Web Services Policy 1.5 - Guidelines for Policy Assertion Authors is intended to provide guidance for Assertion Authors that will work with the Web Services Policy 1.5 - Framework [Web Services Policy Framework] and Web Services Policy 1.5 - Attachment [Web Services Policy Attachment] specifications to create domain specific assertions. The focus of this document is to provide best practices and patterns to follow as well as illustrate the care needed in using WS-Policy to achieve the best possible results for interoperability. It is a complementary guide to using the specifications.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
This is a W3C Working Group Note of the Web Services Policy 1.5 - Guidelines for Policy Assertion Authors specification, developed by the members of the Web Services Policy Working Group, which is part of the W3C Web Services Activity.
A list of changes in this version of the document and a diff-marked version against the previous version of this document are available. Please send comments about this document to the email@example.com mailing list (public archive) with a subject that is prefaced with [ws-policy-guidelines].
Publication as a Working Group Note does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
2. List of Best Practice Statements
3. What is an Assertion?
4. Who is involved in authoring Assertions?
4.1 Roles and Responsibilities
4.1.1 Assertion Authors
5. General Guidelines for Assertion Authors
5.1 Assertions and Their Target Use
5.2 Authoring Styles
5.3 Considerations when Modeling New Assertions
5.3.1 Minimal approach
5.3.2 QName and XML Information Set representation
5.3.3 Self Describing Messages
5.3.4 Single Domains
5.3.5 Order of Behaviors
5.4 Comparison of Nested and Parameterized Assertions
5.4.1 Assertions with Parameters
5.4.2 Nested Assertions
5.5 Designating Ignorable Behavior
5.5.1 Ignorable behavior in authoring
5.5.2 Ignorable behavior at runtime
5.6 Designating Optional Behaviors
5.6.1 Optional behavior at runtime
5.7 Considerations for Policy Attachment
5.7.1 General Guidelines
5.7.2 Considerations for Policy Attachment in WSDL
5.7.3 Considerations for Policy Attachment in UDDI
5.8 Interrelated domains
6. Versioning Policy Assertions
6.1 Referencing Policy Expressions
6.2 Evolution of Assertions (Versioning and Compatibility)
6.3 Supporting New Policy Subjects
B. XML Namespaces
D. Acknowledgements (Non-Normative)
E. Changes in this Version of the Document (Non-Normative)
F. Web Services Policy 1.5 - Guidelines for Policy Assertion Authors Change Log (Non-Normative)
The WS-Policy specification defines a policy to be a collection of policy alternatives. Each policy alternative is a collection of policy assertions. The Web Services Policy 1.5 - Framework provides a flexible framework to represent consistent combinations of behaviors from a variety of domains. A policy assertion is a machine readable metadata expression that identifies behaviors required for Web services interactions. Web Services Policy 1.5 - Guidelines for Policy Assertion Authors is a resource primarily for Assertion Authors and provides guidelines on the use of Web Services Policy 1.5 - Framework and Web Services Policy 1.5 - Attachment specifications to create and use domain specific assertions to enable interoperability.
WS-Policy Assertions communicate the requirements and capabilities of a web service by adhering to the specification, WS-Policy Framework. To enable interoperability of web services different sets of WS-Policy Assertions need to be defined by different communities based upon domain-specific requirements of the web service.
The focus of these guidelines is to capture best practices and usage patterns for practitioners. It is a complementary guide to the Framework and Attachments specifications and the Primer. It is intended to provide non-normative guidelines for WS-Policy Assertion Authors who need to know the features of the language and understand the requirements for describing policy assertions. Some of the guidance for WS-Policy Assertion Authors can also be helpful for those who use the policy assertions created by Assertion Authors.
This document assumes a basic understanding of XML, Namespaces in XML, WSDL, SOAP and the Web Services Policy language.
This is a non-normative document and does not provide a definitive specification of the Web Services Policy framework. B. XML Namespaces lists all the namespace prefixes that are used in this document. (XML elements without a namespace prefix are from the Web Services Policy XML Namespace.)
As a companion document to the primer, this document also follows the Socratic style of beginning with a question, and then answering the question.
The following Best Practices appear in this document with discussion and examples, and are summarized here for quick reference:
17. Consider entire message exchange pattern when specifying Assertions that represent optional behavior related to a subset of that message exchange pattern when considering appropriate policy subject attachment points
An assertion is a piece of metadata that describes a capability related to a specific domain that has chosen to express their capabilities via the WS-Policy expressions. Sets of domain-specific assertions are typically defined in a dedicated specification that describes their semantics, applicability and scoping requirements as well as their data type definition using XML Schema [XML Schema Structures].
Policy assertions representing shared and visible behaviors are useful pieces of metadata to enable interoperability and tooling for automation. The key to understanding when to design policy assertions is to have clarity on the characteristics of a behavior represented by a policy assertion. Some useful ways to discover relevant behaviors are to ask questions like the following:
Is this behavior a requirement?
Is the behavior visible?
A visible behavior refers to a requirement that manifests itself on the wire. Web services provide interoperable machine-to-machine interaction among disparate systems. Web service interoperability is the capability of disparate systems to exchange data using common data formats and protocols supporting characteristics such as messaging, security, reliability and transaction. Such data formats and protocols manifest on the wire. Providers and requesters rely on wire messages conforming to such formats and protocols to achieve interoperability.
If an assertion describes a behavior that does not manifest on the wire then the assertion will not impact the interoperability of wire messages, but may still be relevant to enabling an interoperable interaction. For example, a provider may not wish to interact unless a client can accept an assertion describing provider behavior. An example is an assertion that describes the privacy notice information of a provider and the associated regulatory safeguard in place on the provider's side. For cases where the provider does not intend the assertion to impact interoperability it may mark it as ignorable.
