Web Services Policy 1.5 - Guidelines for Policy Assertion Authors

4.1.1 Assertion Authors

Assertion Authors are part of a community that chooses to exploit the WS-Policy Framework by creating their own specifications to define a set of assertions that express the capabilities and constraints of that target domain. The WS-Policy Framework is based on a declarative model, meaning that it is incumbent on the Assertion Authors to define both the semantics of the assertions as well as the scope of their target domain in their specification. The set of metadata for any particular domain will vary in the granularity of assertion specification required. It is the intent of this document to help communities utilize the framework in such a way that multiple WS-Policy domains can co-exist and consumers and providers can utilize the framework consistently across domains.

When using the WS-Policy Framework, any Assertion Authors defining new WS-Policy assertions must adhere to the MUST's and SHOULD's in the specification and should review the conformance section of the specification.

Assertion Authors should also specify a policy subject. For instance, if a policy assertion were to be used with WSDL, an assertion description should specify a WSDL policy subject.

An example of a domain specification that follows these practices is the WS-SecurityPolicy specification [WS-SecurityPolicy]. The WS-SecurityPolicy authors have defined the scope of their target domain (security) as follows:

"This document [WS-SecurityPolicy] defines a set of security policy assertions for use with the WS-Policy framework with respect to security features provided in WSS: SOAP Message Security, WS-Trust and WS-SecureConversation. This document takes the approach of defining a base set of assertions that describe how messages are to be secured. Flexibility with respect to token types, cryptographic algorithms and mechanisms used, including using transport level security is part of the design and allows for evolution over time. The intent is to provide enough information for compatibility and interoperability to be determined by web service participants along with all information necessary to actually enable a participant to engage in a secure exchange of messages."

An example of scoping individual assertions to policy subjects is also provided by the WS-Security Policy specification in Appendix A.

4.1.2 Consumers

A consumer of WS-Policy Assertions can be any entity that is capable of parsing a WS-Policy XML expression and selecting one alternative from the policy. This selected alternative is then used to govern the creation of a message to send to the subject to which the policy alternative was attached. The WS-Policy Attachment specification defines a set of attachment mechanisms for use with common web service subjects: WSDL definitions [WSDL 1.1, WSDL 2.0 Core Language], and UDDI directory entries [UDDI API 2.0, UDDI Data Structure 2.0, UDDI 3.0].

In the degenerate case, a human could read the XML and determine if a message could be constructed conformant to the advertised policy.

It is expected that consumers of WS-Policy will include a wide range of client configurations, from stand alone client applications to "active" web service requesters that are capable of adapting to the constraints and capabilities expressed in a WS-Policy document and modifying their own configurations dynamically.

4.1.3 Providers

A provider who expresses capabilities and requirements of a Web service as policies can be any web service implementation that can specify its on-the-wire message behavior as a policy expression that conforms to the Web Services Policy 1.5 - Framework [Web Services Policy Framework] and Web Services Policy 1.5 - Attachment [Web Services Policy Attachment] specifications. The Web Services Policy 1.5 - Attachment specification has defined a set of subjects and an extensible mechanism for attaching policies to web services subjects.

When deploying services with policies it is useful for providers to anticipate how to evolve their services capabilities over time. If forward compatibility is a concern in order to accommodate compatibility with different and potentially new clients, providers should refer to 6. Versioning Policy Assertions and Web Services Policy Primer that describes service and policy assertion evolution.

5.3.1 Minimal approach

New Assertion Authors are encouraged to try to not overload assertions. A single assertion indicates a single behavior. Sets of assertions can by grouped by an operator "All". This indicates that there is a relationship between the assertions.

If grouping is utilized, choices between such groupings can be indicated by an "ExactlyOne" one" operator. This basic set of operators allows Assertion Authors a wide range of options for expressing the possible combinations of assertions within their domain.

