ZyXEL Communications ZyXEL ZyWALL P1, User Manual

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ZyWALL P1 User’s Guide
292 Appendix D PPTP
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.
Figure 163 PPTP Protocol Overview
Microsoft includes PPTP as a part of the Windows OS. In Microsoft’s implementation, the
computer, 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
computer and an ANT.