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|OASIS Releases Extensible Resource Identifier (XRI) Specification for Review.|
Members of the OASIS Extensible Resource Identifier (XRI) Technical Committee have approved a set of draft documents for public review and invite feedback through 30-April-2005.
This OASIS TC was chartered to define a URI-compatible identifier scheme and resolution protocol for an "extensible, location-, application-, and transport-independent identification that provides addressability not just of resources, but also of their attributes and versions." The work was expected to build upon the XNS Addressing Specification published by the XNS Public Trust Organization (XNSORG).
According to the XRI Version 2.0 specification summary, this suite of specifications defines "a syntax and resolution protocol for abstract identifiers — identifiers that are independent of a specific location, domain, application, or protocol. The XRI specifications build directly on the foundation provided by the URI (Uniform Resource Identifier) and IRI (Internationalized Resource Identifier) specifications pf IETF RFC 3986 and 3987. Unlike URNs (Uniform Resource Names, RFC 2141), XRIs support both persistent and reassignable abstract identifiers. In addition they offer several other features required for a uniform abstract identification layer, including unlimited federation, cross-references, and global context symbols."
The XRI specification suite presented for public review contains four parts. A non-normative document Introduction to XRIs summarizes the uses and features of XRIs and is intended to serve as an entry point for the XRI 2.0 suite. It clarifies what XRIs are in terms of the supported Uniform Abstract Identification Layer. It provides examples of the types of problems XRIs are designed to solve, for example: The Broken Links Problem (Persistent Identification); The Multiple Authority Problem (Federated Identification); The N-Squared Mapping Problem (Shared Identification); The Exploding Addresses Problem (Simplified Identification); The Bootstrap Discovery Problem (Metadata Identification); The Public Identifier Problem (Privacy-Protected Identification); The Future-Proofing Problem (Extensible Identification).
The XRI Syntax document defines the normative ABNF grammar for XRIs, transformations from XRIs into IRIs and URIs, relative XRI references, and normalization/comparison. XRI syntax "extends generic IRI syntax in the following four ways: (1) Persistent and reassignable segments; unlike generic URI syntax, XRI syntax allows the internal components of an XRI reference to be explicitly designated as either persistent or reassignable; (2) Cross-references, which allow XRI references to contain other XRI references or IRIs as syntactically-delimited sub-segments; (3) Global context symbols, or shorthand symbols for establishing the abstract global context of an identifier; (4) Standardized federation, in which federated identifiers are those delegated across multiple authorities, such as DNS names; XRI syntax standardizes federation of both persistent and reassignable identifiers at any level of the path."
The document on Extensible Resource Identifier (XRI) Resolution "defines an HTTP/HTTPS-based resolution protocol for XRIs, including both generic and trusted resolution. Because XRIs may be used across a wide variety of communities and applications (as database keys, filenames, directory keys, object IDs, XML IDs etc.), no single resolution mechanism may prove appropriate for all XRIs. However, in the interest of promoting interoperability, this specification defines a standard framework for XRI resolution consisting of two parts: Generic Resolution defines the basic two-phase resolution protocol, including the structure of XRI Descriptors (XRIDs), the construction of HTTP(S) URIs for XRI authorities, the use of XRI synonyms for redirects, IRI authorities, local access service, HTTP headers, and HTTP caching. Trusted Resolution specifies how generic resolution is extended to create a chain of trust between the participating authorities using SAML assertions. It defines the additional XRID elements needed, the client and server behavior, and the special requirements of authorities offering trusted resolution.
The Extensible Resource Identifier (XRI) Metadata specification "defines five global context symbol (GCS) characters that may be used to begin an XRI authority segment for the purpose of expressing the abstract global context of an identifier. One of these GCS characters, $, is reserved for identifiers for which the authority is a standards specification. A second key XRI feature, cross-references allows XRIs to be embedded within other XRIs in order to share identifiers across contexts. This syntactic feature allows XRIs in the $ namespace to be used as metadata to describe an XRI itself. Such metadata would include language tags, date/time stamps, version identifiers, and annotations."
XRI Language Metadata ($l) "permits expressing the human language in which an XRI, or an XRI sub-segment, is intended to be understood, interpreted, or pronounced. Date/time Metadata ($d) provides a standard means of representing the date and time that an XRI was assigned to a resource. Version Metadata ($v) defines a standard means of representing the version of an XRI or XRI sub-segment. Annotation Metadata ($-) enables XRI producers to provide human-readable annotations in an XRI, or an XRI sub-segment, without affecting resolution or comparison."
