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Network Working Group                                        L. Masinter
Internet-Draft                                                     Adobe
Intended status: Experimental                           January 21, 2012
Expires: July 24, 2012


         The 'tdb' and 'duri' URI schemes, based on dated URIs
                      draft-masinter-dated-uri-10

Abstract

   This document defines two URI schemes.  The first, 'duri' (standing
   for "dated URI"), identifies a resource as of a particular time.
   This allows explicit reference to the "time of retrieval", similar to
   the way in which bibliographic references containing URIs are often
   written.

   The second scheme, 'tdb' ( standing for "Thing Described By"),
   provides a way of minting URIs for anything that can be described, by
   the means of identifying a description as of a particular time.
   These schemes were posited as "thought experiments", and therefore
   this document is designated as Experimental.

Note

   This document is not a product of any working group.  Many of the
   ideas here have been discussed since 2001.  Versions of this document
   have been discussed on the mailing list <uri@w3.org>.  Previous
   versions have couched 'tdb' and 'tdb' as URN namespaces, used
   different syntax for timestamps, and handled URIs with fragment
   identifiers differently.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 24, 2012.



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Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Overview and Requirements  . . . . . . . . . . . . . . . . . .  4
     1.1.  Persistent identifiers . . . . . . . . . . . . . . . . . .  4
     1.2.  URIs for anything  . . . . . . . . . . . . . . . . . . . .  5
   2.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Semantics  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  'duri' Semantics . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  'tdb' Semantics  . . . . . . . . . . . . . . . . . . . . .  7
     3.3.  Timestamp Semantics  . . . . . . . . . . . . . . . . . . .  8
   4.  Use as a Locator . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Hierarchy  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Additional Considerations  . . . . . . . . . . . . . . . . . .  9
     6.1.  Embedded URI schemes . . . . . . . . . . . . . . . . . . .  9
     6.2.  Useful timestamps  . . . . . . . . . . . . . . . . . . . . 10
     6.3.  Free assignment  . . . . . . . . . . . . . . . . . . . . . 10
     6.4.  Resolution . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.5.  Ambiguous Resources  . . . . . . . . . . . . . . . . . . . 11
     6.6.  Other Ambiguities  . . . . . . . . . . . . . . . . . . . . 11
     6.7.  Why Names with Semantics?  . . . . . . . . . . . . . . . . 12
     6.8.  Avoiding MetaData  . . . . . . . . . . . . . . . . . . . . 12
     6.9.  Avoiding 'duri' and 'tdb'  . . . . . . . . . . . . . . . . 12
     6.10. 'tdb' and levels of indirection  . . . . . . . . . . . . . 13
   7.  URI Specification Templates  . . . . . . . . . . . . . . . . . 13
     7.1.  'duri' Scheme Template . . . . . . . . . . . . . . . . . . 13
     7.2.  'tdb' Scheme Template  . . . . . . . . . . . . . . . . . . 14
   8.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 15
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     11.2. Informative References . . . . . . . . . . . . . . . . . . 16



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   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17


















































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1.  Overview and Requirements

   This document is not a product of any working group.  Many of the
   ideas here have been discussed since 2001.  The practical application
   of the URI schemes defined here is uncertain, but enough interest has
   been expressed in having a stable reference for these concepts that
   it seems worthwhile to publish these, if only as "experimental".

   Versions of this document have been discussed on the mailing list
   <uri@w3.org> and <www-tag@w3.org>.  Previous versions have couched
   'tdb' and 'tdb' as URN namespaces, used different syntax for
   timestamps, and handled URIs with fragment identifiers differently.
   The variations are discussed in line.

   The URI schemes defined here attempted to demonstrate ways of
   addressing several related problems:

1.1.  Persistent identifiers

   [RFC1737] defined 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 which are intended to provide
   persistence, as well as discussions of how to make DNS-based naming
   systems persistent through some allocation of persistent DNS names.
   However, guarantees of both uniqueness and persistence are difficult.

