ZyXEL Communications Parental Control Gateway HS100/HS100W - Manual (Page 320)

HomeSafe User’s Guide
C-2 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.
Diagram C-2 PPTP Protocol Overview
Microsoft includes PPTP as a part of the Windows OS. In Microsoft’s implementation, the PC,
and hence the HomeSafe, 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.
Diagram C-3 Example Message Exchange between PC and an ANT
PPP Data Connection