High Quality Content by WIKIPEDIA articles! The Extensible User Interface Protocol, or XUP, is a proposed web standard. XUP is a SOAP-based protocol for communicating events in a user interface, where the user interface is described by an XML document. The specification does not limit what format the XML document is in, or what event model is used for communicating over XUP. Examples given in the specification for possible user interface languages include XHTML, Wireless Markup Language, and XUL.