If an assertion has no wire or message-level visible behavior then the interacting participants may require some sort of additional mechanism to indicate compliance with the assertion and to enable dispute resolution. Introducing an additional non-repudiation mechanism adds unnecessary complexity to processing a policy assertion.
Does the behavior apply to two or more Web service participants?
A shared behavior refers to a requirement that is relevant to an interoperable Web service interaction and involves two or more participants. If an assertion only describes one participant's behavior the assertion may still be relevant to enabling an interoperable interaction. An example is the use of logging or auditing by the provider. If an assertion only describes one participant's behavior then the assertion may be marked as ignorable (indicating it does not impact interoperability). An ignorable policy assertion is ignored for lax policy intersection. If an assertion is not an ignorable assertion then it is deemed important for agreement between both parties.
Does the behavior have an implied scoping to a policy subject such as service, endpoint, operation and message?
Is there a requirement that a choice must be made for successful interaction?
Sometimes providers and requesters are required to engage in certain behaviors. The use of optimization and reliable messaging are two examples.
There are already many examples in the industry that adhere to the above practices, such as Web Services Reliable Messaging Policy Assertion and WS-SecurityPolicy. Some common characteristics from these documents may be considered as best practices for new Assertion Authors:
Specify both the syntax and the semantics of the assertions
If nested or parameterized assertions are defined, be clear about their usage
Describe the policy subjects the assertions can be attached to.
In this document we will explain why these practices should be followed so that the assertion developers defining such a specification will be well informed and able to adequately specify assertions for their domain.
It is expected that consumers of the metadata specified by the Assertion Authors will also benefit from understanding these practices as it will help them utilize the assertions in the context of the WS-Policy framework. A result of following the best practices will be an assertion specification that describes a contract for the consumers and providers of the capabilities and constraints of the domain.
In order for the policy framework to enable communities to express their own domain knowledge, it is necessary to provide basic functionality that all domains could exploit and then allow points of extension where authors of the various WS-Policy assertions for a particular domain can provide additional semantics.
Below we capture some of the characteristics of the roles and responsibilities for the authors, consumers and providers.
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.
Assertion authors should review the conformance sections of the WS-Policy Framework and Attachment specifications and an assertion must adhere to all the constraints contained in the Framework and Attachment specifications.
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.
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.
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.
As Assertion Authors begin the task of inventing XML dialects to represent policy assertions they can take advantage of WS-Policy building on XML principles and XML Schema validation in their design. WS-Policy relies on the QName of a policy assertion being an XML element but allows Assertion Authors to optionally provide additional semantics through nesting assertions, or specifying assertion parameters. This section covers several aspects of assertion design and provides some answers to the following questions:
What is the intended use of the policy assertion?
Which authoring style will be used?
Is this a new policy domain? Does it need to compose with other domains?
How complex are the assertions?
Is there a need to consider nesting?
Do optional behaviors need to be represented?
Assertion Authors should understand the functionality that the WS-Policy framework provides and apply the knowledge of the policy framework processing when defining the set of assertions.
Assertions can be simple or they can be complex. Assertion Authors may choose to specify multiple peer assertions, each carrying the semantic of a particular behavior, or they may choose to specify assertions that contain assertion parameters and/or nested policy expressions (nested assertions), where each nested assertion of which relates to an aspect of the behavior, yet encapsulated within a single assertion. There are advantages to simplifying the set of assertions. The ultimate goal of policy is to enable interoperability. By keeping assertion design as simple as possible, Assertion Authors will more likely be able to meet that objective.
Assertion Authors need to have a specific goal in mind for the assertions that they author. Assertion specifications should include a detailed specification of the assertion’s semantics and a set of valid policy subjects to which the assertion maybe attached. The specification should also include the scope of the assertion in the context of a particular policy subject. For example, an assertion with Endpoint Policy Subject can be scoped to a given message exchange with that endpoint, or it can be scoped to all messages exchanged with that endpoint. The former case permits a client to select a different alternative with each successive message exchange. Finally, the ability to combine individual assertions may also need to be considered. For example, if an assertion applies to the SOAP protocol, it would be necessary to consider how its presence must interact with other policy assertions that are defined for security.
Assertion Authors should include the following items in an assertion specification:
The definition of the assertion's semantic (See best practice 7. Specify Semantics Clearly).
The specification of the set of valid policy subjects to which an assertion may be attached (See best practice 25. Specify WSDL Policy Subject(s)).
The scope of the assertion in the context of a particular policy subject (See best practices in Section 5.7 Considerations for Policy Attachment).
Any composition considerations if the assertion is used with other assertions in a context (See best practice 31. Specify Composition with Related Assertions).
The WS-Policy Attachment specification defines a number of different policy subjects to which an assertion can be attached. For attaching to WSDL subjects see 5.7 Considerations for Policy Attachment for more detail. Additionally, the framework provides for the means to extend the set of policy subjects beyond the set of subjects defined in the WS-Policy Attachment specification.
Assertion authors should define assertions for behaviors that are relevant to compatibility assessment, such as web service protocols that manifest on the wire.
Assertion authors may define assertions that are not related to compatibility assessment. These assertions may be used to accurately describe behaviour, even if they do not affect compatibility. WS-Policy has the wsp:Ignorable attribute that may be used for indicating assertions that are not related to compatibility assessment, described in 5.5 Designating Ignorable Behavior
WS-Policy supports two different authoring styles, compact form and normal form. A compact form is one in which an expression consists of three constructs: an attribute to decorate an assertion (to indicate whether it is required or optional), semantics for recursively nested policy operators, and a policy reference/inclusion mechanism. A policy expression in the compact form can be translated into its normal form using the policy normalization algorithm described in the Web Service Policy Framework (see section 4.3 Compact Policy Expression).
