Zyxel-communications P-335WT Manual do Utilizador descarregar pdf (Página 432)

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P-335 Series User’s Guide

432 Appendix C 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 256 PPTP Protocol Overview

Microsoft includes PPTP as a part of the Windows OS. In Microsoft’s implementation, the

computer, 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

computer and an ANT.

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