Names and Addresses, URIs, URLs, URNs, URCs

Addressing is one of the fundamental technologies in the web. URIs, or Uniform Resouce Identifiers, are the technology for addressing documents on the web. It is an extensible technology: there are a number of existing addressing schemes, and more may be incorporated over time.

This is the overview of addressing (URL, URI) info at W3C.

W3C Position Statement: Statement of requirements, directions, plan.
Background: Introductory and background materials are collected below.
Glossary of Terms: Acronym soup explained. A bit of shared context for related discussions.
Specs and Drafts: For reference and review
UR* Discussion Forums and Archives: Look over the archives to see what others have written. Then ask questions, make suggestions, participate!
Schemes: http:, ftp:, irc:, etc. Need to look one up? Thinking of suggesting a new one?

UR* Terms

The basic picture looks like this:

	 _______________________________________________________
	|							|
	|	 _______________	 _______________	|
	|	|  ftp:		|	|  urn:		|	|
	|	|  gopher:	|	|  fpi: ?	|	|
	|	|  http:	|	|  path:	|	|
	|	|  etc		|	|		|	|
	|	|_______________|	|_______________|	|
	|		URLs			URNs		|
	|_______________________________________________________|
				   URIs

URI: Uniform Resource Idenifier. The generic set of all names/addresses that are short strings that refer to objects. See URI spec.
URL: Uniform Resource Locators. Exactly what consitutes a locator as opposed to a name is basically lack of persistence, but this is a much discussed point and impossible to define precisely. In practice, the set of schmes referring to existing protocolls, listed in the URL specification .
URN: Uniform Resource Name. 1. Any URI which is not a URL. 2. A particular scheme which is currently (1991,2,3,4,5) under development by the IETF, which should provide for the resolution using internet protocols of names which have a greater persistence than that currently assiated with internet host names or organizations. When defined, a URN(2) will be an example of a URI.

URC: Uniform Resource Citation. A set of attribute/value pairs describing an object. Some of the values may be URIs of various kinds. Others may include, for example, authorship, publisher, datatype, date, copyright status and shoe size. Not normally discussed as a short string, but a set of fields and values with some defined free formatting.
URC is a mechanism of resource description, which can be seen as an instance of the general problem of knowledge representation.

Specifications, Drafts, and Reports

A note on the status of URL specs

While URLs are one of the most stable technologies in the web, the specs for them need a little explanation:

RFC1808: the "relative URL" spec. This one is both authoritative (i.e. it's been through the IETF standards process) and accurate on the matter of the syntax of URLs. Its grammar is complete and consistent, and there are several clarifying examples.
On the other hand, it does not discuss any of the actual URL schemes (such as HTTP, FTP, ...)
RFC1738 - the "URL standard". This one is authoritative, and it discusses each of the URL schemes (known at that time), but its grammar has some mistakes, and it doesn't cover several aspects of URL syntax, such as relative URLs and fragment identifiers
RFC1630 - the "URI spec". This documents the designer's intent, before it was revised by the standards process. It was written by Tim Berners-Lee, but has only informational status in the IETF. It discusses all aspects of URLs (relative, etc.) though it has some mistakes in the grammar etc.

-- connolly, May 1996

In order of age, newest first:

26 July 1995 Internet Draft: The Path URN Specification

Authors: D. LaLiberte, M. Shapiro

Internet Draft archive copy (plain text)
LaLiberte's hypertext at NCSA

June 1995 Proposed Standard: Relative Uniform Resource Locators R. Fielding

(RFC 1808) (text, 34950 bytes)

... When embedded within a base document, a URL in its absolute form may contain a great deal of information which is already known from the context of that base document's retrieval, including the scheme, network location, and parts of the url-path. In situations where the base URL is well-defined and known to the parser (human or machine), it is useful to be able to embed URL references which inherit that context rather than re-specifying it in every instance.

December 1994 Proposed Standard: Uniform Resource Locators (URL) T. Berners-Lee, L. Masinter, M. McCahill

(RFC 1738) (text, 51348 bytes)

This document specifies a Uniform Resource Locator (URL), the syntax and semantics of formalized information for location and access of resources via the Internet.

The published spec is materially the same as this working draft, as of March 1994:

December 1994 Informational Functional Requirements for Uniform Resource Names , K. Sollins, L. Masinter

This document specifies a minimum set of requirements for a kind of Internet resource identifier known as Uniform Resource Names (URNs). ...

RFC 1737 (text, 16337 bytes)

June 1994 Informational Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web T. Berners-Lee.

The URI specification simply defines the syntax for encoding arbitrary naming or addressing schemes, and has a list of such schemes. See:

URI specification: Contents list and list of available forms

RFC 1630

Dec 1994 Unpublished Stable Network File URLs as a Mechanism for Uniform Naming Terry Winograd

Early draft version 12/2/93 -- Fire away!

Discussion and Archives

www-talk: A mailing list for technical discussion of web architecture and technology, with a hypertext archive (now searchable! Thanks EIT guys!)
IETF Working Group: This is where URL standardization used to takes place. For current status, see Roy Fielding's URI WG Archive, and the archive of the uri mailing list.
URN WG: archive

Background and Related Resources

A Beginner's Guide to URLs: by the NCSA SDG writers (originally by Marc Andreesen?). Nice short intro, though it's not precisely accurate nor up to date.
Design Issues: TimBL's original discussion of design issues involved
Resource Discovery and Reliable Links: Connolly's notes on the subject. Should become part of Collaboration and Knowledge Representation.
Hypernews on URNs: By Daniel LaLiberte at NCSA. Also, hypernews on URCs.
URI background: by Ron Daniel

Connolly
TimBL
last update by $Author: connolly $ on $Date: 1996/05/24 16:29:10 $
Created 1990