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rfc:rfc3305

Network Working Group M. Mealling, Ed. Request for Comments: 3305 R. Denenberg, Ed. Category: Informational W3C URI Interest Group

                                                           August 2002
    Report from the Joint W3C/IETF URI Planning Interest Group:

Uniform Resource Identifiers (URIs), URLs, and Uniform Resource Names

            (URNs): Clarifications and Recommendations

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

 This document, a product of the W3C Uniform Resource Identifier (URI)
 Interest Group, addresses and attempts to clarify issues pertaining
 to URIs.  This document addresses how URI space is partitioned and
 the relationship between URIs, URLs, and URNs, describes how URI
 schemes and URN namespaces ids are registered, and presents
 recommendations for continued work on this subject.

Mealling & Denenberg Informational [Page 1] RFC 3305 URIs, URLs, and URNs August 2002

Table of Contents

 1.      The W3C URI Interest Group . . . . . . . . . . . . . . . .  2
 2.      URI Partitioning . . . . . . . . . . . . . . . . . . . . .  2
 2.1     Classical View . . . . . . . . . . . . . . . . . . . . . .  3
 2.2     Contemporary View  . . . . . . . . . . . . . . . . . . . .  3
 2.3     Confusion  . . . . . . . . . . . . . . . . . . . . . . . .  3
 3.      Registration . . . . . . . . . . . . . . . . . . . . . . .  4
 3.1     URI Schemes  . . . . . . . . . . . . . . . . . . . . . . .  4
 3.1.1   Registered URI schemes . . . . . . . . . . . . . . . . . .  4
 3.1.2   Unregistered URI Schemes . . . . . . . . . . . . . . . . .  4
 3.1.2.1 Public Unregistered Schemes  . . . . . . . . . . . . . . .  4
 3.1.2.2 Private Schemes  . . . . . . . . . . . . . . . . . . . . .  5
 3.1.3   Registration of URI Schemes  . . . . . . . . . . . . . . .  5
 3.1.3.1 IETF Tree  . . . . . . . . . . . . . . . . . . . . . . . .  5
 3.1.3.2 Other Trees  . . . . . . . . . . . . . . . . . . . . . . .  5
 3.2     URN Namespaces . . . . . . . . . . . . . . . . . . . . . .  5
 3.2.1   Registered URN NIDs  . . . . . . . . . . . . . . . . . . .  5
 3.2.2   Pending URN NIDs . . . . . . . . . . . . . . . . . . . . .  6
 3.2.3   Unregistered NIDs  . . . . . . . . . . . . . . . . . . . .  7
 3.2.4   Registration Procedures for URN NIDs . . . . . . . . . . .  7
 4.      Additional URI Issues  . . . . . . . . . . . . . . . . . .  7
 5.      Recommendations  . . . . . . . . . . . . . . . . . . . . .  8
 6.      Security Considerations  . . . . . . . . . . . . . . . . .  8
 7.      Acknowledgements . . . . . . . . . . . . . . . . . . . . .  8
         References . . . . . . . . . . . . . . . . . . . . . . . .  9
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . 10
         Full Copyright Statement . . . . . . . . . . . . . . . . . 11

1. The W3C URI Interest Group

 In October, 2000 the W3C formed a planning group whose mission was to
 evaluate the opportunities for W3C work in the area of Uniform
 Resource Identifiers (URIs) and to develop a proposal for continued
 work in this area.  The Interest Group was composed of W3C members
 and invited experts from the IETF to participate as well.  This
 document is a set of recommendations from this group, to the W3C and
 the IETF for work that can and should continue in this area.

2. 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.

Mealling & Denenberg Informational [Page 2] RFC 3305 URIs, URLs, and URNs August 2002

2.1 Classical View

 During the early years of discussion of web identifiers (early to mid
 90s), people assumed that an identifier 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 the 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, URN, and additional subspaces to be defined.  The
 only such additional space ever proposed was Uniform Resource
 Characteristics (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 of these two classes.

2.2 Contemporary View

 Over time, the importance of this additional level of hierarchy
 seemed to lessen; the view became that an individual scheme did not
 need to be cast into one of a discrete set of URI types, such as
 "URL", "URN", "URC", etc.  Web-identifier schemes are, in general,
 URI schemes, as 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 is it 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.

