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<h1>URIs, URLs, and URNs: Clarifications and Recommendations
1.0</h1>

<h2>Report from the joint W3C/IETF URI Planning Interest Group</h2>

<h2>W3C Note 21 September 2001</h2>

<dl>
<dt>This version:</dt>

<dd><a
href="http://www.w3.org/TR/2001/NOTE-uri-clarification-20010921/">http://www.w3.org/TR/2001/NOTE-uri-clarification-20010921/</a></dd>

<dt>Latest version:</dt>

<dd><a
href="http://www.w3.org/TR/uri-clarification">http://www.w3.org/TR/uri-clarification</a></dd>

<dt>by:</dt>

<dd>URI Planning Interest Group, W3C/IETF<br />
(see <a href="#acks">Acknowledgements</a>)</dd>
</dl>

<p class="copyright"><a
href="http://www.w3.org/Consortium/Legal/ipr-notice-20000612#Copyright">
Copyright</a>&#160;&#169;&#160;2001&#160;<a
href="http://www.w3.org/"><abbr
title="World Wide Web Consortium">W3C</abbr></a><sup>&#174;</sup>
(<a href="http://www.lcs.mit.edu/"><abbr
title="Massachusetts Institute of Technology">MIT</abbr></a>, <a
href="http://www.inria.fr/"><abbr lang="fr"
title="Institut National de Recherche en Informatique et Automatique">
INRIA</abbr></a>, <a href="http://www.keio.ac.jp/">Keio</a>), All
Rights Reserved. W3C <a
href="http://www.w3.org/Consortium/Legal/ipr-notice-20000612#Legal_Disclaimer">
liability</a>, <a
href="http://www.w3.org/Consortium/Legal/ipr-notice-20000612#W3C_Trademarks">
trademark</a>, <a
href="http://www.w3.org/Consortium/Legal/copyright-documents-19990405">
document use</a>, and <a
href="http://www.w3.org/Consortium/Legal/copyright-software-19980720">
software licensing</a> rules apply.</p>
</div>

<hr />
<div>
<h2><a id="abstract" name="abstract">Abstract</a></h2>

<p>This paper addresses and attempts to clarify two issues
pertaining to URIs, and presents recommendations. Section 1
addresses how URI space is partitioned and the relationship between
URIs, URLs, and URNs. Section 2 describes how URI schemes and URN
namespace ids are registered. Section 3 mentions additional
unresolved issues not considered by this paper and section 4
presents recommendations.</p>
</div>

<div>
<h2><a id="status" name="status">Status of this Document</a></h2>

<p><em>This section describes the status of this document at the
time of its publication. Other documents may supersede this
document. The latest status of this document series is maintained
at the W3C in a <a href="http://www.w3.org/TR/">list of current W3C
technical reports</a> at http://www.w3.org/TR/ .</em></p>

<p>This is a report from the W3C/IETF URI Planning Interest Group,
for review by W3C members, the IETF community, and other interested
parties. We invite review and discussion of our recommendations for
future work in the IETF and/or W3C. Please address your feedback to
uri@w3.org, a mailing list with <a
href="http://lists.w3.org/Archives/Public/uri/">public
archive</a>.</p>

<p>This document has been produced as part of the <a
href="http://www.w3.org/Addressing/Activity">W3C URI
Activity</a>.</p>
</div>

<div class="toc">
<h2><a id="contents" name="contents">Table of Contents</a></h2>

<ol class="toc">
<li><a href="#uri-partitioning">URI Partitioning</a>

<ol>
<li><a href="#classical">Classical View</a></li>

<li><a href="#contemporary">Contemporary View</a></li>

<li><a href="#confusion">Confusion</a></li>
</ol>
</li>

<li><a href="#registration">Registration</a>

<ol>
<li><a href="#uri-schemes">URI Schemes</a>

<ol>
<li><a href="#registered-uri-schemes">Registered URI
schemes</a></li>

<li><a href="#unregistered-uri-schemes">Unregistered URI
Schemes</a>

<ol>
<li><a href="#public-unregistered-uri-schemes">Public Unregistered
Schemes</a></li>

<li><a href="#private-unregistered-uri-schemes">Private
Schemes</a></li>
</ol>
</li>

