< draft-mealling-uri-ig-01.txt		draft-mealling-uri-ig-02.txt >
			A new Request for Comments is now available in online RFC libraries.
	Network Working Group URI Interest Group		RFC 3305
	Internet-Draft W3C/IETF
	Expires: March 25, 2002 September 24, 2001
	URIs, URLs, and URNs: Clarifications and Recommendations
	Report from the joint W3C/IETF URI Planning Interest Group
	draft-mealling-uri-ig-01.txt
	Status of this Memo
	This document is an Internet-Draft and is in full conformance with
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	Copyright Notice
	Copyright (C) The Internet Society (2001). All Rights Reserved.
	Abstract
	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.
	Questions concerning this document should be directed to the
	uri@w3.org mailing list.
	Table of Contents
	1. URI Partitioning . . . . . . . . . . . . . . . . . . . . . 3
	1.1 Classical View . . . . . . . . . . . . . . . . . . . . . . 3
	1.2 Contemporary View . . . . . . . . . . . . . . . . . . . . 3
	1.3 Confusion . . . . . . . . . . . . . . . . . . . . . . . . 4
	2. Registration . . . . . . . . . . . . . . . . . . . . . . . 4
	2.1 URI Schemes . . . . . . . . . . . . . . . . . . . . . . . 4
	2.1.1 Registered URI schemes . . . . . . . . . . . . . . . . . . 4
	2.1.2 Unregistered URI Schemes . . . . . . . . . . . . . . . . . 4
	2.1.2.1 Public Unregistered Schemes . . . . . . . . . . . . . . . 4
	2.1.2.2 Private Schemes . . . . . . . . . . . . . . . . . . . . . 5
	2.1.3 Registration of URI Schemes . . . . . . . . . . . . . . . 5
	2.1.3.1 IETF Tree . . . . . . . . . . . . . . . . . . . . . . . . 5
	2.1.3.2 Other Trees . . . . . . . . . . . . . . . . . . . . . . . 5
	2.2 URN Namespaces . . . . . . . . . . . . . . . . . . . . . . 5
	2.2.1 Registered URN NIDs . . . . . . . . . . . . . . . . . . . 5
	2.2.2 Pending URN NIDs . . . . . . . . . . . . . . . . . . . . . 6
	2.2.3 Unregistered NIDs . . . . . . . . . . . . . . . . . . . . 6
	2.2.4 Registration Procedures for URN NIDs . . . . . . . . . . . 7
	3. Additional URI Issues . . . . . . . . . . . . . . . . . . 7
	4. Recommendations . . . . . . . . . . . . . . . . . . . . . 7
	5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 8
	References . . . . . . . . . . . . . . . . . . . . . . . . 9
	Author's Address . . . . . . . . . . . . . . . . . . . . . 10
	Full Copyright Statement . . . . . . . . . . . . . . . . . 11
	1. URI Partitioning
	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.
	1.1 Classical View
	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, "http:" 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 two classes.
	1.2 Contemporary View
	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
	"urn:isbn:n-nn-nnnnnn-n" 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.
	1.3 Confusion
	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. "URI Syntax" (RFC 2396)
	[12]), another that talks about URL schemes (e.g. "Registration
	Procedures for URL Schemes" (RFC 2717) [1]), and yet another that
	talks of URN schemes ("Architectural Principles of URN Resolution"
	(RFC 2276) [13]) 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.
	2. Registration
	This section examines the state of registration of URI schemes and
	URN namespaces and the mechanisms by which registration currently
	occurs.
	2.1 URI Schemes
	2.1.1 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 [14]. 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.
	2.1.2 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.
	2.1.2.1 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 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 IANA list.
	2.1.2.2 Private Schemes
	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.
	2.1.3 Registration of URI Schemes
	"Registration Procedures for URL Scheme Names" (RFC 2717) [1]
	specifies procedures for registering scheme names, and points to
	"Guidelines for new URL Schemes" (RFC 2718) [2] which supplies
	guidelines. RFC 2717 describes an organization of schemes into
	"trees".
	2.1.3.1 IETF Tree
	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.
	2.1.3.2 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.
	2.2 URN Namespaces
	A URN namespace is identified by a "Namespace ID", NID, which is
	registered with IANA (see Section 2.2.4).
