Question: In a Network Domain Security (NDS) Authentication Framework (AF), what is a requirement for the `extraCerts` field in the PKIMessage of an Initialization Response?
Option 1: It is optional and should only contain the operator root certificate.
Option 2: It is mandatory and must include the RA/CA certificate(s) and, if applicable, intermediate certificates up to the operator root certificate, in addition to the operator root certificate.
Option 3: It can be empty if the base station already possesses all necessary certificates.
Option 4: It should only contain the generated base station certificate.
Explanation: This is correct because the `extraCerts` field must be populated with specific certificates to ensure a complete trust chain for authentication. Including the operator root certificate, RA/CA certificate(s), and any necessary intermediate certificates ensures that the base station can properly validate the authenticity of the response. This requirement enhances security by preventing potential man-in-the-middle attacks or unauthorized access.