Web Services Metadata Exchange (WS-MetadataExchange)

1 Composable Architecture

The Web services specifications (WS-*) are designed to be composed with each other to provide a rich set of tools for the Web services environment. This specification specifically relies on other Web services specifications to provide secure, reliable, and/or transacted message delivery and to express Web service metadata.

2 Introduction

Web services use metadata to describe what other endpoints need to know to interact with them. Specifically, [WS-Policy] describes the capabilities, requirements, and general characteristics of Web services; [WSDL11] describes abstract message operations, concrete network protocols, and endpoint addresses used by Web services; XML Schema [XMLSchema - Part 1], [XMLSchema - Part 2] describes the structure and contents of XML-based messages received by and sent by Web services.

To bootstrap communication with Web services this specification defines how metadata can be treated as [WS-Transfer] resources for retrieval purposes, how metadata can be embedded in Web service endpoint references, and how Web service endpoints can OPTIONALLY support a request-response interaction for the retrieval of metadata. When the type of metadata sought is clearly known, e.g., [WS-Policy], a requester MAY indicate that only that type is to be returned; where additional types of metadata are being used, or are expected, or when a requester needs to retrieve all of the metadata relevant to subsequent interactions with an endpoint, a requester MAY indicate that all available metadata, regardless of their types, are expected.

The mechanisms defined herein are intended for the retrieval of metadata (i.e., Web service description information) only. They are not intended to provide a general purpose query or retrieval mechanism for other types of data associated with a Web service, such as state data, properties and attribute values, etc.

(01)  <s11:Envelope
(02)     xmlns:s11='http://schemas.xmlsoap.org/soap/envelope/'
(03)     xmlns:wsa='http://www.w3.org/2005/08/addressing'>
(04)     xmlns:wst='http://www.w3.org/2009/12/ws-tra'>
(05)    <s11:Header>
(06)      <wsa:Action>
(07)        http://www.w3.org/2009/12/ws-tra/Get
(08)      </wsa:Action>
(09)      <wsa:To>http://services.example.org/stockquote/metadata</wsa:To>
(10)      <wsa:ReplyTo>
(11)        <wsa:Address>http://client.example.org</wsa:Address>
(12)      </wsa:ReplyTo>
(13)      <wsa:MessageID>
(14)        urn:uuid:1cec121a-82fe-41da-87e1-3b23f254f128
(15)      </wsa:MessageID>
(16)    </s11:Header>
(17)    <s11:Body>
(18)      <wst:Get/>
(19)    </s11:Body>
(20)  </s11:Envelope>

