Appendix Ipptp - ZyXEL Communications ZyWALL 5 User Manual

ZyWALL 5 Internet Security Appliance
Appendix I
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 I-1 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.
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 remote PPTP Client initializes the PPTP connection, 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 Access Concentrator) and the PPTP user. The PNS is the box that hosts both the PPP
and the PPTP stacks and forms one end of the PPTP tunnel. The PAC is the box that dials/answers the
phone calls and relays the PPP frames to the PNS. The PPTP user is not necessarily a PPP client (can
be a PPP server too). Both the PNS and the PAC must have IP connectivity; however, the PAC must
in addition have dial-up capability. The phone call is between the user and the PAC and the PAC
tunnels the PPP frames to the PNS. The PPTP user is unaware of the tunnel between the PAC and the
PNS.
PPTP I-1