Huawei v200r001 II. Control message and data message, III. Two typical L2TP tunnel modes

User Manual - Configuration Guide (Volume 3)	Chapter 2
Versatile Routing Platform	Configuration of L2TP

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

L2TP uses HELLO message to detect the connectivity of a tunnel. When the tunnel is idle for some time, LAC and/or LNS begin to transmit HELLO message to opposite end. If not receiving a reply to HELLO message for some time, the tunnel will be cleared up.

II. 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. And the data message is used to encapsulate PPP frame and transmit in the tunnel. The transmission of control message is reliable, while that of data message is not. If data message is lost, it will not be transmitted again. L2TP supports flow control and congestion control of control message instead of those of data message.

L2TP is transmitted in the form of UDP message. L2TP registers UDP1701 port, which is only used for initial tunnel establishment. Originating side of L2TP tunnel randomly selects an idle port (it is unnecessarily 1701) and transmits a message to 1701 port of receiving side. After receiving the message, the receiving side randomly selects an idle port (it is unnecessarily 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 added with L2TP header, PPP frame is encapsulated into UDP message and transmitted on TCP/IP network.

III. Two typical L2TP tunnel modes

zOriginated by remote dial-up users. Remote system accesses LAC via PSTN/ISDN, then LAC originates the request of establishing channel connection to LNS via Internet. Dial-up user addresses are distributed by LNS. The authentication and charging of remote dial-up users can be completed by the agent at LAC side or completed at LNS side.

zDirectly originated by LAC clients (the users who locally support L2TP protocol). Here, LAC clients directly originate the request of channel connection to LNS without separate LAC equipment. Here, the distribution of LAC client addresses and AAA authentication are completed by LNS.

2-3