(01)  <s11:Envelope
(02)      xmlns:s11='http://schemas.xmlsoap.org/soap/envelope/'
(03)      xmlns:wsa='http://www.w3.org/2005/08/addressing'
(04)      xmlns:mex='http://www.w3.org/2009/12/ws-mex'
(05)      xmlns:wsp='http://www.w3.org/ns/ws-policy'
(06)      xmlns:wst='http://www.w3.org/2009/12/ws-tra'
(07)      xmlns:wsdl='http://schemas.xmlsoap.org/wsdl/'>
(08)    <s11:Header>
(09)      <wsa:Action>
(10)        http://www.w3.org/2009/12/ws-tra/GetResponse
(11)      </wsa:Action>
(12)      <wsa:To>http://client.example.org</wsa:To>
(13)      <wsa:RelatesTo>
(14)        urn:uuid:1cec121a-82fe-41da-87e1-3b23f254f128
(15)      </wsa:RelatesTo>
(16)    </s11:Header>
(17)    <s11:Body>
(18)      <wst:GetResponse>
(19)        <mex:Metadata>
(20)          <mex:MetadataSection Dialect='http://schemas.xmlsoap.org/wsdl/'>
(21)            <wsdl:definitions
(22)                name='StockQuote'
(23)                targetNamespace='http://services.example.org/stockquote'
(24)                xmlns:tns='http://services.example.org/stockquote'
(25)                xmlns:msgs='http://services.example.org/stockquote/schemas'
(26)                xmlns:wsoap='http://schemas.xmlsoap.org/wsdl/soap/'
(27)                xmlns:xs='http://www.w3.org/2001/XMLSchema'>
(28)              <wsdl:import
(29)                  namespace='http://services.example.org/stockquote'
(30)                  location='http://services.example.org/stockquote/schemas' />
(31)              <wsdl:portType name='StockQuotePortType'>
(32)                <wsdl:operation name='GetLastTradePrice'>
(33)                  <wsdl:input message='msgs:GetLastTradePriceInput'
(34)                              name='GetLastTradePriceInput'/>
(35)                  <wsdl:output message='msgs:GetLastTradePriceOutput'
(36)                               name='GetLastTradePriceOutput'/>
(37)                </wsdl:operation>
(38)              </wsdl:portType>
(39)              <wsdl:binding name='StockQuoteBinding'
(40)                    type='tns:StockQuotePortType'>
(41)                <wsp:PolicyReference
(42)                    URI='http://services.example.org/stockquote/policy' />
(43)                <wsoap:binding style='document'
(44)                    transport='http://schemas.xmlsoap.org/soap/http'/>
(45)                <wsdl:operation name='GetLastTradePrice'>
(46)                  <wsoap:operation soapAction='http://services.example.org/stockquote/GetLastTradePrice' />
(47)                  <wsdl:input name='GetLastTradePriceInput'>
(48)                    <wsoap:body use='literal'/>
(49)                  </wsdl:input>
(50)                  <wsdl:output name='GetLastTradePriceOutput'>
(51)                    <wsoap:body use='literal'/>
(52)                  </wsdl:output>
(53)                </wsdl:operation>
(54)              </wsdl:binding>
(55)              <wsdl:service name='StockQuoteService'>
(56)                <wsdl:port name='StockQuotePort'
(57)                           binding='tns:StockQuoteBinding' >
(58)                  <wsoap:address
(59)                      location='http://services.example.org/stockquote' />
(60)                </wsdl:port>
(61)              </wsdl:service>
(62)            </wsdl:definitions>
(63)          </mex:MetadataSection>
(64)          <mex:MetadataSection
(65)              Dialect='http://www.w3.org/2001/XMLSchema'
(66)              Identifier='http://services.example.org/stockquote/schemas'>
(67)            <mex:Location>
(68)              http://services.example.org/stockquote/schemas
(69)            </mex:Location>
(70)          </mex:MetadataSection>
(71)          <mex:MetadataSection
(72)              Dialect='http://www.w3.org/ns/ws-policy'
(73)              Identifier='http://services.example.org/stockquote/policy'>
(74)            <mex:MetadataReference>
(75)              <wsa:Address>
(76)                http://services.example.org/stockquote/policy
(77)              </wsa:Address>
(78)            </mex:MetadataReference>
(79)          </mex:MetadataSection>
(80)        </mex:Metadata>
(81)      </wst:GetResponse>
(82)    </s11:Body>
(83)  </s11:Envelope>

(01)  <s11:Envelope
(02)      xmlns:s11='http://schemas.xmlsoap.org/soap/envelope/'
(03)      xmlns:wsa='http://www.w3.org/2005/08/addressing'
(04)      xmlns:mex='http://www.w3.org/2009/12/ws-mex' >
(05)    <s11:Header>
(06)      <wsa:To>http://services.example.org/stockquote</wsa:To>
(07)      <wsa:Action>
(08)        http://www.w3.org/2009/12/ws-mex/GetMetadata
(09)      </wsa:Action>
(10)      <wsa:MessageID>
(11)        urn:uuid:73d7edfc-5c3c-49b9-ba46-2480caee43e9
(12)      </wsa:MessageID>
(13)      <wsa:ReplyTo>
(14)        <wsa:Address>http://client.example.org</wsa:Address>
(15)      </wsa:ReplyTo>
(16)    </s11:Header>
(17)    <s11:Body>
(18)      <mex:GetMetadata>
(19)        <mex:Dialect URI="http://www.w3.org/ns/ws-policy"
(20)          Identifier="http://services.example.org/stockquote/policy"/>
(21)      </mex:GetMetadata>
(22)    </s11:Body>
(23)  </s11:Envelope>

