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• LNS—An L2TP network server (LNS) functions as both the L2TP server and the PPP end
system. It is usually an edge device on an enterprise network.
An LNS is the other endpoint of an L2TP tunnel and is a peer to the LAC. It is the logical
termination point of a PPP session tunneled by the LAC. The L2TP extends the termination
point of a PPP session from a NAS to an LNS, logically.
L2TP message types and encapsulation architecture
L2TP supports the following types of messages:
• Control messages—Used to establish, maintain, and delete tunnels and close sessions.
Control messages are transmitted over a reliable control channel, which supports flow control
and congestion control.
• Data messages—Used to encapsulate PPP frames and transmit the frames over a tunnel.
Data messages are transmitted over an unreliable data channel that lacks flow control and
congestion control, and retransmission mechanisms.
Control messages and data messages share the same header format. The Type field in the L2TP
header identifies whether a message is a control message or a data message.
Figure 89 sh
ows the relationship between the PPP frame, control channel, and data channel. PPP
frames are transferred over unreliable L2TP data channels. Control messages are transferred within
reliable L2TP control channels.
Figure 89 L2TP architecture
Figure 90 L2TP packet encapsulation structure
Figure 90 depicts the encapsulation structure of an L2TP data packet between the LAC and the LNS.
Usually, L2TP data is transferred as UDP packets. The well-known UDP port for L2TP is 1701,
though this is only used in the tunnel creation stage. The L2TP tunnel initiator selects an idle port (not
necessarily 1701) to send a packet to port 1701 of the receiver. After receiving the packet, the
receiver also selects an idle port (not necessarily 1701 either) to return a packet to the specified port
of the initiator. Then, the two parties use the negotiated ports to communicate until the tunnel is
disconnected.
L2TP tunnel and session
The following types of connections are present between an LNS and an LAC:
• Tunnel—A tunnel corresponds to a LNS-LAC pair, and comprises a control connection and one
or more sessions.
• Session—A session corresponds to one PPP data stream between an LNS and a LAC and is
multiplexed on a tunnel. A session can be set up only after the tunnel is created.
Multiple L2TP tunnels can be established between an LNS and an LAC.
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