Question: What is the format of the username part of the keyname NAI for ERP exchange with an ER server located in the 3GPP AAA Server?
Option 1: The username part is the IMSI of the UE.
Option 2: The username part is the MSISDN of the UE.
Option 3: The username part is the EMSK name as defined in IETF RFC 6696.
Option 4: The username part is the Root NAI of the UE.
Explanation: According to the specification, for ERP exchange with an ER server located in the 3GPP AAA Server, the username part of the keyname NAI shall be the EMSK name as defined in IETF RFC 6696. This is a specific requirement for the username part when interacting with the 3GPP AAA Server.