(01)   <s11:Envelope
(02)      xmlns:s11='http://schemas.xmlsoap.org/soap/envelope/'
(03)      xmlns:wsa='http://www.w3.org/2005/08/addressing'
(04)      xmlns:wsp='http://www.w3.org/ns/ws-policy'
(05)      xmlns:mex='http://www.w3.org/2009/12/ws-mex'>
(06)    <s11:Header>
(07)      <wsa:To>http://client.example.org</wsa:To>
(08)      <wsa:Action>
(09)        http://www.w3.org/2009/12/ws-mex/GetMetadataResponse
(10)      </wsa:Action>
(11)      <wsa:RelatesTo>
(12)        urn:uuid:73d7edfc-5c3c-49b9-ba46-2480caee43e9
(13)      </wsa:RelatesTo>
(14)    </s11:Header>
(15)    <s11:Body>
(16)      <mex:GetMetadataResponse>
(17)        <mex:Metadata>
(18)          <mex:MetadataSection
(19)              Dialect='http://www.w3.org/ns/ws-policy'
(20)              Identifier='http://services.example.org/stockquote/policy'>
(21)            <wsp:Policy>
(22)              <wsp:ExactlyOne>
(23)                <!-- Policy alternatives -->
(24)              </wsp:ExactlyOne>
(25)            </wsp:Policy>
(26)          </mex:MetadataSection>
(27)        </mex:Metadata>
(28)      </mex:GetMetadataResponse>
(29)    </s11:Body>
(30)  </s11:Envelope>

3 Notation

http://www.w3.org/2009/12/ws-mex

<s:Envelope>
  <s:Header>
    [Headers]
    <wsa:Action>[Action]</wsa:Action>
    ...
  </s:Header>
  <s:Body>[Body]</s:Body> 
</s:Envelope>

4 Metadata Resources

A resource is a Web service that is addressable by an endpoint reference [WS-Addressing] and can be represented by an XML Infoset. The resource's representation can be retrieved using the Get operation defined in [WS-Transfer].

When the representation of a resource is mex:Metadata, as defined in 5 Web Services Metadata, or any other document format (e.g. [XMLSchema - Part 1],[XMLSchema - Part 2], [WSDL11], [WS-Policy]) for which a mex:MetadataSection/@Dialect has been defined, then the resource is referred as 'metadata resource'. The representation of a metadata resource MAY be retrieved and/or updated as any other [WS-Transfer] resource. Specifically, the representation of a metadata resource MUST be retrievable through a [WS-Transfer] Get operation.

A Web service endpoint MAY have one or more associated metadata resources. A Web service endpoint MAY also support direct retrieval of metadata by requesters using a GetMetadata operation directed to the Web service endpoint itself, as described in 6.2 Get Metadata.

A metadata resource MAY support other operations defined by [WS-Transfer], such as Put (e.g. to allow update of non-static metadata by authorized agents), or other resource management and access specifications (e.g. HTTP, WS-ResourceFramework).

5 Web Services Metadata

The Web service Metadata element is a collection of metadata units such as [WSDL11] definitions, [XMLSchema - Part 1] documents, [WS-Policy] expressions, etc. Each unit corresponds to metadata for a given scope, domain, or namespace. The collection of units is represented by a Metadata element, and units within the collection are represented by Metadata Section elements.

To facilitate processing, Metadata Sections are tagged with a @Dialect and (OPTIONALLY) @Identifier of the metadata unit. To ensure scalability, a unit of metadata MAY be included in-line within its Metadata Section or MAY be included by reference, either an endpoint reference [WS-Addressing] (Metadata Reference) or a URL (Location).

The outline for the Metadata element is:

<mex:Metadata ...>
  <mex:MetadataSection Dialect="xs:anyURI"
                       Identifier="xs:anyURI"? ...>
  (
    <mex:MetadataReference ...>
      endpoint-reference-type
    </mex:MetadataReference>
    |
    <mex:Location> xs:anyURI </mex:Location>
    |
    DialectSpecificElement
  )
  </mex:MetadataSection> *
  xs:any*
</mex:Metadata>

The following describes additional constraints on the outline listed above:

/mex:Metadata

This contains one Metadata Section child for each distinct unit of metadata. When there is a large amount of metadata, the children SHOULD contain Metadata References or Locations instead of the actual information.

/mex:Metadata/mex:MetadataSection

This represents a single unit of metadata (e.g., a WSDL definitions, an XML Schema document) for a given scope, domain, or namespace. A Metadata Section contains exactly one child element, either the embedded XML for the metadata unit, an endpoint reference to a Metadata Resource for the metadata unit, or a URL to metadata unit.

/mex:Metadata/mex:MetadataSection/@Dialect

This indicates the format and version of the metadata unit contained in this Metadata Section (e.g., WSDL version 1.1). Dialect is an absolute IRI.

This specification defines the following values for Dialect; other specifications SHOULD define values for Dialect for their metadata format(s).