An Introduction to XRIs. OASIS Working Draft Version 04. 14-March-2005. 25 pages. Document identifier: 'xri-intro-V2.0-wd-04'. Edited by Drummond Reed (Cordance) and Dave McAlpin (Epok). Contributors: Fen Labalme (PlaNetwork), Mike Lindelsee (Visa International), and Gabe Wachob (Visa International).
Abstract: "This document is a non-normative introduction to the uses and features of XRIs. It is intended to accompany the XRI 2.0 suite of specifications including Extensible Resource Identifier (XRI) Syntax 2.0, Extensible Resource Identifier (XRI) Resolution 2.0, and Extensible Resource Identifier (XRI) Metadata 2.0."
Extensible Resource Identifier (XRI) Syntax V2.0. OASIS Committee Draft Version 01. 14-March-2005. Document identifier: 'xri-syntax-V2.0-cd-01'. 34 pages. Edited by Drummond Reed (Cordance) and Dave McAlpin (Epok). Contributors: Peter Davis (Neustar), Nat Sakimura (NRI), Mike Lindelsee (Visa International), and Gabe Wachob (Visa International).
Abstract: "This document is the normative technical specification for XRI generic syntax. For a nonnormative introduction to the uses and features of XRIs, see Introduction to XRIs. For the HTTP-based XRI resolution protocol, see Extensible Resource Identifier (XRI) Resolution V2.0. For the set of XRIs defined to provide metadata about other XRIs, see Extensible Resource Identifier (XRI) Metadata V2.0.
Extensible Resource Identifier (XRI) Resolution V2.0. OASIS Committee Draft Version 01. 14-March-2005. Document identifier: 'xri-resolution-V2.0-cd-01'. 54 pages. Edited by Gabe Wachob (Visa International). Contributors: Drummond Reed (Cordance), Dave McAlpin (Epok), Chetan Sabnis (Epok), Peter Davis (Neustar), and Mike Lindelsee (Visa International).
Abstract: "This document defines both a standard and a trusted HTTP-based resolution mechanism for Extensible Resource Identifiers (XRIs), specifically XRIs conforming to Extensible Resource Identifier (XRI) Syntax V2.0 or higher. For a non-normative introduction to the uses and features of XRIs, see the Introduction to XRIs. For the set of XRIs defined to provide metadata about other XRIs, see the Extensible Resource Identifier (XRI) Metadata V2.0.
Extensible Resource Identifier (XRI) Metadata V2.0. OASIS Committee Draft Version 01. 14-March-2005. Document identifier: 'xri-metadata-V2.0-cd-01.' 18 pages. Edited by Drummond Reed (Cordance). Contributors: Dave McAlpin (Epok), Nat Sakimura (NRI), Mike Lindelsee (Visa International), and Gabe Wachob (Visa International).
Abstract: "This document is the normative technical specification for XRI metadata. It is a companion specification to Extensible Resource Identifier (XRI) Syntax V2.0 and Extensible Resource Identifier (XRI) Resolution V2.0. For a nonnormative introduction to the uses and features of XRIs, see the Introduction to XRIs.
Increasingly, there is a demand for distributed directory services that enable the identification of any type of resource, both those directly on the network and those abstract from it, and the sharing of data across domains, enterprises, and applications.
Meeting this need requires an extensible, location-, application-, and transport-independent identification scheme that provides addressability not just of resources, but also of their attributes and versions. The scheme should support both persistent and reassignable identifiers that can be optimized for either human usage or machine efficiency. It should ideally impose no limits on the underlying directory, data, or delegation models and thus be interoperable across the greatest possible number of systems and domains.
The purpose of this committee is to define a URI-compatible identifier scheme and resolution protocol that meets these requirements. The XRI scheme will be a superset of the URI scheme defined by RFC 2396 and RFC 2396bis. For resources that need to be persistently identified and linked, the XRI scheme will also incorporate syntax meeting the functional requirements for URNs (Uniform Resource Names) as described in RFC 1737. Lastly, the XRI scheme will be fully internationalized following the recommendations of the W3C work on IRIs (Internationalized Resource Identifiers).
