L2TP message types and encapsulation structure
L2TP uses the following types of messages:
Control messages—Used to establish, maintain, and delete L2TP tunnels and sessions. Control messages are transmitted over a reliable control channel, which supports flow control and congestion control.
Data messages—Used to encapsulate PPP frames, as shown in Figure 21. Data messages are transmitted over an unreliable data channel and are not retransmitted when packet loss occurs. Data messages can use sequence numbers to reorder packets that are disordered during transport.
Figure 21: Data message format
As shown in Figure 22, both control messages and data messages are encapsulated in UDP datagrams.
Figure 22: L2TP encapsulation structure
