HTML 4.0 Sourcebook
(Publisher: John Wiley & Sons, Inc.)
Author(s): Ian S. Graham
ISBN: 0471257249
Publication Date: 04/01/98

The nfs URL scheme has been proposed as the foundation of a distributed, Web-based filesystem. NFS is a distributed filesystem architecture, commonly used on UNIX systems, and nfs URLs provide a way of referencing NFS-accessible resources. Netscape Navigator supports nfs URLs, but only via specially equipped proxy servers.

Also of interest are the various z39.50 URL schemes. Z39.50 is an information retrieval protocol commonly used by indexed databases such as libraries. Z39.50 URLs provide a standard syntax for encoding Z39.50-format database queries within a URL.

Other schemes, such as irc (for Internet Relay Chat sessions), vemmi (for a distributed multimedia/videotex standard), whois++ (for referencing WHOIS++ servers), and tv (for referencing TV broadcasts!), have also been proposed. These schemes are not discussed here—information about these proposals can be found in the references at the end of the chapter.

Naming Schemes for the Web

The URL scheme is the only naming scheme currently used on the Web, but it is not the only such scheme—several others have been proposed and are likely to be implemented in the future. This brief section summarizes the names and general features of the different proposed schemes. The “Proposed URL Schemes and Updates” portion of the “References” section at the end of this chapter provide pointers to additional discussions on these topics.

Uniform Resource Names: URN

Uniform Resource Names, or URNs, are designed to be a location-independent way of referencing an object. Thus, a URN would not specify the location of the desired resource, but would specify a generic name. The software processing the name would then locate the named object at the closest or most accessible location, using a name lookup service.

The specification of URNs is still being developed by the URN working group of the IETF; the working group home page, at www.ietf.org/html-charters/urn-charter.html, lists the standards documents arising from this group’s efforts. You should visit this site for up-to-date information.

Uniform Resource Locator: URL

Uniform Resource Locators, or URLs, are protocol- and location- specific schemes for referencing resources on the Internet. This is currently the only implemented mechanism for referencing resources on the Internet.

Uniform Resource Identifier: URI

Uniform Resource Identifiers, or URIs, generically represent any naming schemes used to reference resources on the Internet. Thus, both URLs and URNs fall under the category of URIs. The names URI and URL are often used synonymously, but this is only correct in the absence of a defined naming scheme for URNs.

Uniform Resource Citation: URC

Uniform Resource Citations, or URCs, were designed to be collections of attribute/value pairs that describe a particular object (referenced using a URI). Some of the values in these pairs could also be URIs. A URC can act in many ways, for example as a cross-indexing resource for a large resource collection or simply as a collection of references to related data. The specification for URCs is not complete, so that they are not in current use. Indeed, the URC idea is being completely rethought, with the intention of using the new eXtensible Markup Language (XML) as the tool for recording URC-like data within an XML catalog document. Additional information can be found in the Addressing and XML sections of the World Wide Web Consortium (W3C) site, at www.w3.org.

References

The URI specifications originate from the various working groups currently developing Internet and Web standards. In most cases, the Internet protocols or standards are formalized in documents known as Requests for Comments, or RFCs. Once approved by the appropriate Internet Engineering Task Force (IETF), these documents are officially numbered, giving rise to the referenced RFC numbers quoted here. Note that the draft documents that precede an RFC, with names like “draft-masinter-url-data-03.txt,” evolve with time, each new version having a new version number (e.g., “draft-masinter-url-data-04.txt,” and so on). Thus, if you cannot locate the draft document listed here, try looking for an updated version with a higher version number.

Overviews and Defined URL Schemes

Proposed URL Schemes and Updates

Protocol and Data Format Specifications

URI Specification

ds.internic.net/rfc/rfc1630.txt

URNs: Uniform Resource names