L2TP tunnel and session
The following types of connections are present between an LNS and an LAC:
Tunnel—A tunnel corresponds to a LNS-LAC pair, and comprises a control connection and one or more sessions.
Session—A session corresponds to one PPP data stream between an LNS and a LAC and is multiplexed on a tunnel. A session can be set up only after the tunnel is created.
Multiple L2TP tunnels can be established between an LNS and an LAC.
Both control messages and PPP frames are transferred on the tunnel. L2TP uses hello packets to check a tunnel's connectivity. The LAC and the LNS regularly send hello packets to each other. If no response packet is received within a given amount of time, the tunnel is torn down.