L2Tp Tunnel And Session; L2Tp Tunneling Modes And Tunnel Establishment Process - H3C SR8800-F Configuration Manual

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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.
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Data messages—Used to encapsulate PPP frames, as shown in
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 74 Data message format
As shown in
datagrams.
Figure 75 L2TP encapsulation structure

L2TP tunnel and session

An L2TP tunnel is a virtual point-to-point connection between an LAC and an LNS. Multiple L2TP
tunnels can be established between an LNS and an LAC. An L2TP tunnel can carry one or more
L2TP sessions. Each L2TP session corresponds to a PPP session and is multiplexed on an L2TP
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

L2TP tunneling modes include NAS-initiated, client-initiated, and LAC-auto-initiated.
NAS-initiated tunneling mode
As shown in
LAC initiates a tunneling request to the LNS over the Internet.
Figure 76 NAS-initiated tunneling mode
A NAS-initiated tunnel has the following characteristics:
Figure 75, both control messages and data messages are encapsulated in UDP
Figure 76, a remote system dials in to the LAC through a PPPoE/ISDN network. The
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Figure 74. Data messages