L2TP protocol

The Layer Two Tunnel Protocol (L2TP) was designed to encapsulate PPP (Point-to-Point Protocol) packets on layers 2 or 3 (IP) of the OSI model. Typically, a Layer 2 connection is established between a user and a Network Access Server (NAS), over which PPP enables the transport of numerous protocols (IP, IPX, AppleTalk, etc.) over a point-to-point link. The NAS is therefore the same endpoint for both the Layer 2 connection and the PPP session. L2TP allows these two functions to be separated by moving the PPP session termination point to another point on the network, called LNS or L2TP Network Server. The NAS will then generally play the role of LAC or L2TP Access Concentrator. The LAC and LNS are the two ends of the L2TP tunnel created to transport PPP sessions to a specific point on the network. Only the LAC and LNS are aware of the L2TP tunnel, so data transport is as transparent as possible for users and applications.

L2TP was developed on the basis of existing protocols such as PPTP (Point-to-Point Tunnel Protocol) and L2F (Layer Two Forwarding), retaining only their advantages. This makes it possible to interconnect networks of the same type via a network that doesn't support the protocol used (for example, two non-IP networks can communicate using L2TP via an IP network). This reduces costs by avoiding the need to connect to a remote NAS, instead using a shared infrastructure such as Frame Relay or the Internet. In addition, as the termination point for PPP sessions is no longer necessarily at the NAS level, this allows a PPP Multilink Protocol (MPP) set to terminate at the LNS level, and thus to recover all its channels on different NASes.

L2TP also meets VPN and security needs: when used over IP, L2TP enables tunneling over the Internet and the creation of VPNs. Users can connect to their corporate network via an L2TP tunnel (with authentication) and retrieve their remote profile. In addition, he or she can be assigned an IP address on the company network. This facilitates IP address management and avoids unnecessary cluttering of network routing tables, as the LNS aggregates all these addresses and announces the necessary prefixes.

On the other hand, one of L2TP's main drawbacks is the size of its encapsulation. The L2TP protocol adds a maximum 14-byte header, but if used for tunneling over the Internet, the protocol stacking required for this type of operation increases the number of additional bytes due to the various successive encapsulations (IP/UDP/L2TP/PPP/IP) to 50.