# L2TP Terminology

Table 74 describes the basic terms for L2TP.

Table 74: L2TP Terms

Term | Description
--- | ---
Attribute value pair (AVP) | Combination of a unique attribute—represented by an integer—and a value containing the actual value identified by the attribute.
LAC | L2TP access concentrator (LAC)—a node that acts as one side of an L2TP tunnel endpoint and is a peer to the LNS. An LAC sits between an LNS and a remote system and forwards packets to and from each.
Call | A connection (or attempted connection) between a remote system and an LAC.
LNS | L2TP network server (LNS)—a node that acts as one side of an L2TP tunnel endpoint and is a peer to the LAC. An LNS is the logical termination point of a PPP connection that is being tunneled from the remote system by the LAC.
Peer | In the L2TP context, refers to either the LAC or LNS. An LAC’s peer is an LNS, and vice versa.
Proxy authentication | Authentication data from the PPP client that is sent from the LNS as part of a proxy LCP. Data might include attributes such as authentication type, authentication name, and authentication challenge.
Proxy LCP | LCP (Link Control Protocol) negotiation that is performed by the LAC on behalf of the LNS. Proxy sent by the LAC to the LNS containing attributes such as the last configuration attributes sent and received from the client.
Remote system | An end-system or router attached to a remote access network, which is either the initiator or recipient of a call.
Session | A logical connection created between the LAC and the LNS when an end-to-end PPP connection is established between a remote system and the LNS. NOTE: There is a one-to-one relationship between established L2TP sessions and their associated PPP connections.
Tunnel | A connection between an LAC-LNS pair consisting of a control connection and 0 or more L2TP sessions.