Question: What action does a User Equipment (UE) take when it detects an out-of-range sequence number in the Authentication Token (AUTN) received from the network during the Security Mode 6 (SM6) procedure?
Option 1: The UE sends a REGISTER message to the Proxy-Call/Session Control Function (P-CSCF) indicating synchronization failure, potentially using an existing Security Association (SA).
Option 2: The UE initiates a new authentication request to the network by sending a fresh Authentication Request message.
Option 3: The UE silently discards the AUTN and waits for a retry from the network without taking any proactive action.
Option 4: The UE attempts to correct the sequence number before responding back to the network.
Explanation: When a UE encounters an out-of-range sequence number in the AUTN during SM6, it signals this synchronization failure to the P-CSCF via a REGISTER message. This message may utilize an already established SA for transmission. Upon receiving this notification, the P-CSCF proceeds to delete any newly created SAs, addressing the synchronization issue. This process ensures security and synchronicity between the UE and the network are maintained.