An 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.
Contents
- Features
- Composition of an Extensible Resource Identifier
- Resolving an Extensible Resource Identifier
- Proxy resolvers and the HXRI
- Other examples of XRI 20 syntax
- Applications
- Licensing
- References
The XRI 2.0 specifications were rejected by OASIS, a failure attributed to the intervention of the W3C Technical Architecture Group which recommended against using XRIs or taking the XRI specifications forward. 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, but whose limitations the XRI Technical Committee was formed specifically to address.
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).
Features
=, @, +, $, 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. See also: Global context registries.
Composition of an Extensible Resource Identifier
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.
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).
Resolving an Extensible Resource Identifier
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.
Proxy resolvers and the HXRI
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.
Other examples of XRI 2.0 syntax
(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/!12Example 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)Applications
Examples of applications being developed using XRI infrastructure include:
Licensing
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.)
Dr Phillip Hallam-Baker, the VeriSign representative in OASIS argued 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 had 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. Opposition from VeriSign and companies that had connections to Hallam-Baker was instrumental in ensuring the defeat of the proposal to adopt the specifications.