L2TP(Layer 2 Tunneling Protocol)

L2TP(Layer 2 Tunneling Protocol) is an industrial standard Internet tunnel agreement, function and the PPTP protocol roughly similar, such as network data flow can also be to encrypt. But there are also differences, such as the PPTP requirements for IP networks, network L2TP request packet oriented point-to-point connections, Use a single PPTP L2TP use multiple tunnel, tunnel, L2TP provide baotou compression, tunnel validation, while the PPTP no support.

Brief introduction

L2TP agreement is draft, by IETF Ascend, Cisco and Microsoft, 3COM companies such as the participation of bilevel formulation tunnel agreement, which combines the PPTP and L2F two second tunnel agreement advantages, to numerous companies, has become the IETF accepted by relevant layer 2 channel agreement industrial standard, based on Microsoft point-to-point tunnel PPTP protocol () and Cisco layer 2 protocol (L2F yoursuccess forwarding, being an Internet service provider and companies use make this virtual private network operations can through the Internet.

Main application

In VPN connections to set L2TP connection, method with the PPTP VPN configuration, also in VPN attribute window "network" TAB, VPN type on "L2TP IPSec VPN" can.

Second tunnel protocol (L2TP) is used to integrate multi-protocol dial-up service to the existing Internet service providers points. The PPP defines multi-protocol cross second point-to-point link a encapsulation mechanism. Specifically, the user through the use of numerous technical one (e.g. dialling plants, ISDN, ADSL, etc) to acquire second connected to the network access server (NAS), and then in this connection to run on PPP. In this configuration, the second ZhongDuanDian and PPP session end point in the same physical device (such as: NAS).

L2TP expanded the PPP models, allowing the second and the PPP end in different by packet switching network interconnection equipment to. Through L2TP, users in the second connected to a visit concentrators, (e.g. modem pool, ADSL DSLAM etc), then the concentrators, will separate the PPP frame tunnel to NAS. Such, can put the PPP packet of practical processing process and L2 connected end separated.

For such of separation, its apparent a benefit is that L2 connection can be in a (local) circuit concentrators, termination, then on through sharing network as frame relay circuit or Internet extended logic PPP session, instead of the NAS on termination. From the user's perspective, directly on the NAS termination L2 connection and use L2TP no functional difference. L2TP protocol is used to solve the "more connected couplet selected groups of separation" problems. Many links PPP, generally used to concentrate ISDN B channel, need constitute more than all the links to bind channel in a single network access server (NAS) on combination. Because L2TP makes the PPP session may occur in receiving session with the physical point outside of the position, it is used to make all the gateway appeared at a single NAS and allows multiple links operation, even in physical contact scattered in different physical location on the NAS.

L2TP web Server (LNS said L2TP a Server), is the PPP end system used for processing L2TP protocol Server part of the equipment. It as L2TP tunnel on the other side of the endpoints, is the LAC end-to-end equipment, was LAC tunnel transmission PPP session logic termination endpoints.

Information types

L2TP use the following two information types, namely control information and data information. Control information used in tunnel and call the set up, maintain and cleared. Data information for packaging tunnel carried by the PPP frame. Control information using L2TP in a reliable control channels to ensure sent. When there is a packet is lost no forwarding data information.