Appendix A. L2TPv3 Expanded Coverage

Layer 2 Tunnel Protocol version 3 (L2TPv3) is an extension of regular L2TP that allows transport of data-link traffic other than PPP. The differences between version 3, defined in RFC 3931, and its previous incarnation are relatively minor and generally involve making the protocol less PPP specific. RFC 3931 lists the major differences from the original specification (RFC 2661, which we probably now need to refer to as L2TPv2):

Separation of all PPP-related AVPs [attribute-value pair], references, etc., including a portion of the L2TP data header that was specific to the needs of PPP.

Transition from a 16-bit Session ID and Tunnel ID to a 32-bit Session ID and Control Connection ID, respectively.

Extension of the Tunnel Authentication mechanism to cover the entire control message rather than just a portion of certain messages.

The L2TPv3 protocol consists of components to bring up, maintain, and tear down sessions. It can also multiplex different Layer 2 streams into a tunnel.

The RFC defines three different deployment models for L2TPv3, which are differentiated by the level of protocol processing on each tunnel endpoint. In all three cases, the deployment models refer to an L2TP access concentrator (LAC), which performs cross-connect function in the Layer 2 domain, and the L2TP network server (LNS), which terminates and processes Layer 3. The difference between the deployment models really concerns matching Layer 2 and Layer 3 across an L2TP tunnel. Figure A-1 should make this clearer.

Figure A-1. L2TP Deployment Models

The deployment models are as follows:

1. LAC/LNS model This is the classic dial/DSL scenario, in which the LAC performs a Layer 2 cross-connect function and tunnels PPP traffic to an LNS. The LNS terminates the Layer 2 traffic and routes IP traffic. The PPP session is started by a client that connects to the LAC, and the LAC then begins tunnel and session setup with the LNS.

2. LAC/LAC Each L2TP endpoint performs a Layer 2 cross-connect. This is the scenario for pseudowires. In this model, either endpoint can begin session establishment with its peer.

3. LNS/LNS Two Layer 3 hosts use L2TP as a tunnel interface (for example, when a PC uses an L2TP client and begins a Layer 3 tunnel to another LNS over IP). This mode is also known as voluntary tunneling.

The L2TP protocol has a control- and data-plane stack, as shown in Figure A-2. The next two sections review each one in more detail.

Figure A-2. L2TP Control- and Data-Plane Stack (from RFC 3931)