partitioning . . . . . . . . . . . . . . . . . . . . .  3
   1.1     Classical View . . . . . . . . . . . . . . . . . . . . . .  3
   1.2     Contemporary View  . . . . . . . . . . . . . . . . . . . .  3
   1.3     Confusion  . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.      Registration . . . . . . . . . . . . . . . . . . . . . . .  4
   2.1     URI Schemes  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.1.1   Registered URI schemes . . . . . . . . . . . . . . . . . .  4
   2.1.2   Unregistered URI Schemes . . . . . . . . . . . . . . . . .  4
   2.1.2.1 Public Unregistered Schemes  . . . . . . . . . . . . . . .  4
   2.1.2.2 Private Schemes  . . . . . . . . . . . . . . . . . . . . .  5
   2.1.3   Registration of URI Schemes  . . . . . . . . . . . . . . .  5
   2.1.3.1 IETF Tree  . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.1.3.2 Other Trees  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.2     URN Namespaces . . . . . . . . . . . . . . . . . . . . . .  5
   2.2.1   Registered URN NIDs  . . . . . . . . . . . . . . . . . . .  5
   2.2.2   Pending URN NIDs . . . . . . . . . . . . . . . . . . . . .  6
   2.2.3   Unregistered NIDs  . . . . . . . . . . . . . . . . . . . .  6
   2.2.4   Registration Procedures for URN NIDs . . . . . . . . . . .  7
   3.      Additional URI Issues  . . . . . . . . . . . . . . . . . .  7
   4.      Recommendations  . . . . . . . . . . . . . . . . . . . . .  7
   5.      Acknowledgements . . . . . . . . . . . . . . . . . . . . .  8
           References . . . . . . . . . . . . . . . . . . . . . . . .  9
           Author's Address . . . . . . . . . . . . . . . . . . . . . 10
           Full Copyright Statement . . . . . . . . . . . . . . . . . 11

























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1. URI Partitioning

   There is some confusion in the web community over the partitioning of
   URI space, specifically, the relationship among the concepts of URL,
   URN, and URI.  The confusion owes to the incompatibility between two
   different views of URI partitioning, which we call the "classical"
   and "contemporary" views.

1.1 Classical View

   During the early years of discussion of web identifiers (early to mid
   90s), people assumed that an identifer type would be cast into one of
   two (or possibly more) classes.  An identifier might specify the
   location of a resource (a URL) or its name (a URN) independent of
   location.  Thus a URI was either a URL or a URN.  There was
   discussion about generalizing this by addition of a discrete number
   of additional classes; for example, a URI might point to metadata
   rather than the resource itself, in which case the URI would be a URC
   (citation).  URI space was thus viewed as partitioned into subspaces:
   URL and URN, and additional subspaces, to be defined.  The only such
   additional space ever proposed was URC and there never was any buy-
   in; so without loss of generality it's reasonable to say that URI
   space was thought to be partitioned into two classes: URL and URN.
   Thus for example, "http:" was a URL scheme, and "isbn:" would
   (someday) be a URN scheme.  Any new scheme would be cast into one or
   the other of these two classes.

1.2 Contemporary View

   Over time, the importance of this additional level of hierarchy
   seemed to lessen; the view became that an individual scheme does not
   need to be cast into one of a discrete set of URI types such as
   "URL", "URN", "URC", etc.  Web-identifer schemes are in general URI
   schemes; a given URI scheme may define subspaces.  Thus "http:" is a
   URI scheme.  "urn:" is also a URI scheme; it defines subspaces,
   called "namespaces".  For example, the set of URNs of the form
   "urn:isbn:n-nn-nnnnnn-n" is a URN namespace.  ("isbn" is an URN
   namespace identifier.  It is not a "URN scheme" nor a "URI scheme").

   Further according to the contemporary view, the term "URL" does not
   refer to a formal partition of URI space; rather, URL is a useful but
   informal concept: a URL is a type of URI that identifies a resource
   via a representation of its primary access mechanism (e.g., its
   network "location"), rather than by some other attributes it may
   have.  Thus as we noted, "http:" is a URI scheme.  An http URI is a
   URL.  The phrase "URL scheme" is now used infrequently, usually to
   refer to some subclass of URI schemes which exclude URNs.




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1.3 Confusion

   The body of documents (RFCs, etc) covering URI architecture, syntax,
   registration, etc., spans both the classical and contemporary
   periods.  People who are well-versed in URI matters tend to use "URL"
   and "URI" in ways that seem to be interchangable.  Among these
   experts, this isn't a problem.  But among the Internet community at
   large, it is.  People are not convinced that URI and URL mean the
   same thing, in documents where they (apparently) do.  When one sees
   an RFC that talks about URI schemes (e.g.  "URI Syntax" (RFC 2396)
   [12]), another that talks about URL schemes (e.g.  "Registration
   Procedures for URL Schemes" (RFC 2717) [1]), and yet another that
   talks of URN schemes ("Architectural Principles of URN Resolution"
   (RFC 2276) [13]) it is natural to wonder what's the difference, and
   how they relate to one another.  While RFC 2396 1.2 attempts to
   address the distinction between URIs, URLs and URNs, it has not been
   successful in clearing up the confusion.

