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TCP/IP Guide by Charles M. Kozierok

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PAP and CHAP Frame Formats

For links where security is important, PPP provides two optional authentication protocols, PAP and CHAP. These are used during initial link setup by the LCP to deny PPP connections to unauthorized devices.

PAP and CHAP are control protocols and thus use the same basic control protocol frame format described earlier in this section. However, since they have a very different purpose than LCP and many of the other control protocols, they use a distinct set of frames with their own unique set of Code values. PAP uses three different control frame types, and CHAP uses four. Let's look at how PAP and CHAP frames are constructed.

PPP PAP Control Frame Formats

PAP's three control frames are constructed as shown in Tables Table 12-8 ...

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