The two forms of a policy expression are semantically
equivalent. When multiple alternatives are present in a policy, the
normal form may express the choices more explicitly. On the other
hand, the compact form may be more readable for humans when an
assertion is marked as optional using the
attribute. A policy processor may normalize a policy expression
originally authored in compact form at any time without changing
the semantics of the policy. In general, it is not possible to
guarantee in what form a policy expression would be when it is
processed. As a result, the description for a policy assertion
should not depend on the style used to author a policy expression
that contains the assertion.
The semantics of an assertion should be independent of the form (compact or normal form) of policy expressions that contain the assertion.
In the example below, the policy expression is shown in its two
forms, compact and normal. In compact form, the
wsrmp:RMAssertion assertion is augmented by the
wsp:Optional="true" attribute. While the compact form
of the expression might be more human readable, the semantics of
the particular assertion are independent of the form and of the
presence (or absence) of the
Example 5-1. Policy Expression in Compact Form
<wsp:Policy xmlns:wsp='http://www.w3.org/ns/ws-policy' xmlns:sp='http://docs.oasis-open.org/ws-sx/ws-securitypolicy/200702' xmlns:wsrmp='http://docs.oasis-open.org/ws-rx/wsrmp/200608'> <wsrmp:RMAssertion wsp:Optional="true"/> <wsp:ExactlyOne> <wsp:All> <sp:TransportBinding> <wsp:Policy> <sp:TransportToken> <wsp:Policy> <sp:HttpsToken> <wsp:Policy> <sp:RequireClientCertificate/> </wsp:Policy> </sp:HttpsToken> </wsp:Policy> </sp:TransportToken> </wsp:Policy> </sp:TransportBinding> </wsp:All> </wsp:ExactlyOne> </wsp:Policy>
Example 5-2. Policy Expression in Normal Form
<wsp:Policy xmlns:wsp='http://www.w3.org/ns/ws-policy' xmlns:sp='http://docs.oasis-open.org/ws-sx/ws-securitypolicy/200702' xmlns:wsrmp='http://docs.oasis-open.org/ws-rx/wsrmp/200608'> <wsp:ExactlyOne> <wsp:All> <wsrmp:RMAssertion/> <sp:TransportBinding> <wsp:Policy> <sp:TransportToken> <wsp:Policy> <sp:HttpsToken> <wsp:Policy> <sp:RequireClientCertificate/> </wsp:Policy> </sp:HttpsToken> </wsp:Policy> </sp:TransportToken> </wsp:Policy> </sp:TransportBinding> </wsp:All> <wsp:All> <sp:TransportBinding> <wsp:Policy> <sp:TransportToken> <wsp:Policy> <sp:HttpsToken> <wsp:Policy> <sp:RequireClientCertificate/> </wsp:Policy> </sp:HttpsToken> </wsp:Policy> </sp:TransportToken> </wsp:Policy> </sp:TransportBinding> </wsp:All> </wsp:ExactlyOne> </wsp:Policy>
When creating a new policy domain, it is important to understand how policy expressions are used by a framework implementation that has followed the specifications.
The examples given in this document are based on existing assertions such as WS-SecurityPolicy and WS-RM Policy. These policy expressions represent web services message exchange requirements, but policy authoring can be done by individual groups that wish to represent web services application requirements and deployments that wish to reuse the WS-Policy framework in order to enable dynamic negotiation of business requirements and capabilities at runtime.
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" 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.
Assertion Authors should start with a simple working assertion that allows assertion parameter extensibility.
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.
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.
Assertion Authors should use a unique QName to identify a distinct behavior.
Assertion authors should provide an XML schema definition to specify the syntax of an assertion. A reader-friendly description such as an XML outline (see below) is also useful.
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>
Assertion authors should clearly and completely specify the semantics of a policy assertion.
An assertion description should include guidance as to the use of (or constraint against the use of) the wsp:Ignorable attribute to indicate whether or not the behavior indicated by the QName may be ignored by policy intersection.
An Assertion Author should document, in the XML outline and/or schema for the assertion, whether or not the assertion allows for the use of the wsp:Ignorable attribute.
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
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.
An assertion's XML outline and/or schema definition should allow the use of the wsp:Optional attribute so as to enable policy authors to compose compact policy expressions.
For example, consider the following two equivalent policy expressions:
Example 5-5. Normal form expression:
<wsp:Policy> <wsp:ExactlyOne> <wsp:All> <wsam:Addressing> <wsp:Policy/> </wsam:Addressing> </wsp:All> <wsp:All> </wsp:All> </wsp:ExactlyOne> </wsp:Policy>
Example 5-6. Compact form expression:
<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.
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.
Assertion Authors should not define policy assertions to represent information that is necessary to understand a message.
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.
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.
A policy alternative is a collection of zero or more policy assertions. Assertions within a policy alternative are not ordered.
The order of assertions in a policy alternative and order in which behaviors (indicated by assertions) are applied are two distinct concepts. The order of assertions in a policy alternative has no bearing on the order in which behaviors are applied.
Specifying the order in which behaviors are applied is outside the scope of the Web Services Policy Framework. However, the Framework says that assertion authors can write assertions that indicate the order in which behaviors are applied.
According to the SOAP processing model, the order of headers and body processing (for behaviors such as addressing, security, reliability and transaction) is at the discretion of the SOAP node and SOAP-based protocols may be used to control the order of processing.
The Web Services Security specification provides producers with
a choice of signing a message before encrypting or signing a
message after encrypting. That is, WS-Security 1.1, section 8 says,
lines 1173-1183 - says "Finally, if a producer wishes to sign a
message before encryption, then following the ordering rules laid
out in section 5, "Security Header", they SHOULD first prepend the
signature element to the
wsse:Security header, and
then prepend the encryption element, ... Likewise, if a producer
wishes to sign a message after encryption, they SHOULD first
prepend the encryption element to the
header, and then prepend the signature element."
The Web Services Security Policy specification provides assertions which let users control whether to sign the message before encrypting or sign it after encrypting.
In the example below, the SignBeforeEncrypting assertion requires producers to sign a message before encrypting.
