3Com MSR 50 Series Configuration Manual page 1603

AN LNS, an end system of an L2TP tunnel, is the peer of an LAC. It is the logical
termination point of a PPP session that is tunneled by the LAC.
Technical specifications
1 L2TP architecture
Figure 448 shows the relationship between the PPP frame, control channel, and
data channel. PPP frames are transferred over the unreliable L2TP data channels
while control messages are transferred within the reliable L2TP control channels.
Figure 448 L2TP architecture
PPP frame
L2TP data message
L2TP data channel
Packet transmission network
Figure 449 L2TP packet encapsulation structure
IP header
UDP header
Public network
Figure 449 depicts the encapsulation structure of an L2TP data packet between
the LAC and the LNS. Usually, L2TP data is transferred in the form of user data
protocol (UDP) packets. The well-known UDP port for L2TP is 1701, which is only
used in the initial tunnel creation stage. The L2TP tunnel initiator selects an idle
port (which is unnecessarily 1701 either) to send a packet to port 1701 of the
receiver. After receiving the packet, the receiver also selects an idle port (which is
unnecessarily 1701, too) to return a packet to the specified port of the initiator.
From then on, the two parties use the negotiated ports to communicate until the
tunnel is disconnected.
2 Tunnel and session
Two types of connections are present between an LNS and an LAC: tunnel and
session
A tunnel corresponds to an LNS-LAC pair.
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A session is multiplexed on a tunnel and represents a PPP session carried within
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the tunnel.
Multiple L2TP tunnels can exist between an LNS-LAC pair. A tunnel consists of a
control connection and one or more sessions.
A session can be set up only after a tunnel is created successfully. A session
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corresponds to one PPP data stream between the LAC and the LNS.
Both control messages and PPP frames are transferred on the tunnel.
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L2TP uses Hello packets to check the connectivity of a tunnel. The LAC and LNS
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regularly send Hello packets to each other. If no response packet is received in a
certain period of time, the tunnel is torn down.
L2TP control message
L2TP control channel
L2TP header PPP header
L2TP Overview 1603
IP header
Data
Private network