Question: What is the format of the Alternative Network Access Identifier (NAI) as specified in IETF RFC 4282 for use in a 3GPP network?
Option 1: <any_username>@REALM, where REALM is "unreachable.3gppnetwork.org"
Option 2: <IMSI_derived_username>@3gppnetwork.org
Option 3: <null_string>@unreachable.3gppnetwork.org
Option 4: <any_non_null_string>@3gppnetwork.org
Explanation: The Alternative NAI is specified to take the form of 'any_username@REALM' as per IETF RFC 4282, with the REALM part being "unreachable.3gppnetwork.org". This ensures that the Alternative NAI is not routable from any AAA server and contains a username part not derived from the IMSI.