DIAMETER RAA DIAMETER AAA SIP 200 OK SIP 200 OK SIP 200 OK SIP ACK SIP ACK SIP ACK Figure 3.6. Establishment of VoLTE session: terminating side VoLTE and ViLTE 5) The I-CSCF entity adds the header Via, does not add a Record Route and forwards the request to the S-CSCF entity. The subsequent requests, therefore, do not pass through the I-CSCF entity. 6) The S-CSCF entity responds to the I-CSCF entity with 100 Trying message that allows for blocking the retransmission timer of the INVITE request. 7) The S-CSCF entity verifies that the types of media proposed by Alice's UA entity correspond to the services offered to Bob. The S-CSCF entity adds a header Record Route containing its own identity. The S-CSCF entity modifies the initial request, replacing Bob's URI with his IP address. The link between the URI and the IP address was created at registration. The S-CSCF entity adds Bob's URI to the header P-Called-Party- - ID SO that Bob knows which URI the INVITE request refers to. The S-CSCF entity transfers the request to the P-CSCF entity. The IP address of the P-CSCF entity was learnt at registration (information contained in the header Path). 8) The P-CSCF entity responds to the S-CSCF entity with 100 Trying message that allows for blocking the retransmission timer of the INVITE request. 9) The P-CSCF entity adds a header Record Route containing its own identity and forwards the request to Bob's UA entity, whose IP address is included in the URI of the request. 10) Bob's UA entity entity responds to the P-CSCF entity with 100 Trying message that allows for blocking the retransmission timer of the INVITE request. Bob's UA entity stores the different headers Record Route, which it will later use to route subsequent requests. Basic Procedures 11) Bob's UA entity sends a response 183 Session Progress to Alice's UA entity. The response is routed on the basis of the Via headers received from the request INVITE. The response 183 Session Progress contains the Record Route headers received from the request INVITE. This enabl