ZyXEL Communications Prestige 314 PLUS - Manual (Page 207)

Prestige 314 PLUS Broadband Sharing Gateway with 4-Port Switch
PPTP E
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
Prestige 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.
Diagram 4 PPTP Protocol Overview
Microsoft includes PPTP as a part of the Windows OS. In Microsoft’s implementation, the PC, and hence the
Prestige, is the PNS that requests the PAC (the ANT) to place an outgoing call over AAL5 to an RFC 2364
server.
Control & PPP connections
Each PPTP session has distinct control connection and PPP data connection.
Call Connection
The control connection runs over TCP. Similar to L2TP, a tunnel control connection is first established
before call control messages can be exchanged. Please note that a tunnel control connection supports multiple
call sessions.
The following diagram depicts the message exchange of a successful call setup between a PC and an ANT.