1.1 Example

Example 1-1 illustrates a security policy expression using assertions defined in WS-SecurityPolicy [ WS-SecurityPolicy ]:

                              
(01) <wsp:Policy
        xmlns:sp="http://schemas.xmlsoap.org/ws/2005/07/securitypolicy"
        xmlns:wsp="http://www.w3.org/ns/ws-policy" >
(02)   <wsp:ExactlyOne>
(03)     <wsp:All>
(04)        <sp:SignedParts/>
(05)           <sp:Body/>
(06)        </sp:SignedParts/>
(07)     </wsp:All>
(08)     <wsp:All>
(09)        <sp:EncryptedParts/>
(10)           <sp:Body/>
(11)        </sp:EncryptedParts/>
(12)     </wsp:All>
(13)   </wsp:ExactlyOne>
(14) </wsp:Policy>

            

Lines (03-06) represent one policy alternative for signing a message body.

Lines (08-11) represent a second policy alternative for encrypting a message body.

Lines (02-13) illustrate the ExactlyOne policy operator. Policy operators group policy assertions into policy alternatives. A valid interpretation of the policy above would be that an invocation of a Web service will either sign or encrypt the message body.

2.1 Notational Conventions

This specification uses the following syntax within normative outlines:

• The syntax appears as an XML instance, but values in italics indicate data types instead of literal values.

• Characters are appended to elements and attributes to indicate cardinality:

  • "?" (0 or 1)

  • "*" (0 or more)

  • "+" (1 or more)

• The character "|" is used to indicate an exclusive choice between alternatives.

• The characters "(" and ")" are used to indicate that contained items are to be treated as a group with respect to cardinality or choice.

• This document relies on the XML Information Set [ XML Information Set ]. Information item properties are indicated by the style [infoset property] .

• XML namespace prefixes (see Table 2-1 ) are used to indicate the namespace of the element or attribute being defined.

• The ellipses characters "…" are used to indicate a point of extensibility that allows other Element or Attribute Information Items.

Elements and Attributes defined by this specification are referred to in the text of this document using XPath 1.0 [XPATH 1.0] expressions. Extensibility points are referred to using an extended version of this syntax:

• An element extensibility point is referred to using {any} in place of the element name. This indicates that any element name can be used, from any namespace, unless specified otherwise such as in Section 4.3.3 Policy Operators .

• An attribute extensibility point is referred to using @{any} in place of the attribute name. This indicates that any attribute name can be used, from any namespace.

Normative text within this specification takes precedence over normative outlines, which in turn take precedence over the XML Schema [ XML Schema Structures ] descriptions.

2.2 Extensibility

Within normative outlines, in this specification, ellipses (i.e., "…") indicate a point of extensibility that allows other Element or Attribute Information Items. Information Items MAY be added at the indicated extension points but MUST NOT contradict the semantics of the element information item indicated by the [parent] or [owner] property of the extension. In this context, if an Attribute Information Item is not recognized, it SHOULD be ignored. If an Element Information Item is not recognized, it MUST be treated as a policy assertion, unless specified otherwise such as in Section 4.3.4 Policy References .

2.3 XML Namespaces

This specification uses a number of namespace prefixes throughout; they are listed in Table 2-1 . Note that the choice of any namespace prefix is arbitrary and not semantically significant (see [ XML Namespaces ]).

All information items defined by this specification are identified by the XML namespace URI [ XML Namespaces ] http://www.w3.org/ns/ws-policy . A normative XML Schema [ XML Schema Structures , XML Schema Datatypes ] document can be obtained indirectly by dereferencing the namespace document at the WS-Policy 1.5 namespace URI.

It is the intent of the W3C Web Services Policy Working Group that the Web Services Policy 1.5 - Framework and Web Services Policy 1.5 - Attachment XML namespace URI will not change arbitrarily with each subsequent revision of the corresponding XML Schema documents as the specifications transition through Candidate Recommendation, Proposed Recommendation and Recommendation status. However, should the specifications revert to Working Draft status, and a subsequent revision, published as a WD, CR or PR draft, results in non-backwardly compatible changes from a previously published WD, CR or PR draft of the specification, the namespace URI will be changed accordingly.

Under this policy, the following are examples of backwards compatible changes that would not result in assignment of a new XML namespace URI:

• Addition of new global element, attribute, complexType and simpleType definitions.

• Addition of new elements or attributes in locations covered by a previously specified wildcard.

• Modifications to the pattern facet of a type definition for which the value-space of the previous definition remains valid or for which the value-space of the vast majority of instances would remain valid.

• Modifications to the cardinality of elements (i.e. modifications to minOccurs or maxOccurs attribute value of an element declaration) for which the value-space of possible instance documents conformant to the previous revision of the schema would still be valid with regards to the revised cardinality rule.

2.4 Terminology

The keywords " MUST ", " MUST NOT ", " REQUIRED ", " SHALL ", " SHALL NOT ", " SHOULD ", " SHOULD NOT ", " RECOMMENDED ", " MAY ", and " OPTIONAL " in this document are to be interpreted as described in RFC 2119 [ IETF RFC 2119 ].

