The Fifth Corollary of Identifiers

[This is the fifth of seven proposed “Corollaries of Identifiers” to Kim Cameron’s Laws of Identity.]

Kim’s Fifth Law is the one that most directly explains his use of the term “metasystem”:

5. The Law of Pluralism

A universal identity system MUST channel and enable the internetworking of multiple identity technologies run by multiple identity providers.

Again, the corollary from an identifier standpoint falls out nicely:

5a. The Corollary of Identifier Plurality

A universal identifier system MUST channel and enable the internetworking of multiple identifier schemes run by multiple identifier authorities.

This corollary highlights the architectural parallels between TCP/IP as an internetworking protocol and a universal identity metasystem as an “interidentity protocol”. TCP/IP solved the problem of interoperable network packet exchange by providing a way to map local LAN protocols to a common internetworking protocol. The LAN protocols themselves did not need to change; only mappings to the internetworking protocol needed to be added.

The Fifth Corollary postulates that the same solution will be required for the identifiers in a universal identity metasystem. In other words, identifiers designed for local “islands” of identity can’t be expected to provide cross-domain interoperability any more than a LAN protocol could be expected to produce the Internet. Instead, we need a universal identifier metasystem — a “TCP/IP of identifiers”.

A first temptation might be to say: “We already have it – URIs.” It’s hard to argue with the most successful identifiers in history, and URIs have been as integral to the success of the Web as IP addresses were to the Internet. However the Fifth Corollary adds an interesting new requirement when it comes to a universal identity metasystem: the need to “channel and enable the internetworking of multiple identifier schemes operated by multiple identifier authorities”.

While this might sound like what URIs do today, in fact interoperability is limited to a handful of broadly supported URI schemes (chiefly HTTP/HTTPS, but also mailto, ftp, and a few others). In addition, generic URI syntax uses a “single hierarchy” structure, i.e., a URI always represents a single identifier authority. There is no standard way in generic URI syntax to express “cross hierarchy” relationships, a directory concept known as polyarchy. Yet those of us working on the OASIS XRI TC have found polyarchy (which we call cross-references) as essential to the “internetworking of multiple identifier schemes” as TCP/IP packet exchange is to the internetworking of multiple LAN protocols.

For example, the following URIs might represent myself as a personal authority, and my employer, Cordance Corporation, as an organizational authority, respectively:

With URI syntax, each of these authorities may be the root of its own infinitie hierarchy of local resources, e.g.:

But there is no standard HTTP URI syntax for referring across these hierarchies, i.e., for referencing “the resource known by the URI ‘’ in the context of the resource known as ‘’.”

With XRIs, this is easy:


XRIs that use global context symbols (and therefore make the “xri://” prefix optional) can make cross-references even more compact:


While there are other features that (in my humble opinion as XRI TC chair) qualify XRI as the “TCP/IP of identifiers”, cross-references is the feature that speaks most directly to interoperability. To my knowledge, the XRI scheme is the first URI-compatible identifier scheme that permits the use of all valid URIs (and all valid XRIs) within the context of a single resolvable identifier.

If you don’t already have my email address, please send comments via my i-name contact page at =Drummond


About Drummond Reed

Internet entrepreneur in identity, personal data, and trust frameworks
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