H3C Technologies H3C SecPath F5020 - Manual (Page 32)

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L2TP message types and encapsulation structure
L2TP uses the following types of messages:
• Control messages—Used to establish, maintain, and delete L2TP tunnels and sessions. Control
messages are transmitted over a reliable control channel, which supports flow control and
congestion control.
• Data messages—Used to encapsulate PPP frames, as shown in Figure 5 . Data messages are
transmitted over an unreliable data channel and are not retransmitted when packet loss occurs.
Data messages can use sequence numbers to reorder packets that are disordered during transport.
Figure 5 Data message format
As shown in Figure 6 , both control messages and data messages are encapsulated in UDP datagrams.
Figure 6 L2TP encapsulation structure
L2TP tunnel and session
An L2TP tunnel is a virtual point-to-point connection between an LAC-LNS pair. Multiple L2TP tunnels can
be established between an LNS and an LAC. A tunnel can carry one or more L2TP sessions. Each L2TP
session corresponds to a PPP session and is multiplexed on a tunnel. An L2TP session is established
between the LAC and LNS when an end-to-end PPP session is established between a remote system and
the LNS. Data frames for the PPP session are transmitted over the tunnel between the LAC and LNS.
L2TP tunneling modes and tunnel establishment process
Three tunneling modes are available: NAS-initiated, client-initiated, and LAC-auto-initiated.
NAS-initiated tunneling mode
As shown in Figure 7 , a remote system dials in to the LAC through a PPPoE/ISDN network, and the LAC
initiates a tunneling request to the LNS over the Internet.