ZyXEL Communications ZYWALL10 - Manual (Page 241)

ZyWALL 10 Internet Security Gateway
PPTP C
Appendix B
PPTP
What is PPTP?
PPTP (Point-to-Point Tunneling Protocol) is a Microsoft proprietary protocol (RFC 2637 for PPTP is
informational only) to tunnel PPP frames.
How can we transport PPP frames from a PC to a broadband modem over Ethernet?
A solution is to build PPTP into the ANT (ADSL Network Termination) where PPTP is used only over the
short haul between the PC and the modem over Ethernet. For the rest of the connection, the PPP frames are
transported with PPP over AAL5 (RFC 2364). The PPP connection, however, is still between the PC and the
ISP. The various connections in this setup are depicted in the following diagram. The drawback of this
solution is that it requires one separate ATM VC per destination.
Diagram 3 Transport PPP frames over Ethernet
PPTP and the ZyWALL
When the ZyWALL is deployed in such a setup, it appears as a PC to the ANT (ADSL Network
Termination).
In Windows VPN or PPTP Pass-Through feature, the PPTP tunneling is created from Windows 95, 98 and
NT clients to an NT server in a remote location. The pass-through feature allows users on the network to
access a different remote server using the ZyWALL's Internet connection. In NAT mode, the ZyWALL is
able to pass the PPTP packets to the internal PPTP server (i.e. NT server) behind the NAT. Users need to
forward PPTP packets to port 1723 by configuring the server in Menu 15.2 - Server Set Setup. In the case
above as the PPTP connection is initialized by the remote PPTP Client, the user must configure the PPTP
clients. The ZyWALL initializes the PPTP connection hence, there is no need to configure the remote PPTP
clients.
PPTP Protocol Overview
PPTP is very similar to L2TP, since L2TP is based on both PPTP and L2F (Cisco’s Layer 2 Forwarding).
Conceptually, there are three parties in PPTP, namely the PNS (PPTP Network Server), the PAC (PPTP