Network Working Group L. Masinter
Internet-Draft Adobe
Intended status: Informational July 12, 2009
Expires: January 13, 2010
The "tdb" URI scheme: denoting described resources
draft-masinter-dated-uri-06
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Abstract
This document defines a URI scheme, "tdb" ( standing for "Thing
Described By"). It provides a semantic hook for allowing anyone at
any time to mint a URI for anything that they can describe. Such
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URIs may include a timestamp to fix the description at a given date
or time.
This URI scheme may reduce the need to define define new URN
namespaces merely for the purpose of creating stable identifiers. In
addition, they provide a ready means for identifying "non-information
resources" by semantic indirection -- a way of creating a URI for
anything.
Note
This document is not a product of any working group. Many of the
ideas here have been discussed since 2001. This document has been
discussed on the mailing list <uri@w3.org>. Previous versions have
couched "tdb" as a URN namespace, and included a "duri" scheme for
fixing date without indirection, which seems unnecessary. It was
originally written as a thought experiment as a way of resolving the
use/mention problem in semantic web applications, but may have other
uses.
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Table of Contents
1. Overview and Requirements . . . . . . . . . . . . . . . . . . 4
1.1. Easy assignment of permanent identifiers . . . . . . . . . 4
1.2. Persistent identifiers . . . . . . . . . . . . . . . . . . 4
1.3. URIs for abstractions . . . . . . . . . . . . . . . . . . 5
2. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Semantics . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Use as a Locator . . . . . . . . . . . . . . . . . . . . . . . 7
5. Hierarchy . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Timestamps in tdb URIs . . . . . . . . . . . . . . . . . . . . 7
7. Additional Considerations . . . . . . . . . . . . . . . . . . 8
7.1. URI schemes for the description resource . . . . . . . . . 8
7.2. Useful timestamps . . . . . . . . . . . . . . . . . . . . 9
7.3. Free assignment . . . . . . . . . . . . . . . . . . . . . 10
7.4. Resolution . . . . . . . . . . . . . . . . . . . . . . . . 10
7.5. Why Names with Semantics? . . . . . . . . . . . . . . . . 10
7.6. Avoiding MetaData . . . . . . . . . . . . . . . . . . . . 10
7.7. Avoiding tdb . . . . . . . . . . . . . . . . . . . . . . . 10
7.8. tdb and levels of indirection . . . . . . . . . . . . . . 11
8. URI Specification Template . . . . . . . . . . . . . . . . . . 11
9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 12
10. Security Considerations . . . . . . . . . . . . . . . . . . . 12
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
12.1. Normative References . . . . . . . . . . . . . . . . . . . 13
12.2. Informative References . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Overview and Requirements
The tdb URI scheme here solves several related problems:
1.1. Easy assignment of permanent identifiers
The URN specification [RFC1737] 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
[RFC1737] 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" [COOL]. 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 [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).
One might use a URI such as "mailto:" email address to identify a
person, or a "http:" URI to identify an abstract comment. However,
this leaves the question of how one might identify, within the same
context, both the system mailbox and the person to which it is
assigned, or the web page at a http URI and the concept it describes.
The "tdb" URI scheme allows ready assignment of URIs for abstractions
that are distinguished from the media content that describes them.
The goal, then, of the "tdb" URI scheme 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.
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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.
2. Syntax
A tdb URI takes the form:
duri:<timestamp>:<URI>
Where <timestamp> is s sequence of digits representing a date and
time (Section 6) and <URI> is any valid URI.
3. Semantics
The tdb URI scheme is intended to be useful for describing entities,
concepts, abstractions, and other items which may not themselves be
network accessible resources, but have been at some point described
by network accessible resources.
The meaning of a duri is "the resource (or fragment) that was
identified by the <encoded-URI> (after hex decoding) at the very last
instant of the date(time) given".
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, one might use "tdb:2008:http://www.ietf.org" as a
persistent identifier for the Internet Engineering Task Force, as
described by the "http://www.ietf.org" as of the very last instant of
the year 2008.
The "tdb" namespace differs from the 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 "tag", "info", "cid", "mid" or related
schemes, the identification is not dependent on the context of use.
The "tdb" scheme can be thought of as adding a level of semantic
indirection to URI resolution.
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4. Use as a Locator
A tdb URI is not a resource locator in a practical sense. It allows
one to know that a resource was described at some point in time, but
whether the description is still available, or whether that
description is still meaningful, is ambiguous.
5. Hierarchy
The "thing descibed by" a network 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.
6. Timestamps in tdb URIs
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.
While one could imagine using tdb without a timestamp, 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": it fixes the time of access of the
resource, for variable descriptions, and it fixes the time of
interpretation, for descriptions whose meaning (in natural language)
might vary.
