 SIP 183 Session Progress message is forwarded without changing the SDP message to the IBCF-4 instance. 8.2.2. Session establishment for optimal routeing The session establishment procedure for the OMR routeing of the RTP streams, relating to the outgoing call, is shown in Figure 8.7. 1) Alice's UA entity transmits to the P-CSCF entity the SIP INVITE request, whose SDP message contains the characteristics (IP address and port number) of the RTP stream. 236 VoLTE and ViLTE INVITE SIP: el:+4687197378; SIP/2.0 Route: <sip:pcscf1.visitedVA.net;I lr> Content-Type : application/sdp Content-Length: ( C=IN IP4 192.0.2.1 m=audio 49170 RTP/AVP 96 97 Alice's visited network (VisitedA.net) Alice's home network Alice' visited network (VisitedA net) (HomeA.net) P-CSCF IBCF-1 IBCF-2 S-CSCF IBCF-3 IBCF-4 IBCF-5 INVITE INVITE INVITE INVITE INVITE INVITE INVITE INVITE SIP 183 SIP 183 SIP 183 SIP 183 SIP 183 SIP 183 SIP 183 SIP 183 SIP 200 SIP 200 SIP 200 SIP 200 SIP 200 SIP 200 SIP 200 SIP 200 SIP ACK SIP ACK SIP ACK SIP ACK SIP ACK SIP ACK SIP ACK SIP ACK Figure 8.7. Session establishment for optimal routeing originating side 2) The P-CSCF entity removes its URI identity in the Route header and adds that of the S-CSCF entity of the home network, indicating in the iotl parameter the direction of request (visitedA-homeA). The P-CSCF entity selects the IBCF entity of the visited network (IBCF-1 instance) and transfers the SIP INVITE request, whose Feature- Caps header contains the URI identity of the TRF entity. Roaming 237 INVITE SIP: tel:+4687197378; SIP/2.0 Route: <sip:scscf1.operatorHA.net;lr;iotl=visitedA- homeA> Feature- Caps:*;+g.3gpp.trf="<sip:trf1.visitedV.net;iotl=homeA- visitedA>" Content-Type: application/sap Content-Length: (...) C=IN IP4 192.0.2.1 m=audio 49170 RTP/AVP 96 97 3) The IBCF-1 instance selects the IBCF entity of the home network (IBCF-2 instance) and transfers the SIP INVITE message, whose SDP message replaces the RTP stream characteristics received from Alice's UA entity with those provided by the TrGW-1 e