Example 5-7. SignBeforeEncrypting assertion
<wsp:Policy> <sp:AsymmetricBinding> <wsp:Policy> <sp:IncludeTimestamp /> <sp:SignBeforeEncrypting /> <sp:EncryptSignature /> <sp:ProtectTokens /> <wsp:Policy/> </sp:AsymmetricBinding> <wsam:Addressing>...</wsam:Addressing> </wsp:Policy>
In the example below, the EncryptBeforeSigning assertion requires producers to sign a message after encrypting.
Example 5-8. EncryptBeforeSigning assertion
<wsp:Policy> <sp:AsymmetricBinding> <wsp:Policy> <sp:IncludeTimestamp /> <sp:EncryptBeforeSigning /> <sp:EncryptSignature /> <sp:ProtectTokens /> <wsp:Policy/> </sp:AsymmetricBinding> <wsam:Addressing>...</wsam:Addressing> </wsp:Policy>
There are two different ways to provide additional information in an assertion beyond its type: assertion parameters and nested policy expressions. We cover these two cases below followed by a comparison of these approaches targeting when to use either of the two approaches.
The main consideration for choosing between use of parameters or nested policy expressions is that the framework intersection algorithm processes nested policy expressions, but does not consider parameters in the algorithm.
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.
Assertion Authors should represent useful additive information necessary for engaging the behavior represented by a policy assertion as assertion parameters.
In the example below,
sp:Header elements are the two assertion parameters of
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.
Example 5-9. Policy Assertion with Assertion Parameters
<wsp:Policy> <sp:SignedParts> <sp:Body/> <sp:Header/> </sp:SignedParts> </wsp:Policy>
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 represent dependent behaviors that are relevant to a compatibility test and apply to the same policy subject using nested policy assertions.
If there is a nested policy expression, then the Assertion Authors should enumerate the nested policy assertions that are allowed.
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.
Assertion authors should only specify domain specific intersection semantics when policy intersection is insufficient.
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
policy assertion can represent these dependent behaviors.
A policy assertion like the
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
Example 5-10. Transport Security Policy Assertion
<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>
sp:AlgorithmSuite is a nested policy assertion
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
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
Example 5-11. Empty Nested Policy Expression
<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.
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 8. Document Ignorable Behavior and 9. Document Use of the Ignorable Attribute in XML to include this guidance in the assertion's definition.
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.
The target scope of an assertion is an important factor for Assertion Authors to consider as it determines the granularity of the scope for which the behavior is to be engaged. For example, if an assertion has a scope of endpoint policy subject the behavior indicated by that assertion applies to all , messages exchanged in both directions (e.g. both request and response messages) with the specific endpoint to which the policy alternative including that assertion is attached.
Certain behaviors might provide in their specification for the optional use of that behavior in the context of a subset of a given interaction. When such 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 in the context of the rest of the interaction such as the outbound message, will be undefined. Therefore, the Assertion Authors are encouraged to consider the implications of attachment of an assertion that indicates such optional behavior at a message policy subject on the interaction as a whole. For example, if reliable messaging (RM) is applied to a request message because the policy attached to the inbound message in a request-response operation had an alternative that incldued RM in its assertions, is the application of RM to the outbound message permitted, even if there is no policy attached to that subject? Leaving the semantics either unspecified or incompletely specified may result in implementations making assumptions that might have undesireable consequences. This is especially important if the assertion is applicable to more than one specific policy subject. The approach taken by WS-RM Policy [Web Services Reliable Messaging Policy Assertion] is to provide for an RM assertion to be attached at either or both message and endpoint policy subjects. In order to eliminate the ambiguity associated with only using a message policy subject, the WS-RM Policy requires a policy to be attached to an endpoint policy subject as well as a message policy subject whenever a policy is attached to a message policy subject. The combination directly addresses the unstated semantic that if RM is used for inbound messages, that it MAY be used for outbound messages, even if the assertion is not attached to the outbound message (and vice-versa).
Best Practice 17: Consider entire message exchange pattern when specifying Assertions that represent optional behavior related to a subset of that message exchange pattern when considering appropriate policy subject attachment points
Assertion Authors should associate assertions that represent optional behavior with the appropriate policy subject and use the smallest possible granularity (See Best Practice 28) to limit the degree to which optional behavior applies.
Behaviors that must be engaged in the context of an interaction must not be marked with wsp:Optional="true". since this creates two alternatives; one with and one without that assertion. This would allow the policy consumer to select the policy alternative that does not contain that assertion, and thus result in an interaction that did not engage the required behavior that was indicated by that assertion.
Assertion Authors should disallow the use of the wsp:Optional attribute on assertions that represent behaviors that must be engaged.
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 ohter 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 the WS-Policy Framework).
When a given behavior may be optional, it must be possible for both message participants to determine that the assertion has been selected by both parties, either out of band or as reflected by the message content.
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 the policy subject representing the inbound message, then it would not be clear to the service provider whether the outbound whether the outbound messages generated by that provider should also utilize that 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.
Although a policy assertion may be constrained to a specific set of policy subjects by Assertion Authors, its semantics should not be dependent upon the mechanism by which the policy expression is attached to a given policy subject. For instance, an assertion "Foo" has the same semantics when attached to an operation policy subject regardless of whether it was attached using XML element policy attachment or the external URI attachment mechanism. Independence from a specific attachment mechanism allows policy tools to choose the most appropriate mechanism to attach a policy without having to analyze the contents of the policy.
The semantics of a policy assertion should not depend on the attachment mechanism used.
For example, a security policy expression can be assigned a key reference and be attached to a UDDI binding or can be embedded in a WSDL document.
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 specify the semantics of multiple instances of the same policy assertion type in the same policy alternative and the semantics of parameters and nested policy (if any) when there are multiple instances of a policy assertion type in the same policy alternative regardless of the mechanism used to attach them to a policy subject.
