IMS Registration signalling flow —
User not registered
R1-2. SIP 401 Unauthorized response
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S-CSCF1 selects an authentication vector for use in the authentication challenge. The authentication vector may be of the form as in TS 33.203 (if IMS AKA is the selected
AV = RANDn||AUTNn||XRESn||CKn||IKn, where:
RAND: random number used to generate the XRES, CK, IK, and part of the AUTN.
It is also used to generate the RES at the UE.
AUTN: Authentication token (including MAC and SQN).