L2TP
An extension to the PPP protocol that enables Layer 2 secure tunneling operation across a Virtual Private Network (VPN). L2TP merges the best features of Microsoft's Point-to-Point Tunneling Protocol (PPTP) and Cisco's Layer 2 Forwarding (L2F) protocol. Like PPTP, L2TP requires that an ISP's routers support the protocol.
More: Layer Two Tunneling Protocol