PPTP: Point to Point Tunneling Protocol

Point-to-Point-Tunneling Protocol (PPTP) is a networking technology that supports multiprotocol virtual private networks (VPN), enabling remote users to access corporate networks securely across the Microsoft Windows NT^®Workstation, Windows^®95, and Windows 98 operating systems and other point-to-point protocol (PPP)-enabled systems to dial into a local Internet service provider to connect securely to their corporate network through the Internet.

Point to Point Tunneling Protocol (PPTP) can also be used to tunnel a PPP session over an IP network. In this configuration the PPTP tunnel and the PPP session runs between the same two machines with the caller acting as a PNS. PPTP uses a client-server architecture to decouple functions which exist in current Network Access Servers and support Virtual Private Networks. PPTP specifies a call-control and management protocol which allows the server to control access for dial-in circuit switched calls originating from a PSTN or ISDN, or to initiate outbound circuit switched connections.

PPTP is implemented only by the PAC and PNS. No other systems need to be aware of PPTP. Dial networks may be connected to a PAC without being aware of PPTP. Standard PPP client software should continue to operate on tunneled PPP links.

PPTP uses an extended version of GRE to carry user PPP packets. These enhancements allow for low-level congestion and flow control to be provided on the tunnels used to carry user data between PAC and PNS. This mechanism allows for efficient use of the bandwidth available for the tunnels and avoids unnecessary retransmisions and buffer overruns. PPTP does not dictate the particular algorithms to be used for this low level control but it does define the parameters that must be communicated in order to allow such algorithms to work.   

Key functions of the Microsoft PPTP have been covered by L2TP , which is the IETF standard protocol for tunneling. 

Protocol Structure - PPTP: Point to Point Tunneling Protocol

16	32 bit
Length	PPTP message type
Magic cookie
Control message type	Reserved 0
Protocol Version	Reserved 1
Framing capability
Bearing capability
Maximum channels	Firmware revision
Host name (64 Octets)
Vendor string (64 Octets)

• Length - Total length in octets of this PPTP message including the entire PPTP header.
• PPTP message type - The message type. Possible values are: 1Control message; 2Management message.
• Magic cookie - The magic cookie is always sent as the constant 0x1A2B3C4D. Its basic purpose is to allow the receiver to ensure that it is properly synchronized with the TCP data stream.
• Control Message Type - Values may be: 

Control Connection Management - 1Start-Control-Connection-Request;  2 Start-Control-Connection-Reply; 3Stop-Control-Connection-Request; 4Stop-Control-Connection-Reply; 5Echo-Request; 6Echo-Reply.

Call Management - 7Outgoing-Call-Request; 8Outgoing-Call-Reply; 9Incoming-Call-Request; 10Incoming-Call-Reply; 11Incoming-Call-Connected; 12Call-Clear-Request; 13Call-Disconnect-Notify

Error Reporting - 14WAN-Error-Notify

PPP Session Control - 15Set-Link-Info.

• Reserved 0 & 1 - Must be set to 0.
• Protocol version - PPTP version number
• Framing Capabilities - Indicating the type of framing that the sender of this message can provide: 1 - Asynchronous Framing supported; 2 - Synchronous Framing supported
• Bearer Capabilities - Indicating the bearer capabilities that the sender of this message can provide: 1 - Analog access supported; 2 - Digital access supported
• Maximum Channels - The total number of individual PPP sessions this PAC can support.
• Firmware Revision - Contains the firmware revision number of the issuing PAC, when issued by the PAC, or the version of the PNS PPTP driver if issued by the PNS.
• Host Name - Containing the DNS name of the issuing PAC or PNS.
• Vendor Name - Containing a vendor specific string describing the type of PAC being used, or the type of PNS software being used if this request is issued by the PNS.

PPTP is defined by PPTP forum led by Microsoft and circulated among IETF community.