2.1 Classical View
During the early years of discussion of web identifiers (early to mid
90s), people assumed that an identifier 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 the 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, URN, and additional subspaces to be defined. The
only such additional space ever proposed was Uniform Resource
Characteristics (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,
"http:" was a URL scheme, and "isbn:" would (someday) be a URN
scheme. Any new scheme would be cast into one of these two classes.
2.2 Contemporary View
Over time, the importance of this additional level of hierarchy
seemed to lessen; the view became that an individual scheme did not
need to be cast into one of a discrete set of URI types, such as
"URL", "URN", "URC", etc. Web-identifier schemes are, in general,
URI schemes, as a given URI scheme may define subspaces. Thus
"http:" is a URI scheme. "urn:" is also a URI scheme; it defines
subspaces, called "namespaces". For example, the set of URNs, of the
form "urn:isbn:n-nn-nnnnnn-n", is a URN namespace. ("isbn" is an URN
namespace identifier. It is not a "URN scheme", nor is it a "URI
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, "http:" 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.
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 interchangeable. Among these
experts, this isn't a problem, but among the Internet community at
large, it is a problem. People are not convinced that URI and URL
mean the same thing, in documents where they (apparently) do. When
one RFC talks about URI schemes (e.g. "URI Syntax" (RFC 2396) ),
another talks about URL schemes (e.g. "Registration Procedures for
URL Schemes" (RFC 2717) ), and yet another talks of URN schemes
("Architectural Principles of URN Resolution" (RFC 2276) ), it is
natural to wonder how they difference, and how they relate to one
another. While RFC 2396, section 1.2, attempts to address the
distinction between URIs, URLs and URNs, it has not been successful
in clearing up the confusion.
This section examines the state of registration of URI schemes and
URN namespaces and the mechanisms by which registration currently
3.1 URI Schemes
3.1.1 Registered URI schemes
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 "http:" is defined
by RFC2616 . The table lists 34 schemes (at time of publication
of this RFC). 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.
3.1.2 Unregistered URI Schemes
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.
18.104.22.168 Public Unregistered Schemes
Dan Conolly's paper, at http://www.w3.org/Addressing/schemes,
provides a list of known public URI schemes, both registered and un-
registered, a total of 85 schemes at time of publication of this RFC.
50 or so of these are unregistered (not listed in the IANA register).
Some of these URI schemes are obsolete (for example, "phone" is
obsolete, superceded by "tel"), while some have an RFC, but are not
included in the IANA list.
22.214.171.124 Private Schemes
It is 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.
3.1.3 Registration of URI Schemes
"Registration Procedures for URL Scheme Names" (RFC 2717) 
specifies procedures for registering scheme names and points to
"Guidelines for new URL Schemes" (RFC 2718) , which supplies
guidelines. RFC 2717 describes an organization of schemes into
"trees". It is important to note that these two documents use the
historical term 'URL' when in fact, they refer to URIs in general.
In fact, one of the recommended tasks in Section 5 is for these
documents to be updated to use the term 'URI' instead of 'URL'.
126.96.36.199 IETF Tree
The IETF tree is intended for schemes of general interest to the
Internet community, and for those which require a substantive review
and approval process. Registration in the IETF tree requires
publication of the scheme syntax and semantics in an RFC.
188.8.131.52 Other Trees
Although RFC 2717 describes "alternative trees", no alternative trees
have been registered to date, although a vendor-supplied tree ("vnd")
is pending. URI schemes in alternative trees will be distinguished
because they will have a "." in the scheme name.
3.2 URN Namespaces
A URN namespace is identified by a "Namespace ID" (NID), which is
registered with IANA (see Section 3.2.4).
