This document specifies the SIP P-Served-User P-header. This header field addresses an issue that was found in the 3rd Generation Partnership Project (3GPP) IMS (IP Multimedia Subsystem) between an S-CSCF (Serving Call Session Control Function) and an AS (Application Server) on the ISC (IMS Service Control) interface. This header field conveys the identity of the served user and the session case that applies to this particular communication session and application invocation. This memo provides information for the Internet community.