604CHAPTER 43: CONFIGURING L2TP

The L2TP header includes the information of tunnel and session IDs, which are used to identify different tunnels and sessions. The messages with the same tunnel ID and different session IDs is multiplexed in one tunnel. Tunnel ID and session ID are distributed to the opposite end of the tunnel.

L2TP detects the connectivity of a tunnel using a Hello message. When the tunnel is idle for some time, LAC and LNS begin to transmit the Hello message to the opposite end. If no response to the Hello message is received for some time, the sessionis cleared up.

Control message and data message

L2TP has two types of messages: control message and data message. The control message is used to establish, maintain and transmit the tunnel and session connection. The data message is used to encapsulate the PPP frame and transmit it in the tunnel. The transmission of a control message is reliable, but data message transmission is not reliable. If a data message is lost, it is not transmitted again. L2TP supports flow control and congestion control only for control messages, not for data messages.

L2TP is transmitted in the form of a UDP message. L2TP registers UDP Port 1701, which is used only for initial tunnel establishment. Originating side of L2TP tunnel randomly selects an idle port (it need not to be 1701) and transmits a message to 1701 port of receiving side. After receiving the message, the receiving side randomly selects an idle port (it need not to be 1701 and transmits a message back to the specified port of the originating side. By now, the selected ports of both sides are selected and remain unchanged during the time segment when the tunnel is connected.

After being transmitted to L2TP and encapsulated with L2TP header, the PPP frame will be eventually encapsulated into UDP messages and transmitted on a TCP/IP network.

IV. Call setup flow of L2TP tunnel

Call setup flow of L2TP tunnel is shown in the following figure:

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