   In most cases, the assurance of persistence is provided through a
   promise of good management practice, such as is encouraged in "Cool
   URIs don't change" [COOL].

   A primary design goal for URIs is that they are intended to mean the
   same thing, no matter in what context they appear (the "Uniform" of
   "Uniform Resource Identifier").  However, even when URIs have
   "Uniform" meaning independent of the context of use, they don't
   usually guarantee stability over time.  Despite best efforts and
   intentions, the mechanisms of resolution are subject to change in
   unpredictable ways: domain names can disappear or be reassigned, name
   resolving organizations can change in structure or responsibility,
   can disappear, merge, or change in other unpredictable ways.



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   The interpretation of most URNs depend significantly on the reliable
   behavior of name assignment and resolution authorities.  The
   authorities are usually individuals or organizations trusted
   initially first to insure the uniqueness of assignment (so that the
   same name is not latter assigned for a different resource), and
   secondly to reliably maintain the link between the name and the
   named.

   However, assignment and resolution authorities (whether individuals
   or organizations) all have a lifetime.  The functioning of
   identifiers as unique holders of meaning depends on having a reliable
   infrastructure for the authority to maintain records, and for anyone
   to contact the authority or the authority's records to determine the
   thing identified.

1.2.  URIs for anything

   The description of URIs [RFC3986] describes a range for 'Resource'
   that is quite broad:

      This specification does not limit the scope of what might be a
      resource; rather, the term "resource" is used in a general sense
      for whatever might be identified by a URI.  Familiar examples
      include an electronic document, an image, a source of information
      with a consistent purpose (e.g., "today's weather report for Los
      Angeles"), a service (e.g., an HTTP-to-SMS gateway), and a
      collection of other resources.  A resource is not necessarily
      accessible via the Internet; e.g., human beings, corporations, and
      bound books in a library can also be resources.  Likewise,
      abstract concepts can be resources, such as the operators and
      operands of a mathematical equation, the types of a relationship
      (e.g., "parent" or "employee"), or numeric values (e.g., zero,
      one, and infinity).

   However, no means is given for constructing URIs with such a range.
   How, then, might one construct a URI that identifies a human being, a
   corporation, or the value 'zero'?

   One might wish to use a URI such as 'mailto' URI to identify a
   person, or use a 'http' URI to identify an abstract concept, with the
   indirection determined by context.  Doing so, however, leaves the
   open the question of how one might identify, within the same context,
   both the system mailbox and the person to which it is assigned; both
   the resource reached via the HTTP protocol as determined by the
   'http' URI and also the concept that resource describes.

   The idea behind the 'tdb' URI scheme was to provide a ready
   assignment of URIs for things, in a way that clearly distinguished



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   the URI for the thing from the URI of the media content that
   described it.  The 'tdb' URI scheme provides a mechanism which is, at
   the same time:

   persistent:  The URI is not subject to reinterpretation over time.

   objective:  What's meant by the URI, or how it's to be interpreted,
      is explicit in the URI, and does not require an authority to
      adjudicate meaning.

   useful for non-networked things:   The scheme allows identification
      of resources outside the network: people, organizations, places,
      things, even abstract concepts.

   without long-term administration requirement:   The mechanism does
      not depend on administrative processes of authorities for reliable
      interpretation over time.


2.  Syntax

   A 'duri' URI takes the form:
        duri:<timestamp>:<embeddedURI>

   and A 'tdb' URI takes a similar form:
        tdb:<timestamp>:<embeddedURI>

   <embeddedURI> is an absoluteURI (as defined in [RFC3986]).

   Whether <embeddedURI> in duri and tdb URIs should allow an embedded
   fra gment identifier was a subject of some discussion; doing so seems
   useful and harmless.

