entity the DIAMETER MAR (Multimedia-Authentication-Request) message to recover the authentication data of the mobile. 7) The S-CSCF entity receives from the HSS entity the DIAMETER MAA (Multimedia-Authentication-Answer) message containing the random number (RAND), the mobile authentication code (RES), the network authentication code (AUTN), the integrity key (IK) and the cipher key (CK) for establishing the IPSec security association. At this stage, the identity of the S-CSCF entity is registered in the HSS entity, and that of the P-CSCF entity in the S-CSCF entity. 8) The S-CSCF entity responds with a 401 Unauthorized message containing the random number (RAND), the network authentication code (AUTN), the integrity key (IK) and the cipher key (CK). The SIP 401 Unauthorized response is transmitted to the I-CSCF entity whose identity is contained in the first Via header. SIP/2.0 401 Unauthorized WWW-Authenticate: Digest t realm="ims.mnc01.mcc208.3gppnetwork.org ", nonce=base 64 (RAND + AUTN + server specific data) algorithm=AKAv1-MD5, ik="00112233445566778899aabbccddeeff". ck="ffeeddccbbaa11223344556677889900" 9) After removing the Via header containing its identity, the I-CSCF entity forwards the response to the P-CSCF entity whose identity is contained in the first Via header. 10) After removing the Via header containing its identity and the keys IK and CK from the header WWW-Authenticate, the P-CSCF entity transfers the response to the Alice's UA entity (or Bob's) whose identity is contained in the first Via header. In the header Security-Server, the P-CSCF entity indicates the parameters of the security association IPSec established with Alice's UA entity. 80 VoLTE and ViLTE SIP/2.0 401 Unauthorized WWW-Authenticate: Digest ealm="ims.mnc01.mcc208.3gppnetwork.org ", , nonce=base64 (RAND + AUTN + server specific data), algorithm=AKAv1-MD5 Security-Server: ipsec-3gpp; q=0.1; alg=hmac-sha-1-96; spi-c=98765432; spi-s=87654321; port-c=8642; port-s=7531 Upon receiving a 401 Unauthorized message, the mobile retrieves 