interconnection with other IMS networks is described in Figure 6.11. Alice's network (HomeA net) Bob's network (HomeB net) P-CSCF S-CSCF I-CSCF S-CSCF P-CSCF RTP flow SIP flow Figure 6.11. Interconnection with IMS network functional architecture of IMS network In case of an outgoing call, the S-CSCF entity detects that the called subscriber belongs to a different domain and forwards the INVITE request to the BGCF entity that retains its search function of the entity in charge of control interconnection. The interconnection border control function (IBCF) is the gateway that allows the access of the SIP flow to another IMS network. The IBCF entity can make the translation of IP addresses and port numbers, corresponding to the network address and port translation (NAPT). The IBCF entity can perform the translation of IP addresses, port numbers and conversion of IPv4 to IPv6, corresponding to the NAPT-PT (Protocol Translation) function. The IBCF entity performs the masking of the topology of the IMS network, The IBCF entity performs the withdrawal of some headers of the SIP message based on the rules established by the operator, corresponding to the function topology hiding interconnect gateway (THIG). Interconnections The transition gateway (TrGW) is the entity that anchors the RTP stream and allows access traffic to another IMS network. The TrGW entity may perform filtering, transcoding and NAPT or NAPT-PT translation of the RTP streams under the control of the IBCF entity. The interconnection between IMS networks is provided by: - the Ici interface between IBCF entities for the SIP flow; - the Izi interface between TrGW entities for the RTP stream. 6.2.2. Session establishment Table 6.1 summarizes the IP addresses and port numbers for the different RTP streams. Alice's UA TrGW (HomeA. net) IP address 192.0.2.1 192.0.2.2 Port number 49170 56743 TrGW (HomeA. net) TrGW (HomeB net) IP address 178.15.1.1 178.15.1.2 Port number 62111 33248 TrGW (HomeB.net) Bob's UA IP address 190.1.15.1 190.1.15.2 Port number 12538 2415