   A <timestamp> in these URI schemes consists of a restricted subset of
   date times, as per [RFC3339].

     timestamp = date [ "T" time "Z" ]
     date       =date-fullyear [ "-" date-month [ "-" date-mday ]]
     time       = time-hour  [ ":" time-minute
                  [ ":" time-second [ time-secfrac ]]]

   where non-terminals "date-fullyear", "date-month", "date-mday",
   "time-hour", "time-minute", "time-second", "time-secfrac" are taken
   from [RFC3339].  The goal is to allow relatively short expressions
   with no ambiguity, but also allow arbitrary precision.  While shorter
   forms are available (e.g., year-only timestamps), it is possible to
   use forms that are exactly compatible with [RFC3339] "date-time" non-
   terminal.  Earlier versions of this document proposed timestamps



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   relative to International Atomic Time [TAI], using a syntax without
   any puncutation at all, based on [RFC2550].  The syntactic variations
   don't matter much because the dates are not generally processed but
   are there to add uniqueness.)


3.  Semantics

3.1.  'duri' Semantics

   The meaning of a 'duri' URI is "the resource that was identified by
   the <embeddedURI> at the the time given".

   For example, "duri:2001:http://www.ietf.org" is a persistent
   identifier to the resource identified by "http://www.ietf.org" as of
   (the end of) 2001.  (Note that during the many years of discussion
   around times within time intervals, various alternatives were
   proposed for whether a timestamp meant "as was stable during the
   entire period" or "at the beginning of the implied interval" vs. "at
   the end".  Part of the question was whether it was reasonable to coin
   a URI using a year number at any time during that year, since the
   resource being identified may not have been active or established at
   the beginning of the year.)

3.2.  'tdb' Semantics

   The 'tdb' URI scheme is intended to be useful for describing
   entities, concepts, abstractions, and other things which may not
   themselves be network accessible resources, but are (or at least have
   been at some point) described by network accessible resources.

   Thus, a 'tdb' URI would be most useful when the <embeddedURI>
   identifies a 'document' of some sort (something a person could read,
   peruse, view, understand), and where the document thus identified
   describes some thing or concept.  The 'tdb' URI itself then
   identifies the subject of that document.  This is similar to the
   common practice of giving a reference for a concept by including a
   pointer to a document phrase that defines the concept.

   For example, one might use
   "tdb:2009:http://en.wikipedia.org/wiki/IETF" as a persistent
   identifier for the Internet Engineering Task Force (at least as
   described by the Wikipedia article as of 2009).

   The 'tdb' URI scheme can be thought of as giving a way to invoke a
   level of semantic indirection to URI resolution.

   Expressed in RDF, one might consider



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       <duri:T:U;> foaf:primaryTopic <tdb:T:U>

   where '-- foaf:primaryTopic --' is read '-- has, as its primary
   topic, --' ([FOAF] term "primaryTopic").

3.3.  Timestamp Semantics

   It is conventional in references and citations in printed works to
   include the date of publication; this practice provides important
   context.  [MANSTYLE].

   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.  There are two ways that
   the date is useful for 'tdb' URIs: it fixes the time of access of the
   resource (and thus time variations of the description), and it fixes
   the time of interpretation, for descriptions whose meaning might
   vary.  Thus, timestamps are useful in 'tdb' even when the resource
   identification does not vary (as with 'data' URIs).

   While normally, in a literary work in natural language which makes a
   reference to another work, both the reference itself and the work
   referenced are dated, e.g., a footnote in an article written in 1967
   might talk about a "private communication" which itself had a date.
   The difference between a URI and a conventional literary reference is
   the desire to be able to extract the URI from its context and still
   retain its meaning.

   The meaning of a timestamp is the interval specified by the
   granularity of the time range indicated, in the UTC time zone, as
   described in [RFC3339].  If necessary, timestamps can include times
   and even fractional times, so that a generator of 'duri' or 'tdb'
   URIs can be arbitrarily precise.

