http://www.ietf.org/rfc/rfc2661.txt Layer 2 Tunneling Protocol A VPN protocol created by Microsoft, Cisco, Ascend, IBM, and 3Com. L2TP, defined in RFC 2661, is designed to tunnel PPP sessions over various network types in a way that is transparent to both users and applications. The protocol relies on the underlying transport to provide packet level encryption, authentication, and integrity services. L2TP by itself is susceptible to network sniffing attacks as most IP traffic except during the tunnel establishment that uses a CHAP-like authentication method.