Question: What is the correct format for an Emergency NAI for Limited Service State when a UE performs an emergency attach and IMSI is not available or not authenticated?
Option 1: imei<IMEI>@emergency.com
Option 2: mac<MAC>@sos.valid
Option 3: imei<IMEI>@sos.invalid or mac<MAC>@sos.invalid
Option 4: username@epc.mncXXX.mccYYY.3gppnetwork.org
Explanation: The correct format for an Emergency NAI for Limited Service State is specified in the 3GPP TS 23.003, which states that it shall take the form of an NAI with the username@realm format, where the exact format is imei<IMEI>@sos.invalid or mac<MAC>@sos.invalid. This is used when a UE performs an emergency attach and IMSI is not available or not authenticated. The use of ".invalid" as the top-level domain is intentional, as it is a reserved domain that does not need to be resolved for routing purposes.