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XML and the Infoset
For all messaging systems, the selection of the unit of information transfer is an important decision. Simply put, a common understanding of exactly what constitutes a message is required. In Web services, a message is an XML document information item as defined by the XML Information Set, or Infoset [XML-Infoset]. The Infoset is an abstract data model that is compatible with the text-based XML 1.0 [XML-10] and is the foundation of all modern XML specifications (XML Schema [XML-Schema], XML Query [XML-Query], and XSLT 2.0 [XSLT-20]). By basing the Web services architecture on the XML Infoset, rather than on a specific representation format, the architecture and core protocol components are compatible with alternative encodings.The Infoset models an XML document in terms of a set of "information items." The set of possible information items generally maps to the various features in an XML document, such as elements, attributes, namespaces, and comments. Each information item has an associated set of properties that provide a more complete description of the item. The eleven types of information items in an XML document are described in Appendix B, "XML Infoset Information Items." Every well-formed XML document consists of exactly one document information item and at least one element information item.In addition to the pure text-based encoding of the Infoset, the Web services architecture also supports an Infoset encoding that allows opaque binary data to be interleaved with traditional text-based markup. The World Wide Web Consortium (W3C) XML-binary Optimized Packaging, or XOP [XOP], format uses multipart MIME [MIME] to allow raw binary data to be included into an XML 1.0 document without resorting to base64 encoding. A companion specification, SOAP Message Transmission Optimization Method, or MTOM [MTOM], then specifies how to bind this format to SOAP. XOP and MTOM are the preferred approach for mixing raw binary with text-based XML and replace the now deprecated SOAP with Attachments (SwA) and WS-Attachments/DIME.
