ATIS 1000098

As specified in IETF RFC 4412, Communications Resource Priority for the Session Initiation Protocol (SIP), the Session Initiation Protocol (SIP) Resource-Priority Header (RPH) field may be used by SIP user agents, including Public Switched Telephone Network (PSTN) gateways and terminals, and SIP proxy servers to influence prioritization afforded to communication sessions, including PSTN calls. As discussed in 3GPP TS 24.229, Technical Specification Group Services and System Aspects; IP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP); Stage 3, where the network has a requirement to prioritize emergency calls, it can use the "esnet" namespace in the Resource-Priority Header field (as defined in IETF RFC 7135, Registering a SIP Resource Priority Header Field Namespace for Local Emergency Communications, to do so. Where the Resource-Priority Header field is used for this purpose, it is inserted by the entity identifying the emergency call, i.e., the Proxy Call Session Control Function (P-CSCF) or the Interconnection Border Control Function (IBCF). There is no usage of this namespace from the User Agent (UA), and when this namespace is used, the trust domain implementation removes it if set by the UA.