While XCAP allows several authorized users or devices to modify the same XML document, XCAP does not provide an effective synchronization mechanism (except polling) to keep resources equivalent between the server and the client. "xcap-diff" event package together with the SIP event notification framework [RFC3265] and the XCAP-diff format [I-D.ietf-simple-xcap-diff], allows a user to subscribe to changes in an XML document, and to receive notifications whenever a change in an XML document takes place.