   If there is any ambiguity of the resource within the range of time
   indicated (for example, if the timestamp consists only of a year, and
   the resource changes over the course of the year), then the last
   available resource state within the the range indicated should be
   used.

   Timestamps are allowed to be specified with as much precision as
   needed.  This keeps most 'duri' and 'tdb' URIs relatively short.


4.  Use as a Locator

   A 'duri' URI is not directly useful as a resource locator, since many
   resources vary their content over time.



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   A 'tdb' URI is not a resource locator in a practical sense, since it
   explicitly requires human interpretation.  However, it allows one to
   know that a resource was described at some point in time; whether the
   description is still available, or whether that description is still
   meaningful, is not guaranteed.


5.  Hierarchy

   For 'tdb', the "thing described by" a resource may bear little
   relationship to the "thing described by" a relative pointer, so the
   'tdb' URI scheme seems to have no use cases for using "/" as a
   hierarchical delimiter.

   However, 'duri' URIs can often be used with relative URI references
   with some amount of reliability.


6.  Additional Considerations

6.1.  Embedded URI schemes

   The 'tdb' scheme is primarily useful when the <embeddedURI>
   identifies an "information resources".

   For example, a 'http' URI might refer to a web page or the subject of
   a web as it was described at the given time.  This can be a way of
   referring to a web site at some time in the past, or an organization
   that has changed, merged, split, or disappeared.

   A 'file' URI with a known-to-be unique host name might also be used
   within a 'tdb' URI, for example,

       tdb:2001-08-14T14:23:27Z:file://this.example.com/c|/temp/test.txt

   This use is primarily focused on providing a unique way of
   identifying something, even if the referent is not widely known.
   (Using 'file' URIs in this way without a fully qualified domain name
   as the authority would not be appropriate, because the interpretation
   is not uniform even at any particular instant.)

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

        tdb:2001:data:,The%20US%20president

   names the concept described by the (text/plain) string "The US
   president" at the end of 2001.  Of course, this practice is only



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   useful if the referent of the data is (or was at the time) unique.
   Since a 'data' URI does not contain a way to designate content-
   language, the string in question should be unambiguous as to its
   language.  In the case of 'data' URIs, there is no assigning
   authority at all; the interpretation of the 'tdb' depend on the
   interpreting community.

   Using 'duri' with an embedded 'urn' might not seem to be too useful,
   but it might be useful where the assignment of names in a URN
   namespace are not, in practice, permanent, and one wants 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.,

         duri:2000:urn:ietf:std:50

   might be used to refer to "the document that the IETF considered to
   be STD 50, at the end of 2000".

   For 'tdb', many URIs identify resources which do not clearly describe
   anything at all.  Even so, some care should be given; for example,
   the home page for an organization might not be as good a resource to
   use to describe an organization as the organization's "about" page or
   an external authority's description of the organization.  It is up to
   the minter of the 'tdb' URI to choose wisely.

6.2.  Useful timestamps

   Timestamps in the future are suspect, because the future content of a
   description resource cannot usually be reliably predicted.
   Timestamps which preceed the availability of the description resource
   should not be used either.  For example, using a http URI with a
   timestamp before the description resource has been created is also
   not recommended.

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

   Note that the use of the "last available state" allows for the
   conventional bibliographic convention that a work published in 2009
   can use "2009" as the date string, to refer to the work in the year
   of publication.

6.3.  Free assignment

   Because of the many possible schemes that can be used in the embedded
   URI, there should be no difficulty in almost any computational



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   process being able to assign 'duri' or 'tdb' URIs 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.

6.4.  Resolution

   There are no direct resolution servers or processes for 'duri' or
   'tdb' URIs.  However, a 'duri' URI 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 [archive].
   And a 'tdb' URI is "resolvable" to the extent that the description
   resource can be accessed and interpreted.

   Clients without access to an Internet archive service might take the
   embedded URI of a 'duri' and attempt resolution of that identifier.
   This will give an approximation whose reliability depends on the what
   has happened in the time since the date indicated.

