IMS Registration signalling flow —
User not registered
R1-1f. SIP REGISTER request: between S-CSCF1 and HSS(7 of 22)
As the REGISTER request arrived without integrity protection to P-CSCF1, S-CSCF1 shall
challenge it. For this, S-CSCF1 requires at least one authentication vector (AV) to be used in the challenge
to the user. If a valid AV is not available, then S-CSCF1 requests at least one AV from the HSS.
S-CSCF1 indicates to the HSS that it has been assigned to serve this user.