http://www.rfc-editor.org/rfc/rfc5263.txt
RFC 5263: Session Initiation Protocol (SIP) Extension for Partial Notification of Presence Information, M. Lonnfors, J. Costa-Requena, E. Leppanen, H. Khartabil2008-09-01T23:00:00-00:00By default, presence delivered using the presence event package for the Session Initiation Protocol (SIP) is represented in the Presence Information Data Format (PIDF). A PIDF document contains a set of elements, each representing a different aspect of the presence being reported. When any subset of the elements change, even just a single element, a new document containing the full set of elements is delivered. This memo defines an extension allowing delivery of only the presence data that has actually changed. [STANDARDS-TRACK]