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17. XML Extensibility in DAV

17. XML Extensibility in DAV​

The XML namespace extension ([REC-XML-NAMES]) is used in this specification in order to allow for new XML elements to be added without fear of colliding with other element names. Although WebDAV request and response bodies can be extended by arbitrary XML elements, which can be ignored by the message recipient, an XML element in the "DAV:" namespace SHOULD NOT be used in the request or response body unless that XML element is explicitly defined in an IETF RFC reviewed by a WebDAV working group.

For WebDAV to be both extensible and backwards-compatible, both clients and servers need to know how to behave when unexpected or unrecognized command extensions are received. For XML processing, this means that clients and servers MUST process received XML documents as if unexpected elements and attributes (and all children of unrecognized elements) were not there. An unexpected element or attribute includes one that may be used in another context but is not expected here. Ignoring such items for purposes of processing can of course be consistent with logging all information or presenting for debugging.

This restriction also applies to the processing, by clients, of DAV property values where unexpected XML elements SHOULD be ignored unless the property's schema declares otherwise.

This restriction does not apply to setting dead DAV properties on the server where the server MUST record all XML elements.

Additionally, this restriction does not apply to the use of XML where XML happens to be the content type of the entity body, for example, when used as the body of a PUT.

Processing instructions in XML SHOULD be ignored by recipients. Thus, specifications extending WebDAV SHOULD NOT use processing instructions to define normative behavior.

XML DTD fragments are included for all the XML elements defined in this specification. However, correct XML will not be valid according to any DTD due to namespace usage and extension rules. In particular:

  • Elements (from this specification) are in the "DAV:" namespace,
  • Element ordering is irrelevant unless otherwise stated,
  • Extension attributes MAY be added,
  • For element type definitions of "ANY", the normative text definition for that element defines what can be in it and what that means.
  • For element type definitions of "#PCDATA", extension elements MUST NOT be added.
  • For other element type definitions, including "EMPTY", extension elements MAY be added.

Note that this means that elements containing elements cannot be extended to contain text, and vice versa.

With DTD validation relaxed by the rules above, the constraints described by the DTD fragments are normative (see for example Appendix A). A recipient of a WebDAV message with an XML body MUST NOT validate the XML document according to any hard-coded or dynamically-declared DTD.

Note that this section describes backwards-compatible extensibility rules. There might also be times when an extension is designed not to be backwards-compatible, for example, defining an extension that reuses an XML element defined in this document but omitting one of the child elements required by the DTDs in this specification.

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