If there is more than one metadata unit with the same Dialect, e.g., more than one XML Schema document, including them all, one per Metadata Section, is explicitly encouraged.

/mex:Metadata/mex:MetadataSection/@Dialect ="http://www.w3.org/2009/12/ws-mex/Dialects/ws-mex"

This Dialect value indicates that the type of the metadata contained within the Metadata Section is itself a mex:Metadata element. When the Dialect value is used in conjunction with mex:MetadataReference or mex:Location, the Dialect value provides the ability to include metadata by reference (an endpoint reference or a URL). Example 7-2 illustrates the use of this Dialect.

/mex:Metadata/mex:MetadataSection/@Identifier

This indicates the Identifier for the metadata unit in this Metadata Section. Identifier is an absolute IRI. If omitted, there is no implied value.

The interpretation of Identifier is Dialect-specific. While the Dialect attribute indicates the metadata format and version of the metadata in the Metadata Section, the Identifier attribute MAY be used to identify a Metadata Section or to just differentiate between Metadata Sections containing the same type of metadata. The value of the Identifier MAY be the same as the value of one of the attributes or elements of the metadata in the Metadata Section (if the metadata is included by value) or in the representation of a metadata resource (if the metadata is included by reference through Metadata Reference or Location). A metadata publisher MAY choose any value for the Identifier. The values of Identifier attributes in multiple Metadata Section elements in the same Metadata element MAY be the same. For well-known metadata formats, it is RECOMMENDED that the value of the Identifier comes from the metadata when that is possible, as the table below shows.

If there is more than one metadata section with the same identifier, e.g., more than one XML Schema in the same target namespace, including them all, one per metadata section, is explicitly encouraged.

/mex:Metadata/mex:MetadataSection/mex:MetadataReference

This is an endpoint reference to a metadata resource and is of type EndpointReferenceType as defined by [WS-Addressing]. The resource MUST support the Get operation [WS-Transfer] to allow the retrieval of the metadata unit for the Metadata Section's Dialect and Identifier (if any). When this element is present, it MUST have no element siblings.

/mex:Metadata/mex:MetadataSection/mex:Location

This contains a URL to metadata, and the metadata MUST be retrievable from that URL using the primary access mechanism for the scheme of the URL. For example, for an HTTP URL, the metadata MUST be retrievable by sending an HTTP GET request to the URL. When this element is present, it MUST have no element siblings.

/mex:Metadata/mex:MetadataSection/DialectSpecificElement

When any element other than Metadata Reference or Location is present, the element is to be interpreted as the representation of the metadata unit associated with the Metadata Section's Dialect and Identifier.

6 Retrieving Metadata

[Action]
  http://www.w3.org/2009/12/ws-mex/GetMetadata

[Body]
  <mex:GetMetadata ... >
    <mex:Dialect URI="xs:anyURI" 
                 Identifier="xs:anyURI" ?
                 Content="xs:anyURI" ? .../> *
    xs:any*
  </mex:GetMetadata> 

[Action]
  http://www.w3.org/2009/12/ws-mex/GetMetadataResponse

[Body]
  <mex:GetMetadataResponse>
    <mex:Metadata ...> ...  </mex:Metadata>
    xs:any*
  </mex:GetMetadataResponse>

7 Metadata in Endpoint References

6 Retrieving Metadata describes two mechanisms that allow requesters to retrieve ("pull metadata") Web services metadata from an endpoint (GetMetadata) or a metadata resource (WS-Transfer Get).

In addition to these mechanisms, the issuer of a service endpoint reference MAY include Web services metadata for that service inside the endpoint reference itself. This mechanism ("push metadata") simplifies the bootstrapping of the service interaction on the requester side by avoiding additional calls to retrieve (pull) the Web service metadata for the service endpoint.

Embedding a mex:Metadata element inside an endpoint reference allows the issuer of the endpoint reference to include metadata by value or by reference, according to the options described in 5 Web Services Metadata. The outline of a [WS-Addressing] endpoint reference which includes Web services metadata is as follows:

<wsa:EndpointReference ...>
    <wsa:Address>xs:anyURI</wsa:Address>
    <wsa:ReferenceParameters> xs:any* lt;/wsa:ReferenceParameters> ?
    <wsa:Metadata>
        <mex:Metadata ...> ... </mex:Metadata>?
        xs:any*
    </wsa:Metadata> ?
    xs:any*
</wsa:EndpointReference>

/wsa:EndpointReference/wsa:Metadata/mex:Metadata

A unique mex:Metadata element appearing as a direct child of the wsa:Metadata endpoint reference element.

