3Com MSR 50 Series Configuration Manual page 1604

1604 C 82: L2TP C
HAPTER ONFIGURATION
3 Control message and data message
L2TP supports two types of messages: control message and data message.
Control messages are intended for establishment and maintenance of tunnels
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and sessions and for transmission control. Control messages are transmitted
over a reliable channel, which supports flow control and congestion control.
Data messages are intended to encapsulate PPP frames to be tunneled. Data
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messages are transmitted over an unreliable channel without flow control and
congestion control. Data messages, if lost, are not to be retransmitted.
Control messages and data messages share the same header structure. An L2TP
header contains a tunnel ID and a session ID, which are used to identify the tunnel
and session. Packets with the same tunnel ID but different session IDs are
multiplexed to the same tunnel. The tunnel ID and session ID in a header are those
of the intended receiver, not the sender.
Two typical L2TP tunnel modes
Figure 450 shows two typical tunnel modes:
Tunnel between a remote system and the LNS.
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Tunnel between an LAC client and the LNS.
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Figure 450 Two typical L2TP tunnel modes
PSDN/ISDN
Remote system
Different tunnel mode has different tunnel initiator:
A tunnel between a remote user and an LNS can be initiated by the remote
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dial-up user. In this mode, the remote system dials in the LAC through PSTN or
ISDN and then the LAC initiates a tunneling request to the LNS over the
Internet. Upon receipt of the tunneling request, the LNS assigns an address to
the dial-up user. Authentication and accounting of the remote dial-up user can
be implemented by either the agent on the LAC side or the LNS.
A tunnel between an LAC client and an LNS is initiated directly by the LAC
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client. In this mode, the LAC client directly initiates a tunneling request to the
LNS, without requiring an independent LAC. The LNS assigns an address to the
LAC client.
L2TP tunnel establishment
Figure 451 shows a typical L2TP network.
LAC client
LAC
Internet
Frame relay
or ATM
LAC
LNS Inner server
Inner server
LNS