3Com 10014299 manual Tunnel and session, Networking diagram of two typical methods of Vpdn

VPDN and L2TP Overview 603

The networking diagram of these two typical methods is illustrated in the following figure:

Figure 180 Networking diagram of two typical methods of VPDN

LAC Client

LAC Internet

Remote

ClientPSTN/ISDN

LAC Internet

HomeLAN

LNS

HomeLAN

LNS

Overview of L2TP The L2TP (Layer 2 Tunneling Protocol) supports transmitting PPP frames by tunneling, and the end of layer 2 data link and the PPP session can reside on different devices, communicating based on packet switching which extends the

PPPmodel. Integrating the respective advantages of L2F protocol and PPTP, L2TP has become the industrial standard of layer 2 tunneling protocol. The architecture of the protocol stack to which the L2TP belongs is illustrated in Figure 181.

Figure 181 L2TP architecture

PPP Frames

L2TP Data Messages

L2TP Data Channel (unreliable)

L2TP Control Messages

L2TP Control Channel (reliable)

Packet Transport (UDP,……)

The L2TP architecture illustrated in Figure 181 describes the relation among PPP frames, control channels and data channels. A PPP frame is first transmitted in the unreliable data channel after being encapsulated with the L2TP header, and then undergoes the packet transmission process of UDP, Frame Relay and ATM. A control message is transmitted in the reliable L2TP control channel.

Tunnel and session

A L2TP tunnel is established between LAC and LNS, which is composed of one control connection and n (n0) sessions. Only one L2TP tunnel can be established between a pair of LAC and LNS. Both control message and PPP data message are transmitted in the tunnel. A session is also established between LAC and LNS, but session establishment must follow the successful establishment of the tunnel (including the exchange of such information as identity protection, L2TP version, frame type and hardware transmission type). One session connection corresponds to one PPP data stream between LAC and LNS.