In example Example 7-1, a [WS-Addressing] endpoint reference contains WSDL metadata by value that identifies the binding used to access the service endpoint:

(01)  <wsa:EndpointReference
(02)      xmlns:wsa='http://www.w3.org/2005/08/addressing'>
(03)    <wsa:Address>http://services.example.org/stockquote</wsa:Address>
(04)    <wsa:Metadata>
(05)      <mex:Metadata xmlns:mex='http://www.w3.org/2009/12/ws-mex'>
(06)        <mex:MetadataSection Dialect='http://schemas.xmlsoap.org/wsdl/'>
(07)          <wsdl:definitions
(08)              name='StockQuote'
(09)              targetNamespace='http://services.example.org/stockquote'
(10)              xmlns:tns='http://services.example.org/stockquote'
(11)              xmlns:wsoap='http://schemas.xmlsoap.org/wsdl/soap/'
(12)              xmlns:wsdl='http://schemas.xmlsoap.org/wsdl/'
(13)              xmlns:wsp='http://www.w3.org/ns/ws-policy'
(14)              xmlns:msgs='http://services.example.org/stockquote/schemas
(15)              xmlns:xs='http://www.w3.org/2001/XMLSchema'>
(16)            <wsdl:import
(17)                namespace='http://services.example.org/stockquote'
(18)                location='http://services.example.org/stockquote/schemas' />
(19)            <wsdl:portType name='StockQuotePortType'>
(20)              <wsdl:operation name='GetLastTradePrice'>
(21)                <wsdl:input message='msgs:GetLastTradePriceInput'
(22)                            name='GetLastTradePriceInput'/>
(23)                <wsdl:output message='msgs:GetLastTradePriceOutput'
(24)                             name='GetLastTradePriceOutput'/>
(25)              </wsdl:operation>
(26)            </wsdl:portType>
(27)            <wsdl:binding name='StockQuoteBinding'
(28)                  type='tns:StockQuotePortType'>
(29)              <wsp:PolicyReference
(30)                  URI='http://services.example.org/stockquote/policy' />
(31)              <wsoap:binding style='document'
(32)                           transport='http://schemas.xmlsoap.org/soap/http'/>
(33)              <wsdl:operation name='GetLastTradePrice'>
(34)                <wsoap:operation soapAction='http://services.example.org/stockquote/GetLastTradePrice' />
(35)                <wsdl:input name='GetLastTradePriceInput'>
(36)                  <wsoap:body use='literal'/>
(37)                </wsdl:input>
(38)                <wsdl:output name='GetLastTradePriceOutput'>
(39)                  <wsoap:body use='literal'/>
(40)                </wsdl:output>
(41)              </wsdl:operation>
(42)            </wsdl:binding>
(43)            <wsdl:service name='StockQuoteService'>
(44)              <wsdl:port name='StockQuotePort'
(45)                         binding='tns:StockQuoteBinding' >
(46)                <wsoap:address
(47)                         location='http://services.example.org/stockquote' />
(48)              </wsdl:port>
(49)            </wsdl:service>
(50)          </wsdl:definitions>
(51)        </mex:MetadataSection>
(52)        <mex:MetadataSection
(53)            Dialect='http://www.w3.org/2001/XMLSchema'
(54)            Identifier='http://services.example.org/stockquote/schemas'>
(55)          <mex:MetadataReference>
(56)            <wsa:Address>
(57)              http://services.example.org/stockquote/schemas
(58)            </wsa:Address>
(59)          </mex:MetadataReference>
(60)        </mex:MetadataSection>
(61)      </mex:Metadata>
(62)    </wsa:Metadata>
(63)  </wsa:EndpointReference>

Lines 4-62 in Example 7-1 show the way a Metadata element can be embedded in the endpoint reference of a service. The Metadata element contains two Metadata Sections. In the first one (lines 6-51) the WSDL of the Web service endpoint is included 'by value'. In the second Metadata Section (lines 52-60) an endpoint reference to a metadata resource is shown. The representation of the metadata resource is an XML Schema as hinted by the Dialect attribute (line 53) and is distinguished from other XML Schemas for that Web service endpoint by the value of Identifier attribute (line 54) which in this case happens to be the same as the value of the targetNamespace attribute of the XML Schema. The [WS-Transfer] Get operation can be used to retrieve the referred XML Schema. Since no metadata is available about the metadata resource, it is assumed that binding-related information was communicated out-of-band (e.g. an application domain-specific specification has defined a binding for that domain) so that the requester could send a [WS-Transfer] Get request to the metadata resource's endpoint, as defined in 8 Bootstrapping Metadata Retrieval.