It requires a good deal of effort to evaluate the capabilities of a domain and capture them in a way that reflects the options of the domain if the domain has a lot of assertions to define. Interoperability testing of new policy domains is recommended to ensure that consumers and providers are able to use the new domain assertions. To facilitate proper progression of an assertion, Assertion Authors should start with a simple working assertion that allows extensibility. As the design work progresses, one may add more parameters or nested policy assertions to meet one's interoperability needs.

New Assertion Authors are encouraged to look at Web Services Reliable Messaging Policy Assertion to see an example of a relatively simple domain that has defined three assertions. Assertion Authors are encouraged to look at WS-SecurityPolicy to see an example of a complex domain that has been decomposed into a set of policy expressions.

5.3.2 QName and XML Information Set representation

Web Services Policy language allows Assertion Authors to invent their own XML dialects to represent policy assertions. The policy language relies only on the policy assertion XML element QName. This QName is unique and identifies the behavior represented by a policy assertion. Assertion Authors have the option to represent an assertion parameter as a child element (by leveraging natural XML nesting) or an attribute of an assertion. The general guidelines on when to use XML elements versus attributes apply. Use a unique QName to identify a distinct behavior.

An example of a specification that provides an XML Outline is the Web Services Reliable Messaging Policy document [Web Services Reliable Messaging Policy Assertion]. The definition of the outline syntax used in that specification is found in its Terminology section (1.1). As an example of the outline syntax in use, the following outline has been copied from the aforementioned specification.

 <wsrmp:RMAssertion [wsp:Optional="true"]? ...> 
   <wsp:Policy >
     [ <wsrmp:SequenceSTR/> |
       <wsrmp:SequenceTransportSecurity/> ] ?
     <wsrmp:DeliveryAssurance/> 
     <wsp:Policy >
        [ <wsrmp:ExactlyOnce/> | 
          <wsrmp:AtLeastOnce/> |
          <wsrmp:AtMostOnce/> ]
        <wsrmp:InOrder/> ?
     </wsp:Policy>
     </wsrmp:DeliveryAssurance> ] ?
    </wsp:Policy>
 </wsrmp:RMAssertion/>
 

The syntax of an assertion can be represented using an XML outline (plus an XML schema document). If the assertion has a nested policy expression then the assertion XML outline can enumerate the nested assertions that are allowed. An example is the following:

 <sp:IssuedToken sp:IncludeToken="xs:anyURI"? ... >
  <sp:Issuer> wsa:EndpointReferenceType</sp:Issuer>?
  <sp:RequestSecurityTokenTemplate TrustVersion="xs:anyURI"? >
    ...
  </sp:RequestSecurityTokenTemplate >
  <wsp:Policy >
    <sp:RequireDerivedKeys /> ?
    <sp:RequireExternalReference /> ?
    <sp:RequireInternalReference /> ?
    ...
  </wsp:Policy> ?
  ...
</sp:IssuedToken>
 
 

To give an example, the WS-ReliableMessaging Policy document specifies attribute extensibility as part of the XML definition, allowing the wsp:Ignorable attribute:

/wsrmp:RMAssertion/@{any}
This is an extensibility mechanism to allow different {extensible} types of information, based on a schema, to be passed.
				

The Policy Framework provides two modes of authoring policy expressions: compact and normal form. One of the mechanisms that the Policy Framework provides to policy authors for purposes of writing compact policy expressions is the wsp:Optional attribute. Assertion Authors should allow for the use of the wsp:Optional attribute in the XML outline and/or schema definition of an assertion as this will allow policy expression authors to compose compact policy expressions.

For example, consider the following two equivalent policy expressions:

<wsp:Policy>
  <wsp:ExactlyOne>
    <wsp:All>
    <wsam:Addressing>
        <wsp:Policy/>
    </wsam:Addressing>
    </wsp:All>
    <wsp:All>
    </wsp:All>
  </wsp:ExactlyOne>
</wsp:Policy>

<wsp:Policy>
    <wsam:Addressing wsp:Optional="true">
        <wsp:Policy/>
    </wsam:Addressing>
</wsp:Policy>

If the assertion author had not provided for the wsp:Optional attribute to be included on the assertion, then policy expression authors would be forced to express the optionality of a behavior as two explicit policy alternatives, one with and one without that assertion when including assertions of that type in their policies.

