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
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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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