L2TP

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In computer networking, the Layer 2 Tunneling Protocol (L2TP) is a tunneling protocol used to support virtual private networks (VPNs).

1 History and future
2 Description
3 L2TP/IPsec
4 L2TP in ADSL networks
5 External links

[edit] History and future

Published in 1999 as proposed standard RFC 2661, L2TP has its origins primarily in two older tunneling protocols for PPP: Cisco's Layer 2 Forwarding (L2F) and Microsoft's Point-to-Point Tunneling Protocol (PPTP). A new version of this protocol, L2TPv3, was published as proposed standard RFC 3931 in 2005. L2TPv3 provides additional security features, improved encapsulation, and the ability to carry data links other than simply PPP over an IP network (e.g., Frame Relay, Ethernet, ATM, etc).

[edit] Description

L2TP acts as a data link layer (layer 2 of the OSI model) protocol for tunneling network traffic between two peers over an existing network (usually the Internet). It is common to carry Point-to-Point Protocol (PPP) sessions within an L2TP tunnel. L2TP does not provide confidentiality or strong authentication by itself. IPsec is often used to secure L2TP packets by providing confidentiality, authentication and integrity. The combination of these two protocols is generally known as L2TP/IPsec (discussed below).

The two endpoints of an L2TP tunnel are called the LAC (L2TP Access Concentrator) and the LNS (L2TP Network Server). The LAC is the initiator of the tunnel while the LNS is the server, which waits for new tunnels. Once a tunnel is established, the network traffic between the peers is bidirectional. To be useful for networking, higher-level protocols are then run through the L2TP tunnel. To facilitate this an L2TP session (or call) is established within the tunnel for each higher-level protocol such as PPP. Either the LAC or LNS may initiate sessions. The traffic for each session is isolated by L2TP, so it is possible to set up multiple virtual networks across a single tunnel.

Be sure to consider MTU when implementing L2TP.

The packets exchanged within an L2TP tunnel are categorised as either control packets or data packets. L2TP provides reliability features for the control packets, but no reliability for data packets. Reliability, if desired, must be provided by the nested protocols running within each session of the L2TP tunnel.

Cisco has the software patent related to L2F and L2TP [1]. It has been assigned patent number 5,918,019 in the United States. [2]

An L2TP tunnel can extend across an entire PPP session or only across one segment of a two-segment session. This can be represented by four different tunneling models, namely voluntary tunnel, compulsory tunnel — outgoing call, compulsory tunnel — remote dial, and L2TP multi-hop connection.[3] [4] [5]

In the voluntary tunnel model, a tunnel is created by the user, typically by the use of an L2TP enabled client which is called as LAC client. The user will send L2TP packets to the Internet Service Provider (ISP) which will forward them on to the LNS. The ISP does not need to support L2TP, it only forwards the L2TP packets between LAC and LNS . The LAC client acts as an L2TP tunnel initiator which effectively resides on the same system as the remote client. The tunnel extends across the entire PPP session from the L2TP client to the LNS.

In the compulsory tunnel model-incoming call, a tunnel is created between ISP LAC and the LNS. The company may provide the remote user with Virtual Private Network(VPN)login account from which he can access the corporate server. As a result the user will send PPP packets to the ISP (LAC) which will encapsulate them in L2TP and tunnel them to the LNS. In the compulsory tunneling cases, the ISP must be L2TP capable. In this model the tunnel only extends across the segment of the PPP session between the ISP and the LNS.

In the compulsory tunnel model-remote dial the home gateway (LNS) initiates a tunnel to an ISP (LAC) (outgoing call) and instructs the ISP to place a local call to the PPP enabled client which is remote user. This model is intended for cases where the remote PPP Answer Client has a permanent established phone number with an ISP. This model is expected to be used when a company with established presence on the Internet needs to establish a connection to a remote office that requires a dial-up link. In this model the tunnel only extends across the segment of the PPP session between the LNS and the ISP.

An L2TP Multi-hop connection is a way of redirecting L2TP traffic on behalf of client LACs and LNSs. A Multi-hop connection is established using an L2TP Multi-hop gateway. A tunnel is established from a client LAC to the L2TP Multi-hop gateway and then another tunnel is established between the L2TP Multi-hop gateway and a target LNS. L2TP traffic between client LAC and LNS is redirected to each other through the gateway.

