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13.2 PPTP
PPTP is a VPN tunneling protocol which encapsulates PPP frames in IP packets for
transmission over a public IP network such as the Internet. PPTP is based on
client/server model. The PPTP client initiates a PPTP connection to the server while
the PPTP server accepts the incoming PPTP connection from the client. PPTP is
often used to implement remote access VPNs over an IP network (such as a
broadband network), to extend the reach of your Intranet.
13.2.1 Introduction to PPTP Implementation
As mentioned earlier, PPTP is used to encapsulate PPP frames in IP packets for
transmission over a public IP network such as the Internet. The PPTP client or server
encapsulates the original user packets inside PPP frames before sending them
through a PPTP tunnel over the Internet; while the peer performs decapsulation firstly,
and then forward the original packets to their intended destinations.
As shown in the following figure, the typical application of PPTP is that some laptop or
desktop computers act as the PPTP client devices, that is, some employees in the
remote branch offices or mobile users (traveling employees, telecommuters, etc.) use
the Windows built-in PPTP client software to initiate PPTP connections to the server;
the Device deployed at the head office acts as a PPTP server device, which accepts
the PPTP incoming connections from the clients. After a PPTP tunnel has been
established between the PPTP client and server, the PPTP server will receive the
PPTP packets from the client firstly, and then perform decapsulation, lastly forward the
original packets to their intended destinations.
Figure 13-1 Typical Application of PPTP
The Device can function as a PPTP client or server; or both, that is, it is the PPTP
client for some tunnels and PPTP server for other tunnels. When the Device functions
as the PPTP client and server at the same time, on one hand, it can receive the
packets from other PPTP client devices; on the other hand, it can transmit the
received packets to other PPTP server devices.
As shown in the following figure, to securely connect an enterprise’s branch office with
its head office, and connect some mobile users with both the branch office and head
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