The 3rd-Generation Partnership Project 3GPP has identified cases where different SIP private header extensions referred to as P- header fields, defined in RFC 7315, need to be included in SIP requests and responses currently not allowed according to RFC 7315. This document updates RFC 7315, in order to allow inclusion of the affected P- header fields in such requests and responses. This document also makes updates for RFC 7315 in order to fix misalignments that occurred when RFC 3455 was updated and obsoleted by RFC 7315.