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.
Assertion Authors should leverage defined attachment models when possible to document the use of the policy assertions they author and ensure that there are no additional semantics implied by the defined attachment models for their assertions.
Assertion Assertion Authors should leverage defined policy subjects when possible to facilitate the deployment of their policy assertions. Common Policy subjects have been defined and used by other policy assertion authors and new policy assertions that leverage these existing subjects will be easier to define and group.
Policy assertion authors should unambiguously identify the appropriate policy subjects for their assertions. If the best practices are followed, and the assertions are scoped according to their subject, then multiple policy domains may be combined without conflict. Each domain should define any limitations at the policy subject level that might impact interoperability.
Assertion Authors should review the policy subjects defined in WS-PolicyAttachments and identify which of the existing policy subjects can be used with the assertions they define. That identification will facilitate the deployment of their policy assertions.
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
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 - Attachment [Web Services Policy Attachment].
Such additional constraints must be clearly specified by the
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 should specify the set of relevant WSDL policy subjects with which the assertion may be associated.
Assertion Authors that utilize WSDL policy subjects need to understand how the assertions will be processed in intersection and merging, and the specific implications of the processing for 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.
To determine the relevant attachment points, Assertion Authors should consider the scope of the attachment point.
For example, an assertion should only be allowed in the portType element if the assertion reasonably applies to any endpoint that ever references that portType. Most of the known policy assertions are designed for the endpoint, operation or message policy subject.
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. The best practice is to choose the most granular WSDL policy subject to which the behavior represented by a policy assertion applies.
Assertion Authors should choose the most granular WSDL policy subject to which the behavior represented by a policy assertion applies.
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.
If an assertion is allowed to be associated with multiple WSDL policy subjects, the assertion author should describe the rules for multiple instances of the same assertion attached to multiple WSDL policy subjects in the same context.
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 subjects may be associated with the RM Policy assertion, and which WSDL 1.1 elements may have RM Policy assertions attached.
If the behavior indicated by an assertion varies when attached to different policy subjects the Assertion Authors should consider the following:
Decompose the semantics with several assertions.
Rewrite a single assertion targeting a specific policy 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.
If an assertion can be attached at multiple attachment points within a policy subject, Assertion Authors should specify a preferred attachment point for the chosen policy subject.
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 type 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.
In general, UDDI protocol messages can be used to save TModel, businessEntity, businessService and bindingTemplate definitions with policies attached. These definitions can also be the target of a "find" protocol message, thus allowing authors to store and retrieve policy assertions. There are two ways to associate policy expressions with UDDI definitions: direct referece, and registering policy as a UDDI TModel. Assertion Authors defining new assertions should consider each approach.
UDDI defines the following policy subjects: Service Provider Policy, Service Policy subject and Endpoint Policy subject.
When defining assertions and recommending a service provider policy subject [uddi:BusinessEntity], or a service policy subject [uddi:buisnessService], assertion authors are scoping the behaviors to the service provider as a whole. When defining assertions and recommending an endpoint policy subject [uddi:bindingTemplate, uddi:tModel], assertion authors are scoping behaviors to a deployed endpoint.
Assertion Authors need to be clear about how assertions defined in their domain may fit with assertions for interrelated domains. Assertion Authors should not duplicate existing assertions and should also make sure that when adding assertions those new assertions are consistent with pre-existing assertions of any interrelated domain.
Assertion authors should clearly specify how an assertion may compose with other related assertions, if any.
A classic example of such an interrelated domain is security, because security tends to cut across all aspects of a solution. Web Services Reliable Messaging Policy Assertions [Web Services Reliable Messaging Policy Assertion] defines additional assertions related to [WS-SecurityPolicy], an interrelated security domain. One such additional assertion specifies the use of transport security to protect a message sequence, for example.
Example 5-12. Reliable Message Sequence Security
<wsrmp:SequenceTransportSecurity [wsp:Optional="true"]? ... />
The Reliable Message Policy specification states "This assertion
is effectively meaningless unless it occurs in conjunction with the
RMAssertion and a
assertion that requires the use of some transport-level security
Assertion Authors need to consider not just the expression of the current set of requirements but how they anticipate new assertions being added to the set. There are three aspects to versioning policy assertions:
Assertion Extensibility. Assertion authors should allow for extensibility (see best practice 5. Start with a Simple Assertion)
Policy Language Extensibility
Over time, the Policy WG or third parties can version or extend the Policy Language with new or modified constructs. These constructs may be compatible or incompatible with previous versions.
The current WS-Policy language Web Services Policy Framework provides extensibility points on 6 elements with a combination of attribute and/or element extensibility:
Policy: element from ##other namespace and any attribute
ExactlyOne, All: element from ##other namespace; no attribute extensibility
PolicyReference: any element and any attribute
PolicyAttachment: element from ##other namespace and any attribute
AppliesTo: any element and any attribute
URI: any attribute
Supporting New Policy Subjects (see Section 6.3 Supporting New Policy Subjects).
The Web Services Policy
Primer illustrates how providers can utilize the
identification mechanism defined in the Policy specification to
expose a complex policy expression as a reusable building block for
other policy expressions by reference. Reuse may also be useful for
domain Assertion Authors, especially those defining complex
assertions utilizing references to policy expressions by nesting.
Statically available parameterized content may also be reused by
different assertions. However, such referencing mechanism is
outside the scope of WS-Policy naming and referencing framework and
other mechanisms could be used. As an example, in Web Services Policy Primer Section
sp:issuedToken assertion utilizes the
sp:RequestSecurityTokenTemplate parameter that
contains necessary information to request a security token. The
contents of the parameter are static and may be reused in different
security scenarios using other referencing mechanisms (these are
outside the scope of the WS-Policy Framework).
