Question: When securing the DIAMETER-based Zn reference point between a BSF (Bootstrapping Server Function) and a NAF (Network Application Function) located in **different operators' networks**, which security protocol is REQUIRED to be used as specified in relevant telecom standards?
Option 1: NDS/IP
Option 2: TLS (Transport Layer Security)
Option 3: IPsec
Option 4: PGP (Pretty Good Privacy)
Explanation: According to the requirements for securing reference points in telecom networks, when the BSF and NAF are located in different operators' networks, the DIAMETER-based Zn' reference point must be secured using TLS. This ensures confidentiality, integrity, and mutual authentication between the entities across different network domains. TLS is specifically mandated for such inter-operator scenarios to maintain security and trust.