3.2.1 Registered URN NIDs
There are two categories of registered URN NIDs:
o Informal: These are of the form, "urn-<number>", where <number> is
assigned by IANA. There are four registered (at time of
publication of this RFC) in this category (urn-1, urn-2, urn-3,
o Formal: The official list of registered NIDs is kept by IANA at
http://www.iana.org/assignments/urn-namespaces. At the time of
publication of this RFC it lists ten registered NIDs:
* 'ietf', defined by "URN Namespace for IETF Documents" (RFC
* 'pin', defined by "The Network Solutions Personal Internet Name
(PIN): A URN Namespace for People and Organizations" (RFC 3043)
* 'issn' defined by "Using The ISSN as URN within an ISSN-URN
Namespace" (RFC 3043) 
* 'oid' defined by "A URN Namespace of Object Identifiers" (RFC
* 'newsml' defined by "URN Namespace for NewsML Resources" (RFC
* 'oasis' defined by "A URN Namespace for OASIS" (RFC 3121) 
* 'xmlorg' defined by "A URN Namespace for XML.org" (RFC 3120)
* 'publicid' defined by "A URN Namespace for Public Identifiers"
(RFC 3151) 
* 'isbn' defined by "Using International Standard Book Numbers as
Uniform Resource Names" (RFC 3187) 
* 'nbn' defined by "Using National Bibliography Numbers as
Uniform Resource Names" (RFC 3188) 
3.2.2 Pending URN NIDs
There are a number of pending URN NID registration requests, but
there is no reliable way to discover them, or their status. It would
be helpful if there were some formal means to track the status of NID
requests such as 'isbn'.
3.2.3 Unregistered NIDs
In the "unregistered" category (besides the experimental case, not
described in this paper), there are entities that maintain namespaces
that, while completely appropriate as URNs, just haven't bothered to
explore the process of NID 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
3.2.4 Registration Procedures for URN NIDs
"URN Namespace Definition Mechanisms" (RFC 2611)  describes the
mechanism to obtain an NID for a URN namespace, which is registered
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 explain 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").
4. Additional URI Issues
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:
o The use of URIs as identifiers that don't actually identify
network resources (for example, they identify an abstract object,
such as an XML namespace, or a physical object such as a book or
even a person).
o IRIs (International Resource Identifiers): the extension of URI
syntax to non-ASCII.
We recommend the following:
1. 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.
2. 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 than "URL schemes"
and, after refinement, moved forward as Best Current Practices in
3. The registration procedures for alternative trees should be
clarified in RFC 2717.
4. Public, but unregistered schemes, should become registered, where
possible. Obsolete schemes should be purged or clearly marked as
5. IANA registration information should be updated:
* Add 'urn' to the list of registered URI schemes with a pointer
to the URN namespace registry.
* Maintain status information about pending registrations (URI
schemes and URN NID requests ).
* 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".
6. Security Considerations
This memo does not raise any known security threats.
The participants in the URI Planning Interest Group are:
o Tony Coates
o Dan Connolly
o Diana Dack
o Leslie Daigle
o Ray Denenberg
o Martin Duerst
o Paul Grosso
o Sandro Hawke
o Renato Iannella
o Graham Klyne
o Larry Masinter
o Michael Mealling
o Mark Needleman
o Norman Walsh
 Petke, R. and I. King, "Registration Procedures for URL Scheme
Names", BCP 35, RFC 2717, November 1999.
 Masinter, L., Alvestrand, H., Zigmond, D. and R. Petke,
"Guidelines for new URL Schemes", RFC 2718, November 1999.
 Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
 Mealling, M., "The Network Solutions Personal Internet Name
(PIN): A URN Namespace for People and Organizations", RFC 3043,
 Rozenfeld, S., "Using The ISSN (International Serial Standard
Number) as URN (Uniform Resource Names) within an ISSN-URN
Namespace", RFC 3044, January 2001.
 Mealling, M., "A URN Namespace of Object Identifiers", RFC 3061,
 Coates, A., Allen, D. and D. Rivers-Moore, "URN Namespace for
NewsML Resources", RFC 3085, March 2001.
 Best, K. and N. Walsh, "A URN Namespace for OASIS", RFC 3121,
 Best, K. and N. Walsh, "A URN Namespace for XML.org", RFC 3120,
 Walsh, N., Cowan, J. and P. Grosso, "A URN Namespace for Public
Identifiers", RFC 3151, August 2001.
 Daigle, L., van Gulik, D., Iannella, R. and P. Faltstrom, "URN
Namespace Definition Mechanisms", BCP 33, RFC 2611, June 1999.
 Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
 Sollins, K., "Architectural Principles of Uniform Resource Name
Resolution", RFC 2276, January 1998.
 Fielding, R., Gettys, J., Mogul, J., Nielsen, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
 Hakala, J. and H. Walravens, "Using International Standard Book
Numbers as Uniform Resource Names", RFC 3187, October 2001.
 Hakala, J., "Using National Bibliography Numbers as Uniform
Resource Names", RFC 3188, October 2001.
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