In computer networking, Layer 2 Tunneling Protocol (L2TP) is a tunneling protocol used to support virtual private networks (VPNs) or as part of the delivery of services by ISPs. It uses encryption ('hiding') only for its own control messages (using an optional pre-shared secret), and does not provide any encryption or confidentiality of content ...

Layer 2 Tunneling Protocol (L2TP) allows L2 frames to be tunneled over an IP network. This document covers the kernel’s L2TP subsystem. It documents kernel APIs for application developers who want to use the L2TP subsystem and it provides some technical details about the internal implementation which may be useful to kernel developers and ...

The necessary setup for tunneling a session with L2TP consists of two steps: (1) Establishing the control connection, and (2) establishing a session as triggered by an incoming call or outgoing call. An L2TP session MUST be established before L2TP can begin to forward session frames.

PPP Frames are passed over an unreliable Data Channel encapsulated first by an L2TP header and then a Packet Transport such as UDP, Frame Relay, ATM, etc. Control messages are sent over a reliable L2TP Control Channel which transmits packets in …

Two Tunneling Protocol (L2TP) encapsulates PPP frames, encrypted and/or compressed, which can be sent over X.25, ATM, Frame Relay or IP networks [2].

L2TP control frames carry messages between L2TP clients/servers and are used to setup / teardown tunnels and sessions. An L2TP client or server is implemented in userspace. Each L2TP tunnel is implemented using a UDP or L2TPIP socket; L2TPIP provides L2TPv3 IP encapsulation (no UDP) and is implemented using a new l2tpip socket family.

L2TP报文头中标记为可选（opt）的字段，是指在数据消息中可选，在控制消息中则是必选的。 类型（Type），取值为“0”时表示数据消息，取值为“1”时表示控制消息。 长度在位标志，取值为“1”时表示报文头中存在长度字段Length。 控制消息中必须为“1”。 顺序字段在位标志，取值为“1”时表示报文头中存在Ns和Nr字段。 控制消息中必须为“1”。 取值为“1”时表示报文头中存在offset size字段。 控制消息中必须为“0”。 优先级（Priority），只用于数据消息。 控制消息中必须为“0”。 版 …

通过在RADIUS为每个用户配置Frame-IP、Frame-Route或指定地址池等参数，在该用户上线时，通过LNS为其分配已经规划好的IP地址。 本文档针对VPN特性，从配置过程和配置举例两方面对特性进行介绍。

二层隧道协议第三版L2TPv3(Layer Two Tunneling Protocol - Version 3)是一种二层隧道技术，可以透传多种二层报文（如PPP、Ethernet、HDLC、ATM等），运用于用户侧的二层接入链路在分组交换网络中透明传递。用户需要在分支和总部或者分支之间建立二层连接时，可以通过部署 ...

Configuring L2TP About L2TP. The Layer 2 Tunneling Protocol (L2TP) is a Virtual Private Dialup Network (VPDN) tunneling protocol. L2TP sets up point-to-point tunnels across a public network (for example, the Internet) and transmits encapsulated PPP …