In the compulsory tunnel model - incoming call, a tunnel is created without any action from the user and without allowing the user any choice.
As a result, the user sends Point-to-Point Protocol packets to the Internet service provider (ISP) (Layer Two Tunneling Protocol (L2TP) access concentrator (LAC)). The ISP encapsulates the packets in L2TP and sends them in a tunnel to the L2TP network server (LNS). In the compulsory tunneling cases, the ISP must be L2TP-capable. In this model, the tunnel extends only across the segment of the PPP session between the ISP and the LNS.