http://www.rfc-editor.org/rfc/rfc7315.txt
RFC 7315: Private Header (P-Header) Extensions to the Session Initiation Protocol (SIP) for the 3GPP, R. Jesske, K. Drage, C. Holmberg2014-07-01T23:00:00-00:00This document describes a set of private header (P-header) Session Initiation Protocol (SIP) fields used by the 3GPP, along with their applicability, which is limited to particular environments. The P-header fields are used for a variety of purposes within the networks that the partners implement, including charging and information about the networks a call traverses. This document obsoletes RFC 3455.