Question: What action does a UE take when receiving a ProSe direct link establishment request from a 5G ProSe remote UE or end UE, specifically for establishing a secure PC5 unicast link?
Option 1: The UE sends an authentication request to the network.
Option 2: The UE allocates a PRTI value and creates a RELAY KEY REQUEST message with specific parameters.
Option 3: The UE initiates a key agreement procedure without allocating a PRTI value.
Option 4: The UE establishes a secure PC5 unicast link directly with the remote UE or end UE.
Explanation: According to the specified procedure, upon receiving a ProSe direct link establishment request, the UE allocates a PRTI value and creates a RELAY KEY REQUEST message. This message includes specific parameters such as SUCI or CP-PRUK ID, relay service code, and nonce_1 received from the remote UE or end UE. The UE then sends this message to initiate the authentication and key agreement procedure for establishing a secure PC5 unicast link.