 resource identifier (URI); - subscription by the mobile and the P-CSCF entity for mobile registration events; VoLTE and ViLTE - notification by the S-CSCF entity of the events concerning the registration of the mobile. The registration process to the IMS network is shown in Figure 3.2. P-CSCF I-CSCF S-CSCF SIP REGISTER SIP REGISTER DIAMETER UAR DIAMETER UAA SIP REGISTER DIAMETER MAR DIAMETER MAA SIP 401 Unauthorized SIP 401 Unauthorized SIP 401 Unauthorized SIP REGISTER SIP REGISTER DIAMETER LIR DIAMETER LIA SIP REGISTER DIAMETER SAR DIAMETER SAA SIP 200 OK SIP 200 OK SIP 200 OK SIP REGISTER DIAMETER UDR DIAMETER UDA SIP SUBSCRIBE SIP 200 OK SIP SUBSCRIBE SIP 200 OK SIP 200 OK SIP SUBSCRIBE SIP 200 OK SIP SUBSCRIBE SIP 200 OK SIP NOTIFY SIP NOTIFY SIP 200 OK SIP 200 OK SIP NOTIFY SIP 200 OK SIP NOTIFY SIP 200 OK Figure 3.2. Mobile registration to IMS network Basic Procedures 1) Alice's user agent (UA) sends to the P-CSCF entity an initial REGISTER request containing her private identity (alice_private@homeA.net) in the parameter username in the header Authorization. The header To contains the public identity of Alice's UA entity (sip: alice@homeA.net). The header Contact contains the IP address of Alice's UA entity. The header Security-Client contains the parameters defining the security association IPSec established with the P-CSCF entity. The headers Require and Proxy-Require contain the value sec- agree, indicating that the security mechanism based on IPSec is required. The header Proxy-Require is addressed to the P-CSCF entity. The header Require is addressed to the S-CSCF entity in case the request is transmitted directly to it. REGISTER sip:ims.mnc01.mcc208.3gppnetwork.org SIP/2.0 To: <sip:alice@homeA.net> Contact: <sip:192.0.2.101> Authorization: Digest Bername="alice_private@homeA.net" realm=" ims.mnc01.mcc208.3gppnetwork.org ", nonce="", uri="sip:ims.mnc01.mcc208.3gppnetwork.org",response=" Security-Client: ipsec-3gpp; alg=hmac-sha-1-96; spi- - c=23456789; spi-s=12345678; port-c=2468; port-s=1357 Requir