The L2TP Protocol is used for integrating multi-protocol dial-up services into existing Internet Service Providers Point of Presence. PPP defines an encapsulation mechanism for transporting multiprotocol packets across layer 2 (L2) point-to-point links. Typically, a user obtains a L2 connection to a Network Access Server (NAS) using one of a number of techniques (e.g., dialup POTS, ISDN,ADSL, etc.) and then runs PPP over that connection. In such a configuration, the L2 termination point and PPP session endpoint reside on the same physical device (i.e., the NAS).

L2TP extends the PPP model by allowing the L2 and PPP endpoints to reside on different devices interconnected by a packet-switched network. With L2TP, a user has an L2 connection to an access concentrator (e.g., modem bank, ADSL DSLAM, etc.), and the concentrator then tunnels individual PPP frames to the NAS. This allows the actual processing of PPP packets to be divorced from the termination of the L2 circuit.

One obvious benefit of such a separation is that instead of requiring the L2 connection terminate at the NAS, the connection may terminate at a (local) circuit concentrator, which then extends the logical PPP session over a shared infrastructure such as frame relay circuit or the Internet. From the user"s perspective, there is no functional difference between having the L2 circuit terminate in a NAS directly or using L2TP.

This protocol may also be used to solve the "multilink hunt-group splitting" problem. Multilink PPP, often used to aggregate ISDN B channels, requires that all channels composing a multilink bundle be grouped at a single Network Access Server (NAS). Because L2TP makes a PPP session appear at a location other than the physical point at which the session was physically received, it can be used to make all channels appear at a single NAS, allowing for a multilink operation even when the physical calls are spread across distinct physical NASs.

L2TP utilizes two types of messages, control messages and data messages. Control messages are used in the establishment, maintenance and clearing of tunnels and calls. Data messages are used to encapsulate PPP frames being carried over the tunnel. Control messages utilize a reliable Control Channel within L2TP to guarantee delivery (see section 5.1 for details). Data messages are not retransmitted when packet loss occurs.

Protocol Structure - L2TP: Layer 2 Tunneling Protocol

Related protocols:PPP, PPTP, L2F, ATM, Frame Relay, UDP

Sponsor Source:L2TP is defined by IETF (http://www.ietf.org) in RFC 2661

Reference:http://www.javvin.com/protocol/rfc2661.pdf: Layer Two Tunneling Protocol "L2TP