5.3.3 Self Describing Messages

WS-Policy is intended to communicate the requirements, capabilities and behaviors of nodes that provide the message's path, not specifically to declare properties of the message semantics. One of the advantages of Web services is that an XML message can be stored and later examined (e.g. as a record of a business transaction) or interpreted by an intermediary; however, if information that is necessary to understand a message is not available, these capabilities suffer.

Policy assertions should not be used to express the semantics of a message. Rather, if a property is required to understand a message, it should be communicated in the message, or be made available by some other means (e.g., being referenced by a URI in the message) instead of being communicated as a policy element. Note that there are other specifications that target specification of semantics of a message, such as SAWSDL.

If the messages could not be made self describing by utilizing additional properties present in the message as required by the assertion, it would be necessary to determine the behaviors engaged at runtime by additional means. A general protocol that aids in determining such behaviors may be utilized, however a standard protocol for this purpose is currently not available to ensure interoperability. Thus, a private protocol should be used with care.

Another approach is to use of the assertion to selectively apply to subjects. For example, a dedicated endpoint may be allocated to ensure the engagement of a behavior that is expressed by a policy assertion. This approach can be considered when messages cannot be self describing.

Policy assertions should not be used to express the semantics of a message. Firstly, an assertion type indicates a runtime behavior. Secondly, Assertion Authors need to indicate how the runtime behavior represented in the assertion type can be inferred or indicated from a message at runtime. If there is a need for the behavior to be represented in a persistent way or if there is a need for additional data or metadata that is present in a message to be persisted, it should be incorporated into the assertion design or in the message itself. In essence, the Assertion Authors should consider how to make messages self describing when utilizing their assertions by specifying additional properties, headers, etc. that must be present in a message as part of their assertion design.

For example, if the details of a message's encryption ( e.g., the cipher used, etc) are expressed in policy that isn't attached to the message, it isn't possible to later decipher it. This is very different from expressing, in policy, what ciphers (and so forth) are supported by a particular endpoint, or those that are required in a particular message; the latter are the intended uses of the WS-Policy framework.

5.3.4 Single Domains

When considering the creation of a new domain of policy assertions, it is important to identify whether or not the domain is self-contained or at least if a subset of the domain can be well defined. A domain that expresses a broad set of capabilities will also need to have a community supporting implementations of these capabilities to provide value to the consumers. Ultimately it is the consumers and providers that will determine whether a particular set of assertions correctly characterize a domain. A new community should avoid duplicating assertions that have already been defined as this will create ambiguity not clarification. New Assertion Authors should focus on creating assertions for those specific constraints and capabilities that do not overlap with other domains but that communicate new functionality.

The model advocated for new assertion development is a cooperative marketplace [some might say it is an "opt-in" model]. The providers of services need to find value in the set of assertions or they will not include the assertions in their service descriptions.

It is the responsibility of the Assertion Authors to avoid duplication of assertions. A review by a broad community is the best way to ensure that the granularity of a set of domain assertions is appropriate.

5.4.1 Assertions with Parameters

Policy assertion parameters are the opaque payload of an assertion. Parameters carry additional useful information for engaging the behavior described by an assertion and are preserved through policy processing such as normalization, merge and policy intersection. Requesters may use policy intersection to select a compatible policy alternative for an interaction. Assertion parameters do not affect the outcome of policy intersection unless the assertion specifies domain specific processing for policy intersection.

In the XML representation of a policy assertion, the child elements and attributes of the assertion excluding child elements and attributes from the policy language namespace name are the assertion parameters.

