http://www.rfc-editor.org/rfc/rfc6993.txt
RFC 6993: Instant Messaging and Presence Purpose for the Call-Info Header Field in the Session Initiation Protocol (SIP), P. Saint-Andre2013-07-01T23:00:00-00:00This document defines and registers a value of "impp" ("instant messaging and presence protocol") for the "purpose" header field parameter of the Call-Info header field in the Session Initiation Protocol (SIP).