This TC's work may be influenced by the general architecture described in XNS and specifically by the XNS Addressing Specification. The XNS specifications published by the XNS Public Trust Organization (XNSORG) have been contributed to the TC for consideration in the committee's work. XNS is licensed under royalty-free (RF) terms...
In no event shall this Technical Committee finalize or approve any technical specification if it believes that the use, distribution, or implementation of such specification would necessarily require the unauthorized infringement of any third party rights known to the Technical Committee, and such third party has not agreed to provide necessary license rights on perpetual, royalty-free, non-discriminatory terms.
The scope of the XRI TC is limited to the definition of the XRI identifier scheme, one or more resolution protocols (in particular, a generic resolution protocol with secure resolution extensions), XRI metadata (special XRI identifiers used for describing other XRI identifiers), and documents supporting implementation and interoperability of XRIs.
A separate OASIS TC (XRI Data Interchange - XDI) is expected to define an XML schema and an associated Web service for exchanging data and metadata identified by XRIs.
Audience: Since the work of this TC is at the level of Web identifiers and resolution protocols, the audience is very broad. XRIs and XRI resolution is intended to used by developers and implementers of Web, Web Services, and Semantic Web technologies both within and across enterprise boundaries. It is particularly relevant to architects of trusted computing, XML namespaces, ontology designers, and other elements of Web services infrastructure..." [adapted from the revised XRI TC Charter]
Through 2005-04-29, feedback had been received mainly on two topics (other than corrections to the ABNF):
Intellectual property issues:
The selection of a new URI scheme: The W3C Technical Architecture Group (TAG) reported that it had reviewed the XRI 2.0 documents and published its comments, (excerpted in part):
Summary: The TAG has reviewed the XRI 2.0 documents and requirements
surrounding the use of XRI. At this time, it is the opinion of the
TAG that the case has not been made for a new URI scheme, rather that
the requirements can be addressed very well using the http URI scheme
and existing implementations of HTTP and DNS.
The TAG has reviewed the XRI 2.0 documents and while we do
understand the case for an abstraction layer, we also believe that
this can be provided with the http scheme and existing HTTP and DNS
The recommendations that we have documented in Architecture of the World
Wide Web, Volume One state that "A specification SHOULD reuse an
existing URI scheme (rather than create a new one) when it provides
the desired properties of identifiers and their relation to
resources." In this case, a properly managed and supported use of
the existing http scheme, based on the excellent analysis in your
documents, does have the desired properties and can provide the same
functionality without the loss of interoperability which would
accompany a new scheme.
An abstraction layer which uses current technologies could be deployed
much more quickly, as creating a new URI scheme would require the
whole web to implement these technologies before they would achieve
Addendum to TAG Review of XRI 2.0, from Henry S. Thompson, May 13, 2005, by and on behalf of the W3C Technical Architecture Group (TAG):
This note is an addendum to our review of the XRI proposal. We offer this partly in reply to an earlier comment from Dave McAlpin, who clarified that although "XRI borrows heavily from generic URI syntax" never-the-less "XRI in its native form can't be a URI scheme".
The TAG feels strongly that this clarification does not undermine our review as written. On the contrary, a key principle of Web Architecture is that resources should be named with URIs, so finding that XRIs are in fact not intended as URIs adds to rather than reduces our concern.
Furthermore, by using a syntax that is so nearly identical to that of URIs, the XRI proposal risks causing confusion. How will casual or even expert users know which such strings are intended for use on the Web and which not? In that respect, this seems the worst of all possible worlds, and we would therefore not only re-iterate our original advice, that the functionality XRIs seek to deliver can best be delivered by using http: URIs, but add the advice that URIs are the core of the Web, and you should use them: Good practice: Identify with URIs — 'To benefit from and increase the value of the World Wide Web, agents should provide URIs as identifiers for resources.'
See also the April 19, 2005 posting and the March 28, 2005 posting.
Two postings from Gabe Wachob summarize issues raised in the feedback:
- "Clarifications". April 26, 2005. - "In order to address misconceptions that have arisen during the formal review process for the XRI specifications, I'd like to make some clarifications based on our (Visa) understanding of the XRI TC and its work..." In the IPR issues, see: (a) the new location for the IPR Agreement between OneName and XNSORG, referenced (but now 404'd) in the TC Charter; (b) the related matter of fees for registering global identifiers ('i-names') per the instructions at the 2idi identity services provider web site
- "Moving forward after comment period". May 03, 2005.
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