Over time, there may be multiple equivalent behaviors emerging in the Web Service interaction space. Examples of such multiple equivalent behaviors are WSS: SOAP Message Security 1.0 vs. 1.1 and WS-Addressing August 2004 version vs. WS-Addressing W3C Recommendation [WS-Addressing Core]. These equivalent behaviors are mutually exclusive for an interaction. Such equivalent behaviors can be modeled as independent assertions.
Assertion Authors should use independent assertions for modeling different versions of a behavior.
The policy expression in the example below requires the use of WSS: SOAP Message Security 1.0.
Example 6-1. Message-level Security and WSS: SOAP Message Security 1.0
<Policy> <sp:Wss10>…</sp:Wss10> </Policy>
The policy expression in the example below requires the use of WSS: SOAP Message Security 1.1. These are multiple equivalent behaviors and are represented using distinct policy assertions.
Example 6-2. Message-level Security and WSS: SOAP Message Security 1.1
<Policy> <sp:Wss11>…</sp:Wss11> </Policy>
The best practice 25. Specify WSDL Policy Subject(s) specifies that policy authors should define the set of policy subjects to which policy assertions can be attached. Over time, new policy subjects may need to be defined. When this occurs, policy Assertion Authors may update the list of policy subjects supported by an assertion.
When the assertion's semantics do not change to invalidate any of the original policy subjects but new policy subjects need to be added, it may be possible to use the same assertion to designate the additional policy subjects without a namespace change. For example, a policy assertion for a protocol that is originally designed for endpoint policy subject may add message policy subject to indicate finer granularity in the attachment provided that endpoint policy subject is also retained in its design. When new policy subjects are added it is incumbent on the authors to retain the semantic of the policy assertion.
Security considerations are discussed in the Web Services Policy Framework document.
The table below lists XML Namespaces that are used in this document. The choice of any namespace prefix is arbitrary and not semantically significant.
||[SOAP 1.2 Messaging Framework (Second Edition)]|
||[Web Services Policy Framework, Web Services Policy Attachment]|
||[Web Services Reliable Messaging Policy Assertion]|
This document is the work of the W3C Web Services Policy Working Group.
Members of the Working Group are (at the time of writing, and by alphabetical order): Dimitar Angelov (SAP AG), Abbie Barbir (Nortel Networks), Charlton Barreto (Adobe Systems Inc.), Sergey Beryozkin (IONA Technologies, Inc.), Vladislav Bezrukov (SAP AG), Toufic Boubez (Layer 7 Technologies), Symon Chang (BEA Systems, Inc.), Paul Cotton (Microsoft Corporation), Doug Davis (IBM Corporation), Jacques Durand (Fujitsu Limited), Ruchith Fernando (WSO2), Christopher Ferris (IBM Corporation), William Henry (IONA Technologies, Inc.), Frederick Hirsch (Nokia), Maryann Hondo (IBM Corporation), Ondrej Hrebicek (Microsoft Corporation), Steve Jones (Layer 7 Technologies), Tom Jordahl (Adobe Systems Inc.), Paul Knight (Nortel Networks), Philippe Le Hégaret (W3C/MIT), Mark Little (JBoss Inc.), Mohammad Makarechian (Microsoft Corporation), Ashok Malhotra (Oracle Corporation), Jonathan Marsh (WSO2), Arnaud Meyniel (Axway Software), Jeff Mischkinsky (Oracle Corporation), Dale Moberg (Axway Software), Anthony Nadalin (IBM Corporation), David Orchard (BEA Systems, Inc.), Sanjay Patil (SAP AG), Manjula Peiris (WSO2), Fabian Ritzmann (Sun Microsystems, Inc.), Daniel Roth (Microsoft Corporation), Tom Rutt (Fujitsu Limited), Sanka Samaranayake (WSO2), Felix Sasaki (W3C/Keio), Yakov Sverdlov (CA), Asir Vedamuthu (Microsoft Corporation), Sanjiva Weerawarana (WSO2), Ümit Yalçinalp (SAP AG), Prasad Yendluri (webMethods, Inc.).
Previous members of the Working Group were: Jeffrey Crump, Glen Daniels, Jong Lee, Monica Martin, Bob Natale, Eugene Osovetsky, Bijan Parsia, Skip Snow, Seumas Soltysik, Mark Temple-Raston.
The people who have contributed to discussions on firstname.lastname@example.org are also gratefully acknowledged.
A list of major changes since the Working Draft dated 28 September, 2007 is below:
Clarified the target audience for this document (see 1. Introduction).
Moved 20. Semantics Independent of Attachment Mechanisms from section 5.1 Assertions and Their Target Use to section 5.7.1 General Guidelines.
Added a new section: 5.3.5 Order of Behaviors.
Dropped an incorrect ignorable example in section 5.3.2 QName and XML Information Set representation.
Rewrote section: 5.6 Designating Optional Behaviors.
Updated C. References.