(01)  <wsa:EndpointReference
(02)      xmlns:wsa='http://www.w3.org/2005/08/addressing'>
(03)    <wsa:Address>http://services.example.org/stockquote</wsa:Address>
(04)    <wsa:Metadata>
(05)      <mex:Metadata xmlns:mex='http://www.w3.org/2009/12/ws-mex'>
(06)        <mex:MetadataSection
(07)            Dialect='http://www.w3.org/2009/12/ws-mex'>
(08)          <mex:MetadataReference>
(09)            <wsa:Address>
(10)              http://services.example.org/stockquote/metadata
(11)            </wsa:Address>
(12)            <wsa:Metadata>
(13)              <mex:Metadata>
(14)                <mex:MetadataSection
(15)                  Dialect='http://schemas.xmlsoap.org/wsdl/'>
(16)                  <!-- Reference to WS-Transfer WSDL wst:Resource portType
(17)                     plus binding -->
(18)                </mex:MetadataSection>
(19)              </mex:Metadata>
(20)            </wsa:Metadata>
(21)          </mex:MetadataReference>
(22)        </mex:MetadataSection>
(23)      </mex:Metadata>
(24)    </wsa:Metadata>
(25)  </wsa:EndpointReference>

Example 7-2 shows an example of a Web service endpoint reference in which the Metadata element contains a single Metadata Reference element to a metadata resource (lines 8-21) the representation of which is mex:Metadata as hinted by the value of the Dialect attribute (line 7). The Metadata Reference contains a Metadata element (lines 13-19) which contains the necessary metadata for interacting with the metadata resource through [WS-Transfer] operations. Since the Metadata Reference is an endpoint reference to a [WS-Transfer] resource, the embedded metadata includes the [WS-Transfer] WSDL portType and the necessary binding information for communicating with that resource.

8 Bootstrapping Metadata Retrieval

This specification provides several mechanisms to aid service endpoints and service requesters in bootstrapping the interaction. In particular, the mechanisms described in 7 Metadata in Endpoint References allow issuers of endpoint references to include sufficient protocol binding information to allow requesters to issue a Get request against a metadata resource, or a GetMetadata request against a service endpoint, in order to retrieve all the information needed to use the service.

When that information is not available, however, requesters will rely on contextual or out-of-band information in order to choose the protocol binding most appropriate to use in retrieving Web service metadata. Furthermore, specific protocol bindings for metadata retrieval MAY be defined by communities within the context of particular application domains.

9 Exposing WSDL for Operations Implicitly Defined by a Policy Assertion

An endpoint MAY indicate that it supports a specific feature by including the feature's policy assertion within its WSDL. By doing so, the endpoint is indicating that corresponding operations (if any) are supported by that endpoint even though they do not explicitly appear in its WSDL. An example of this is an endpoint that indicates the use of WS-Transfer [WS-Transfer] by the use of the wstrp:TransferResource WS-Policy [WS-Policy] assertion.

An endpoint MAY choose to expose the WSDL of the policy defined feature by using the http://schemas.xmlsoap.org/wsdl/ dialect and the dialect identifier of the target namespace of the feature.

The Feature WSDL, the WSDL associated with these implicit operations, can be annotated to indicate any endpoint specific metadata that might be needed by clients interacting with this service. For example, the WSDL can have policy assertions that indicate a particular security mechanism used to protect the feature's operations supported by this endpoint. When a Feature WSDL does not provide a concrete endpoint, the consumer MUST use the concrete aspects of the endpoint's WSDL.

10 Security Considerations

It is strongly RECOMMENDED that the communication between Web services be secured using the mechanisms described in WS-Security [WS-Security]. In order to properly secure messages, the body and all relevant headers need to be included in the signature. Specifically, any standard messaging headers, such as those from WS-Addressing [WS-Addressing], need to be signed with the body in order to "bind" the two together.

