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1 I-Names and I-Numbers

The first problem XRIs are designed to solve is persistent addressing - how to maintain an address that does not need to change no matter how often the contact data for a person or organization changes. XRIs accomplish this by adding a new layer of addressing over the existing IP numbering and DNS naming layers used on the Internet today. In fact, the XRI addressing layer consists of two layers that mirror the two layers below it (DNS domain names and Internet IP numbers). In order of abstraction, these two layers are:

• I-numbers — machine-friendly identifiers (similar to IP addresses) that are registered to a resource (person, organization, application, file, digital object, etc.) and never reassigned. This means an i-number can always be used to address a network representation of the resource as long it remains available somewhere on the network. I-numbers, like IP addresses, are designed to be very efficient for network routers to process and resolve.

• I-names — human-friendly identifiers that (in most cases) resolve to an i-number, but are much easier for people to use (just as a domain name is much easier to use than an IP address). Though typically long-lived, i-names differ from i-numbers in one critical way: they may be transferred or reassigned to another resource by their owner. For example, a company that changes its corporate name could sell its old i-name to another company, while both companies could retain their original i-number.

XRIs are backwards compatible with the DNS and IP addressing systems, so it is possible for domain names and IP addresses to used as i-names (or, in rare cases, as i-numbers). Like DNS names, XRIs can also be delegated, i.e., nested multiple levels deep, just like the directory names on a local computer file system. For example, a company can register a top-level (global) i-name for itself and then assign second- or lower-level (community) i-names to its divisions, employees, etc.

2 Other XRI Features

Besides persistence, XRIs also support three other key features not available in conventional URIs, DNS domain names, or IP addresses:

1. Cross-references. An XRI can contain another XRI (or a URI), to any level of nesting. (This is one of the key features that makes XRIs so attractive for XDI data sharing, see below.)

1. Peer-to-peer addressing — syntax supporting the ability for any two network nodes to assign each other XRIs and perform cross-resolution.

1. Global context symbols — a simple, human-friendly way to indicate the global context of an i-name or i-number using a single symbol (=, @, +, $, or !).

Lastly, XRIs are fully internationalized following the W3C Internationalized Resource Identifier (IRI) specifications.

XRIs form the basis for the XDI trusted data sharing protocol under development by the OASIS XDI Technical Committee.

3 Examples

(Note that none of these show the prefix "xri://", which is optional in XRIs.)

Example i-names:

Example i-numbers:

Example of mixed i-names and i-numbers (XRI allows any combination of the two):