Question: In SIP Digest based authentication, how does a UE typically respond when receiving a challenge with the `stale` parameter set to `TRUE` in the `WWW-Authenticate` header?
Option 1: The UE ignores the stale parameter and proceeds with the original registration request.
Option 2: The UE retries the REGISTER request with a new response, computing the Digest over the newly provided nonce.
Option 3: The UE sends an error message to the S-CSCF indicating it cannot handle stale challenges.
Option 4: The UE prompts the user to re-enter their username and password.
Explanation: When the `stale` parameter is set to `TRUE`, it indicates a synchronization failure detected by the S-CSCF, prompting the UE to refresh its authentication attempt with a new nonce. This ensures the UE's response remains valid and secure, addressing potential synchronization issues or replay attacks. By retrying with a new Digest computed over the latest nonce, the UE aligns with the network's expected security posture, facilitating successful registration.