In the example below, sp:Body and sp:Header elements are the two assertion parameters of the sp:SignedParts policy assertion (this assertion requires the parts of a message to be protected). These two parameters identify the parts of a wire message that should be protected. These parameters carry additional useful information for engaging the behavior.

<wsp:Policy>
  <sp:SignedParts>
    <sp:Body/>
    <sp:Header/>
  </sp:SignedParts>
</wsp:Policy>

5.4.2 Nested Assertions

The framework provides the ability to "nest" policy assertions. For domains with a complex set of options, nesting provides one way to indicate dependent elements within a behavior. In particular, when assertion authors define an assertion type that allows nested policy expression, it is important to also define the semantics of that assertion when it contains an empty nested policy expression (for example: <wsam:Addressing><wsp:Policy/></wsam:Addressing>).

The following design questions below can help to determine when to use nested policy expressions:

• Are these assertions designed for the same policy subject?

• Do these assertions represent dependent behaviors?

If the answers are yes to both of these questions then leveraging nested policy expressions is something to consider. Keep in mind that a nested policy expression participates in the policy intersection algorithm. If a requester uses policy intersection to select a compatible policy alternative then the assertions in a nested policy expression play a first class role in the outcome. If there is a nested policy expression, an assertion description should declare it and enumerate the nested policy assertions that are allowed. There is one caveat to watch out for: policy assertions with deeply nested policy can greatly increase the complexity of a policy and should be avoided when they are not needed.

Assertion Authors should recognize that the framework can yield multiple assertions of the same type. The QName of the assertion is the only vehicle for the framework to match a specific assertion, NOT the contents of the element. If the assertion is a parameterized assertion the authors must understand that this type of assertion will require additional processing by consumers in order to disambiguate the assertions or to understand the semantics of the name value pairs, complex content, attribute values contribution to the processing. The tradeoff is the generality vs. the flexibility and complexity of the comparison expected for a domain.

If the assertion authors want to delegate the processing to the framework, utilizing nesting should be considered. Otherwise, domain specific comparison algorithms may need to be devised and be delegated to the specific domain handlers that are not visible to the WS-Policy framework. However, domain specific intersection processing reduces interop and increases the burden to implement an assertion.

We will use the WS-SecurityPolicy to illustrate the use of nested assertions.

Securing messages is a complex usage scenario. The WS-SecurityPolicy Assertion Authors have defined the sp:TransportBinding policy assertion to indicate the use of transport-level security for protecting messages. Just indicating the use of transport-level security for protecting messages is not sufficient. To successfully interact with a Web service, the consumer must know not only that transport-level security is required, but also the transport token to use, the secure transport to use, the algorithm suite to use for performing cryptographic operations, etc. The sp:TransportBinding policy assertion can represent these dependent behaviors.

A policy assertion like the sp:TransportBinding identifies a visible behavior that is a requirement. A nested policy expression can be used to enumerate the dependent behaviors on the Transport binding. A nested policy expression is a policy expression that is a child element of another policy assertion element. A nested policy expression further qualifies the behavior of its parent policy assertion.

In the example below, the child Policy element is a nested policy expression and further qualifies the behavior of the sp:TransportBinding policy assertion. The sp:TransportToken is a nested policy assertion of the sp:TransportBinding policy assertion. The sp:TransportToken assertion requires the use of a specific transport token and further qualifies the behavior of the sp:TransportBinding policy assertion (which already requires the use of transport-level security for protecting messages).

<sp:TransportBinding>
  <Policy>
    <sp:TransportToken>
      <Policy>
        <sp:HttpsToken>
          <wsp:Policy/>
        </sp:HttpsToken>
      </Policy>
    </sp:TransportToken>
    <sp:AlgorithmSuite>
      <Policy>
        <sp:Basic256Rsa15/>
      </Policy>
    </sp:AlgorithmSuite>
  </Policy>
</sp:TransportBinding>

The sp:AlgorithmSuite is a nested policy assertion of the sp:TransportBinding policy assertion. The sp:AlgorithmSuite assertion requires the use of the algorithm suite identified by its nested policy assertion (sp:Basic256Rsa15 in the example above) and further qualifies the behavior of the sp:TransportBinding policy assertion.

