3Com 3C13636 Configuration Manual page 1210

3Com Router 3000 Ethernet Family
Configuration Guide
PPP Frame
L2TP Data message
L2TP Data message
(unreliable)
Figure 2-2 Architecture of L2TP protocol
The architecture of L2TP protocol shown above describes the relationship between
PPP frame, control tunnel and data tunnel. PPP frame is transmitted in unreliable L2TP
data channel. Control message is transmitted in reliable L2TP control channel.
Usually L2TP data is carried in UDP packets for transmission. L2TP registers the UDP
port 1701, but this port is only used for the tunnel setup at the early stage. L2TP tunnel
initiator selects an arbitrary port from available ones (unnecessarily being 1701) and
forwards packets to 1701 port of the receiver. After the receiver receives the packets, it
also selects a free port randomly (unnecessarily being 1701) and forwards packets
again to the specified port of the initiator. Thus, ports of the two sides are determined.
They will remain unchanged until the tunnel connection is disconnected.
2) Definitions of tunnel and session
There are two kinds of connections between LNS-LAC pairs: Tunnel connection and
Session connection. Tunnel connections define pairs of LNS and LAC while Session
connections are multiplexed in a Tunnel connection to present PPP sessions in it.
Several L2TP tunnels can be created between a LNS-LAC pair, which consist of a
control connection, and one or several Sessions. Session connections can be set up
only after tunnels are created successfully (including such information exchange as ID
protection, L2TP version, frame type, hardware transmission type, etc.). Each session
connection corresponds to a PPP data stream between LAC and LNS. Both control
messages and PPP data packets are transmitted in the tunnels.
L2TP uses Hello packets to check the connectivity of a tunnel. LAC and LNS forward
Hello packets to peer ends at regular intervals. If no response to Hello packet is
received within a certain period, the tunnel will be cleared.
3) Definitions of control message and data message
There are two kinds of messages in L2TP: control messages and data messages.
Control messages are used for the setup, maintenance and transmission control of
tunnel and session connections, while data messages are for PPP frame encapsulation
and transmission in tunnels. The transmission of control messages is reliable,
delivering flow and congestion control. On the contrary, the transmission of data
messages is unreliable, meaning it lacks mechanisms of retransmission, flow control,
and congestion control.
L2TP Control message
Packet transmission packet (UDP,......)
3Com Corporation
Chapter 2 Configuration of L2TP
L2TP Control tunnel
(reliable)
2-3