A tunneling protocol that operates at the Data Link Layer (Layer 2) of the OSI Reference Model. PPTP is a proprietary technique that encapsulates Point-toPoint Protocol (PPP) frames in Internet Protocol (IP) packets using the Generic Routing Encapsulation (GRE) protocol. Packet filters provide access control, end-to-end and server-to-server. PPTP was initially conceived by Ascend, developed by Microsoft, and described in IETF RFC 2637. See also Data Link Layer, frame, GRE, IETF, IP, IPsec, L2TP, OSI Reference Model, packet, PPP, protocol, server, SOCKS, and tunneling.