Setting aside the details of using transport-level security, a policy-aware client that recognizes this policy assertion can engage transport-level security and its dependent behaviors automatically. This means the complexity of security usage is absorbed by a policy-aware client and hidden from Web service application developers.

Assertion Authors should note the effect of nested policy expressions on policy intersection in their nested policy design. The result of intersecting an assertion that contains an empty nested policy expression with an assertion of the same type without a nested policy expression is that the assertions are not compatible. Therefore, when providers require dependent behaviors these behaviors should be explicitly specified as assertions in a nested policy expression. When the definition of an assertion allows for nested dependent behaviors, but the use of the assertion only contains an empty nested policy expression, this specific use indicates the specification of no nested dependent behaviors. This use must not be interpreted as being compatible with "any" of the nested dependent behaviors that are allowed by the assertion, unless otherwise specified by the assertion definition.

As an example, WS-Security Policy defines sp:HttpToken assertion to contain three possible nested elements, sp:HttpBasicAuthentication, sp:HttpDigestAuthentication and sp:RequireClientCertificate. When the HttpToken is used with an empty nested policy in a policy expression by a provider, it will indicate that none of the dependent behaviors namely authentication or client certificate is required.

<sp:TransportToken> 
  <wsp:Policy> 
	<sp:HttpsToken> 
	  <wsp:Policy/> 
	</sp:HttpsToken> 
  </wsp:Policy> 
</sp:TransportToken>

A non-anonymous client who requires authentication or client certificate will not be able to use this provider solely on the basis of intersection algorithm alone.

5.5.1 Ignorable behavior in authoring

The Policy Framework provides an intersection algorithm that has two defined modes for processing (lax and strict). The Framework also defines an attribute (wsp:Ignorable) that can be used to influence whether assertions are part of the compatibility assessment between two alternatives. [see Web Services Policy Framework and Web Services Policy Primer]. Assertion authors should consider whether the behavior represented by the Assertion they are defining can be safely ignored for the purposes of intersection, and should follow 9. Document Ignorable Behavior and 10. Document Use of the Ignorable Attribute in XML to include this guidance in the assertion's definition.

5.5.2 Ignorable behavior at runtime

Regardless of whether the assertion allows the ignorable attribute, assertion authors should indicate the semantic of the runtime behavior in the description of the assertion.

As said in section 3.4.1 Strict and Lax Policy Intersection in Web Services Policy Primer, "Regardless of the chosen intersection mode, ignorable assertions do not express any wire-level requirements on the behavior of consumers - in other words, a consumer could choose to ignore any such assertions that end up in the resulting policy after intersection, with no adverse effects on runtime interactions." Therefore, any assertion that is marked with ignorable should not impose any wire-level requirements on the part of consumers. Assertion Authors are reminded that regardless of whether an assertion is marked as ignorable, policy consumers using strict intersection will not 'ignore' the assertion.

5.6.1 Optional behavior at runtime

Since optional behaviors indicate optionality for both the provider and the consumer, behaviors that must always be engaged by a consumer must not be marked as "optional" with a value "true" since this would allow the consumer to select the policy alternative that does not contain the assertion, and thus not engaging the behavior.

The target scope of an optional assertion is an important factor for Assertion Authors to consider as it determines the granularity where the behavior is optionally engaged. For example, if the assertion is targeted for an endpoint policy subject, it is expected to govern all the messages that are indicated by the specific endpoint when optional behavior is engaged . Since the behavior would be applicable to policy subject that is designated, it is important for the Assertion Authors to choose the appropriate level of granularity for optional behaviors, to consider whether a specific message or all messages, etc. are targeted.

