BCF-5, and controls the TrGW-1 entity. The transit and roaming function (TRF) is located in the visited network of the caller (Alice). The TRF entity receives the initial request SIP INVITE from the S-CSCF entity of the home network of the caller and forwards the request to the home network of the called party (Bob). Roaming Roaming interfaces are provided by: - Ici interfaces between the IBCF-1 and-2 IBCF instances, between the IBCF-3 and IBCF-4 instances and between the IBCF-7 and IBCF-8 instances, for the SIP stream; - Izi interface between the GW-3 and TrGW-4 entities, for the RTP stream. The interfaces for the interconnection is provided by: - Ici interface between the IBCF-5 and IBCF-6 instances for the SIP flow; - Izi interface between the TrGW-1 and TrGW-3 entities for the RTP stream. Figure 8.4 provides an example of SDP announcements containing the characteristics (IP addresses, port numbers and domain name) of the RTP stream. INVITE (190.1.15.1, 16789) S-CSCF (178.15.1.1, 62111) Domain (192.0.2.1, 49170) 183 (0.0.0.0, 16511) (192.0.2.4, 16511) Ha operatorH IBCF-2 IBCF-3 TrGW-2 TrGW-2 INVITE (179.14.1.2, 34500) (190.1.15.1, 16789) (178.15.1.1, 62111) (192.0.2.1, 49170) 183 (0.0.0.0, 16511) (192.0.2.4, 16511) Domain Xa operatorX INVITE (178.15.1.1, 62111) (192.0.2.1, 49170) 183 (0.0.0.0, 16511) (192.0.2.4, 16511) IBCF-1 IBCF-4 INVITE TrGW-1 TrGW-1 INVITE (192.0.2.1, 49170) (192.0.2.4, 16511) (192.0.2.1, 49170) (192.0.2.4, 16511) P-CSCF IBCF-5 TrGW-1 RTP flow Domain Va operatorV Figure 8.4. SDP announcements: optimal routeing The SIP INVITE request generated by the various IBCF entities maintains in the SDP message the characteristics of RTP streams VoLTE and ViLTE (IP address, port number and domain name) of the previous announcements. The IBCF-4 instance must detect from the SIP INVITE request received from the IBCF-3 instance that a looping from the home network has been achieved and must implement the OMR routeing for the RTP stream, to provide to the IBCF-5 instance the SDP message generated by Alice