A Policy Assertion identifies a visible domain specific behavior that is a capability or requirement. Typically, the provider of a Web service exposes a Policy Assertion to convey a condition under which the provider offers a Web service. A Policy Assertion is supported by a requestor (or requestors) if and only if the requestor (or requestors) satisfies the corresponding requirement. In most cases: there is more than one participant in the capability or requirement expressed by a Policy Assertion; and the semantics of a Policy Assertion are reflected in messages. For example, say to indicate that messages are protected using mechanisms provided by the underlying transport (say HTTPS), a transport security policy assertion can be used. This assertion is supported by a requestor if and only if the requestor uses transport level security, and the semantics of this assertion are reflected in messages because they are encrypted over the wire.
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