Question: What method is used for authentication between Security Edge Protection Proxy (SEPP) and Service Communication Proxy (SCP) within one Public Land Mobile Network (PLMN) if the PLMN uses protection at the transport layer?
Option 1: Mutual authentication using NDS/IP
Option 2: Authentication provided by the application layer
Option 3: Authentication provided by the transport layer protection solution
Option 4: Physical security measures
Explanation: According to the 3GPP specification, if a PLMN uses protection at the transport layer, authentication between SEPP and SCP within that PLMN shall use the authentication provided by the transport layer protection solution. This ensures secure communication between these entities within the network.