R3303-HP HSR6800 Routers Layer 2 - WAN Configuration Guide

• Session—A session corresponds to one PPP data stream between an LNS and a LAC and is

multiplexed on a tunnel. A session can be set up only after the tunnel is created.

Multiple L2TP tunnels can be established between an LNS and an LAC.

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.

Message types

L2TP uses the following types of messages:

• Control messages—Used to establish, maintain, and delete tunnels and close sessions. Control

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

congestion control.

• Data messages—Used to encapsulate PPP frames and transmit the frames over a tunnel. 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 format. 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 tunneling modes and tunnel establishment process

Three typical L2TP tunneling modes

Typical L2TP tunneling modes include the following:

• NAS-initiated—In Figure 24, a r

emote system dials in to the LAC through a PPPoE/ISDN network,

and the LAC initiates a tunneling request to the LNS over the Internet. The LNS assigns a private IP

address to the remote system. Authentication and accounting of the remote system can be

implemented on the LAC or on the LNS.

Figure 24 NAS-initiated tunneling mode

• Client-initiated—In Figure 25, after being permitted to access the Internet, a remote system running

the L2TP client application (LAC client) directly initiates a tunneling request to the LNS without any

dedicated LAC device. The LNS assigns the LAC client a private IP address.

A LAC client needs a public network address to communicate with the LNS through the Internet.