By 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.