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Network Working Group                                        L. Masinter
Internet-Draft                                Adobe Systems Incorporated
Expires: September 30, 2002                                April 1, 2002


          "duri" and "tdb" URN namespaces based on dated URIs
                      draft-masinter-dated-uri-02

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   This Internet-Draft will expire on September 30, 2002.

Copyright Notice

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

Abstract

   This document defines two namespaces of URNs based on prepending a
   date to an (encoded) URI.  The results are namespaces in which names
   are readily assigned, offer the persistence of reference that is
   required by URNs, but do not require a stable authority to assign the
   name.  The first namespace ("duri") is used to refer to URI-
   identified resources as they appeared at a particular time.  The
   second namespace ("tdb") is useful as a way of creating URNs that
   refer to abstractions that may not themselves be networked resources.

   The definition of these namespaces may reduce the need to define new
   URN namespaces merely for the purpose of creating stable identifiers.




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Note

   This document is not a product of any working group.  Many of the
   ideas here have been discussed for a number of years.  This document
   has been discussed on the mailing list <uri@w3.org>.














































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1. Overview and Requirements

   The URN namespaces defined here solve several related problems.

1.1 Easy URN assignment

   The URN specification [3] allows for many URN namespaces, and many
   have been registered.  However, obtaining an appropriate URN in any
   of the currently defined URN namespaces may be difficult: a number of
   URN namespace registrations have been accompanied by comments that no
   other URN namespace was available for the class of documents for
   which identifiers were wanted.

1.2 Persistent identifiers

   RFC 1737 [3] defines several requirements for Uniform Resource Names.
   In particular, it requires "persistence":

      Persistence: It is intended that the lifetime of a URN be
      permanent.  That is, the URN will be globally unique forever, and
      may well be used as a reference to a resource well beyond the
      lifetime of the resource it identifies or of any naming authority
      involved in the assignment of its name.

   Many people have wondered how to create globally unique and
   persistent identifiers.  There are a number of URI schemes and URN
   namespaces already registered.  However, an absolute guarantee of
   both uniqueness and persistence is very difficult.

   In some cases, the guarantee of persistence comes through a promise
   of good management practice, such as is encouraged in "Cool URLs
   don't change" [2].  However, relying on promise of good management
   practice is not the same as having a design that guarantees
   reliability independent of actual administrative practice.

   A primary design goal for URIs is that they are intended to mean the
   same thing, no matter in what context they appear: a "Uniform" way to
   Identify a Resource.  However, even when URIs have Uniform meaning
   from the point of view of the source of the reference, they don't
   guarantee stability over time.  Despite best efforts and intentions,
   identifying information can change in unpredictable ways: domain
   names can disappear or be reassigned, name assigning organizations
   can change structure, responsibility, disappear, merge, or change in
   unpredictable ways.

   There is a significant dependence in the interpretation of many URNs
   with the concept of "naming authority".  The authority is presumably
   some individual or organization both to insure uniqueness of



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   assignment and also to help with understanding the meaning of the
   link between the name and the named.

   However, authorities, whether individuals or organizations, have a
   lifetime, and must be consulted at some point to understand the
   bindings.  The functioning of names as unique identifiers and holders
   of meaning depends on having a reliable infrastructure of consulting
   the authority or the authorities records to determine the thing
   referenced.

1.3 URIs for abstractions

   The description of URIs [6] describes a range for 'Resource' that is
   quite broad:

      A resource can be anything that has identity.  Familiar examples
      include an electronic document, an image, a service (e.g.,
      "today's weather report for Los Angeles"), and a collection of
      other resources.  Not all resources are network "retrievable";
      e.g., human beings, corporations, and bound books in a library can
      also be considered resources.

   However, most of the URI mechanisms are either quite concrete,
   (including an identification of protocol and protocol parameters for
   connecting to a network communication endpoint), or else quite vague
   about the way in which they are connected to the resource they
   identify.  There are no current URN namespaces (or URI schemes) that
   allow easy assignment of URIs for abstractions.

   The goal, then, of the second URN scheme proposed below is to provide
   a mechanism which is, at the same time:

      permanent: The identity of the resource identified is not subject
      to reinterpretation over time.

      explicitly bound: The mechanism by which the identified resource
      can be determined is explicitly included in the URI.

      useful for non-networked items: Allows identification of resources
      outside the network: people, organizations, abstract concepts.

      no administration: The mechanism does not depend on reliable
      administrative processes of authorities for either assignment or
      interpretation.