<li><a href="#uri-scheme-registration">Registration of URI
Schemes</a>

<ol>
<li><a href="#ietf-tree">IETF Tree</a></li>

<li><a href="#other-trees">Other Trees</a></li>
</ol>
</li>
</ol>
</li>

<li><a href="#urn-namespaces">URN Namespaces</a>

<ol>
<li><a href="#registered-nids">Registered URN NIDs</a></li>

<li><a href="#pending-urn-nids">Pending URN NIDs</a></li>

<li><a href="#unregistered-nids">Unregistered NIDs</a></li>

<li><a href="#nid-registration-process">Registration Procedures for
URN NIDs</a></li>
</ol>
</li>
</ol>
</li>

<li><a href="#additional-uri-issues">Additional URI Issues</a></li>

<li><a href="#recommendations">Recommendations</a></li>
</ol>

<h3>Appendixes</h3>

<ul class="toc">
<li>A <a href="#acks">Acknowledgements</a></li>

<li>B <a href="#bibliography">References</a></li>
</ul>
</div>

<hr />
<div class="body">
<div class="div1">
<h2><a id="uri-partitioning" name="uri-partitioning"></a>1 URI
Partitioning</h2>

<p>There is some confusion in the web community over the
partitioning of URI space, specifically, the relationship among the
concepts of URL, URN, and URI. The confusion owes to the
incompatibility between two different views of URI partitioning,
which we call the "classical" and "contemporary" views.</p>

<div class="div2">
<h3><a id="classical" name="classical"></a>1.1 Classical View</h3>

<p>During the early years of discussion of web identifiers (early
to mid 90s), people assumed that an identifer type would be cast
into one of two (or possibly more) classes. An identifier might
specify the location of a resource (a URL) or its name (a URN)
independent of location. Thus a URI was either a URL or a URN.
There was discussion about generalizing this by addition of a
discrete number of additional classes; for example, a URI might
point to metadata rather than the resource itself, in which case
the URI would be a URC (citation). URI space was thus viewed as
partitioned into subspaces: URL and URN, and additional subspaces,
to be defined. The only such additional space ever proposed was URC
and there never was any buy-in; so without loss of generality it's
reasonable to say that URI space was thought to be partitioned into
two classes: URL and URN. Thus for example, "<code>http:</code>"
was a URL scheme, and "<code>isbn:</code>" would (someday) be a URN
scheme. Any new scheme would be cast into one or the other of these
two classes.</p>
</div>

<div class="div2">
<h3><a id="contemporary" name="contemporary"></a>1.2 Contemporary
View</h3>

<p>Over time, the importance of this additional level of hierarchy
seemed to lessen; the view became that an individual scheme does
not need to be cast into one of a discrete set of URI types such as
"URL", "URN", "URC", etc. Web-identifer schemes are in general URI
schemes; a given URI scheme may define subspaces. Thus
"<code>http:</code>" is a URI scheme. "<code>urn:</code>" is also a
URI scheme; it defines subspaces, called "namespaces". For example,
the set of URNs of the form "<code>urn:isbn:n-nn-nnnnnn-n</code>"
is a URN namespace. ("<code>isbn</code>" is an URN namespace
identifier. It is not a "URN scheme" nor a "URI scheme").</p>

<p>Further according to the contemporary view, the term "URL" does
not refer to a formal partition of URI space; rather, URL is a
useful but informal concept: a URL is a type of URI that identifies
a resource via a representation of its primary access mechanism
(e.g., its network "location"), rather than by some other
attributes it may have. Thus as we noted, "<code>http:</code>" is a
URI scheme. An http URI is a URL. The phrase "URL scheme" is now
used infrequently, usually to refer to some subclass of URI schemes
which exclude URNs.</p>
</div>

