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Page 101 / 141 Scroll up to view Page 96 - 100
ZyXEL P-330W User’s Guide
101
Appendix A PPPoE
Figure 65
Single-Computer per Router Hardware Configuration
How PPPoE Works
The PPPoE driver makes the Ethernet appear as a serial link to the computer and the computer
runs PPP over it, while the modem bridges the Ethernet frames to the Access Concentrator
(AC).
Between the AC and an ISP, the AC is acting as a L2TP (Layer 2 Tunneling Protocol)
LAC (L2TP Access Concentrator) and tunnels the PPP frames to the ISP.
The L2TP tunnel is
capable of carrying multiple PPP sessions.
With PPPoE, the VC (Virtual Circuit) is equivalent to the dial-up connection and is between
the modem and the AC, as opposed to all the way to the ISP.
However, the PPP negotiation is
between the computer and the ISP.
P-330W as a PPPoE Client
When using the P-330W as a PPPoE client, the computers on the LAN see only Ethernet and
are not aware of PPPoE.
This alleviates the administrator from having to manage the PPPoE
clients on the individual computers.
Figure 66
P-330W as a PPPoE Client
Page 102 / 141
ZyXEL P-330W User’s Guide
Appendix B PPTP
102
Appendix B
PPTP
What is PPTP?
PPTP (Point-to-Point Tunneling Protocol) is a Microsoft proprietary protocol (RFC 2637 for
PPTP is informational only) to tunnel PPP frames.
How can we transport PPP frames from a computer to a broadband
modem over Ethernet?
A solution is to build PPTP into the ANT (ADSL Network Termination) where PPTP is used
only over the short haul between the computer and the modem over Ethernet. For the rest of
the connection, the PPP frames are transported with PPP over AAL5 (RFC 2364) The PPP
connection, however, is still between the computer and the ISP. The various connections in
this setup are depicted in the following diagram. The drawback of this solution is that it
requires one separate ATM VC per destination.
Figure 67
Transport PPP frames over Ethernet
PPTP and the P-330W
When the P-330W is deployed in such a setup, it appears as a computer to the ANT.
In Windows VPN or PPTP Pass-Through feature, the PPTP tunneling is created from
Windows 95, 98 and NT clients to an NT server in a remote location. The pass-through feature
allows users on the network to access a different remote server using the P-330W's Internet
connection. In SUA/NAT mode, the P-330W is able to pass the PPTP packets to the internal
PPTP server (i.e. NT server) behind the NAT. You need to configure port forwarding for port
1723 to have the P-330W forward PPTP packets to the server. In the case above as the remote
PPTP Client initializes the PPTP connection, the user must configure the PPTP clients. The P-
330W initializes the PPTP connection hence; there is no need to configure the remote PPTP
clients.
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103
Appendix B 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 68
PPTP Protocol Overview
Microsoft includes PPTP as a part of the Windows OS. In Microsoft’s implementation, the
computer, and hence the P-330W, 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.
Page 104 / 141
ZyXEL P-330W User’s Guide
Appendix B PPTP
104
Figure 69
Example Message Exchange between Computer and an ANT
PPP Data Connection
The PPP frames are tunneled between the PNS and PAC over GRE (General Routing
Encapsulation, RFC 1701, 1702). The individual calls within a tunnel are distinguished using
the
Call ID
field in the GRE header.
Page 105 / 141
ZyXEL P-330W User’s Guide
105
Appendix B PPTP

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