R3166-R3206-HP High-End Firewalls VPN Configuration Guide-6PW101

Both control messages and PPP frames are transferred on the tunnel. L2TP uses hello packets to check a

tunnel’s connectivity. The LAC and the LNS regularly send hello packets to each other. If no response

packet is received within a given amount of time, the tunnel is torn down.

Control message and data message

L2TP supports two types of messages: Control messages and data messages.

• Control messages are used in establishment, maintenance, and clearing of tunnels and sessions.

Control messages are transmitted over a reliable control channel, which supports flow control and

congestion control.

• Data messages encapsulate PPP frames to be tunneled. Data messages are transmitted over an

unreliable data channel that lacks flow control and congestion control, and retransmission

mechanisms.

Control messages and data messages share the same header structure. The Type field in the L2TP header

identifies whether a message is a control message or a data message. The tunnel ID and session ID fields

in the L2TP header identify the tunnel and session respectively. 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 the

intended receiver’s, not those of the sender.

L2TP tunnel modes and tunnel establishment process

Two typical L2TP tunnel modes

Typical L2TP tunnel modes include NAS-initiated and client-initiated.

• NAS-initiated

In this mode, a remote system dials in the LAC through a PPPoE/ISDN network, and the LAC initiates a

tunneling request to the LNS over the Internet, as shown in Figure 57. T

he LNS will assign the remote

system an IP address. Authentication and accounting of remote systems can be implemented on the LAC

by an agent or on the LNS.

Figure 57 NAS-initiated tunnel mode

• Client-initiated

In this mode, a remote system running the L2TP client application initiates a tunneling request to the LNS

directly without any dedicated LAC device. The LNS will assign an L2TP client an IP address.