User Manual - Configuration Guide (Volume 3)
Versatile Routing Platform Chapter 2
Configuration of L2TP
2-3
L2TP header includes the information of Tunnel ID and Session ID, which are us ed t o
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. W hen 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 clear ed up.
II. Control message and data message
L2TP has two types of messages: control message and data message. The c ontrol
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 t unnel.
The transmission of control message is reliable, while that of data m essage is n ot. 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 r andomly
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 r andomly 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 rem ain
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
z Originated by remote dial-up users. Remote system accesses LAC via
PSTN/ISDN, then LAC originates the request of establishing channel c onnection
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.
z Directly 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.