Which type of messages encapsulates PPP frames in L2TP?

The correct choice is data messages. In the context of L2TP (Layer 2 Tunneling Protocol), data messages are specifically designed to carry encapsulated PPP (Point-to-Point Protocol) frames between the endpoints of an L2TP tunnel. This is critical for enabling the transfer of data over the tunnel that was originally structured in a PPP format.

Data messages are essential in providing a mechanism for wrapping the PPP frames so they can be transmitted through the L2TP tunnel, ensuring that the original data structure and integrity of the frame are maintained throughout the process. This capability allows for the routing of PPP frames over a network, making it highly useful for services like VPNs (Virtual Private Networks) which require secure data transmission.

Control messages, on the other hand, are involved in managing the setup and teardown of the tunnel but do not carry the PPP data itself. Authentication messages refer to the security checks that may occur prior to the establishment of a tunnel, while establishment messages are focused on the negotiation and management aspects of the tunnel connections, rather than the actual transportation of data.