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2. Namespace definitions

2.1 "duri" namespace

   It is traditional in convention references and citations in printed
   works to include the date of publication; this practice serves the
   important purpose that the context of the naming can be determined.

   The "duri" URN namespace takes the form:
        urn:duri:<date>:<encoded-URI>

    where <date> is a digit string corresponding to a date (Section 4),
   and an <encoded-URI> is an absolute URI-reference [6]  in which any
   character excluded from URN syntax has been escaped (Section 3).

   The meaning of a duri is "the resource (or fragment) that was
   identified by the <encoded-URI> (after hex decoding) at the very
   first instant of the date(time) given".

   For example, 'urn:duri:2001:http://www.ietf.org' is a persistent
   identifier to 'http://www.ietf.org' as of the very first moment of
   the year 2001.  A duri may not be a resource locator in a practical
   sense, because the time of location has passed.  However, is an
   acceptable resource identifier, and fulfills all of the requirements
   for URNs [3].

2.2 "tdb" namespace

   The second URN namespace defined is a parallel space which is useful
   for describing entities, concepts, abstractions, and other items
   which are not themselves network accessible resources, but have been
   at some point described by network accessible resources.

   The "tdb" namespace designates the "thing described by" a resource at
   a given URI at the given time.  This URN namespace is described by
   'tdb', e.g.,

           urn:tdb:<date>:<encoded-URI>

    with the same syntactic rules as 'duri'.

   The intent is to use the inversion of "is a document about".  It is
   common practice to give a reference for a concept by including a
   pointer to a document, segment, phrase that defines the concept.
   "tdb" attempts to capture this practice in URI space.

   For example, "urn:tdb:2001:http://www.ietf.org" can be used to
   designate the Internet Engineering Task Force organization, at least



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   as it was described by or referenced by its home page at the first
   instant of 2001.

   The "tdb" namespace differs from most other mechanisms for
   identifying abstractions because the designation of what is actually
   identified by the tdb doesn't depend on knowing the intention of the
   "assigner" of the identifier.  Unlike many of the alternatives
   proposed, the identification is not dependent on the context of use.

   The "tdb" namespace can be thought of as following another level of
   indirection to URI resolution.  While one could imagine using 'tdb'
   without a date, it would leave the possibility that a reference that
   is unambiguous at one time might become ambiguous at some other time.






































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3. Encoding URIs

   Both "duri" and "tdb" URN namespaces require that some characters in
   the URI references be encoded.

3.1 Characters that must be encoded

   The characters that must be encoded are:

   o  All characters marked excluded in RFC 2141 [1], section 2.4
      These are excluded because they are not allowed in URNs.

                   \"&<>[]^`{|}~

   o  The character "#"
      Note that the <encoded-URI> of a "duri" or "tdb" can include a
      fragment identifier, but the "#" character used to delimit it must
      be encoded.

   o  The character "%"
      The encoded-URI can itself contain encoded characters, which are
      encoded with the same method.  To insure that decoding happens at
      the right level of processing, the "%" itself must be encoded.

   Unfortunately, there are many cases where there is a double encoding
   of characters, first to construct the embedded URI itself and second
   to then embed the URI within the tdb or duri URN.

3.2 Hierarchy and unencoded /

   The URN specification [3] discourages the use of "/" in URNs because,
   in general, there is no good interpretation of hierarchy and relative
   URIs for assigned names.  However, for the particular case of duris
   (at least), there seems to be no good reason to avoid the "/" because
   it corresponds fairly naturally (in many cases) to the hierarchy of
   the original space.

   Note that because of this, "duri" URNs can actually be used with
   relative URI references with some amount of reliability.  (The double
   encoding of previously encoded URI characters causes some problems.)











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4. Dates

   A <date> is a simple expression of date, optional time, with
   arbitrary precision.  The goal is to allow relatively short
   expressions of dates with no ambiguity, and with arbitrary precision.
   (The idea for this syntax came from [4].)

     date = year [ month [ day [ hour [ minute [ second [ fraction ]]]]]]

      year     = 4digit
      month    = 2digit
      day      = 2digit
      hour     = 2digit
      minute   = 2digit
      second   = 2digit
      fraction = *digit

   The representation of a date or time refers to the very first instant
   of the given date, so that, for example, 1999 and 199901010000 are
   equivalent.  If necessary, dates can include times and even
   fractional times, so that a generator of duris can be arbitrarily
   precise.