When optional behaviors are indicated by attaching assertions with only one side of an interaction, such as an inbound message of a request-response, the engagement of the rest of the interaction will be undefined. Therefore, the Assertion Authors are encouraged to consider how the attachment on a message policy subject on a response message should be treated when optional behaviors are specified for message exchanges within a request response for response messages, using message policy subject. Leaving the semantics not specified or incompletely specified may result in providers making assumptions. Similarly, if engagement of a behavior is only specified for an outbound message, the Assertion Authors should consider describing the semantics if the incoming messages also utilized the behavior. This is especially important if the assertion is applicable to more than one specific policy subject. One approach that is currently taken by WS-RM Policy [Web Services Reliable Messaging Policy Assertion] is to introduce both message and endpoint policy subjects for one of its assertions and require the use of endpoint policy subject when message policy subject is used via attachment.

Behaviors must be engaged with respect to messages that are targeted to the provider so that the provider can determine that the optional behavior is engaged. In other words, the need for self describing messages [5.3.3 Self Describing Messages ] should not be forgotten. An explicit, out of band mechanism might be necessary to enable a client to indicate that the optional behavior is engaged. (Such an out of band mechanism is outside the scope of WS-Policy Framework).

The Web Services Policy Primer document contains an example that outlines the use of MTOM as an optional behavior that can be engaged by a consumer. Related to this behavior is an assertion that identifies the use of MIME Multipart/Related serialization [MTOMPolicy]. Policy-aware clients that recognize and engage this policy assertion will use Optimized MIME Serialization for messages.

The semantics of the MTOM assertion declare that the behavior must be reflected in messages by requiring that they use an obvious wire format (MIME Multipart/Related serialization). Thus, this optional behavior is self describing. For example, an inbound message to a web service that requires MTOM must adhere to Optimized MIME Serialization. By examining the message, the provider can determine whether the policy alternate that contains the MTOM assertion is being obeyed ( Best Practice: Indicate use of an Optional Assertion).

Note that if a MTOM assertion were only bound to an inbound message endpoint, then it would not be clear whether the outbound message from the provider would also utilize the behavior. Thus this assertion should be associated at the granularity of an entire message exchange. The semantics of the assertion should specify this to avoid inappropriate assumptions by implementations. This is important for an optional assertion where it may not be clear whether it is to apply in a message exchange when optionally used in part of that exchange (Best Practice: Consider entire message exchange pattern when specifying Assertions that may be optional).

5.7.1 General Guidelines

The Policy attachment mechanism used to communicate the policy assertions should not affect or imply additional semantics in the interpretation of Policy alternatives. If it did, each policy assertion would need to be written with different (and possibly unknown) attachment mechanisms in mind. Since multiple attachment mechanisms may be used, a policy alternative created during the process of calculating an effective policy can contain multiple instances of the same policy assertion type ( i.e., the SignedParts assertion). It is therefore also important for the policy authors to define what it means if multiple assertions are present.

Assertion authors should review sections 3.2 and 4.5 of the Policy Framework Web Services Policy Framework for more detail on the processing for multiple assertions of the same type.

An example of this practice is the Reliable Messaging Policy Assertion document [Web Services Reliable Messaging Policy Assertion]. In the Sequence STR Assertion (section 2.5.1) the Reliable Messaging Policy Assertion authors state that "The STR assertion defines the requirement that an RM Sequence MUST be bound to an explicit token that is referenced from a wsse:SecurityTokenReference in the CreateSequence message. This assertion MUST apply to [Endpoint Policy Subject]. This assertion MUST NOT be used for an endpoint that does not also use the RM assertion". This is illustrative of how the domain assertion author can specify additional constraints and assumptions for attachment and engagement of behavior in addition to the capabilities specified in Web Services Policy 1.5 - AttachmentWS-PolicyAttachment [Web Services Policy Attachment]. Such additional constraints must be clearly specified by the assertion authors.

