[1] http://tinyurl.com/ot5x5#Policy_References

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.

Perhaps adding



  This optional URI attribute specifies the Retrieval Algorithm being used to resolve an external policy expression identified by ./@URI.


Note that this is modeled after the DigestAlgorithm.  You would not provide @Digest without specifying the @DigestAlgorithm used to calculate it.

@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 expression.