Network Working Group L. Masinter
Internet-Draft Adobe
Intended status: Informational October 22, 2010
Expires: April 25, 2011
The 'tdb' and 'duri' URI schemes, based on dated URIs
draft-masinter-dated-uri-07
Abstract
This document defines two URI schemes. The first, 'duri' (standing
for "dated URI"), allows indicating a URI as of a particular date
(and time). This allows explicit reference to the "time of
retrieval", similar to the way in which bibliographic references
containing URIs are used.
The second scheme, 'tdb' ( standing for "Thing Described By"),
provides a way of using a way of minting URIs for anything that can
be described, with the ability to fix the description to a given date
or time. The 'tdb' URI scheme may reduce the need to define define
new URN namespaces merely for the purpose of creating stable
identifiers for concepts or abstractions: it provides 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' and 'tdb' as URN namespaces.
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."
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This Internet-Draft will expire on April 25, 2011.
Copyright Notice
Copyright (c) 2010 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
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Table of Contents
1. Overview and Requirements . . . . . . . . . . . . . . . . . . 4
1.1. Persistent identifiers . . . . . . . . . . . . . . . . . . 4
1.2. URIs for abstractions . . . . . . . . . . . . . . . . . . 5
2. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. 'duri' Syntax . . . . . . . . . . . . . . . . . . . . . . 6
2.2. tdb Syntax . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. encoded-URI encoding . . . . . . . . . . . . . . . . . . . 6
2.4. Timestamp 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 . . . . . . . . . . . . . . . . . . . . . . . 9
5. Hierarchy . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Additional Considerations . . . . . . . . . . . . . . . . . . 9
6.1. Embedded URI schemes . . . . . . . . . . . . . . . . . . . 9
6.2. Useful timestamps . . . . . . . . . . . . . . . . . . . . 10
6.3. Free assignment . . . . . . . . . . . . . . . . . . . . . 11
6.4. Resolution . . . . . . . . . . . . . . . . . . . . . . . . 11
6.5. Why Names with Semantics? . . . . . . . . . . . . . . . . 11
6.6. Avoiding MetaData . . . . . . . . . . . . . . . . . . . . 11
6.7. Avoiding 'duri' and 'tdb' . . . . . . . . . . . . . . . . 12
6.8. 'tdb' and levels of indirection . . . . . . . . . . . . . 12
7. URI Specification Templates . . . . . . . . . . . . . . . . . 13
7.1. 'duri' Scheme Template . . . . . . . . . . . . . . . . . . 13
7.2. tdb Scheme Template . . . . . . . . . . . . . . . . . . . 13
8. IANA considerations . . . . . . . . . . . . . . . . . . . . . 14
9. Security Considerations . . . . . . . . . . . . . . . . . . . 14
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1. Normative References . . . . . . . . . . . . . . . . . . . 15
11.2. Informative References . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Overview and Requirements
The URI schemes defined here address several related problems:
1.1. 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
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.
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1.2. 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.
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
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2.1. 'duri' Syntax
A 'duri' URI takes the form:
duri:<timestamp>:<encoded-URI>
where <timestamp> is s sequence of digits representing a date and
time (Section 2.4) and <encoded-URI> is an absolute URI-reference
[RFC3986] in which any reserved character other than "/" have been
percent-encoded (Section 2.3). Note that the URI which has been
encoded MAY include a fragment identifier.
2.2. tdb Syntax
A 'tdb' URI takes a similar form:
tdb:<timestamp>:<encoded-URI>
with the same syntax.
2.3. encoded-URI encoding
The following characters must be encoded within <encoded-URI>:
o The character "#"
Note that the URI encoded by <encoded-URI> can include a fragment
identifier; the "#" character used to delimit it must be encoded.
This feature is intended for use with 'tdb', where the fragment
identified might contain the description. Including an encoded
"#" with a 'duri' is not as useful, since the fragment identifier
might as well be applied to the 'duri' itself.
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, this means there are 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' URI.
2.4. Timestamp syntax
A timestamp in these URI schemes consists of a restricted subset of
date times, as per [RFC3339]. The goal is to allow relatively short
expressions with no ambiguity, but also with arbitrary precision.
timestamp = date [ "T" time "Z" ]
date =date-fullyear [ "-" date-month [ "-" date-mday ]]
time = time-hour [ ":" time-minute
[ ":" time-second [ time-secfrac ]]]
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where non-terminals "date-fullyear", "date-month", "date-mday",
"time-hour", "time-minute", "time-second", "time-secfrac" are taken
from [RFC3339]. The goal was to minimize the amount of precision
needed, while retaining the possibility of generating timestamps that
are exactly compatible with [RFC3339] "date-time" non-terminal.