   Dates are interpreted relative to International Atomic Time [7], so
   that there is no ambiguity about time zone.


























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5. Additional Considerations


5.1 Embedded URI schemes

   The intent of "duri" and "tdb" is to use them with embedded URI
   references that identify documents or document fragments.

   For example, use with a "http" URI can be used to refer to a web page
   or the subject of a web site at a given time.  This can be a way of
   referring to a web site at some date in the past, or an organization
   that has changed or merged.

   Local systems that have unique host names can use "file" URIs with
   "tdb", for example,

       urn:tdb:20010814142327:file://this.example.com/c|/temp/test.txt

    since this use is primarily focused on providing a unique way of
   identifying an abstraction, even if the referent of the abstraction
   is broadly known.  (Using 'file:' URIs without a host name is not
   recommended, because the interpretation is not uniform.)

   Some URI schemes are more problematic.  For example, one might think
   that a URN within a URN wouldn't be useful.  But it might be possible
   where the assignment of names in a URN namespace are not, in
   practice, permanent, or that one might want to refer to the
   assignment as of a given date.  In this case, it is possible to use a
   "urn" within a "duri", e.g.,

         urn:duri:2000:urn:ietf:std:50

    might be used to refer to "the document that was STD 50 in effect as
   of the first instant of 2000".  [5]

   One might consider using "tdb" with "data" to designate concepts that
   can be described uniquely briefly inline.  For example,

        urn:tdb:2001:data:,The%2520US%2520president

    names the concept described by the (text/plain) string "The US
   president" at the very first instant of 2001.  (Note the awkward
   double quoting of space as "%20" and then the "%" as "%25".) Of
   course, this practice is only useful if the referent of the data is
   (or was at the time) completely unique.  Since "data" does contain a
   way to designate content-language, the string in question would have
   to not be ambiguous as to its language.  In the case of 'data', there
   is no assigning authority at all; the interpretation of the 'tdb' URN



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   depend on the interpreting community.

5.2 Useful dates

   Dates in the future are suspect, because the meaning of the duri or
   tdb cannot readily be determined in advance reliably.  Dates prior to
   the actual assignment of the resource to the embedded URI (and,
   certainly, dates far in the past) SHOULD NOT be used, because the
   meaning of the reference is left in question.  For example, using
   http URIs before a web service was available at the given URI doesn't
   make much sense.

   However, although these practices are NOT RECOMMENDED, there is no
   assurance that they haven't been used; by itself, a duri/tdb does not
   constitute an assertion that the encoded-URI was available or
   assigned at the date specified.

   Note that the use of the "very first instant" means that a duri/tdb
   using only a year must give a year greater than the first year in
   which the corresponding URI was published; if a web page is published
   in the middle of 2001, then "duri:2001:..." would be inappropriate.

5.3 Free assignment

   Because of the many possible schemes that can be used in the
   <encoded-  URI> portion, there should be no difficulty in almost any
   computational process being able to assign duris or tdbs at will.  Of
   course, it is necessary for there to be some resource which is
   available at some point in time, and to have a clock which is
   accurate to the granularity of the frequency of assignment.

5.4 Resolution

   There no accurate resolution servers for duri or tdb URNs.  However,
   duri might be "resolvable" in the sense that a resource that was
   accessed at a point in time might have the result of that access
   cached or archived in an Internet archive service.  See, for example,
   the "Internet Archive" project [10].  A "tdb" is only resolvable in
   the sense that if the corresponding duri can be resolved, it may be
   possible that the result can be accessed and interpreted.

   Clients without access to an Internet archive service might take the
   decoded <encoded-URI> of a duri and attempt resolution of *that*
   identifier.  This will give an approximation whose reliability
   depends on the amount of time elapsed since the date indicated.






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5.5 Why Names with Semantics?

   There are a number of proposals for URN schemes that create otherwise
   unbound "names", where the URN scheme only provides for uniqueness.
   Neither "duri" nor "tdb" intrinsically have the property that the
   names assigned are without any resolution semantics.  This is
   intentional; it's difficult to create names that carry no semantics
   whatsoever about the authority that assigned the name and the
   intention of the authority for what the name should designate.