2. Registration

   This section examines the state of registration of URI schemes and
   URN namespaces and the mechanisms by which registration currently
   occurs.

2.1 URI Schemes

2.1.1 Registered URI schemes

   The official register of URI scheme names is maintained by IANA, at
   http://www.iana.org/assignments/uri-schemes .  For each scheme, the
   RFC that defines the scheme is listed, for example "http:" is defined
   by RFC2616 [14].  The table currently lists 30 schemes.  In addition,
   there are a few "reserved" scheme names; at one point in time these
   were intended to become registered schemes but have since been
   dropped.

2.1.2 Unregistered URI Schemes

   We distinguish between public (unregistered) and private schemes.  A
   public scheme (registered or not), is one for which there is some
   public document describing it.

2.1.2.1 Public Unregistered Schemes

   Dan Conolly's paper at http://www.w3.org/Addressing/schemes provides
   a list of known, public URI schemes, both registered and un-
   registered, a total of 84 schemes.  50 or so of these are
   unregistered (not listed in the IANA register).  Some may be obsolete



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   (for example, it appears that "phone", is obsolete, superceded by
   "tel").  Some have an RFC, but are not included in the IANA list.

2.1.2.2 Private Schemes

   It's probably impossible to determine all of these, and it's not
   clear that it's worthwhile to try, except perhaps to get some idea of
   their number.  In the minutes of the August 1997 IETF meeting is the
   observation that there may be 20-40 in use at Microsoft, with 2-3
   being added a day, and that WebTV has 24, with 6 added per year.

2.1.3 Registration of URI Schemes

   "Registration Procedures for URL Scheme Names" (RFC 2717) [1]
   specifies procedures for registering scheme names, and points to
   "Guidelines for new URL Schemes" (RFC 2718) [2] which supplies
   guidelines.  RFC 2717 describes an organization of schemes into
   "trees".

2.1.3.1 IETF Tree

   The IETF tree is intended for schemes of general interest to the
   Internet community, and which require a substantive review and
   approval process.  Registration in the IETF tree requires publication
   of the scheme syntax and semantics in an RFC.

2.1.3.2 Other Trees

   Although RFC 2717 describes "alternative trees", no alternative trees
   have been registered to date, although a vendor-supplied tree ("vnd")
   is pending.  URI schemes in alternative trees will be distinguished
   because they will have a "." in the scheme name.

2.2 URN Namespaces

   A URN namespace is identified by a "Namespace ID", NID, which is
   registered with IANA (see Section 2.2.4).

2.2.1 Registered URN NIDs

   There are two categories of registered URN NIDs:

   o  Informal: These are of the form "urn-<number>" where <number> is
      assigned by IANA.  There are three registered in this category
      (urn-1, urn-2, and urn-3).

   o  Formal: The official list of registered NIDs is kept by IANA at
      http://www.iana.org/assignments/urn-namespaces.  Currently it



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      lists eight registered NIDs:

      *  'ietf', defined by "URN Namespace for IETF Documents" (RFC
         2648) [3]

      *  'pin', defined by "The Network Solutions  Personal Internet
         Name (PIN): A URN Namespace for People and  Organizations" (RFC
         3043) [4]

      *  'issn' defined by "Using The ISSN as URN  within an ISSN-URN
         Namespace" (RFC 3043) [4]

      *  'oid' defined by "A URN Namespace of Object  Identifiers" (RFC
         3061) [6]

      *  'newsml' defined by "URN Namespace for  NewsML Resources" (RFC
         3085) [7]

      *  'oasis' defined by "A URN Namespace for  OASIS" (RFC 3121) [8]

      *  'xmlorg' defined by "A URN Namespace for  XML.org" (RFC 3120)
         [9]

      *  'publicid' defined by "A URN Namespace for  Public Identifiers"
         (RFC 3151) [10]


2.2.2 Pending URN NIDs

   There are a number of pending URN NID registration requests but there
   is no reliable way to discover them, or their status.  For example,
   'isbn' and 'nbn' have been approved by the IESG and are in the RFC
   Editor's queue.  'isbn', as a potential URN namespace (or URI
   scheme), in particular has been a source of much speculation and
   confusion over several years.  It would be helpful if there were some
   formal means to track the status of NID requests such as 'isbn'.

