ZyXEL Communications Internet Security Gateway ZyWALL 2 Series User Manual page 558

ZyWALL 2 Series User's Guide
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 H-2 PPTP Protocol Overview
Microsoft includes PPTP as a part of the Windows OS. In Microsoft's implementation, the PC, and hence the
ZyWALL, 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.
H-2 PPTP