Layer 2 Tunneling Protocol : L2TP

From Wikipedia,
Jordan Leonen

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

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).

Description
L2TP acts like 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). L2TP is in fact a layer 5 protocol session layer, and uses the registered UDP port 1701. The entire L2TP packet, including payload and L2TP header, is sent within a UDP datagram. 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. MTU should be considered 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 U.S. Patent 5,918,019 in the United States.

Tunneling Models
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 [2] [3] [4]
voluntary tunnel compulsory tunnel — incoming call compulsory tunnel — remote dial and L2TP multi-hop connection In the voluntary tunnel model, a tunnel is created by the user, typically by the use of an L2TP enabled client which is called the 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 home gateway. The company may provide the remote user with a 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 the remote user. This model is intended for cases where the remote PPP Answer Client has a permanently 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.

L2TP Packet Structure
An L2TP packet consists of :
0 - 15 bit 16 - 31 bit Flags and Version Info Length (opt) Tunnel ID Session ID Ns (opt) Nr (Opt) Offset Size (opt) Offset Pad (Opt)...... Payload data
Field meanings:
Flags and version control flags indicating Data/Control packet and presence of length, sequence, offset fields. Length(optional) Total length of the message in bytes, present only when length flag is set. Tunnel ID Indicates the identifier for the control connection. Session ID Indicates the identifier for a session within a tunnel. Ns(optional) sequence number for this data or control message, beginning at zero and incrementing by one (modulo 2**16) for each message sent. Present only when sequence flag set. Nr(optional) sequence number for expected message to be received. Nr is set to the Ns of the last in-order message received plus one (modulo 2**16). In data messages, Nr is reserved and, if present (as indicated by the S bit), MUST be ignored upon receipt.. Offset Size(optional) Specifies where payload data is located past the L2TP header Actual data within the offset padding is undefined. If the offset field is present, the L2TP header ends after the last byte of the offset padding. This field exists if the offset flag is set. Offset Pad(optional) Variable length Payload data Variable length (Max payload size = Max size of UDP packet - size of L2TP header)

L2TP Packet Exchange

At the time of setup of L2TP connection, many control packets are exchanged between server and client to establish tunnel and session for each direction. One peer requests other peer to assign a specific tunnel and session id through these control packets. Then using this tunnel and session id data packets are exchanged with the compressed PPP frames as payload.
The list of L2TP Control messages exchanged between LAC and LNS, for handshaking before establishing a tunnel and session in voluntary tunneling method are

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 Encapsulating 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.
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.

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 transport