We introduce the following terms that are used throughout this document:

3.1 Policy Assertion

[ Definition : A policy assertion represents a requirement, a capability, or other property of a behavior.] A policy assertion identifies a behavior that is a requirement or capability of a policy subject . [ Definition : A policy subject is an entity (e.g., an endpoint, message, resource, operation) with which a policy can be associated. ] Assertions indicate domain-specific (e.g., security, transactions) semantics and are expected to be defined in separate, domain-specific specifications.

An assertion MAY indicate that it is an ignorable policy assertion (see 4.4 Ignorable Policy Assertions ). [ Definition : An ignorable policy assertion is an assertion that may be ignored for policy intersection (as defined in 4.5 Policy Intersection ).] By default, an assertion is not ignorable for policy intersection.

Assertions are typed by the authors that define them. [ Definition : A policy assertion type represents a class of policy assertions and implies a schema for the assertion and assertion-specific semantics.] The policy assertion type is identified only by the XML Infoset [namespace name] and [local name] properties (that is, the qualified name or QName) of the root Element Information Item representing the assertion. Assertions of a given type MUST be consistently interpreted independent of their policy subjects .

Authors MAY define that an assertion contains a policy expression (as defined in 4. Policy Expression ) as one of its [children] . Nested policy expression(s) are used by authors to further qualify one or more specific aspects of the original assertion. For example, security policy authors may define an assertion describing a set of security algorithms to qualify the specific behavior of a security binding assertion.

The XML Infoset of a policy assertion MAY contain a non-empty [attributes] property and/or a non-empty [children] property. Such properties, excluding the Attribute and Element Information Items from the WS-Policy language XML namespace name are policy assertion parameters and MAY be used to parameterize the behavior indicated by the assertion. [ Definition : A policy assertion parameter qualifies the behavior indicated by a policy assertion .] For example, an assertion identifying support for a specific reliable messaging mechanism might include an attribute information item to indicate how long an endpoint will wait before sending an acknowledgement.

Authors should be cognizant of the processing requirements when defining complex assertions containing policy assertion parameters or nested policy expressions . Specifically, authors are encouraged to consider when the identity of the root Element Information Item alone is enough to convey the requirement or capability.

3.2 Policy Alternative

[ Definition : A policy alternative is a potentially empty collection of policy assertions .] An alternative with zero assertions indicates no behaviors. An alternative with one or more assertions indicates behaviors implied by those, and only those assertions. [ Definition : A policy vocabulary is the set of all policy assertion types used in a policy.] [ Definition : A policy alternative vocabulary is the set of all policy assertion types within the policy alternative .] When an assertion whose type is part of the policy's vocabulary is not included in a policy alternative, the policy alternative without the assertion type indicates that the assertion will not be applied in the context of the attached policy subject. See the example in Section 4.3.1 Optional Policy Assertions

Assertions within an alternative are not ordered, and thus aspects such as the order in which behaviors (indicated by assertions) are applied to a subject are beyond the scope of this specification. However, authors can write assertions that control the order in which behaviours are applied.

A policy alternative MAY contain multiple assertions of the same type. Mechanisms for determining the aggregate behavior indicated by the assertions (and their Post-Schema-Validation Infoset (PSVI) (See XML Schema Part 1 [ XML Schema Structures ]) content, if any) are specific to the assertion type and are outside the scope of this document.

Note: Depending on the semantics of the domain specific policy assertions a combination of the policy assertions can be required to specify a particular behavior.

3.3 Policy

[ Definition : A policy is a potentially empty collection of policy alternatives . ] A policy with zero alternatives contains no choices; a policy with one or more alternatives indicates choice in requirements or capabilities within the policy.

Alternatives are not ordered, and thus aspects such as preferences between alternatives in a given context are beyond the scope of this specification.

Alternatives within a policy may differ significantly in terms of the behaviors they indicate. Conversely, alternatives within a policy may be very similar. In either case, the value or suitability of an alternative is generally a function of the semantics of assertions within the alternative and is therefore beyond the scope of this specification.

3.4 Policies of Entities in a Web Services Based System

Applied to a Web services based system, policy is used to convey conditions on an interaction between entities (requester application, provider service, Web infrastructure component, etc). An interaction involves one or more message exchanges between two entities. It is the responsibility of assertion authors to define the interaction scope of an assertion including any constraints on the policy subjects to which the assertion may be attached and a clear specification of the message (s) within that interaction scope to which the assertion applies.

Any entity in a Web services based system may expose a policy to convey conditions under which it functions. Satisfying assertions in the policy usually results in behavior that reflects these conditions. For example, if two entities - requester and provider - expose their policies, a requester might use the policy of the provider to decide whether or not to use the service. A requester MAY choose any alternative since each is a valid configuration for interaction with the service, but a requester MUST choose only a single alternative for an interaction with a service since each represents an alternative configuration.

A policy assertion is supported by an entity in the web services based system if and only if the entity satisfies the requirement (or accommodates the capability) corresponding to the assertion. A policy alternative is supported by an entity if and only if the entity supports all the assertions in the alternative. And, a policy is supported by an entity if and only if the entity supports at least one of the alternatives in the policy. Note that although policy alternatives are meant to be mutually exclusive, it cannot be decided in general whether or not more than one alternative can be supported at the same time.