<div class="div2">
<h3><a id="confusion" name="confusion"></a>1.3 Confusion</h3>

<p>The body of documents (RFCs, etc) covering URI architecture,
syntax, registration, etc., spans both the classical and
contemporary periods. People who are well-versed in URI matters
tend to use "URL" and "URI" in ways that seem to be interchangable.
Among these experts, this isn't a problem. But among the Internet
community at large, it is. People are not convinced that URI and
URL mean the same thing, in documents where they (apparently) do.
When one sees an RFC that talks about URI schemes (e.g. <a
href="#RFC2396">[RFC 2396]</a>), another that talks about URL
schemes (e.g. <a href="#RFC2717">[RFC 2717]</a>), and yet another
that talks of URN schemes (<a href="#RFC2276">[RFC 2276]</a>) it is
natural to wonder what's the difference, and how they relate to one
another. While RFC 2396 1.2 attempts to address the distinction
between URIs, URLs and URNs, it has not been successful in clearing
up the confusion.</p>
</div>
</div>

<div class="div1">
<h2><a id="registration" name="registration"></a>2
Registration</h2>

<p>This section examines the state of registration of URI schemes
and URN namespaces and the mechanisms by which registration
currently occurs.</p>

<div class="div2">
<h3><a id="uri-schemes" name="uri-schemes"></a>2.1 URI Schemes</h3>

<div class="div3">
<h4><a id="registered-uri-schemes"
name="registered-uri-schemes"></a>2.1.1 Registered URI schemes</h4>

<p>The official register of URI scheme names is maintained by IANA,
at http://www.iana.org/assignments/uri-schemes . For each scheme,
the RFC that defines the scheme is listed, for example
"<code>http:</code>" is defined by <a href="#RFC2616">[RFC
2616]</a>. The table currently lists 30 schemes. In addition, there
are a few "reserved" scheme names; at one point in time these were
intended to become registered schemes but have since been
dropped.</p>
</div>

<div class="div3">
<h4><a id="unregistered-uri-schemes"
name="unregistered-uri-schemes"></a>2.1.2 Unregistered URI
Schemes</h4>

<p>We distinguish between public (unregistered) and private
schemes. A public scheme (registered or not), is one for which
there is some public document describing it.</p>

<div class="div4">
<h5><a id="public-unregistered-uri-schemes"
name="public-unregistered-uri-schemes"></a>2.1.2.1 Public
Unregistered Schemes</h5>

<p>Dan Connolly maintains a <a
href="http://www.w3.org/Addressing/schemes">list</a> of known,
public URI schemes, both registered and un-registered, a total of
84 schemes. 50 or so of these are unregistered (not listed in the
IANA register). Some may be obsolete (for example, it appears that
"<code>phone</code>", is obsolete, superceded by
"<code>tel</code>"). Some have an RFC, but are not included in the
IANA list.</p>
</div>

<div class="div4">
<h5><a id="private-unregistered-uri-schemes"
name="private-unregistered-uri-schemes"></a>2.1.2.2 Private
Schemes</h5>

<p>It's probably impossible to determine all of these, and it's not
clear that it's worthwhile to try, except perhaps to get some idea
of their number. In the minutes of the August 1997 IETF meeting is
the observation that there may be 20-40 in use at Microsoft, with
2-3 being added a day, and that WebTV has 24, with 6 added per
year.</p>
</div>
</div>

<div class="div3">
<h4><a id="uri-scheme-registration"
name="uri-scheme-registration"></a>2.1.3 Registration of URI
Schemes</h4>

<p><a href="#RFC2717">[RFC 2717]</a> specifies procedures for
registering scheme names, and points to <a href="#RFC2718">[RFC
2718]</a> which supplies guidelines. RFC 2717 describes an
organization of schemes into "trees".</p>

<div class="div4">
<h5><a id="ietf-tree" name="ietf-tree"></a>2.1.3.1 IETF Tree</h5>

<p>The IETF tree is intended for schemes of general interest to the
Internet community, and which require a substantive review and
approval process. Registration in the IETF tree requires
publication of the scheme syntax and semantics in an RFC.</p>
</div>

<div class="div4">
<h5><a id="other-trees" name="other-trees"></a>2.1.3.2 Other
Trees</h5>

<p>Although RFC 2717 describes "alternative trees", no alternative
trees have been registered to date, although a vendor-supplied tree
("<code>vnd</code>") is pending. URI schemes in alternative trees
will be distinguished because they will have a "." in the scheme
name.</p>
</div>
</div>
</div>

