xl2tpd is an implementation of the Layer 2 Tunnelling Protocol (RFC 2661). L2TP allows you to tunnel PPP over UDP. Some ISPs use L2TP to tunnel user sessions from dial-in servers (modem banks, ADSL DSLAMs) to back-end PPP servers. Another important application is Virtual Private Networks where the IPsec protocol is used to secure the L2TP connection (L2TP/IPsec, RFC 3193). The L2TP/IPsec protocol is mainly used by Windows and Mac OS X clients. On Linux, xl2tpd can be used in combination with IPsec implementations such as Openswan.
