PPTP is defined in RFC 2637, Point-to-Point Tunneling Protocol (PPTP), and is used to provide VPN connection for home/mobile users to gain secure access to the enterprise network. Encrypted payload is transported over GRE tunnel that is negotiated over TCP control channel. In order for PPTP traffic to pass through NAT, the NAT device must correlate the TCP control channel with the corresponding GRE tunnel. This mechanism is referred to as PPTP ALG.