Point-to-Point Tunneling Protocol (PPTP) is an extension of PPP that takes advantage of the authentication, compression, and encryption mechanisms of PPP. With dial-up remote access, PTTP is the most commonly used protocol but for single client-to-server connections as it allows only a single point-to-point connection per session. It encapsulates PPP frames into IP datagrams for transmission over an IP network.
PPTP tunnels the PPP frame within a Generic Routing Encapsulation (GRE) header using IP protocol 47 and a TCP header using port 1723. For PPTP traffic to pass through a firewall, the firewall must allow TCP port 1723 and IP protocol 47. PPTP is commonly used by Windows clients for nonparallel communications. It allows IP, IPX or NetBEUI traffic to be encrypted and then encapsulated in an IP header. PPTP uses Microsoft Point-to-Point Encryption (MPPE) and compression from PPP. PPTP tunnels must be established by using the same authentication mechanisms as PPP connections (PAP, CHAP, and EAP).