Section 4.3.4 states
"...there is no requirement that the IRI be resolvable; retrieval
mechanisms are beyond the scope of this specification."
I would agree that defining various retrieval mechanisms would be out of
scope, but defining a way to identify a retrieval mechanism
could/should be in scope.
This optional URI attribute specifies the Retrieval Algorithm
being used to resolve an external policy expression identified by
Note that this is modeled after the DigestAlgorithm. You would not
provide @Digest without specifying the @DigestAlgorithm used to calculate
@Digest is opaque and you cannot determine the digest algorithm by
looking at its value.
Likewise, we should treat @URI as opaque and provide an identifier for
the algorithm that can be used to resolve the external policy