Technically, an i-number is one form of an extensible resource identifier (XRI) — an abstract structured identifier standard developed at the Organization for the Advancement of Structured Information Standards for sharing resources and data across domains and applications.
However, the XRI layer is the first architecture that combines a uniform syntax and resolution protocol for both persistent and reassignable identifiers.
Furthermore, all of these forms of XRI synonyms can be resolved using the same http- or https-based resolution protocol.
The results of XRI resolution are an XML document called an XRDS (Extensible Resource Descriptor Sequence).
XRDS documents are the basis for the Yadis identity service discovery protocol that is now part of OpenID.