<div class="div2">
<h3><a id="urn-namespaces" name="urn-namespaces"></a>2.2 URN
Namespaces</h3>

<p>A URN namespace is identified by a "Namespace ID", NID, which is
registered with IANA (see <a
href="#nid-registration-process"><b>2.2.4 Registration Procedures
for URN NIDs</b></a>).</p>

<div class="div3">
<h4><a id="registered-nids" name="registered-nids"></a>2.2.1
Registered URN NIDs</h4>

<p>There are two categories of registered URN NIDs:</p>

<ul>
<li>
<p>Informal: These are of the form "urn-&lt;number&gt;" where
&lt;number&gt; is assigned by IANA. There are three registered in
this category (urn-1, urn-2, and urn-3).</p>
</li>

<li>
<p>Formal: The official list of registered NIDs is kept by IANA at
http://www.iana.org/assignments/urn-namespaces. Currently it lists
eight registered NIDs:</p>

<ul>
<li>
<p>'ietf', defined by <a href="#RFC2648">[RFC 2648]</a>, <cite>URN
Namespace for IETF Documents</cite></p>
</li>

<li>
<p>'pin', defined by <a href="#RFC3043">[RFC 3043]</a>, <cite>The
Network Solutions Personal Internet Name (PIN): A URN Namespace for
People and Organizations</cite></p>
</li>

<li>
<p>'issn' defined by <a href="#RFC3043">[RFC 3043]</a>, <cite>Using
The ISSN as URN within an ISSN-URN Namespace</cite></p>
</li>

<li>
<p>'oid' defined by <a href="#RFC3061">[RFC 3061]</a>, <cite>A URN
Namespace of Object Identifiers</cite></p>
</li>

<li>
<p>'newsml' defined by <a href="#RFC3085">[RFC 3085]</a>, <cite>URN
Namespace for NewsML Resources</cite></p>
</li>

<li>
<p>'oasis' defined by <a href="#RFC3121">[RFC 3121]</a>, <cite>A
URN Namespace for OASIS</cite></p>
</li>

<li>
<p>'xmlorg' defined by <a href="#RFC3120">[RFC 3120]</a>, <cite>A
URN Namespace for XML.org</cite></p>
</li>

<li>
<p>'publicid' defined by <a href="#RFC3151">[RFC 3151]</a>, <cite>A
URN Namespace for Public Identifiers</cite></p>
</li>
</ul>
</li>
</ul>
</div>

<div class="div3">
<h4><a id="pending-urn-nids" name="pending-urn-nids"></a>2.2.2
Pending URN NIDs</h4>

<p>There are a number of pending URN NID registration requests but
there is no reliable way to discover them, or their status. For
example, 'isbn' and 'nbn' have been approved by the IESG and are in
the RFC Editor's queue. 'isbn', as a potential URN namespace (or
URI scheme), in particular has been a source of much speculation
and confusion over several years. It would be helpful if there were
some formal means to track the status of NID requests such as
'isbn'.</p>
</div>

<div class="div3">
<h4><a id="unregistered-nids" name="unregistered-nids"></a>2.2.3
Unregistered NIDs</h4>

<p>In the "unregistered" category (besides the experimental case,
not described in this paper) there are bonafide NIDs that just
haven't bothered to even explore the process of registration.The
most prominent that comes to mind is 'hdl'. In the case of 'hdl',
it has been speculated that this scheme has not been registered
because it is not clear to the owners whether it should be
registered as a URI scheme or as a URN namespace.</p>
</div>

<div class="div3">
<h4><a id="nid-registration-process"
name="nid-registration-process"></a>2.2.4 Registration Procedures
for URN NIDs</h4>

<p><a href="#RFC2611">[RFC 2611]</a> describes the mechanism to
obtain an NID for a URN namespace, which is registered with
IANA.</p>

<p>A request for an NID should describe features including:
structural characteristic of identifiers (for example, features
relevant to caching/shortcuts approaches); specific character
encoding rules (e.g., which character should be used for
single-quotes); RFCs, standards, etc, that explains the namespace
structure; identifier uniqueness considerations; delegation of
assignment authority, including how to become an assigner of
identifiers; identifier persistence considerations; quality of
service considerations; process for identifier resolution; rules
for lexical equivalence; any special considerations required for
conforming with the URN syntax (particularly applicable in the case
of legacy naming systems); validation mechanisms (determining
whether a given string is currently a validly-assigned URN; and
scope (for example,"United States social security numbers").</p>
</div>
</div>
</div>