Different security mechanisms might be desired depending on the frequency of messages. For example, for infrequent messages, public key technologies might be adequate for integrity and confidentiality. However, for high-frequency events, it might be more performant to establish a security context for the events using the mechanisms described in [WS-Trust] and [WS-SecureConversation]. Note that if a shared secret is used it is RECOMMENDED that derived keys be used to strengthen the secret as described in WS-SecureConversation.

Requests for metadata that are not available to anonymous parties are strongly RECOMMENDED to require usage of WS-Security so that the requester can be authenticated and authorized to access the indicated metadata. Similarly, integrity and confidentiality SHOULD be used whenever metadata has restricted access.

Recipients of metadata are RECOMMENDED to validate the signature to authenticate and verify the integrity of the data. Specifically, recipients SHOULD verify that the sender has the right to "speak" for the metadata. This is important because some metadata, such as schemas, have embedded target IRIs that might be outside the scope of the sender.

Additionally, some metadata formats, such as policies [WS-Policy], can have embedded security semantics. These SHOULD be verified using the same considerations outlined in this section.

The following list summarizes common classes of attacks that apply to this protocol and identifies the mechanism to prevent/mitigate the attacks:

• Message alteration - Alteration is prevented by including signatures of the message information using WS-Security.

• Message disclosure - Confidentiality is preserved by encrypting sensitive data using WS-Security.

• Key integrity - Key integrity is maintained by using the strongest algorithms possible (by comparing secured policies - see [WS-Policy] and [WS-SecurityPolicy])

• Authentication - Authentication is established using the mechanisms described in WS-Security and WS-Trust. Each message is authenticated using the mechanisms described in WS-Security

• Accountability - Accountability is a function of the type of and strength of the key and algorithms being used. In many cases, a strong symmetric key provides sufficient accountability. However, in some environments, strong PKI signatures are required.

• Availability - Metadata services are subject to a variety of availability attacks such as application-level denial of service. It is RECOMMENDED that the mechanisms described in WS-Security be considered as mitigations for some forms of attacks. Other attacks, such as network-level denial of service are harder to avoid. Note that both of these classes of attack are outside the scope of this specification.

• Replay - Messages can be replayed for a variety of reasons. To detect and eliminate this attack, mechanisms SHOULD be used to identify replayed messages such as the timestamp/nonce outlined in WS-Security. Alternatively, and OPTIONALLY, other technologies, such as sequencing, can also be used to prevent replay of application messages.

11 WS-Metadata Exchange Metadata

An endpoint MAY indicate that it supports WS-MetadataExchange, or its features, by including the WS-MetadataExchange Policy assertion within its WSDL. By doing so the endpoint is indicating that the corresponding WS-MetadataExchange operations are supported by that endpoint even though they do not explicitly appear in its WSDL (i.e. the WSDL that MAY be retrievable by using a WS-MetadataExchange GetMetadata with a Dialect IRI of http://schemas.xmlsoap.org/wsdl/).

An endpoint MAY choose to expose the WS-MetadataExchange WSDL by using the following WS-MetadataExchange Dialect:

The WS-MetadataExchange WSDL can be annotated to indicate any endpoint specific metadata that might be needed by clients interacting with this service. For example, the WSDL MAY have policy assertions that indicate a particular security mechanism used to protect the WS-MetadataExchange operations supported by this endpoint.

<mex:MetadataExchange ...>
  <mex:MetadataExchangeDialect ...>
    xs:anyURI
  </mex:MetadataExchangeDialect> * 
  <mex:MetadataContent ...> xs:anyURI </mex:MetadataContent> *
  ...
</mex:MetadataExchange>

12 Acknowledgements

This specification has been developed as a result of joint work with many individuals and teams, including: Ashok Malhotra (Oracle Corp.), Asir Vedamuthu (Microsoft Corp.), Bob Freund (Hitachi, Ltd.), Doug Davis (IBM), Fred Maciel (Hitachi, Ltd.), Geoff Bullen (Microsoft Corp.), Gilbert Pilz (Oracle Corp.), Greg Carpenter (Microsoft Corp.), Jeff Mischkinsky (Oracle Corp.), Katy Warr (IBM), Li Li (Avaya Communications), Mark Little (Red Hat), Prasad Yendluri (Software AG), Ram Jeyaraman (Microsoft Corp.), Sreedhara Narayanaswamy (CA), Sumeet Vij (Software AG), Vikas Varma (Software AG), Wu Chou (Avaya Communications), Yves Lafon (W3C).

13 References