[edit] L2TP/IPsec

Because of the lack of confidentiality inherent in the L2TP protocol, it is often implemented along with IPsec. This is referred to as L2TP/IPsec, and is standardized in IETF RFC 3193.

The process of setting up an L2TP/IPsec VPN is as follows:

Negotiation of IPsec Security Association (SA), typically through Internet Key Exchange (IKE). This is carried out over UDP port 500, and commonly uses either a shared password (so-called "pre-shared keys"), public keys, or X.509 certificates on both ends, although other keying methods exist.
Establishment of Encapsulated Security Payload (ESP) communication in transport mode. The IP Protocol number for ESP is 50 (compare TCP's 6 and UDP's 17). At this point, a secure channel has been established, but no tunneling is taking place.
Negotiation and establishment of L2TP tunnel between the SA endpoints. The actual negotiation of parameters takes place over the SA's secure channel, within the IPsec encryption. L2TP uses UDP port 1701.

When the process is complete, L2TP packets between the endpoints are encapsulated by IPsec. Since the L2TP packet itself is wrapped and hidden within the IPsec packet, no information about the internal private network can be garnered from the encrypted packet. Also, it is not necessary to open UDP port 1701 on firewalls between the endpoints, since the inner packets are not acted upon until after IPsec data has been decrypted and stripped, which only takes place at the endpoints. If a firewall or packet filter is integrated into the endpoint itself, however, it will probably be necessary to open port 1701 on that endpoint.

A potential point of confusion in L2TP/IPsec is the use of the terms "tunnel" and "secure channel." Tunnel refers to a channel which allows untouched packets of one network to be transported over another network. In the case of L2TP/IPsec, it allows L2TP/PPP packets to be transported over IP. A secure channel refers to a connection within which the confidentiality of all data is guaranteed. In L2TP/IPsec, first IPsec provides a secure channel, then L2TP provides a tunnel.

[edit] L2TP in ADSL networks

L2TP is often used as a tunneling mechanism to resell ADSL endpoint connectivity. An L2TP tunnel would sit between the user and the ISP the connection would be resold to, so the reselling ISP wouldn't appear as doing the transit.

[edit] External links

[edit] Implementations

Cisco: Cisco L2TP documentation, also read Technology brief from Cisco
Open source and Linux: Linux RP-L2TP project, OpenL2TP project, L2tpns project, L2TPD project (inactive), Linux L2TP/IPSec server
Microsoft: Microsoft L2TP/IPsec VPN Client

[edit] Internet standards and extensions

RFC 2341 Cisco Layer Two Forwarding (Protocol) "L2F". (A predecessor to L2TP)
RFC 2637 Point-to-Point Tunneling Protocol (PPTP). (A predecessor to L2TP)
RFC 2661 Layer Two Tunneling Protocol "L2TP"
RFC 2809 Implementation of L2TP Compulsory Tunneling via RADIUS
RFC 2888 Secure Remote Access with L2TP
RFC 3070 Layer Two Tunneling Protocol (L2TP) over Frame Relay
RFC 3145 L2TP Disconnect Cause Information
RFC 3193 Securing L2TP using IPsec
RFC 3301 Layer Two Tunnelling Protocol (L2TP): ATM access network
RFC 3308 Layer Two Tunneling Protocol (L2TP) Differentiated Services
RFC 3355 Layer Two Tunnelling Protocol (L2TP) Over ATM Adaptation Layer 5 (AAL5)
RFC 3371 Layer Two Tunneling Protocol "L2TP" Management Information Base
RFC 3437 Layer Two Tunneling Protocol Extensions for PPP Link Control Protocol Negotiation
RFC 3438 Layer Two Tunneling Protocol (L2TP) Internet Assigned Numbers: Internet Assigned Numbers Authority (IANA) Considerations Update
RFC 3573 Signaling of Modem-On-Hold status in Layer 2 Tunneling Protocol (L2TP)
RFC 3817 Layer 2 Tunneling Protocol (L2TP) Active Discovery Relay for PPP over Ethernet (PPPoE)
RFC 3931 Layer Two Tunneling Protocol - Version 3 (L2TPv3).
RFC 4045 Extensions to Support Efficient Carrying of Multicast Traffic in Layer-2 Tunneling Protocol (L2TP).