PPTP
The function of the Point-to-Point Tunneling Protocol (PPTP) is to create a secure transmission tunnel between two points on a network. The tunneling functionality that PPTP provides forms the basis for creating multi-protocol virtual private networks (VPNs), which allow users to access remote networks through a secure connection. PPTP works in conjunction with PPP and, as such, uses PPP authentication methods including PAP, CHAP, and MS-CHAP.
To establish a PPTP session between a client and server, a TCP connection known as a PPTP control connection is required to create and maintain the communication tunnel. The PPTP control connection exists between the IP address of the PPTP client and the IP address of the PPTP server, using TCP port 1723 on the server and a dynamically assigned port on the client. It is the function of the PPTP control connection to pass the PPTP control and management messages used to maintain the PPTP communication tunnel between the remote system and the server. Once the PPTP connection is made, it provides a secure channel, or tunnel, using the original PPP connection between the devices.