6.5.  Ambiguous Resources

   There are many URIs which are, unfortunately, not particularly
   "uniform", in the sense that two clients can observe completely
   different content for the same resource, at exactly the same time.
   These resources are not so useful with 'tdb' URIs, since time alone
   is not adequate to identify precisely because the results of access
   depends on other details of the observation (e.g., IP address,
   cookies, HTTP request headers, which physical server responded,
   etc.).

   When using 'duri' with URIs for which result of access varies
   depending on other conditions of access, all a 'duri' URI really says
   is that someone observed something at the given URI at a certain
   time.

6.6.  Other Ambiguities

   Unfortunately, the scope of a URI is always ambiguous as a reference
   point for both documents and things described by them.  When I point
   to a web site (http://www.ietf.org), do I mean just the home page or
   the whole site?  Do I mean just the HTML there, or also the embedded
   images and other things that are displayed?  While "tdb" and "duri"
   attempt to nail down two of the more important areas of ambiguity
   (use/mention and time varying), other dimensions of ambiguity remain.






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6.7.  Why Names with Semantics?

   The 'tdb' URI scheme differs from other URI or URN methods 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 the 'tag' [RFC4151], 'info'
   [RFC4452], 'cid' or 'mid' [RFC2392] schemes, for example, the
   identification does not depend on any authority or process not
   reusing the same identifier at some later point for a different
   concept, or maintaining any records or meaning.  In these other
   schemes, the assigning authority only insures uniqueness at the time
   of minting, with some other agent or process or context providing the
   authority to interpret the meaning of the identifier in the future.
   In this sense, 'duri' and 'tdb' are different, in that it is the
   agreement between the describer (the agent creating the URI) and the
   receiver of the URI (the agent interpreting the URI) to agree upon
   the semantics without any reference to any third party.

6.8.  Avoiding MetaData

   One might consider the timestamp in a 'duri' or 'tdb' URI to be just
   one piece of additional metadata about the URI, and consider adding
   other pieces of metadata as annotation.

   However, the use of the timestamp 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 URI) but merely
   refining.

6.9.  Avoiding 'duri' and 'tdb'

   Many applications of URIs already provide a context of timestamp.
   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.

   Some applications of URIs already implicitly use the level of
   interpretive indirection that is explicit with 'tdb', For example,
   within an ontology language definition, the URIs used for abstract
   concepts, individuals and so forth are generally considered the
   "thing described by" the URI.

   In addition, the 'application/rdf+xml' Media Type [RFC3870] uses the
   fragment identifier resolution as an explicit way of identifying
   things that are described by an RDF document.



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6.10.  '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."  [LOOK]


7.  URI Specification Templates

7.1.  'duri' Scheme Template

   URI scheme name:  duri

   Status:  permanent

   URI scheme syntax:  The syntax is described in detail in Section 2 of
      this document.  Briefly, the syntax is
      duri:<timestamp>:<embeddedURI>
      where <timestamp> is year, year-month or date taken from
      [RFC3339], and <embeddedURI> is an <absoluteURI> from [RFC3986].

   URI scheme semantics:  A URI as of a particular time.  Semantics are
      described in detail in this document.






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   Encoding considerations:  'duri' URIs consist of a prefix followed by
      another URI, and should have the same encoding considerations as
      others.

   Applications/protocols that use this URI scheme name:  Limited: this
      scheme was originally developed as a "thought experiment".

   Interoperability considerations:  The actual interoperability with
      Internet archiving services needs further exploration.

   Security considerations:  See Section 9 of this document.

   Contact:  Larry Masinter tdb:2012:http://larry.masinter.net

   Author/Change controller:  Contact, as above.

   References:  See References of this document.