2.2.3 Unregistered NIDs

   In the "unregistered" category (besides the experimental case, not
   described in this paper) there are bonafide NIDs that just haven't
   bothered to even explore the process of registration.The most
   prominent that comes to mind is 'hdl'.  In the case of 'hdl', it has
   been speculated that this scheme has not been registered because it
   is not clear to the owners whether it should be registered as a URI
   scheme or as a URN namespace.





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2.2.4 Registration Procedures for URN NIDs

   "URN Namespace Definition Mechanisms" (RFC 2611) [11] describes the
   mechanism to obtain an NID for a URN namespace, which is registered
   with IANA.

   A request for an NID should describe features including: structural
   characteristic of identifiers (for example, features relevant to
   caching/shortcuts approaches); specific character encoding rules
   (e.g., which character should be used for single-quotes); RFCs,
   standards, etc, that explains the namespace structure; identifier
   uniqueness considerations; delegation of assignment authority,
   including how to become an assigner of identifiers; identifier
   persistence considerations; quality of service considerations;
   process for identifier resolution; rules for lexical equivalence; any
   special considerations required for conforming with the URN syntax
   (particularly applicable in the case of legacy naming systems);
   validation mechanisms (determining whether a given string is
   currently a validly-assigned URN; and scope (for example,"United
   States social security numbers").

3. Additional URI Issues

   There are additional unresolved URI issues, not considered by this
   paper, which we hope will be addressed by a follow-on effort.  We
   have not attempted to completely enumerate these issues, however,
   they include (but are not limited to) the following:

   o  The use of URIs as identifiers that don't actually identify
      network resources (for example they identify an abstract object
      such as an XML schema, or a physical object such as a book or even
      a person).

   o  IRIs (International Resource Identifiers): the extension of URI
      syntax to non-ASCII.


4. Recommendations

   We recommend the following:

   1.  The W3C and IETF should jointly develop and endorse a model for
       URIs, URLs and URNs consistent with the '"Contemporary View"
       described in section 1, and which considers the additional URI
       issues listed or alluded to in section 3.

   2.  RFCs such as 2717 ("Registration Procedures for URL Scheme
       Names") and 2718 ("Guidelines for new URL Schemes") should both



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       be generalized to refer to "URI schemes" rather that "URL
       schemes" and, after refinement, moved forward as Best Current
       Practice in IETF.

   3.  The registration procedures for alternative trees should be
       clarified in RFC 2717.

   4.  Public but unregistered schemes should become registered, where
       possible.  Obsolete schemes should be purged or clearly marked as
       obsolete.

   5.  IANA registration information should be updated:

       *  Add 'urn' to the list of registered URI schemes with a pointer
          to the URN namespace registry.

       *  Maintain status information about pending registrations (URI
          schemes and URN NID requests ).

       *  Insure that it is clear that the page is the official
          registry, e.g., by adding a heading to the effect "This is the
          Official IANA Registry of URI Schemes".


5. Acknowledgements

   The participants in the URI Planning Interest Group are:

   o  Tony Coates

   o  Dan Connolly

   o  Diana Dack

   o  Leslie Daigle

   o  Ray Denenberg

   o  Martin DÂ’rst

   o  Paul Grosso

   o  Sandro Hawke

   o  Renato Iannella

   o  Graham Klyne




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   o  Larry Masinter

   o  Michael Mealling

   o  Mark Needleman

   o  Norman Walsh

References

   [1]   Petke, R. and I. King, "Registration Procedures for URL Scheme
         Names", BCP 35, RFC 2717, November 1999.

   [2]   Masinter, L., Alvestrand, H., Zigmond, D. and R. Petke,
         "Guidelines for new URL Schemes", RFC 2718, November 1999.

   [3]   Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
         August 1999.

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

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

   [6]   Mealling, M., "A URN Namespace of Object Identifiers", RFC
         3061, February 2001.

   [7]   Coates, A., Allen, D. and D. Rivers-Moore, "URN Namespace for
         NewsML Resources", RFC 3085, March 2001.

   [8]   Best, K. and N. Walsh, "A URN Namespace for OASIS", RFC 3121,
         June 2001.

   [9]   Best, K. and N. Walsh, "A URN Namespace for XML.org", RFC 3120,
         June 2001.

   [10]  Walsh, N., Cowan, J. and P. Grosso, "A URN Namespace for Public
         Identifiers", RFC 3151, August 2001.

   [11]  Daigle, L., van Gulik, D., Iannella, R. and P. Faltstrom, "URN
         Namespace Definition Mechanisms", BCP 33, RFC 2611, June 1999.

   [12]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
         Resource Identifiers (URI): Generic Syntax", RFC 2396, August
         1998.



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

   [14]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H., Masinter, L.,
         Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
         HTTP/1.1", RFC 2616, June 1999.


Author's Address

   URI Planning Interest Group
   W3C/IETF







































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Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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