Supporting third-party PEPs communicating with XACML policy engine

This note describes a deployment scenario that we are increasingly
encountering.
It highlights some area where we believe XACML 2.0 may need extension.

We describe the scenario using terminology taken from Sec 3.1 of
[xacml-2.0-core].

SCENARIO

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An enterprises utilizes a variety of applications and COTS to access
resources.

Each component includes an embedded PEP for access control. Some
components may include some resource management aspects
and so include some part of a context-handler as part of the PEP
('enhanced PEP").

New components and devices with embedded PEPs are being acquired by the
enterprise over time.

The enterprise has an existing access control policy infrastructure
based on XACML, including one more PDPs.

(1)
What protocol should the enterprise require the PEPs to implement? One
strategy is to use the