PPTP

The Point-to-Point Tunneling Protocol (PPTP) is an obsolete method for implementing virtual private networks, with many known security issues. PPTP uses a control channel over and a GRE tunnel operating to encapsulate packets.

The PPTP specification does not describe encryption or authentication features and relies on the Point-to-Point Protocol being tunneled to implement security functionality. However, the most common PPTP implementation shipping with the Microsoft Windows product families implements various levels of authentication and encryption natively as standard features of the Windows PPTP stack. The intended use of this protocol is to provide security levels and remote access levels comparable with typical VPN products.

A specification for PPTP was published in July 1999 as RFC 2637 and was developed by a vendor consortium formed by Microsoft, Ascend Communications (today part of Alcatel-Lucent), 3Com, and others. PPTP has not been proposed nor ratified as a standard by the Internet Engineering Task Force.

A PPTP tunnel is instantiated by communication to the peer on port 1723. This TCP connection is then used to initiate and manage a second GRE tunnel to the same peer. The PPTP GRE packet format is non standard, including an additional acknowledgement field replacing the typical routing field in the GRE header. However, as in a normal GRE connection, those modified GRE packets are directly encapsulated into IP packets, and seen as IP protocol number 47. The GRE tunnel is used to carry encapsulated PPP packets, allowing the tunnelling of any protocols that can be carried within PPP, including , NetBEUI and IPX.

In the Microsoft implementation, the tunneled PPP traffic can be authenticated with , , MS-CHAP v1/v2 .

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