A timestamp SHOULD be supplied, since the network resources which
provide descriptions can also change over time. The timestamp is
allowed to be quite broad -- only a year -- or with as much precision
as needed. This keeps "tdb" URIs relatively short. To avoid
ambiguity, a single instant has been chosen -- for tdb this is "the
last possible instant of the indicated range".
A timestamp in the tdb scheme is a simple expression of date,
optional time, with arbitrary precision. The goal is to allow
relatively short expressions with no ambiguity, but also with
arbitrary precision. (Other date formats were considered, but
arbitrary precision syntactic simplicity of only using digits time
zones not.)
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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 (open interval)
instant just before the end of the given date/time range at the
resolution supplied. 199912 is "just before" 1999, but 19991231 falls
between them. If necessary, timestamps can include times and even
fractional times, so that a generator of tdbs can be arbitrarily
precise.
Timestamps are interpreted relative to International Atomic Time
(TAI) [TAI]. The syntax and semantics are similar to those in
[RFC2550]; in particular, using TAI avoids ambiguity about time zones
and difficulties with leap seconds.
There are actually two dates to consider, with "tdb". There is the
date that the resource is obtained, and there is the date that the
description it makes is read, understood, and used to denote.
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.
7. Additional Considerations
7.1. URI schemes for the description resource
The "tdb" scheme is intended for use with resources which have
retrievable resources that describe something else -- these
"description resources" are intended as "information resources".
For example, use with a "http" URI can be used to refer to the
subject of a web page (at 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.
Local systems that have known-to-be unique host names can use "file"
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URIs with "tdb", for example,
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 not widely known. (Using 'file:' URIs in this way without a fully
qualified domain name would not be appropriate, because the
interpretation is not uniform.)
One might consider using "tdb" with "data" 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 very last instant of 2001. Of course, this
practice is only useful if the referent of the data is (or was at the
time) completely unique. Since "data" does not 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' depend on
the interpreting community.
Many URIs identify resources which do not clearly describe anything
at all. The "home page" for an organization isn't nearly as good a
resource to use to describe an organization as the organization's
"about" page. But it is up to the minter of the tdb URI to choose
wisely.
7.2. Useful timestamps
Timestamps far in the future are suspect, because the future content
of a description resource cannot usually 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 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 does not
constitute an assertion that the description resource was available
or assigned at the date specified.
Note that the use of the "very last instant" 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.
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7.3. Free assignment
Because of the many possible schemes that can be used in the <URI>
portion, there should be no difficulty in almost any computational
process being able to assign 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.
7.4. Resolution
There no resolution servers or processes for tdb URI. However, a tdb
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 the "tdb" is
"resolvable" in the sense that the description resource can be
accessed and interpreted.
7.5. Why Names with Semantics?
There are a number of URI and URN schemes that create otherwise
unbound "names", where the scheme only provides for uniqueness, with
some other agent or process or context providing the authority to
interpret the meaning of the identifier at some point in the future.
"tdb" is different, in that it is the agreement between the describer
(the agent creating the tdb URI) and the receiver of the URI (the
agent interpreting the tdb URI) to agree upon the semantics without
any reference to any third party.
7.6. Avoiding MetaData
One might consider the date in a 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 date in a tdb URI 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.
7.7. Avoiding 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
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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
[namespaces] as a reference to the "thing described by" the URI used.
7.8. 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]
8. URI Specification Template
URI scheme name: tdb
Status: permanent
URI scheme syntax: Briefly, the syntax is
tdb:<date>:<URI>
The syntax is described in this document.
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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: This scheme
was designed to 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, but this should not in itself cause
interoperability difficulties.
Security considerations: See Section 10 of this document.
Contact: Larry Masinter tdb:2009:http://larry.masinter.net
Author/Change controller: as above
References: See References of this document.
9. IANA considerations
This document includes a URI scheme registration (Section 8 that
should be entered into the IANA registry of URI schemes as a
permanent registration (once approved.)
10. 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.
11. 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 Al Gilman, Aaron
Swartz, Brian McBride, Stuart Williams, Michael Mealling, Ray
Denenberg and Pat Hayes.
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12. References
12.1. Normative References
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 3986,
January 2005.
[TAI] Bureau International des Poids et Mesures, "International
Atomic Time".
[namespaces]
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>.
12.2. Informative References
[COOL] Berners-Lee, T., "Cool URIs don't change", 1998,
<http://www.w3.org/Provider/Style/URI.html>.
[LOOK] Carroll, L., "Through the Looking Glass", 1872, <http://
www.literature.org/authors/carroll-lewis/
through-the-looking-glass/chapter-08.html>.
[RFC1737] Sollins, K., "Functional Requirements for Uniform Resource
Names", RFC 1737, December 1994.
[RFC2550] Glassman, S., Manasse, M., and J. Mogul, "Y10K and
Beyond", RFC 2550, April 1 1999.
[archive] Kahle, B., "Preserving the Internet", Scientific
American , March 1997,
<http://www.sciam.com/0397issue/0397kahle.html>.
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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