Layer 2 Tunneling Protocol
Layer 2 Tunneling Protocol (L2TP) is a combination of the Point-to-Point Tunneling Protocol (PPTP) and Layer 2 Forwarding (L2F), a technology proposed by Cisco Systems, Inc. L2TP encapsulates PPP frames that are sent over IP, X.25, Frame Relay, and ATM networks. The payloads of encapsulated PPP frames can be encrypted/compressed. When sent over the Internet, L2TP frames are encapsulated as User Datagram Protocol (UDP) messages, as shown in Figure 24.6.
Figure 24.6. Structure of the L2TP packet.
L2TP frames include L2TP connection maintenance messages and tunneled data. L2TP connection maintenance messages include only ...
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