ZyXEL Communications Corporation PRESTIGE310S Manuel D’Utilisation
Prestige 310 Broadband Sharing Gateway
PPTP
E
Appendix A: 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.
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.
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.
PPTP and the Prestige
When the Prestige 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 Prestige's Internet connection. In SUA mode, the Prestige 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 15.1. In the case above as the PPTP connection
is initialized by the remote PPTP Client, the user must configure the PPTP clients. For the Prestige the PPTP
connection is initialized by the Prestige and hence, there is no need to configure the remote PPTP clients.
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 Prestige's Internet connection. In SUA mode, the Prestige 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 15.1. In the case above as the PPTP connection
is initialized by the remote PPTP Client, the user must configure the PPTP clients. For the Prestige the PPTP
connection is initialized by the Prestige and 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.
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.