<div class="div1">
<h2><a id="additional-uri-issues"
name="additional-uri-issues"></a>3 Additional URI Issues</h2>

<p>There are additional unresolved URI issues, not considered by
this paper, which we hope will be addressed by a follow-on effort.
We have not attempted to completely enumerate these issues,
however, they include (but are not limited to) the following:</p>

<ul>
<li>
<p>The use of URIs as identifiers that don't actually identify
network resources (for example they identify an abstract object
such as an XML schema, or a physical object such as a book or even
a person).</p>
</li>

<li>
<p>IRIs (International Resource Identifiers): the extension of URI
syntax to non-ASCII.</p>
</li>
</ul>
</div>

<div class="div1">
<h2><a id="recommendations" name="recommendations"></a>4
Recommendations</h2>

<p>We recommend the following:</p>

<ol>
<li>
<p>The W3C and IETF should jointly develop and endorse a model for
URIs, URLs and URNs consistent with the '"Contemporary View"
described in section 1, and which considers the additional URI
issues listed or alluded to in section 3.</p>
</li>

<li>
<p>RFCs such as 2717 ("Registration Procedures for URL Scheme
Names") and 2718 ("Guidelines for new URL Schemes") should both be
generalized to refer to "URI schemes" rather that "URL schemes"
and, after refinement, moved forward as Best Current Practice in
IETF.</p>
</li>

<li>
<p>The registration procedures for alternative trees should be
clarified in RFC 2717.</p>
</li>

<li>
<p>Public but unregistered schemes should become registered, where
possible. Obsolete schemes should be purged or clearly marked as
obsolete.</p>
</li>

<li>
<p>IANA registration information should be updated:</p>

<ul>
<li>
<p>Add 'urn' to the list of registered URI schemes with a pointer
to the URN namespace registry.</p>
</li>

<li>
<p>Maintain status information about pending registrations (URI
schemes and URN NID requests ).</p>
</li>

<li>
<p>Insure that it is clear that the page is the official registry,
e.g., by adding a heading to the effect "This is the Official IANA
Registry of URI Schemes".</p>
</li>
</ul>
</li>
</ol>
</div>
</div>

<div class="back">
<div class="div1">
<h2><a id="acks" name="acks">A Acknowledgements</a></h2>

<p>The participants in the URI Planning Interest Group are:</p>

<ul>
<li>Tony Coates</li>

<li>Dan Connolly</li>

<li>Diana Dack</li>

<li>Leslie Daigle</li>

<li>Ray Denenberg</li>

<li>Martin D&#252;rst</li>

<li>Paul Grosso</li>

<li>Sandro Hawke</li>

<li>Renato Iannella</li>

<li>Graham Klyne</li>

<li>Larry Masinter</li>

<li>Michael Mealling</li>

<li>Mark Needleman</li>

<li>Norman Walsh</li>
</ul>
</div>

<div class="div1">
<h2><a id="bibliography" name="bibliography">B References</a></h2>

<dl>
<dt class="label"><a id="RFC2717" name="RFC2717">RFC 2717</a></dt>

<dd>IETF (Internet Engineering Task Force). <cite><a
href="http://www.ietf.org/rfc/rfc2717.txt">Registration Procedures
for URL Scheme Names</a></cite>, ed. R. Petke and I. King.
1999.</dd>

<dt class="label"><a id="RFC2718" name="RFC2718">RFC 2718</a></dt>

<dd>IETF (Internet Engineering Task Force). <cite><a
href="http://www.ietf.org/rfc/rfc2718.txt">Guidelines for new URL
Schemes</a></cite>, ed. L. Masinter, H. Alvestrand, D. Zigmond, and
R. Petke. 1999.</dd>

<dt class="label"><a id="RFC2648" name="RFC2648">RFC 2648</a></dt>

<dd>IETF (Internet Engineering Task Force). <cite><a
href="http://www.ietf.org/rfc/rfc2648.txt">A URN Namespace for IETF
Documents</a></cite> ed. R. Moats. 1999.</dd>

