L2Tp Tunnel And Session; L2Tp Tunneling Modes And Tunnel Establishment Process - HPE FlexNetwork MSR series 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.
•
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 21 Data message format
L2TP header
As shown in
datagrams.
Figure 22 L2TP encapsulation structure
IP header
(Public network)

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 23 NAS-initiated tunneling mode
Remote system
Host A
A NAS-initiated tunnel has the following characteristics:
L2TP data massage
PPP frame
IP header
PPP header
(Private network)
Figure 22, both control messages and data messages are encapsulated in UDP
UDP header
Figure 23, a remote system dials in to the LAC through a PPPoE/ISDN network. The
Dial-in
PPPoE/ISDN
LAC
Device A
RADIUS server A
Data
L2TP control or data message
LAC initiated
L2TP tunnel
Internet
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Figure 21. Data messages
Private
network
LNS
Device B
RADIUS server B