7.2.  'tdb' Scheme Template

   URI scheme name:  tdb

   Status:  permanent

   URI scheme syntax:  The syntax is described in detail in Section 2 of
      this document.  Briefly, the syntax is
      tdb:<timestamp>:<embeddedURI>
      where <timestamp> is year, year-month or date taken from
      [RFC3339], and <embeddedURI> is an <absoluteURI> from [RFC3986].

   URI scheme semantics:  Semantic indirection at indicated date.
      Semantics are described in detail in this document.

   Encoding considerations:  'tdb' URIs consist of a prefix followed by
      another URI, and should have the same encoding considerations as
      others.

   Applications/protocols that use this URI scheme name:  Limited: This
      scheme was originally designed as a "thought experiment", as a way
      resolve some of the use/mention ambiguities in semantic web
      applications that wish to "denote" concepts and other ideas and
      not just access resources over the Internet.

   Interoperability considerations:  Existing semantic web applications
      may have other means of fixing meaning at a particular time or
      semantic indirection, and do not fix description by time.





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   Security considerations:  See Section 9 of this document.

   Contact:  Larry Masinter tdb:2012:http://larry.masinter.net

   Author/Change controller:  as above

   References:  See References of this document.


8.  IANA considerations

   This document includes two URI scheme registrations (Section 7 that
   should be entered into the IANA registry of URI schemes as a
   permanent registration (once approved).


9.  Security Considerations

   'tdb' identifiers are not any more reliable because they have dates.
   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.


10.  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 those who have
   severely critiqued various versions of this document in the past,
   including Jonathan Rees, Nathan, Alan Ruttenberg, Alfred Hines,
   Herbert Van de Sompel, Al Gilman, Aaron Swartz, Brian McBride, Stuart
   Williams, Michael Mealling, Ray Denenberg and Pat Hayes.


11.  References

11.1.  Normative References

   [RFC3339]  Klyne, G. and C. Newman, "Date an Time on the Internet:
              Timestamps", RFC 3339, July 2002.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 3986,
              January 2005.







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11.2.  Informative References

   [COOL]     Berners-Lee, T., "Cool URIs don't change", 1998,
              <http://www.w3.org/Provider/Style/URI.html>.

   [FOAF]     Brickley, D., "FOAF Vocabulary Specification 0.98",
              August 2010,
              <http://xmlns.com/foaf/spec/#term_primaryTopic>.

   [LOOK]     Carroll, L., "Through the Looking Glass", 1872, <http://
              www.literature.org/authors/carroll-lewis/
              through-the-looking-glass/chapter-08.html>.

   [MANSTYLE]
              The University of Chicago, "The Chicago Manual Of Style
              Online: Chicago-Style Citation Quick Guide", 2012, <http:/
              /www.chicagomanualofstyle.org/tools_citationguide.html>.

   [MEMENTO]  VandeSompel, H., Nelson, M., and R. Sanderson, "HTTP
              framework for time-based access to resource states --
              Memento", December 2011,
              <http://tools.ietf.org/html/draft-vandesompel-memento>.

   [RFC1737]  Sollins, K., "Functional Requirements for Uniform Resource
              Names", RFC 1737, December 1994.

   [RFC2392]  Levinson, E., "Content-ID and Message-ID Uniform Resource
              Locators", RFC 2392, August 1998.

   [RFC2550]  Glassman, S., Manasse, M., and J. Mogul, "Y10K and
              Beyond", April 1999, <http://tools.ietf.org/html/rfc2550>.

   [RFC3870]  Swartz, A., "application/rdf+xml Media Type Registration",
              RFC 3870, September 2004.

   [RFC4151]  Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
              RFC 4151, October 2005.

   [RFC4452]  Van de Sompel, H., Hammond, T., Neylon, E., and S. Weibel,
              "The "info" URI Scheme for Information Assets with
              Identifiers in Public Namespaces", RFC 4452, April 2006.

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

   [archive]  Kahle, B., "Preserving the Internet", Scientific
              American , March 1997.




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Author's Address

   Larry Masinter
   Adobe
   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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