Note that an entity may be able to support a policy even if the entity does not understand the type of each assertion in the vocabulary of the policy ; the entity only has to understand the type of each assertion in the vocabulary of a policy alternative the entity supports. This characteristic is crucial to versioning and incremental deployment of new assertions because this allows a provider's policy to include new assertions in new alternatives while allowing entities to continue to use old alternatives in a backward-compatible manner.

4.1 Normal Form Policy Expression

To facilitate interoperability, this specification defines a normal form for policy expressions that is a straightforward XML Infoset representation of a policy, enumerating each of its alternatives that in turn enumerate each of their assertions . The schema outline for the normal form of a policy expression is as follows:

            (01) <wsp:Policy … >
(02)   <wsp:ExactlyOne>
(03)     ( <wsp:All> ( <
            
              Assertion
            
             …> … </
            
              Assertion
            
            > )* </wsp:All> )*
(04)   </wsp:ExactlyOne>
(05) </wsp:Policy> 

          

The following describes the Element Information Items defined in the schema outline above:

If an assertion in the normal form of a policy expression contains a nested policy expression , the nested policy expression MUST contain at most one policy alternative (see 4.3.2 Policy Assertion Nesting ).

To simplify processing and improve interoperability, the normal form of a policy expression SHOULD be used where practical.

For example, the following is the normal form of a policy expression.

            (01) <wsp:Policy
        xmlns:sp="http://schemas.xmlsoap.org/ws/2005/07/securitypolicy"
        xmlns:wsp="http://www.w3.org/ns/ws-policy" >
(02)   <wsp:ExactlyOne>
(03)     <wsp:All>
(04)        <sp:SignedParts/>
(05)           <sp:Body/>
(06)        </sp:SignedParts/>
(07)     </wsp:All>
(08)     <wsp:All>
(09)        <sp:EncryptedParts/>
(10)           <sp:Body/>
(11)        </sp:EncryptedParts/>
(12)     </wsp:All>
(13)   </wsp:ExactlyOne>
(14) </wsp:Policy>

          

Lines (03-07) and Lines (08-11) express the two alternatives in the policy. If the first alternative is selected, the message body needs to be signed [ WS-SecurityPolicy ] is supported; conversely, if the second alternative is selected, the message body needs to be encrypted.

4.2 Policy Identification

A policy expression MAY be associated with an IRI [ IETF RFC 3987 ]. The schema outline for attributes to associate an IRI is as follows:

            (01) <wsp:Policy ( Name="
            
              xs:anyURI
            
            " )?
(02)                 ( wsu:Id="
            
              xs:ID
            
            " | xml:id="
            
              xs:ID
            
            " )?
(03)             … >
(04)   …
(05) </wsp:Policy>

          

The following describes the Attribute Information Items listed and defined in the schema outline above:

The following example illustrates how to associate a policy expression with the absolute IRI "http://www.example.com/policies/P1" :

            (01) <wsp:Policy
        Name="http://www.example.com/policies/P1"
        xmlns:wsp="http://www.w3.org/ns/ws-policy" >
(02)   <!-- Details omitted for readability -->
(03) </wsp:Policy>

          

The following example illustrates how to associate a policy expression with the IRI-reference "#P1" :

            (01) <wsp:Policy
        wsu:Id="P1"
        xmlns:wsp="http://www.w3.org/ns/ws-policy"
        xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd" >
(02)   <!-- Details omitted for readability -->
(03) </wsp:Policy>

          

4.3 Compact Policy Expression

To express a policy in a more compact form while still using the XML Infoset, this specification defines three constructs: an attribute to decorate an assertion , semantics for recursively nested policy operators, and a policy reference/inclusion mechanism. Each sub section below describes a construct and its equivalent normal form. To interpret a compact expression in an interoperable form, a policy expression in the compact form can be converted (see Section 4.3.6 Normalization ) to the normal form (see Section 4.1 Normal Form Policy Expression ).

A policy expression consists of a wsp:Policy wrapper element and zero or more child and descendent elements.

              (01) <
              
                Assertion
              
               ( wsp:Optional="
              
                xs:boolean
              
              " )? …> … </
              
                Assertion
              
              >

            

                (01) <wsp:ExactlyOne>
(02)   <wsp:All> <
                
                  Assertion
                
                 …> … </
                
                  Assertion
                
                > </wsp:All>
(03)   <wsp:All />
(04) </wsp:ExactlyOne>

              

                (01) <wsp:ExactlyOne>
(02)   <wsp:All> <
                
                  Assertion
                
                 …> … </
                
                  Assertion
                
                > </wsp:All>
(03) </wsp:ExactlyOne>

              

              (01) <wsp:Policy
        xmlns:sp="http://schemas.xmlsoap.org/ws/2005/07/securitypolicy"
        xmlns:wsp="http://www.w3.org/ns/ws-policy" >
(02)   <sp:IncludeTimestamp wsp:Optional="true" />
(03) </wsp:Policy>

            

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