TCP/IP Explained
An easy to understand, comprehensive, and high-level introduction to TCP/IP.
TABLE OF CONTENTS TCP/IP Explained
Section 22.4: PPP Authentication Protocols
22.4 PPP Authentication Protocols
RFC 1334 defines two authentication protocols for use in a PPP environment, namely the Password Authentication Protocol (PAP), and the Challenge Handshake Authentication Protocol (CHAP). However, while these protocols are the most common in a PPP environment, due to the nature of PPP, other proprietary protocols (beyond the scope of this book) could also be used.
22.4.1 The Password Authentication Protocol (PAP)
The Password Authentication Protocol is a simple method by which a peer can learn the identity of its partner. The actual process takes place only after the initial Link Establishment phase has been completed, and takes the form of a two-way handshake. Essentially, after completion of the Link Establishment phase, the peer continually sends an Identification/Password pair until they are either acknowledged or the link is terminated.
As we saw in section 22.3.4, authentication is requested through a LCP option. For PAP the Type field contains a value of 3, the Length field 4, and the 16 bit Authentication Protocol field the value C023 16. The Data field is then unused. The PAP then travels in a PPP packet with a Protocol field value of C023 16. The general format of a PAP packet is shown in Figure 22-28.
The Code field can contain the values 1, 2, or 3 indicating an Authenticate Request, Positive Acknowledgement (Ack), or Negative Acknowledgement (Nak) respectively. The Identifier field is a unique...
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