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Chapter 9 Link Protocol Configuration
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Following descriptions summarize PPP frame fields illustrated in
Figure 73.
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Flag—A single byte that indicates the beginning or end of a
frame. The flag field consists of binary sequence 01111110.
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Address—a single byte that contains binary sequence
1111111, the standard broadcast address. PPP does not
assign individual station addresses.
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Control—a single byte that contains binary sequence
00000011, which calls for transmission of user data in an
unsequenced frame. A connectionless link service similar to
that of Logical Link Control (LLC) Type 1 is provided.
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Protocol—two bytes that identify protocol encapsulated in
information field of frame. The most up-to-date values of
protocol field are specified in most recent Assigned Numbers
Request for Comments (RFC).
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Data—Zero or more bytes that contain datagram for the
protocol specified in the protocol field. The end of the
information field is found by locating the closing flag
sequence and allowing 2 bytes for the FCS field. The default
maximum length of the information field is 1,500 bytes. By
prior agreement, consenting PPP implementations can use
other values for the maximum information field length.
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Frame check sequence (FCS)—normally 16 bits (2 bytes). By
prior agreement, consenting PPP implementations can use a
32-bit (4-byte) FCS for improved error detection. The LCP
can negotiate modifications to the standard PPP frame
structure. Modified frames, however, always will be clearly
distinguishable from standard frames.
LCP can negotiate modifications to standard PPP frame structure.
Modified frames, however, always must be clearly
distinguishable from standard frames.
PPP Authentication Protocols
Point − to − Point Protocol (PPP) currently supports two
authentication protocols: Password Authentication Protocol (PAP)
and Challenge Handshake Authentication Protocol (CHAP). Both
are specified in RFC 1334 and are supported on synchronous and
asynchronous interfaces.
Background