Public review begins for XRI 2.0

OASIS has opened a fifteen-day public review of the Extensible Resource Identifier (XRI) Resolution version 2.0 Committee Draft. XRI provides a uniform syntax for abstract structured identifiers. The public review ends 26 March 2008.

Because XRIs may be used across a wide variety of communities and applications (as Web addresses, database keys, filenames, object IDs, XML IDs, tags, etc.), no single resolution mechanism may prove appropriate for all XRIs. However, in the interest of promoting interoperability, this specification defines a simple generic resource description format called XRDS (Extensible Resource Descriptor Sequence), a standard protocol for requesting XRDS documents using HTTP(S) URIs, and standard protocol for resolving XRIs using XRDS documents and HTTP(S) URIs.

Both generic and trusted versions of the XRI resolution protocol are defined (the latter using HTTPS (RFC 2818) and/or signed SAML assertions. In addition, an HTTP(S) proxy resolution service is specified both to provide network-based resolution services and for backwards compatibility with existing HTTP(S) infrastructure.

Resolution is the function of dereferencing an identifier to a set of metadata describing the identified resource. For example, in DNS, a domain name is typically resolved using the UDP protocol into a set of resource records describing a host. If the resolver does not have the answer cached, it will start by querying one of the well-known DNS root nameservers for the fully qualified domain name. Since domain names work from right to left, and the root nameservers know only about top level domains, they will return the NS (name server) records for the top-level domain. The resolver will then repeat the same query to those name servers and 'walk down the tree' until the domain name is fully resolved or an error is encountered. A simple non-recursing resolver will rely on a recursing nameserver to do this work. For example, it will send a query for the fully qualified domain name to a local nameserver. If the nameserver doesn't have the answer cached, it will resolve the domain name and return the results back to the resolver (and cache the results for subsequent queries).

XRI resolution follows this same architecture except at a higher level of abstraction, i.e., rather than using UDP to resolve a domain name into a text-based resource descriptor, it uses HTTP(S) to resolve an XRI into an XML-based resource descriptor called an XRDS document.

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