Maxtor PPP user guide

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Networks

Tech Note #5

Point-to-Point Protocol (PPP)

Summary

This document describes the use of PPP in the Matrox iSwitch. It contains the following:



A list of products that support this feature.



A description of PPP and its features.

Applicability

This feature is available in the following products:



All of the Matrox iSwitch product line.

PPP

What Is It?

PPP is the standard protocol for remote access connectivity and serial point-to-point links. PPP is
used to send data packets over a serial line, such as a modem using an analog telephone line. The
popularity of PPP is partly because it is a suite of protocols. The main components of PPP are

the encapsulation scheme (how it forms a data packet), the authentication mechanism, the
Link Control Protocol (LCP) and the Network Control Protocol (NCP).

Encapsulation

PPP uses a variant of the High Speed Data Link Control (HDLC) protocol to frame the data from
the network layer. The frame sent over the physical link may be either: synchronous or
asynchronous, dedicated or circuit-switched. The framing includes a Frame Check Sequence
(FCS) that allows for error detection.

Authentication

Before establishing a session between end points, it may be desirable to authenticate the identity
of one or both end points. This is particularly true in the case of remote dial-up connections. The

Password Authentication Protocol (PAP) is a mechanism to provide a username and its
corresponding password. The drawback to PAP is that the username and password are

transmitted as clear text (not encrypted) and are only sent during the initial link setup between
the client and the remote site.

The Challenge Handshake Authentication Protocol (CHAP) overcomes both of these limitations.

CHAP protects against eavesdroppers by encrypting the password before transmission to
the remote site. It also protects against the PPP connection being “stolen” by randomly issuing

a challenge to which the remote end point must respond.

© Matrox Networks 1998

TechNote #5 - PPP Page 2 of 3

LCP

LCP allows for the management of communication functions. A link may be
established/terminated and negotiate configuration options between the two end points. Typical
configuration options include encapsulation options, optimal packet sizes, authentication type,
magic numbers and so on.

NCP

The NCP is actually a series of independently defined protocols that encapsulate network layer
protocols, such as TCP/IP, IPX, Appletalk, DECnet and OSI. A separate RFC details each
network layer protocol's specific needs. For example, the Matrox iSwitch is TCP/IP based and
requires negotiation of options such as IP address and Van Jacobson compression. Several RFC
documents specify PPP. Those most applicable to the Matrox iSwitch are:

• 1661 - The Point-to-Point Protocol.

• 1662 - PPP in HDLC-like Framing.

• 1172 - The Point-to-Point Protocol Initial Configuration Options.

• 1334 - PPP Authentication.

• 1332 - The PPP Internet Protocol Control Protocol (IPCP).

• 1990 – The PPP Multilink Protocol.

How Does It Work?

For a detailed explanation of how PPP and its related protocols work, please refer to the RFCs
listed above.

The Matrox iSwitch uses PPP to establish a connection to the Internet via an ISP.

• Initially there is no physical link between the Matrox iSwitch and the ISP. When the LAN

accesses the Internet, the Matrox iSwitch automatically dials the ISP. The modems negotiate
a physical link and establish a carrier. Then the PPP process begins. The Matrox iSwitch PPP
process moves from the closed state to the LCP negotiation phase.

• In the LCP negotiation phase both the Matrox iSwitch and the ISP suggest certain

configuration parameters which the other side may accept or reject. The parameters
negotiated depend partially upon the particular ISP. Typical parameters would be ACCM,
compression types, packet sizes and authentication.

• If the Matrox iSwitch negotiates the parameters successfully with the ISP and establishes a

successful link, PPP moves to the authentication phase. The Matrox iSwitch sends the
username and password information. If the authentication fails the link will terminate and
PPP will return to the closed state. Otherwise, PPP transitions to the ready phase and the
Matrox iSwitch negotiates IPCP options with the ISP.

• During the IPCP phase, the ISP assigns the Matrox iSwitch an IP address to use on the

Internet. Other configuration data that can be exchanged includes the IP address of the ISP,
whether Van Jacobson Header Compression is used, and so on.

© Matrox Networks 1998