IMS Registration signalling flow —
User not registered
R1-3d. SIP REGISTER request, with credentials: between I-CSCF1 and HSS(18 of 22)
Upon receiving an integrity protected REGISTER request carrying the authentication challenge
response, S-CSCF1 checks that the expected response (calculated by S-CSCF1 using XRES and
other parameter as defined in RFC 3310) matches the received challenge response. If the check is
successful then the user has been authenticated and the public user identity is registered in the S-CSCF.