2.6 Optional Policy Assertion

Through a customer survey program, Contoso learns that a significant number of their customers prefer to use the Optimized MIME Serialization (as defined in the MTOM specification) for sending and receiving messages. Contoso adds optional support for the Optimized MIME Serialization and expresses this optional behavior in a machine-readable form.

To indicate the use of optimization using the Optimized MIME Serialization, Contoso uses the mtom:OptimizedMimeSerialization policy assertion (see the example below).

<mtom:OptimizedMimeSerialization />

The mtom:OptimizedMimeSerialization element is a policy assertion. (The prefix mtom is used here to denote the Optimized MIME Serialization Policy namespace.) This assertion identifies the use of MIME Multipart/Related serialization for as required for request and response messages. Policy-aware clients can recognize this policy assertion and engage Optimized MIME Serialization for messages. The semantics of this assertion are reflected in messages: they use an optimized wire format (MIME Multipart/Related serialization).

Like Contoso’s optional support for Optimized MIME Serialization, there are behaviors that may be engaged (in contrast to must be engaged) for a Web service interaction. A service provider will not fault if these behaviors are not engaged. Policy assertions can be marked optional to represent behaviors that may be engaged for an interaction. A policy assertion is marked as optional using the wsp:Optional attribute. Optional assertions represent the capabilities of the service provider as opposed to the requirements of the service provider.

In the example below, the Optimized MIME Serialization policy assertion is marked optional. This policy expression allows the use of optimization and requires the use of addressing and one of transport- or message-level security. If a client sends an optimized (MTOM) message, this will be indicated by characteristics associated by using such an optimized message, including a wire format that is a Multipart/Related message and a content-type header of "application/xop+xml" for the outer package. In this case, the response message will also be optimized, also having a Multipart/Related message and content-type header of "application/xop+xml".  Note that when optimized messages are used, the Multipart/Related message can have a single part containing the primary SOAP envelope.

<All>
  <mtom:OptimizedMimeSerialization wsp:Optional="true"/>
  <wsap:UsingAddressing />
  <ExactlyOne>
    <sp:TransportBinding>…</sp:TransportBinding>
    <sp:AsymmetricBinding>…</sp:AsymmetricBinding>
  </ExactlyOne>
</All>

Contoso is able to meet their customer needs by adding optional support for the Optimized MIME Serialization. An optional policy assertion represents a behavior that may be engaged.