Point-to-point tunneling protocol (PPTP) is a protocol from Microsoft for virtual private networks. A specification was published as an "informational" RFC in 1999. It has not been ratified by the IETF. The protocol is considered insecure and as such should not be used.
PPTP is implemented as a PPP session over Generic Routing Encapsulation (GRE). Authentication is usually by MSCHAP-v2, and supplies key material for the subsequent Microsoft Point-to-Point Encryption (MPPE) applied to data packets.
PPTP uses a default port [1] (http://www.iana.org/assignments/port-numbers) of 1723 for communicating across a network.
External links
Point-to-Point Tunneling Protocol (PPTP), RFC 2637, July 1999; [2] (http://www.ietf.org/rfc/rfc2637.txt?number=2637)
FAQ on PPTP from Microsoft; [3] (http://www.microsoft.com/ntserver/ProductInfo/faqs/PPTPfaq.asp)
FAQ on security flaws in Microsoft's implementation, Bruce Schneier, 1998; [4] (http://www.schneier.com/pptp-faq.html)
Poptop, a PPTP Server for Linux; [5] (http://www.poptop.org/)
PPTP Client, a Linux, FreeBSD and NetBSD client; [6] (http://pptpclient.sourceforge.net/)
Additionally, a layer between the network and data link layer is needed for ARP and RARP.
It also suffers from being designed for simple network setups having only a single data link layer (for example an ADSL user tunneling into a corporate network could have IP over PPTP over IP over PPPoA over the ADSL link)
One example of where the OSI model is better used is showing where SSL/TLS fits in.
Confirm with your Internet Service Provider (ISP) that it's Okay to make incoming VPN calls to your Windows XP VPN server (that is, that they don't block incoming VPN traffic).
If you have a "broadband router" (DSL or cable), make sure that all PPTP connections are sent to the Windows XP Professional computer (PPTP is a type of VPN connection based on the "tunneling protocol" it uses).
Make sure there are two network cards on the Windows XP machine if you want to be able to connect to the rest of the network through the VPN!
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