3 Relation to XML

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All SOAP messages are encoded using XML (see [7] for more information on XML).

A SOAP application SHOULD include the proper SOAP namespace on all elements and attributes defined by SOAP in messages that it generates. A SOAP application MUST be able to process SOAP namespaces in messages that it receives. It MUST discard messages that have incorrect namespaces (see section 4.4) and it MAY process SOAP messages without SOAP namespaces as though they had the correct SOAP namespaces.

SOAP defines two namespaces (see [8] for more information on XML namespaces):

• The SOAP envelope has the namespace identifier "http://schemas.xmlsoap.org/soap/envelope/"
• The SOAP serialization has the namespace identifier "http://schemas.xmlsoap.org/soap/encoding/"

A SOAP message MUST NOT contain a Document Type Declaration. A SOAP message MUST NOT contain Processing Instructions. [7]

SOAP uses the local, unqualified "id" attribute of type "ID" to specify the unique identifier of an encoded element. SOAP uses the local, unqualified attribute "href" of type "uri-reference" to specify a reference to that value, in a manner conforming to the XML Specification [7], XML Schema Specification [11], and XML Linking Language Specification [9].

With the exception of the SOAP mustUnderstand attribute (see section 4.2.3) and the SOAP actor attribute (see section 4.2.2), it is generally permissible to have attributes and their values appear in XML instances or alternatively in schemas, with equal effect. That is, declaration in a DTD or schema with a default or fixed value is semantically equivalent to appearance in an instance.