L2TP (Layer 2 Tunneling Protocol) is a network protocol primarily used to support Virtual Private Networks (VPNs) and to facilitate secure data transmission over public networks such as the internet. It operates at the data link layer (Layer 2) of the OSI model, encapsulating data packets to create a tunnel between two endpoints—typically a client device and a VPN server.
The two main components in an L2TP connection are: (1) L2TP Access Concentrator (LAC): The entry point for the tunnel, usually at the client or ISP side. (2) L2TP Network Server (LNS): The endpoint that receives, decapsulates, and forwards the data to the target network.
Transport: L2TP packets are typically transmitted over UDP, which helps avoid certain network issues like TCP meltdown. L2TP does not provide encryption or strong authentication by itself. For security, it is almost always paired with IPsec (Internet Protocol Security), which adds encryption, authentication, and integrity checks. This combined protocol is commonly referred to as L2TP/IPsec