ed message DIAMETER IDR. 9.2.2.2. Session establishment at originating side The procedure for establishing the session implementing e-SRVCC function for an outgoing call is given in Figure 9.13. 1) The establishment of the session is started by the SIP INVITE request transmitted by Alice's UA entity to Bob's UA entity. The body of the SDP message contains the characteristics of the RTP stream of the offer of Alice's UA entity: - IPv4 address (192.1.1.1); - port number (3456). Service Centralization and Continuity P-CSCF S-CSCF HomeB.net SIP INVITE SIP 100 SIP INVITE SIP 100 SIP INVITE SIP INVITE SIP 100 SIP 100 SIP INVITE SIP 100 SIP INVITE SIP 100 SIP 183 SIP 183 SIP 183 SIP 183 SIP 183 SIP 183 SIP PRACK SIP PRACK SIP PRACK SIP PRACK SIP PRACK SIP PRACK SIP 200 SIP 200 SIP 200 SIP 200 SIP 200 SIP 200 SIP UPDATE SIP UPDATE SIP UPDATE SIP UPDATE SIP UPDATE SIP UPDATE SIP 200 SIP 200 SIP 200 SIP 200 SIP 200 SIP 200 SIP 180 SIP 180 SIP 180 SIP 180 SIP 180 SIP 180 SIP 200 SIP 200 SIP 200 SIP 200 SIP 200 SIP 200 SIP ACK SIP ACK SIP ACK SIP ACK SIP ACK SIP ACK Figure 9.13. Session establishment for service continuity originating side VoLTE and ViLTE The Route headers contain the identities of the P-CSCF and S-CSCF entities, Alice's UA entity does not know the identity of the ATCF entity which, however, is located between the P-CSCF and S-CSCF entities. 2) As the SIP 200 OK response to the SIP REGISTER request contains the header Feature - Caps with parameter +g. 3 gpp atcf, the P-CSCF entity forwards the SIP INVITE request to the ATCF entity. 3) Upon receipt of the SIP INVITE message, the ATCF entity reserves the resource with the ATGW entity providing the anchor point for the RTP stream. The ATCF entity transfers to the S-CSCF entity the SIP INVITE request whose associated SDP message, sent to the domain HomeB net, is that of ATGW entity to replace that of Alice's UA entity: - Alice's IP address (192.1.1.1) is replaced by the value communicated by the ATGW entity (200.1.1.2); - the port number (3456) is replaced by th