Question: What determines the structure of a Fast Re-authentication Network Access Identifier (NAI) when the 3GPP AAA server returns an incomplete NAI for EAP-SIM or EAP-AKA?
Option 1: The MCC and MNC of the Selected PLMN always dictate the realm, regardless of the permanent user identity.
Option 2: The username part is taken from the fast re-authentication identity, combined with the realm from the permanent user identity (which could be a root or decorated NAI).
Option 3: The Fast Re-authentication NAI always mirrors the format of the IMSI, incorporating the MCC and MNC directly into the username part.
Option 4: The structure is determined by the type of EAP method used (EAP-SIM or EAP-AKA), with each having a distinct formatting rule.
Explanation: When the 3GPP AAA server does not return a complete NAI, the Fast Re-authentication NAI is constructed by combining the username part from the returned fast re-authentication identity with the realm portion from the permanent user identity. This ensures continuity and correct identification within the network, especially in scenarios involving different PLMNs (Public Land Mobile Networks). The permanent user identity's realm provides the necessary context for authentication, while the username part from the AAA server ensures the uniqueness of the identifier.