|20060829||UY||Created first draft based on agreed outline and content|
|20061013||UY||Editorial fixes (suggested by Frederick), fixed references, bibl items, fixed dangling pointers, created eds to do|
|20061018||MH||Editorial fixes for readability, added example for Encrypted parts|
|20061030||UY||Fixes for Paul Cotton's editorial comments (20061020)|
|20061031||UY||Fixes for Frederick's editorial comments (20061025)|
|20061031||UY||Optionality discussion feedback integration|
|20061115||MH||First attempt at restructuring to include primer content|
|20061120||MH||Restructure to address action items 64,77, which refer to bugzilla 3705 and F2F RESOLUTION 3792|
|20061127||ASV||Updated the list of editors. Added Frederick and Umit to the list of editors. Editors' action 86.|
|20061128||MH||Replaced section in Lifecycle with pointer to the text in the primer: related to action 77|
|20061129||FJH||Editorial revision (editorial actions 84 and 90) - includes suggestions from Asir: Part 1 and Part 2.|
|20061129||ASV||Formatted examples in 6.2 Evolution of Assertions (Versioning and Compatibility).|
|20061218||FS||Formatted examples in 5.2 Authoring Styles and scenario section.|
|20061219||TIB||Editorial revision: most parts of editorial action 96. Remaining editorials to be reviewed.|
|20061220||TIB||Editorial revision: completed missing parts of editorial action 96 after editorial reviews by co-editors.|
|20061226||MH||Editorial revision: reconciled terms related to "Assertion Authors" 106 and bug http://www.w3.org/Bugs/Public/show_bug.cgi?id=3983|
|20070104||UY||Resolution of Issue 3982 Based on Minutes for resolution, Minor formatting for consistent use of the term "Assertion Author"|
|20070104||UY||Resolution of Issue 3980|
|20070108||ASV||Reset Section E. Changes in this Version of the Document.|
|20070122||PY||Completed action item: 127 Resolution for issue 4197|
|20070130||UY||Completed action item: 144. Resolution for issues 3985 and 3986|
|20070130||UY||Completed action item: 137. Resolution for issue 4198|
|20070130||UY||Completed action item: 119. Resolution for issue 4141|
|20070130||UY||Completed action item: 126. Resolution for issue 4188|
|20070130||UY||Fixed SAWSDL ref name|
|20070131||FJH||Fixed numerous spelling and typo errors. Implement resolution for issue 3953 as noted in message 90 and amended as noted in message 217. Changes correspond to editor's action 152.|
|20070221||MH||Partial implementation for issue 4072 in response to editor's action 154 . NOTE ALSO- I needed to put back in the "prefix" entity defintion [line7] to get the build to work.|
|20070306||ASV||Implemented partial resolution for issue 3987. Related editorial action is 153.|
|20070308||DBO||Changed "lifecycle" spec references to versioning to fix build.|
|20070314||FJH||Implemented resolution for issue 4072 as outlined in proposal. Editorial action 204.|
|20070314||FJH||Implemented resolution for issue 3987 as outlined in proposal. Editorial action 203.|
|20070315||ASV||Implemented the resolution for issue 3979. Editors' action 198.|
|20070315||ASV||Implemented the resolution for issue 3981. Editors' action 205.|
|20070315||FJH||Implemented resolution for issue 4035 as outlined in proposal. Editorial action 197.|
|20070319||MH||Implemented resolution for issue 4073 in response to editor's action 199 as outlined in proposal .|
|20070320||ASV||Implemented the resolution for issue 4319. Editors' action 206.|
|20070320||ASV||Implemented the resolution for issue 3990. Editors' action 210.|
|20070320||ASV||Implemented the resolution for issue 4212. Editors' action 207.|
|20070321||ASV||Updated section E. Changes in this Version of the Document.|
|20070329||DBO||Changed all <p>Best Practice: to <p role="practice">|
|20070416||DBO||Updated 6.2 and 6.3 for issue 3989. Note, removed one best practice that was a dup.|
|20070423||FJH||Updated 5.5 Designating Optional Behaviors for issue 3989. Added informative reference for MTOMPolicy. Added two best practices, one is similar to G16 but focused on optional. Revised practice that was there.|
|20070425||MH||Updated 5.3 "Considerations when Modeling New Assertions" related to issue 3989. [Editorial Action 229] Restructured text to follow examples|
|20070425||TIB||Updated 5.2 Authoring Styles for issue 3989 and editors' action item 227|
|20070426||PY||Editorial changes to align with the OASIS WS-SecurityPolicy specification. For issue 4318. Editors' action 245.|
|20070427||FJH||Updated 5.5.1 Optional behavior in Compact authoring adding G7 and G8 for issue 3989 and editors' action item 250 as noted in message 69. Also replaced TBD in section 2 with descriptive text."|
|20070501||ASV||Reset Section E. Changes in this Version of the Document.|
|20070507||PY||Updated 5.6 WSDL guidelines section, to follow the new format and added G15, G16, G17 and G18. Accounts for parts of resolution for issue 3989 corresponding to editors' action items 232, 253, and 256.|
|20070507||TIB||Updated 5.1 Assertions and their Target Use for issue 3989 and editors' action item 227.|
|20070508||MH||Updated Section 5 for adding guidelines G9, G10 on ignorable, and G5 , G6 (general) to address editors' action items 251. 256.|
|20070511||PY||Updated 5.6 WSDL guidelines section to add G19 identified in AI 256 (now G24). Accounts for parts of resolution for issue 3989 corresponding to editors' action item 256 - now complete.|
|20070513||ASV||Updated Section 5.4.1 to use the new format re issue issue 3989. Editors' action 230.|
|20070514||ASV||Updated Section 5.4.2 to use the new format re issue issue 3989. Editors' action 230. Collapsed Section 5.4.2 and 5.4.3.|
|20070514||ASV||Added G11 and G13 to Section 5.4.1 and 5.4.2 re issue issue 3989. Editors' action 252 and 255.|
|20070516||PY||Editorial change to section 5.7 to place best practices after the associated explanatory text and to fix grammar.|
|20070518||PY||Ensured Best Practices G1, G3 and G20 of original IBM/MS Contribution are reflected.|
|20070518||PY||Updated Appendix E, Changes in this Version of the Document (E. Changes in this Version of the Document).|
