L2TP

Layer 2 Tunneling Protocol is a standard used to enable VPNs (virtual private networks) to function on an existing network, usually the internet. L2TP commonly carries PPP (Point-to-Point Protocol) sessions within it’s tunnel. When used on it’s own, L2TP does not offer authentication or confidentiality. For this reason, IPsec (Internet Protocol Security) is often used to provide these packets with authentication, confidentiality and integrity. The combination of these two protocols is referred to as L2TP/IPsec. The end points of an L2TP tunnel are known as the LAC (L2TP Access Concentrator) and the LNS (L2TP Network Sever). The LAC acts as the tunnel’s initiator while the LNS awaits new tunnels. When a tunnel has been established, network traffic between users becomes bidirectional. For more useful purposes, more sophisticated protocols are run through the L2TP tunnel. To facilitate this process, an L2TP session is established within the tunnel to support the higher-level protocol. From there, the LAC or LNS may initiate a session. L2TP isolates traffic for each session, enabling multiple VPNs to be established over a single tunnel.