L2TP - Layer 2 Tunneling Protocol

L2TP Attributes Summary

Projected L2TP standard was made available in the year 1999 by means of RFC 2661. It was originated primarily from two different tunneling protocols, named as: Point-to-Point communication protocol and PPTP (Point to Point Tunneling protocol). In other words, L2TP (Layer 2 Tunnel Protocol) is an up-and-coming IETF (Internet Engineering Task Force) standard that came in front with the traits of two on-hand tunneling protocols, named as: Cisco’s L2F (Layer 2 Forwarding) and Microsoft’s PPTP (Point-to-Point Tunneling Protocol). L2TP protocol is actually an expanded form of the PPP (a significant constituent for VPNs).

VPNs (virtual private networks) may let the user to connect to the corporate intranets/extranets. VPNs provides cost-effective networking but long-established dial-up networks hold up only registered IP (internet protocol) addresses, which are used to limit the applications types for VPNs. The main reasons for L2TP utilization is its support to multiple protocols along with holding of unregistered and privately directed IP addresses.

L2TP may be used as a part of ISP (internet service provider) delivery of services. But in such cases, it may remain powerless in providing any kind of encryption service for having privacy feature, etc. That’s why it is usually dependant upon an encryption offering protocol.

But L2TPv3 is branded as the latest version of under discussion protocol, which was introduced in RFC 3931(2005). And this most up-to-date version offered added security features, enhanced encapsulation, along with the capacity to take data links, etc.

Packet’s structure for L2TP

An L2TP packet is made up of different fields as: flags and version information (0-15 bits) field, length (16-31bits) field but it is an optional field, Tunnel ID (0-15 bits) field, session ID (16-31 bits) field, Ns (0-15 bits) optional field, Nr (16-31 bits) optional field, offset size (0-15 bits) optional field, offset pad (16-31 bits) optional field and payload data field of variable length.

Packet’s exchange in case of L2TP

At L2TP connection set up time, lots of control packets may be swapped between server side and client side in order to create tunnel and session so to be used for every direction. With control packets help, one peer may request to other peer for the assignment of a particular tunnel plus session id so data packets by using them (tunnel and session id) can make exchanges with the PPP frames.

Further that L2TP control messages list is exchanged in connecting LAC and LNS, for the purpose of handshaking previous to the establishment of a tunnel plus session.

L2TP tunnel models

An L2TP tunnel may make bigger across the complete PPP session or else across simply one part of a session with two segments. Different tunneling models can be used to represent this state of affairs and these models are named as: voluntary tunnel model, compulsory tunnel model (for incoming call), compulsory tunnel model (for remote dial up connection plus L2TP multi-hop connections).