The editors of the Extensible Resource Identifier (XRI) Syntax and Resolution Specification have issued a call for public comment on Working Draft version 07 produced by the OASIS Technical Committee. The XRI TC was chartered to define a URI scheme and a corresponding URN namespace for distributed directory services that enable the identification of resources (including people and organizations) and the sharing of data across domains, enterprises, and applications. The Extensible Resource Identifier (XRI) notation "provides a standard means of abstractly identifying a resource independent of any given concrete representation of that resource or, in the case of a completely abstract resource, independent of any representation at all. XRIs are defined similarly to URIs in Uniform Resource Identifiers (URI): Generic Syntax (RFC 2396) but contain additional syntactical elements and extend the unreserved character set to include characters beyond those allowed in generic URIs. To accommodate applications that expect generic URIs, rules are defined that allow an XRI to be transformed into a conformant URI as defined by RFC 2396. Since a revision of RFC 2396 is currently a work in progress, the XRI scheme also incorporates some simplifications and enhancements to generic URI syntax as proposed in the revised specification (RFC2396bis)." The XRI scheme is formally defined by an ABNF grammar presented in Appendix A.

Bibliographic Information

Extensible Resource Identifier (XRI) Syntax and Resolution Specification. Edited by Gabe Wachob (Visa International), Drummond Reed (OneName), Dave McAlpin (Epok), Mike Lindelsee (Visa International), Peter Davis (Neustar), and Nat Sakimura (NRI). OASIS Technical Committee Working Draft. Version 07. Document identifier: wd-xri-specification-07. 29-July-2003. 49 pages. Produced by members of the OASIS OASIS Extensible Resource Identifier Technical Committee.

XRI Requirements and Glossary. Version 1.0. 12-June-2003. 28 pages. Document identifier: xri-requirements-and-glossary-v1.0. Edited by Gabe Wachob, Drummond Reed, Marc LeMaitre, Dave McAlpin, and Davis McPherson. "This document describes architectural motivations and requirements for development of the Extensible Resource Identifier (XRI) specifications. It also includes a normative glossary of terms used in this document and other XRI deliverables."

XRI Background and Motivation

Internet architecture today is based primarily on two layers of identifiers... The first layer, IP (Internet Protocol) addressing, defines the Internet itself. IP was developed to standardize packet exchange between local area networks, a task that required a layer of globally unique identifiers for every network segment and host. Since the goal was highly efficient packet routing, IP addresses were designed to be very machine-friendly -- a series of decimal numbers (IPv4) or hex characters (IPv6) representing fixed-byte addressing segments.

A second layer, the DNS (Domain Name System), was subsequently developed to provide a name service for IP hosts. This abstraction layer solved two problems: (a) it provided human-friendly identifiers for IP-addressable hosts, making them much easier for people to remember and use, and (b) it allowed Internet hosts or users to have a logical identity that transcended a particular IP address.

These two layers of identifiers, when combined with local area network identifiers, can uniquely identify any resource on the Internet. Tim Berners-Lee and other architects took full advantage of this when creating the World Wide Web. They developed an identifier syntax originally called URL (Uniform Resource Locator) and now called URI (Uniform Resource Identifier) that allowed a combination of DNS names, IP addresses, and local identifiers to serve as a hyperlink between resources. The syntactic rules for URI schemes (e.g., HTTP URIs, FTP URIs, email URIs, etc.) were most recently specified in IETF RFC 2396 in August 1998.

The phenomenal success of the Web meant that URIs became the fastest-growing new address in history. As the Web grew, it encountered the problem of links breaking because the resource referenced by a URI changed its location on the network. Berners-Lee and others recognized that solving this problem would require another level of abstraction -- a layer of persistent URIs that would remain the same even when the resources they referenced changed their locations. They called this new type of location-independent identifier a URN (Uniform Resource Name). The URI scheme for URNs was specified by IETF RFC 2141 in May 1997.

Since the completion of the IETF URN work, a number of new technologies have appeared for modeling human semantics and data exchange relationships over the Internet, including the Semantic Web, Topic Maps, Web services, digital identity, and digital rights management. While many of these technologies require persistent identifiers, they have also generated a number of other new requirements for abstract identifiers that are not addressed by URNs. These requirements form the primary motivations for XRIs...

The overall goal of the XRI specifications is to establish a standard syntax and resolution protocol for fully abstract identifiers -- in short, to enable a third layer of Internet identifiers similar to the DNS naming and IP addressing layers that exist today...

The potential for this new layer goes beyond just the Internet. With the growing convergence of the Web with other networks such as wired and wireless phone networks, satellite networks, package delivery networks, etc., an XRI can serve as a true unified address -- a single abstract identifier that can be resolved (with the appropriate data protections) to any concrete address or attribute associated with the target resource. Unified addresses represent an enormous potential savings in labor -- both in people spending time looking up phone numbers, fax numbers, email addresses, etc., and in developers spending time coding and testing routines to locate and verify the current address of a target resource." [adapted from the XRI Requirements and Glossary document]