5.6 Avoiding MetaData

   One might consider the date in a duri/tdb to be just one piece of
   additional metadata about the encoded-URI, and consider adding other
   pieces of metadata as annotation.

   However, the use of the date in a duri/tdb is intended primarily as a
   mechanism of accomplishing uniqueness over time.  No other bit of
   metadata or description readily fills that purpose.  Further, the
   date is not descriptive (an assertion about the encoded-URI) but
   merely refining.

5.7 Avoiding duri and tdb

   Many applications of URIs already provide a context of date.  For
   example, one could imagine a hypertext system where the URIs
   contained within a document were intended to refer to the resources
   as of the date of the enclosing document.  This would be a reasonable
   interpretation of URIs within an Internet archive system, for
   example.

   And some applications of URIs arguably already contain the level of
   interpretive indirection that is explicit with "tdb".  For example,
   one might consider the use of URIs as namespace names within XML [8]
   as a reference to the "thing described by" the URI used.

5.8 tdb and RDF

   The Resource Description Framework [9] is an XML-based framework for
   describing assertions.  RDF uses URIs to identify the objects being
   described and XML-based tags to describe the relationships between
   them.

   The relations in RDF, however, may already provide for the "thing
   described by" indirection.  The example in Section 3.2.1 of [9]
   suggests the model for the sentence "The students in course 6.001 are
   Amy, Tim and Mary" might be written in RDF/XML as




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     <rdf:RDF>
      <rdf:Description about="http://mycollege.edu/courses/6.001">
        <s:students>
          <rdf:Bag>
        <rdf:li resource="http://mycollege.edu/students/Amy"/>
        <rdf:li resource="http://mycollege.edu/students/Tim"/>
        <rdf:li resource="http://mycollege.edu/students/Mary"/>
         </rdf:Bag>
        </s:students>
      </rdf:Description>
     </rdf:RDF>

    but the resources listed are web pages (served by HTTP) and the
   class and students are the "things described by" those web pages.

   Of course, RDF contains mechanisms that allow this to be said
   directly.

5.9 tdb and levels of indirection

   The "tdb" scheme introduces a level of semantic indirection.  The
   puzzles and confusions about use and mention, name and reference, and
   levels of indirection have been puzzling and amusing for quite a
   while.

      "It's long," said the Knight, "but it's very, very beautiful.
      Everybody that hears me sing it--either it brings tears into their
      eyes, or else--"
      "Or else what?" said Alice, for the Knight had made a sudden
      pause.
      "Or else it doesn't, you know.  The name of the song is called
      'Haddock's Eyes.'"
      "Oh, that's the name of the song, is it?" Alice said, trying to
      feel interested.
      "No, you don't understand," the knight said, looking a little
      vexed.  "That's what the name is called.  The name really is 'The
      Aged Aged Man.'"
      "Then I ought to have said 'That's what the song is called'?"
      Alice corrected herself.
      "No, you oughtn't: that's quite another thing! The song is called
      'Ways and Means': but that's only what it's called, you know!"
      "Well, what is the song, then?" said Alice, who was by this time
      completely bewildered.
      "I was coming to that," the Knight said.  "The song really is 'A-
      sitting On A Gate': and the tune's my own invention." [11]






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6. URN Specification Templates

6.1 duri Specification Template

   o  Namespace ID:
      "duri" requested.

   o  Registration Information:
      Registration Version: 1
      Registration Date: 2001-08-19

   o  Declared registrant of the namespace:
      Larry Masinter

   o  Declaration of syntactic structure:
      Briefly, the syntax is
      urn:duri:<date>:<encoded-URI>
      The syntax is described in this document.

   o  Relevant ancillary documentation:
      (See References of this document)

   o  Identifier uniqueness considerations:
      Uniqueness is guaranteed by the structure of adding a designation
      of a specific instant to a URI.  However, URIs with ambiguous
      interpretation at any given instant (e.g., "file" URIs without a
      given host name) will not be unique.

   o  Identifier persistence considerations:
      The designation of a dated URI is completely persistent for all
      time.

   o  Process of identifier assignment:
      Any date can be used with any URI independently by anyone.

   o  Process of identifier resolution:
      Identifiers can only be resolved approximately.  See Section 4.3.

   o  Conformance with URN Syntax:
      Note that the use of "/" for hierarchy, while discouraged in the
      URN specification, is allowed in duris.

   o  Rules for Lexical Equivalent:
      For dates, YYYY is equivalent to YYYY01, YYYYMM is equivalent to
      YYYYMM01, while YYYYMMDD is equivalent to YYYYMMDD0...  followed
      by any number of 0's.  In considering equivalence of the encoded
      URI, if two duris with equivalent dates contain lexically
      equivalent URIs, the duris are equivalent.



