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.
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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 email@example.com, a mailing list with public archive.
This document has been produced as part of the W3C URI Activity.
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.
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, "
was a URL scheme, and "
isbn:" would (someday) be a URN
scheme. Any new scheme would be cast into one or the other of these
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
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 "
is a URN namespace. ("
isbn" is an URN namespace
identifier. It is not a "URN scheme" nor a "URI scheme").
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 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. [RFC 2396]), another that talks about URL schemes (e.g. [RFC 2717]), and yet another that talks of URN schemes ([RFC 2276]) 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.
This section examines the state of registration of URI schemes and URN namespaces and the mechanisms by which registration currently occurs.
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 [RFC
2616]. 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
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.
Dan Connolly maintains a list 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
phone", is obsolete, superceded by
tel"). Some have an RFC, but are not included in the
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.
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.
A URN namespace is identified by a "Namespace ID", NID, which is registered with IANA (see 2.2.4 Registration Procedures for URN NIDs).
There are two categories of registered URN NIDs:
Informal: These are of the form "urn-<number>" where <number> is assigned by IANA. There are three registered in this category (urn-1, urn-2, and urn-3).
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:
'ietf', defined by [RFC 2648], URN Namespace for IETF Documents
'pin', defined by [RFC 3043], The Network Solutions Personal Internet Name (PIN): A URN Namespace for People and Organizations
'issn' defined by [RFC 3043], Using The ISSN as URN within an ISSN-URN Namespace
'oid' defined by [RFC 3061], A URN Namespace of Object Identifiers
'newsml' defined by [RFC 3085], URN Namespace for NewsML Resources
'oasis' defined by [RFC 3121], A URN Namespace for OASIS
'xmlorg' defined by [RFC 3120], A URN Namespace for XML.org
'publicid' defined by [RFC 3151], A URN Namespace for Public Identifiers
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'.
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.
[RFC 2611] describes the mechanism to obtain an NID for a URN namespace, which is registered with IANA.
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").
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:
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).
IRIs (International Resource Identifiers): the extension of URI syntax to non-ASCII.
We recommend the following:
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.
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.
The registration procedures for alternative trees should be clarified in RFC 2717.
Public but unregistered schemes should become registered, where possible. Obsolete schemes should be purged or clearly marked as obsolete.
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".
The participants in the URI Planning Interest Group are: