The TCP/IP Guide - PPP Authentication Protocol (PAP, CHAP) Frame Formats

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The TCP/IP Guide

The TCP/IP Guide
9 TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9 TCP/IP Network Interface Layer (OSI Data Link Layer) Protocols
           9 TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)
                9 Point-to-Point Protocol (PPP)
                     9 PPP Protocol Frame Formats

PPP Link Control Protocol (LCP) Frame Formats

PPP Multilink Protocol (MP) Frame Format

PPP Authentication Protocol (PAP, CHAP) Frame Formats
(Page 3 of 3)

PPP CHAP Control Frame Formats

The four CHAP frame types are formatted as shown in Table 40. As you can see in that table, the Challenge and Response frames use one message format, illustrated in Figure 39, while Success and Failure frames use a different one, shown in Figure 40.

**Table 40: PPP Challenge Handshake Authentication Protocol (CHAP) Formats**
Frame Type	Code Field	Identifier Field	Length Field	Data Field
*Challenge*	1	New value generated for each frame.	5 + Length of challenge text + Length of name
*Response*	2	Copied from the Identifier field of the Challenge frame for which this is a reply.	5 + Length of Value + Length of Name
*Success*	3	Copied from the Identifier field of the Response frame for which this is a reply.	4 (or more if extra data is included).	May contain an arbitrary, implementation-dependent Message field to indicate to the user whether authentication was successful or failed.
*Failure*	4		4 (or more if extra data is included).

**Figure 39: PPP CHAP Challenge and Response Frame Format**

**Figure 40: PPP CHAP Success and Failure Frame Format**

PPP Link Control Protocol (LCP) Frame Formats

PPP Multilink Protocol (MP) Frame Format

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