|20070520||ASV||Added Best Practice 31. Specify Composition with Related Assertions (from the IBM and MS Contribution to 5.8 Interrelated domains. Added an ed note that Section 5.8 Interrelated domains needs to be re-structured.|
|20070520||ASV||Added Best Practice 11. Assertions should not describe message semantics (from the IBM and MS Contribution to 5.3.3 Self Describing Messages .|
|20070520||ASV||Added an ed note that Section 5.5 Designating Ignorable Behavior looks incomplete.|
|20070520||ASV||Added an ed note in Section 5.1 Assertions and Their Target Use that there is an open issue against Best Practice G2.|
|20070524||DBO||Editorial changes to align with the W3C WS-Addressing Metadata specification. For issue 4375. Editors' action 284.|
|20070529||PY||Implemented Resolution for issue 4573. Apply "Best Practices" consistently.|
|20070529||PY||Implemented Resolution in Editors' action 290. Consistent use of Assertion Authors.|
|20070529||PY||Implemented Resolution in Editors' action 291. Consistent use of should in place of must in the best practice statements.|
|20070529||PY||Implemented Resolution in Editors' action 294.|
|20070530||PY||Implemented Resolution in Editors' action 303.|
|20070530||PY||Implemented Resolution in Editors' action 304.|
|20070530||PY||Implemented Resolution in Editors' action 305.|
|20070530||PY||Implemented Resolution in Editors' action 306.|
|20070530||PY||Implemented Resolution in Editors' action 307.|
|20070530||PY||Implemented Resolution in Editors' action 308.|
|20070601||TIB||Implemented Resolution in Editors' actions 310 and 311.|
|200706013||MH||Implemented Resolution in Editors' actions 292 and 293.|
|200706016||ASV||Implemented Editors' action 289.|
|20070616||ASV||Implemented the resolution for issue 4074. Editors' action 286.|
|200706018||ASV||Implemented Editors' action 295.|
|200706018||TIB||Implemented place holder for Editors' action 249 for locking the document.|
|20070713||FJH||Restructured and updated 5.8 Interrelated domains to use Architecture of WWW format and add example, according to Editors' action 309. Updated the WSDL 20 reference [WSDL 2.0 Core Language] and WS-SecurityPolicy reference [WS-SecurityPolicy] for issue 4831. Editors' action 326|
|20070717||FJH||Implemented the resolution for issue 4853. Editors' action 333.|
|20070717||FJH||Implemented the resolution for issue 4852. Editors' action 332.|
|20070717||DBO||Implemented partial resolution, section 5.5 updates, for issue 4662, Editors' action 332 #2.|
|20070718||ASV||Implemented the resolution for issue 3988. Editors' action: 338, drop Section 7 Scenario and a worked example|
|20070718||ASV||Implemented the resolution for issue 3978. Editors' action: 339, drop Section 6 Applying Best Practices for Policy Attachment|
|20070718||DBO||Implemented the resolution for issue 4661, 4662, 4861. Editors' action: 342, 346.|
|20070718||DBO||Implemented the resolution for issue 4664. Editors' action: 343.|
|20070718||DBO||Implemented the resolution for issue 4566. Editors' action: 249, 328.|
|20070718||FJH||Implemented the resolution for issue 4862. Editors' action: 348.|
|20070718||FJH||Implemented the resolution for issue 4654. Editors' action: 340. Add new section 5.7.1 General Guidelines.|
|20070718||FJH||Updated Web Services Reliable Messaging Policy reference [Web Services Reliable Messaging Policy Assertion] and WS-Addressing Metadata reference [WS-Addressing Metadata]. Editors' action 331.|
|20070719||FJH||Implemented the resolution for issue 4859. Editors' action: 335.|
|20070727||ASV||Implemented the resolution for issue 4660. Editors' action: 342.|
|20070727||ASV||Implemented the resolution for issue 4695. Editors' action: 347.|
|20070727||ASV||Updated Section E. Changes in this Version of the Document.|
|20070806||FS||Updated references for draft publication.|
|20070912||PY||Implemented the resolution for issue 5041. Editors' action: 356.|
|20070912||FJH||Implemented the resolution for issue 5043. Editors' action 357.|
|20070913||TIB||Implemented the resolution for issue 4861. Editors' action 353 with the caveats and clarifications expressed in message 2007Sep-0002.|
|20070921||MH||Implemented the resolution for issue 5044. Editors' action 358.|
|20070921||ASV||Updated references [Web Services Policy Framework] and [Web Services Policy Attachment].|
|20070921||ASV||Reset Section E. Changes in this Version of the Document.|
|20071017||FJH||Implemented the resolution for issue 5128. Editors' action 371.|
|20071017||FJH||Implemented the resolution for issue 5185. Editors' action 373.|
|20071024||FJH||Implemented the resolution for issue 5184. Editors' action 372. Implemented as originally proposed b,c,e,f,h,j. Implemented as amended a, k,l. Did not implement g which was not processed by WG.|
|20071024||TIB||Implemented the resolution for issue 5186. Editors' action 374.|
|20071026||MH||Implemented the resolution for issue 5206. Editors' action 379.|
|20071026||MH||Implemented the resolution for issue 5189. Editors' action 381.|
|20071026||ASV||Fixed typos (re 5206 and editors' action 379). s/Example 1/SignBeforeEncrypting assertion/ and s/Example 2/EncryptBeforeSigning assertion/ in 5.3.5 Order of Behaviors.|
|20071026||ASV||Fixed incorrect changes (re 5184 and editors' action 372). a) s/Supporting New Policy Subjects/(see Section 6.3 Supporting New Policy Subjects)/ and b) s/6.3 Supporting New Policy Subjects (see Section 6.3 Supporting New Policy Subjects)/6.3 Supporting New Policy Subjects/.|
|20071026||ASV||Applied missed changes (re 5186 and editors' action 374) to B. XML Namespaces and C. References. Fixed SOAP 1.2, Web Services Reliable Messaging, Web Services Reliable Messaging Policy Assertion, WS-SecurityPolicy and WS-Trust references.|
|20071026||ASV||Updated E. Changes in this Version of the Document.|
|20071029||ASV||Incorporated Chris' proposed resolution for issue 5218: drop the second sentence in 21. Describe Semantics of Multiple Assertions of Same Type, "If there are multiple instances of a policy assertion type in the same policy alternative without parameters and nested policies, these have the same meaning as a single assertion of the type within the policy alternative." Editors' action 384.|