With native support in every major OS except Windows 3.x and DOS, PPP is the de facto standard for remote-access communications, and it is relatively simple. Described in STD 51, PPP is designed to provide a simple network connection between two end points over which other network protocols, such as IP, IPX and AppleTalk, are transported. While dial-up communications is our focus here, PPP is not limited to dial-up remote access (including ISDN); it is also found in many point-to-point connections and dial-up VPN (virtual private network) protocols, such as PPTP, L2F and L2TP.

Overturning the Right Stone Prior to Windows98, there was no easy way to see if compression was enabled or what authentication protocol was in use with PPP. In fact, most OSes try to hide PPP from the end user. And even when it is possible to get PPP traces from common OSes, interpreting them is another matter.

When you connect using Dial-up Networking, Windows95 generates a file in the \WINDOWS directory called PPPLOG.TXT. This file contains a very high-level decode of the PPP negotiation showing what layers and options the client received and accepted, and whether you are properly authenticated. However, if you want to see which IP addresses were negotiated, you need to brush up on your HEX skills. Windows98 writes a similar log, but it dumps the packet data--including your user name and password if you're not using a secure algorithm such as CHAP (Challenge Handshake Authorization Protocol)--into the log file as well. You'll want to disable and delete PPP logging when you've finished debugging. PPP dialers on other OSes typically resolve the addressing into dotted quad notation.

Third-party PPP stacks such as FTP Software's OnNet Kernel 3.6 beta offer more features for PPP dialing and tracing in the user interface. For example, when negotiating PPP, OnNet Kernel reports the progress of the PPP connection in plain English. Unix systems also provide extensive logging facilities.

Linking Up There are four steps to a PPP connection: LCP (Link Control Protocol) negotiation, link authentication and quality management, NCP (Network Control Protocol) negotiation and link termination. Here we look at the first three steps. LCP sets up the PPP link as well as negotiated parameters, such as packet sizes and compression. Link quality and authentication is negotiated within LCP. LCP is negotiated in both directions--there is no client/server relationship in the normal sense.

NCP sets up the Layer 3 protocols and encompasses several protocols specific to each network protocol, such as IPCP (IP Control Protocol) and IPXCP (IPX Control Protocol).

To troubleshoot PPP connections, you need to know a little about the protocol. Four messages in PPP have special meaning: Configuration Request (Conf-Req), Configuration Acknowledge (Conf-Ack), Configuration Not-Acknowledge (Conf-Nak) and Configuration Reject (Conf-Rej) (see "PPP Messages Decoded,").