<dt class="label"><a id="RFC3043" name="RFC3043">RFC 3043</a></dt>

<dd>IETF (Internet Engineering Task Force). <cite><a
href="http://www.ietf.org/rfc/rfc3043.txt">The Network Solutions
Personal Internet Name (PIN): A URN Namespace for People and
Organizations</a></cite> ed. M. Mealling. 2001.</dd>

<dt class="label"><a id="RFC3044" name="RFC3044">RFC 3044</a></dt>

<dd>IETF (Internet Engineering Task Force). <cite><a
href="http://www.ietf.org/rfc/rfc3044.txt">Using The ISSN
(International Serial Standard Number) as URN (Uniform Resource
Names) within an ISSN-URN Namespace</a></cite> ed. S. Rozenfeld.
2001.</dd>

<dt class="label"><a id="RFC3061" name="RFC3061"></a>RFC 3061</dt>

<dd>IETF (Internet Engineering Task Force). <cite><a
href="http://www.ietf.org/rfc/rfc3061.txt">A URN Namespace of
Object Identifiers</a></cite> ed. M. Mealling, 2001.</dd>

<dt class="label"><a id="RFC3085" name="RFC3085">RFC 3085</a></dt>

<dd>IETF (Internet Engineering Task Force). <cite><a
href="http://www.ietf.org/rfc/rfc3085.txt">URN Namespace for NewsML
Resources</a></cite> ed. A. Coates, D. Allen, and D. Rivers-Moore.
2001.</dd>

<dt class="label"><a id="RFC3121" name="RFC3121">RFC 3121</a></dt>

<dd>IETF (Internet Engineering Task Force). <cite><a
href="http://www.ietf.org/rfc/rfc3121.txt">A URN Namespace for
OASIS</a></cite> ed. K. Best and N. Walsh. 2001.</dd>

<dt class="label"><a id="RFC3120" name="RFC3120">RFC 3120</a></dt>

<dd>IETF (Internet Engineering Task Force). <cite><a
href="http://www.ietf.org/rfc/rfc3120.txt">A URN Namespace for
XML.org</a></cite> ed. K. Best and N. Walsh. 2001.</dd>

<dt class="label"><a id="RFC3151" name="RFC3151">RFC 3151</a></dt>

<dd>IETF (Internet Engineering Task Force). <cite><a
href="http://www.ietf.org/rfc/rfc3151.txt">A URN Namespace for
Public Identifiers</a></cite>, ed. N. Walsh, J. Cowan, and P.
Grosso. 2001.</dd>

<dt class="label"><a id="RFC2611" name="RFC2611">RFC 2611</a></dt>

<dd>IETF (Internet Engineering Task Force). <cite><a
href="http://www.ietf.org/rfc/rfc2611.txt">URN Namespace Definition
Mechanisms</a></cite>, ed. L. Daigle, R. Iannella, and P.
Faltstrom. 1999.</dd>

<dt class="label"><a id="RFC2396" name="RFC2396"></a>RFC 2396</dt>

<dd>IETF (Internet Engineering Task Force). <cite><a
href="http://www.ietf.org/rfc/rfc2396.txt">Uniform Resource
Identifiers (URI): Generic Syntax</a></cite>, ed. T. Berners-Lee,
R. Fielding, and L. Masinter. 1998.</dd>

<dt class="label"><a id="RFC2276" name="RFC2276">RFC 2276</a></dt>

<dd>IETF (Internet Engineering Task Force). <cite><a
href="http://www.ietf.org/rfc/rfc2276.txt">Architectural Principles
of Uniform Resource Name Resolution</a></cite>, ed. K. Sollins.
1998.</dd>

<dt class="label"><a id="RFC2616" name="RFC2616">RFC 2616</a></dt>

<dd>IETF (Internet Engineering Task Force). <cite><a
href="http://www.ietf.org/rfc/rfc2616.txt">Hypertext Transfer
Protocol -- HTTP/1.1</a></cite>, ed. R. Fielding, J. Gettys, J.
Mogul, et. al. 1999.</dd>
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