5.7.2 Considerations for Policy Attachment in WSDL

A behavior identified by a policy assertion applies to the associated policy subject. If a policy assertion is to be used within WSDL, Assertion Authors should specify a WSDL policy subject.

The specific WSDL policy subject is determined with respect to a behavior as follows:

• If the behavior applies to any message exchange using any of the endpoints offered by a service then the subject is the service policy subject.

• If the behavior applies to any message exchange made using an endpoint then the subject is the endpoint policy subject.

• If the behavior applies to any message exchange defined by an operation then the subject is the operation policy subject.

• If the behavior applies to an input message then the subject is the message policy subject - similarly for output and fault message WSDL policy subjects.

Assertion Authors that wish to utilize WSDL policy subjects need to understand how the assertions will be processed in an intersection and merging, and the specific implications of the processing for considering a specific attachment point and policy subject. This topic is considered in detail in Web Services Policy Primer

For a given WSDL policy subject, there may be several attachment points. For example, there are three attachment points for the endpoint policy subject: the port, binding and portType element. Assertion Authors should identify the relevant attachment point when defining a new assertion.

In using WSDL attachment, it should be noted that the service policy subject is a collection of endpoint policy subjects. The endpoint policy subject is a collection of operation WSDL policy subjects and so on. As a result, the WSDL policy subjects compose naturally. It is quite tempting to associate the identified behavior to a broader policy subject than to a fine granular policy subject. For instance, it is convenient to attach a supporting token assertion (defined by the Web Services Security Policy specification) to an endpoint policy subject instead of a message policy subject. However such policy attachments to WSDL policy subjects of broader scope and granularity should be done only after careful evaluation.

For authoring convenience, Assertion Authors may allow the association of an assertion to multiple WSDL policy subjects within the same context of use (e.g in the same WSDL description). If an assertion is allowed to be associated with multiple WSDL policy subjects as is possible with WSDL, then the Assertion Authors have the burden to describe the rules when multiple instances of the same assertion are attached to different WSDL policy subjects in order to avoid non-interoperable behavior.

To give one example, section 2.3 of the Web Services Reliable Messaging Policy Assertion specification [Web Services Reliable Messaging Policy Assertion] gives rules on which WSDL policy subjectsSubjects may be associated with the RM Policy assertion, and which WSDL 1.1 elements may have RM Policy assertions attached.

If the capability may imply different semantics with respect to attachment points, the Assertion Authors should consider the following:

• Decompose the semantics with several assertions.

• Rewrite a single assertion targeting a specific subject.

Since many attachment points are available in WSDL, it would be necessary for Assertion Authors to recommend a preferred attachment point. One approach would be to identify different attachment points in a policy subject, choose the most granular policy subject that the behavior applies to and specify that as a preferred attachment point. However, this approach only works if the policy subject is a true WSDL construct other than some other protocol concept that is layered over WSDL message exchanges. For example, as described previously the WS-RM Policy is a capability that governs a target endpoint's capability to accept message sequences that are beyond single message exchange. Therefore, its semantics encompass the cases when message level WSDL policy subjects may be used as attachment but also considers the case when sequences are present. In addition, when the policy assertions do not target wire-level behaviors but rather abstract requirements, this technique does not apply.

Assertion Authors that utilize WSDL policy subjects need to understand how the assertions will be processed in merging and the specific implications of a result where multiple assertions of the assertion typesame are in an alternative, in the merged policy. For example, consider the SignedParts assertion defined in WS-SecurityPolicy 1.2. The definition of SignedParts assertion explicitly permits multiple SignedParts assertions to be present within a policy alternative, and declares it to be equivalent to a single SignedParts assertion containing the union of all specified message parts. So, if a SignedParts assertion is specified in a WSDL binding at the input message level and subsequently an additional SignedParts assertion is specified at the WSDL endpoint policy subject level, then the effective policy at the endpoint could have more than one SignedParts assertion in the same alternative. However, the clear semantics defined by the SignedParts assertion enable processing of the multiple occurrences properly.