	2.2.1 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 three registered in this category
	(urn-1, urn-2, and urn-3).
	o 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 "URN Namespace for IETF Documents" (RFC
	2648) [3]
	* 'pin', defined by "The Network Solutions Personal Internet
	Name (PIN): A URN Namespace for People and Organizations" (RFC
	3043) [4]
	* 'issn' defined by "Using The ISSN as URN within an ISSN-URN
	Namespace" (RFC 3043) [4]
	* 'oid' defined by "A URN Namespace of Object Identifiers" (RFC
	3061) [6]
	* 'newsml' defined by "URN Namespace for NewsML Resources" (RFC
	3085) [7]
	* 'oasis' defined by "A URN Namespace for OASIS" (RFC 3121) [8]
	* 'xmlorg' defined by "A URN Namespace for XML.org" (RFC 3120)
	[9]
	* 'publicid' defined by "A URN Namespace for Public Identifiers"
	(RFC 3151) [10]
	2.2.2 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. 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'.
	2.2.3 Unregistered NIDs
	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.
	2.2.4 Registration Procedures for URN NIDs
	"URN Namespace Definition Mechanisms" (RFC 2611) [11] 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").
	3. 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 schema, 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.
	4. Recommendations
	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 that "URL
	schemes" and, after refinement, moved forward as Best Current
	Practice in IETF.
	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
	obsolete.
	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".
	5. Acknowledgements
	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 D’rst
	o Paul Grosso
	o Sandro Hawke
	o Renato Iannella
	o Graham Klyne
	o Larry Masinter
	o Michael Mealling
	o Mark Needleman
	o Norman Walsh
	References
	[1] Petke, R. and I. King, "Registration Procedures for URL Scheme
	Names", BCP 35, RFC 2717, November 1999.
	[2] Masinter, L., Alvestrand, H., Zigmond, D. and R. Petke,
	"Guidelines for new URL Schemes", RFC 2718, November 1999.
	[3] Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
	August 1999.
	[4] Mealling, M., "The Network Solutions Personal Internet Name
	(PIN): A URN Namespace for People and Organizations", RFC 3043,
	January 2001.
	[5] Rozenfeld, S., "Using The ISSN (International Serial Standard
	Number) as URN (Uniform Resource Names) within an ISSN-URN
	Namespace", RFC 3044, January 2001.
	[6] Mealling, M., "A URN Namespace of Object Identifiers", RFC
	3061, February 2001.
	[7] Coates, A., Allen, D. and D. Rivers-Moore, "URN Namespace for
	NewsML Resources", RFC 3085, March 2001.
	[8] Best, K. and N. Walsh, "A URN Namespace for OASIS", RFC 3121,
	June 2001.
	[9] Best, K. and N. Walsh, "A URN Namespace for XML.org", RFC 3120,
	June 2001.
	[10] Walsh, N., Cowan, J. and P. Grosso, "A URN Namespace for Public
	Identifiers", RFC 3151, August 2001.
	[11] Daigle, L., van Gulik, D., Iannella, R. and P. Faltstrom, "URN
	Namespace Definition Mechanisms", BCP 33, RFC 2611, June 1999.
	[12] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
	Resource Identifiers (URI): Generic Syntax", RFC 2396, August
	1998.
	[13] Sollins, K., "Architectural Principles of Uniform Resource Name
	Resolution", RFC 2276, January 1998.
	[14] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., Masinter, L.,		Title: Report from the Joint W3C/IETF URI Planning
	Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --		Interest Group: Uniform Resource Identifiers
	HTTP/1.1", RFC 2616, June 1999.		(URIs), URLs, and Uniform Resource Names (URNs):
			Clarifications and Recommendations
			Author(s): M. Mealling, Ed., R. Denenberg, Ed.
			Status: Informational
			Date: August 2002
			Mailbox: michael@verisignlabs.com, rden@loc.gov
			Pages: 11
			Characters: 21793
			Updates/Obsoletes/SeeAlso: None
	Author's Address		I-D Tag: draft-mealling-uri-ig-02.txt
	URI Planning Interest Group		URL: ftp://ftp.rfc-editor.org/in-notes/rfc3305.txt
	W3C/IETF
	Full Copyright Statement		This document, a product of the W3C Uniform Resource Identifier (URI)
			Interest Group, addresses and attempts to clarify issues pertaining to
			URIs. This document addresses how URI space is partitioned and the
			relationship between URIs, URLs, and URNs, describes how URI schemes
			and URN namespaces ids are registered, and presents recommendations
			for continued work on this subject.
	Copyright (C) The Internet Society (2001). All Rights Reserved.		This memo provides information for the Internet community. It does
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