L2TP (tunneling) support for PPP connections
Layer 2 Tunneling Protocol (L2TP) is a tunneling protocol that extends Point-to-Point Protocol (PPP) to support a link layer tunnel between a requesting L2TP client (L2TP Access Concentrator or LAC) and a target L2TP server endpoint (L2TP Network Server or LNS).
Layer Two Tunneling Protocol
By using Layer Two Tunneling Protocol (L2TP) tunnels, it is possible to separate the location at which the dial-up protocol ends and where the access to the network is provided. That is why L2TP is also referred to as Virtual PPP.
These
figures illustrate three different tunneling implementations of L2TP.
Figure 1. PPP virtual initiator or PPP virtual terminator
Figure 2. PPP dial initiator or PPP virtual terminator
Figure 3. PPP virtual dial or PPP virtual answer
The L2TP protocol is documented as a Request for Comment (RFC) standard, RFC-2661. An L2TP tunnel can extend across an entire PPP session or only across one segment of a two-segment session. This can be represented by four different tunneling models.
- Voluntary tunnel
In the voluntary tunnel model, a tunnel is created by the user, typically by the use of a Layer Two Tunneling Protocol (L2TP)-enabled client. - Compulsory tunnel model - incoming call
In the compulsory tunnel model - incoming call, a tunnel is created without any action from the user and without allowing the user any choice. - Compulsory tunnel model - remote dial
In the compulsory tunnel model - remote dial, the home gateway (L2TP network server (LNS)) initiates a tunnel to an Internet service provider (ISP) (LAC) and instructs the ISP to place a local call to the Point-to-Point Protocol (PPP) answer client. - L2TP multi-hop connection
A Layer Two Tunneling Protocol (L2TP) multi-hop connection is a way of redirecting L2TP traffic on behalf of client L2TP access concentrators (LACs) and L2TP network servers (LNSs).
Parent topic: Connection alternatives