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   o  Validation mechanism:
      Dates should be reasonable and meet the syntactic requirements.
      The URI encoded within should meet the syntactic requirements of
      the URI scheme used.

   o  Scope: Global.


6.2 tdb Specification Template

   o  Namespace ID:
      "tdb" requested.

   o  Registration Information:
      Registration Version: 1
      Registration Date: 2001-08-19

   o  Declared registrant of the namespace:
      Larry Masinter

   o  Declaration of syntactic structure:
      Briefly, the syntax is
      urn:tdb:<date>:<encoded-URI>
      The syntax is described in this document.

   o  Relevant ancillary documentation:
      (See References of this document)

   o  Identifier uniqueness considerations:
      Uniqueness is guaranteed by the structure of adding a designation
      of a specific instant to a URI.  However, URIs with ambiguous
      interpretation at any given instant (e.g., "file" URIs without a
      given host name) will not be unique.

   o  Identifier persistence considerations:
      The designation of a dated URI is completely persistent for all
      time, although the intent of a resource that is no longer
      available will be hard to discern.

   o  Process of identifier assignment:
      Any date can be used with any URI independently by anyone.

   o  Process of identifier resolution:
      Resolution of "tdb" identifiers requires interpreting the resource
      identified by the corresponding "duri".  See Section 2.2 of this
      document.

   o  Rules for Lexical Equivalent:



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      As with "duri", see Section 6.1.

   o  Conformance with URN Syntax:
      As with "duri", see Section 6.1.

   o  Validation mechanism:
      As with "duri", see Section 6.1.

   o  Scope:
      Global.









































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7. IANA considerations

   This document includes two URN NID registrations (sections 5.1 and
   5.2) that should be entered into the IANA registry of URN NIDs.















































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8. Security Considerations

   "duri" and "tdb" names are not any more reliable because they are
   dated.  URIs don't contain enough information to supply the authority
   for deciding what was or wasn't at a given URI at a given date.














































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9. Acknowledgements

   There have been many discussions over several years on the
   relationship of URLs, URNs, URIs, resources and resource identifiers,
   with many contributions.  Particular thanks to Aaron Swartz, Brian
   McBride and Stuart Williams for their recent comments, and to Michael
   Mealling, who encouraged me to pursue this.












































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References

   [1]   Moats, R., "URN Syntax", RFC 2141, May 1997.

   [2]   Berners-Lee, T., "Cool URLs don't change"", 1998.

   [3]   Sollins, K., "Functional Requirements for Uniform Resource
         Names", RFC 1737, December 1994.

   [4]   Glassman, S., Manasse, M. and J. Mogul, "Y10K and Beyond", RFC
         2550, April 1 1999.

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

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

   [7]   Bureau International des Poids et Mesures, "International
         Atomic Time".

   [8]   Bray, T., Hollander, D. and A. Layman, "Namespaces in XML", W3C
         Recommendation REC-xml-names, January 1999, <http://www.w3.org/
         TR/REC-xml-names>.

   [9]   Lassila, O. and R. Swick, "Resource Description Framework (RDF)
         Model and Syntax Specification", W3C Recommendation REC-rdf-
         syntax, February 1999, <http://www.w3.org/TR/REC-rdf-syntax>.

   [10]  archive.org, "The Internet Archive: Building an 'Internet
         Library'", <http://www.archive.org>.

   [11]  Carroll, L., "Through the Looking Glass", 1872.


Author's Address

   Larry Masinter
   Adobe Systems Incorporated
   345 Park Ave
   San Jose, CA  95110
   US

   Phone: +1 408 536 3024
   EMail: LMM@acm.org
   URI:   http://larry.masinter.net




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Full Copyright Statement

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Acknowledgement

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



















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