2.3 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 interchangeable.  Among these
 experts, this isn't a problem, but among the Internet community at

Mealling & Denenberg Informational [Page 3] RFC 3305 URIs, URLs, and URNs August 2002

 large, it is a problem.  People are not convinced that URI and URL
 mean the same thing, in documents where they (apparently) do.  When
 one RFC talks about URI schemes (e.g. "URI Syntax" (RFC 2396) [12]),
 another talks about URL schemes (e.g. "Registration Procedures for
 URL Schemes" (RFC 2717) [1]), and yet another talks of URN schemes
 ("Architectural Principles of URN Resolution" (RFC 2276) [13]), it is
 natural to wonder how they difference, and how they relate to one
 another.  While RFC 2396, section 1.2, attempts to address the
 distinction between URIs, URLs and URNs, it has not been successful
 in clearing up the confusion.

3. Registration

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

3.1 URI Schemes

3.1.1 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 lists  34 schemes (at time of publication
 of this RFC).  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.

3.1.2 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.

3.1.2.1 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 85 schemes at time of publication of this RFC.
 50 or so of these are unregistered (not listed in the IANA register).
 Some of these URI schemes are obsolete (for example, "phone" is
 obsolete, superceded by "tel"), while some have an RFC, but are not
 included in the IANA list.

Mealling & Denenberg Informational [Page 4] RFC 3305 URIs, URLs, and URNs August 2002

3.1.2.2 Private Schemes

 It is 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.

3.1.3 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".  It is important to note that these two documents use the
 historical term 'URL' when in fact, they refer to URIs in general.
 In fact, one of the recommended tasks in Section 5 is for these
 documents to be updated to use the term 'URI' instead of 'URL'.

3.1.3.1 IETF Tree

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

3.1.3.2 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.

3.2 URN Namespaces

 A URN namespace is identified by a "Namespace ID" (NID), which is
 registered with IANA (see Section 3.2.4).

3.2.1 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 four registered (at time of
    publication of this RFC) in this category  (urn-1, urn-2,  urn-3,
    and urn-4).

Mealling & Denenberg Informational [Page 5] RFC 3305 URIs, URLs, and URNs August 2002

 o  Formal: The official list of registered NIDs is kept by IANA at
    http://www.iana.org/assignments/urn-namespaces.  At the time of
    publication of this RFC it lists ten 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]

  • 'isbn' defined by "Using International Standard Book Numbers as

Uniform Resource Names" (RFC 3187) [15]

  • 'nbn' defined by "Using National Bibliography Numbers as

Uniform Resource Names" (RFC 3188) [16]

3.2.2 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.  It would
 be helpful if there were some formal means to track the status of NID
 requests such as 'isbn'.

Mealling & Denenberg Informational [Page 6] RFC 3305 URIs, URLs, and URNs August 2002

3.2.3 Unregistered NIDs

 In the "unregistered" category (besides the experimental case, not
 described in this paper), there are entities that maintain namespaces
 that, while completely appropriate as URNs, just haven't bothered to
 explore the process of NID 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.

3.2.4 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 explain 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").

4. 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 namespace, 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.

Mealling & Denenberg Informational [Page 7] RFC 3305 URIs, URLs, and URNs August 2002

5. 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 be
    generalized to refer to "URI schemes", rather than "URL schemes"
    and, after refinement, moved forward as Best Current Practices in
    the 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".

6. Security Considerations

 This memo does not raise any known security threats.

7. Acknowledgements

 The participants in the URI Planning Interest Group are:
 o  Tony Coates
 o  Dan Connolly
 o  Diana Dack

Mealling & Denenberg Informational [Page 8] RFC 3305 URIs, URLs, and URNs August 2002

 o  Leslie Daigle
 o  Ray Denenberg
 o  Martin Duerst
 o  Paul Grosso
 o  Sandro Hawke
 o  Renato Iannella
 o  Graham Klyne
 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.

Mealling & Denenberg Informational [Page 9] RFC 3305 URIs, URLs, and URNs August 2002

 [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.
 [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.
 [15] Hakala, J. and H. Walravens, "Using International Standard Book
      Numbers as Uniform Resource Names", RFC 3187, October 2001.
 [16] Hakala, J., "Using National Bibliography Numbers as Uniform
      Resource Names", RFC 3188, October 2001.

Authors' Addresses

 Michael Mealling
 VeriSign, Inc.
 21345 Ridgetop Circle
 Sterling, VA  20166
 US
 EMail: michael@verisignlabs.com
 Ray Denenberg
 Library of Congress
 Washington, DC  20540
 US
 EMail: rden@loc.gov

Mealling & Denenberg Informational [Page 10] RFC 3305 URIs, URLs, and URNs August 2002

Full Copyright Statement

 Copyright (C) The Internet Society (2002).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
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 or assist in its implementation may be prepared, copied, published
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Acknowledgement

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

Mealling & Denenberg Informational [Page 11]

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