User Authentication Failure

IMS-AKA determines user authentication failure as either:

• IK incorrect—If the REGISTER includes a bad IK, the P-CSCF detects this and discards the packet at the IPSEC layer. In this case, the REGISTER never reaches the S-CSCF.
• XRES incorrect—In this case, the REGISTER reaches the S-CSCF. The S-CSCF detects the incorrect XRES, the S-CSCF sends a 4xxx Auth_Failure message back to the UA via IMS.