PPP Authentication Protocol (PAP, CHAP) Frame Formats
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For links where security is important, PPP provides two optional authentication protocols. These are used during initial link setup by the Link Control Protocol (LCP) to deny PPP connections to unauthorized devices. The two authentication protocols are the Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP).
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. I thought it might be useful to see how PAP and CHAP frames are constructed. PAP uses three different control frame types, and CHAP uses four. To save space and show how the frames compare to each other, I will show them in one table for each protocol instead of seven tables.
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