3. Semantics
3.1. 'duri' Semantics
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 meaning of a 'duri' URI is "the resource that was identified by
the <encoded-URI> (after hex decoding) at the date(time) given".
For example, "duri:2001:http://www.ietf.org" is a persistent
identifier to "http://www.ietf.org" as of 2001. A 'duri' URI may not
be a resource locator in a practical sense: the time of location has
not yet arrived or has passed.
3.2. 'tdb' 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.
A 'tdb' URI is intended to be used where the <encoded-URI> identifies
a 'document' (something a person could read, peruse, understand) or a
fragment thereof, where the document describes some thing or concept.
The 'tdb' URI itself then identifies the subject of that document.
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" in 2008.
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 "tag", "info", "cid", "mid"
or related schemes, the identification is not dependent on the
context of use. The 'tdb' URI scheme can be thought of as giving a
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way to invoke a level of semantic 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. There are two ways that
the date is useful for 'tdb' URIs: 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.
3.3. Timestamp Semantics
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. 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 resource
state as of the very last instant of 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.
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4. Use as a Locator
A 'duri' URI is not directly useful as a resource locator, since many
resources vary their content over time.
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. Note, however, that double-encoding
of previously encoded URI characters will cause some problems.
6. Additional Considerations
6.1. Embedded URI schemes
The intent of 'duri' and 'tdb' is to use them with embedded URI
references that identify documents or document fragments. That is,
they are most useful for "information resources".
For example, use with a "http" URI can be used to 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.
Local systems that have known-to-be unique host names can use "file"
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.)
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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 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.
Using 'tdb' or '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, 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.,
duri:2000:urn:ietf:std:50
might be used to refer to "the document that the IETF considered to
be STD 50, as of the last instant of 2000".
For 'tdb', 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.
6.2. Useful timestamps
Timestamps far 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 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 "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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6.3. 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 '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" in the sense that the description
resource 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 what has happened in the time since the date
indicated.
6.5. 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.
'duri' and 'tdb' is 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.
6.6. 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.
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6.7. 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
abstract concepts that are described by an RDF document.
6.8. '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]
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7. URI Specification Templates
7.1. 'duri' Scheme Template
URI scheme name: duri
Status: permanent
URI scheme syntax: Briefly, the syntax is
tdb:<timestamp>:<encoded-URI>
The syntax is described in this document.
URI scheme semantics: A URI as of a particular time. Semantics are
described in detail in this document.
Encoding considerations: 'duri' URIs consist of a prefix followed by
another URI, and should have the same encoding considerations as
others. Note discussion of double-encoding.
Applications/protocols that use this URI scheme name: Limited: this
scheme was originally developed as a "thought experiment",
although there is some discussion of using it with Memento
[MEMENTO].
Interoperability considerations: The actual interoperability with
Internet archiving services needs further exploration.
Security considerations: See Section 9 of this document.
Contact: Larry Masinter tdb:2010:http://larry.masinter.net
Author/Change controller: as above
References: See References of this document.
7.2. tdb Scheme Template
URI scheme name: tdb
Status: permanent
URI scheme syntax: Briefly, the syntax is
tdb:<timestamp>:<encoded-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. Note discussion of double-encoding possibilities.
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.
Security considerations: See Section 9 of this document.
Contact: Larry Masinter tdb:2010: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 Alfred Hines, Herbert
Van de Sompel, Al Gilman, Aaron Swartz, Brian McBride, Stuart
Williams, Michael Mealling, Ray Denenberg and Pat Hayes.
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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.
[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>.
11.2. 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>.
[MEMENTO] Memento Development Group, "Memento: Adding Time to the
Web", 2010, <http://mementoweb.org/>.
[RFC1737] Sollins, K., "Functional Requirements for Uniform Resource
Names", RFC 1737, December 1994.
[RFC3870] Swartz, A., "application/rdf+xml Media Type Registration",
RFC 3870, September 2004.
[archive] Kahle, B., "Preserving the Internet", Scientific
American , March 1997,
<http://www.sciam.com/0397issue/0397kahle.html>.
Masinter Expires April 25, 2011 [Page 15]
Internet-Draft The 'tdb' and 'duri' URI schemes October 2010
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
Masinter Expires April 25, 2011 [Page 16]