http://www.rfc-editor.org/rfc/rfc5002.txt
RFC 5002: The Session Initiation Protocol (SIP) P-Profile-Key Private Header (P-Header), G. Camarillo, G. Blanco2007-08-01T23:00:00-00:00This document specifies the SIP P-Profile-Key P-header. This header field is used in the 3rd-Generation Partnership Project (3GPP) IMS (IP Multimedia Subsystem) to provide SIP registrars and SIP proxy servers with the key of the profile corresponding to the destination SIP URI of a particular SIP request. This memo provides information for the Internet community.