's UA entity. The response SIP 183 Session Progress from the IBCF-5 instance contains in the SDP message the characteristics of RTP streams provided by the TrGW-1 entity, which need to be transferred to Alice's UA entity. The IBCF-4 instance replaces the IP address provided by the IBCF-5 instance by the undetermined IP address 0.0.0.0 as the IP address has no meaning in the home and transit networks. The IBCF-4 instance, however, maintains in the SDP message the RTP stream characteristics provided by the IBCF-5 instance. The undetermined IP address is deleted by IBCF-1 instance and the IP address provided by the IBCF-5 instance is restored. 8.2. Procedures 8.2.1. Session establishment for nominal routeing 8.2.1.1. Originating side The session establishment procedure for the nominal routeing of the RTP streams, relating to the outgoing call, is described in Figure 8.5. To simplify the presentation, responses 100 Trying and 180 Ringing and precondition mechanisms are not shown. Table 8.1 summarizes the IP addresses and the port numbers of the RTP flows, established, on the one hand, between Alice's UA entity and the TrGW-1 entity, and on the other hand, between the TrGW-1 and TrGW-2 entities. Roaming 229 P-CSCF IBCF-1 IBCF-2 S-CSCF IBCF-3 SIP INVITE SIP INVITE SIP INVITE SIP INVITE SIP INVITE SIP 183 SIP 183 SIP 183 SIP 183 SIP 183 SIP 200 SIP 200 SIP 200 SIP 200 SIP 200 SIP ACK SIP ACK SIP ACK SIP ACK SIP ACK Figure 8.5. Session establishment for nominal routeing originating side Alice's UA TrGW-1 IP address 192.0.2.1 192.0.2.2 Port number 49170 TrGW-1 TrGW-2 IP address 178.15.1.1 190.1.15.2 Port number 62111 12538 Table 8.1. RTP flow characteristics in the case of nominal routeing originating side 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. INVITE SIP: tel:+4687197378; SIP/2.0 Route: <sip:pcscf1.visitedVA.net;lr> Content-Type application/sdp Content-Length: (...) C=IN IP4 192.0.2.1 m=audi