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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 1 2 3 PPP Multilink Protocol (MP) Frame Format

PPP PAP Control Frame Formats

Let’s start with PAP. Its three control frames are constructed as shown in Table 39. The Authenticate-Request uses one format, illustrated in Figure 37, while the other two frame types use a different format, shown in Figure 38.

Table 39: PPP Password Authentication Protocol (PAP) Frame Formats

Frame Type	Code Field	Identifier Field	Length Field	Data Field
Authenticate-Request	1	New value generated for each frame.	6 + Length of Peer-ID + Length of Password
Authenticate-Ack	2	Copied from the Identifier field of the Authenticate-Request frame for which this is a reply.	5 + Length of included Message.	Contains a one-byte Msg-Length subfield that specifies the length of the Message subfield that follows it. The Message subfield contains an arbitrary string of data whose use is implementation dependent. It may be used to provide an indication of authentication success or failure to the user. If not used, the Msg-Length field is still included, but its value is set to zero.
Authenticate-Nak	3

Figure 38: PPP PAP Authenticate-Ack and Authenticate-Nak Frame Format

PPP Link Control Protocol (LCP) Frame Formats 1 2 3 PPP Multilink Protocol (MP) Frame Format

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