Extensible Resource Identifier (abbreviated XRI) is a scheme and resolution protocol for abstract identifiers compatible with Uniform Resource Identifiers and Internationalized Resource Identifiers, developed by the XRI Technical Committee at OASIS. The goal of XRI is a standard syntax and discovery format for abstract, structured identifiers that are domain-, location-, application-, and transport-independent, so they can be shared across any number of domains, directories, and interaction protocols.
The XRI 2.0 specifications were rejected by OASIS,[1] a failure attributed[2] to the intervention of the W3C Technical Architecture Group which recommended against using XRIs or taking the XRI specifications forward.[3] The core of the dispute is whether the widely interoperable HTTP URIs are capable of fulfilling the role of abstract, structured identifiers, as the TAG believes,[4] but whose limitations the XRI Technical Committee was formed specifically to address.[5]
The designers of XRI believed that, due to the growth of XML, Web services, and other ways of adapting the Web to automated, machine-to-machine communications, it was increasingly important to be able to identify a resource independent of any specific physical network path, location, or protocol in order to:
This work led, by early 2003, to the publication of a protocol based on HTTP(S) and simple XML documents called XRDS (Extensible Resource Descriptor Sequence).
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=, @, +, $, or !) that provide a simple, human-friendly way to indicate the global context of an i-name or i-number. These are not required, but may be used within communities of interest that agree on their meaning and how they are resolved.An XRI starting with "=" is thought of identifying a person. An XRI starting with "@" identifies a company or organization. A starting "+" indicates a generic concept, subject or topic.[6]
A "*" marks a delegation. For example with "=family*name", "=family" delegates the resolving of its sub-XRI "name" to another resolver. This is analogous to DNS' delegating the subdomain resolution to other nameservers (name.family.de: after resolving de, the nameserver responsible for de delegates to the family nameserver, which delegates to the name nameserver).
XRIs are resolved to XRDS documents using the HTTP(S) protocol in the same way as URLs are resolved to Resource records using the DNS protocol. This lookup process can be configured by passing parameters.[7]
An XRI can be transformed into a URI by adding "http://xri.net/" at the beginning and appending the XRI. Internally, the URI now refers to a proxy resolver, which resolves a URI of this kind to an XRDS document. The proxy resolver found under http://xri.net for example can be used to resolve an XRI. So =example becomes http://xri.net/=example. The second form is called an HTTP XRI or HXRI for short. The owner of the XRI =example can tell the proxy resolver what to do, if the HXRI is called. One possible reaction is to do a 302 HTTP redirect to a stored URI.
Further parameters to specify the resolution can be appended to the HXRI, e.g. to get the whole XRDS document or to get service descriptions for this XRI. E.g. if you attach ?_xrd_r=application/xrds+xml to the HXRI, the whole XRDS document is returned. So http://xri.net/=example?_xrd_r=application/xrds+xml returns the whole XRDS for the XRI =example.
Say a library system uses URNs in the ISBN namespace to identify books and DNS subdomains to identify its library branches. HTTP URI syntax does not provide a standard way to express the URN for the book title in the context of the DNS name for the library branch. XRI cross-reference syntax solves this problem by allowing the library (and even automated programs running at the library) to programmatically construct the XRIs necessary to address any book at any branch. Examples:
xri://broadview.library.example.com/(urn:isbn:0-395-36341-1) xri://shoreline.library.example.com/(urn:isbn:0-395-36341-1) xri://northgate.library.example.com/(urn:isbn:0-395-36341-1)
This ability to create structured, self-describing identifiers can be extended to many other uses. For example, say the library wanted to indicate the type of each book available. By establishing a simple XRI dictionary of book types, it can now programmatically construct XRIs that include this metadata,
xri://broadview.library.example.com/(urn:isbn:0-395-36341-1)/(+hardcover) xri://broadview.library.example.com/(urn:isbn:0-395-36341-1)/(+softcover) xri://broadview.library.example.com/(urn:isbn:0-395-36341-1)/(+reference)
(Note that none of these show the prefix "xri://", which is optional in XRIs when they are not in URI normal form, i.e., they have not undergone the specified transformation between XRI format and URI format.)
Example XRIs composed entirely of reassignable segments:
=Mary.Jones @Jones.and.Company +phone.number +phone.number/(+area.code) =Mary.Jones/(+phone.number) @Jones.and.Company/(+phone.number) @Jones.and.Company/((+phone.number)/(+area.code))
Example XRIs composed entirely of persistent segments:
=!13cf.4da5.9371.a7c5 @!280d.3822.17bf.ca48!78d2/!12
Example of XRIs with mixes of persistent and reassignable segments (XRI allows any combination of the two):
=!13cf.4da5.9371.a7c5/(+phone.number) @Jones.and.Company!78d2/!12/(+area.code)
Examples of applications being developed using XRI infrastructure include:
The XRI Technical Committee is chartered under the RF on Limited Terms Mode of the OASIS IPR policy (See http://www.oasis-open.org/committees/xri/ipr.php for more details.)
Some people argue that the use of the technologies employed in XRI are subject to patent claims, that the licensing rights to these patents has been vested in XDI.org, a non-profit organization which has in turn licensed a non-exclusive interest in the use of the patents to companies associated with the original patent holders, despite the above IPR statement.
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