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PPP Background 103

Figure 5-3

PPP Frame Format

The frame structure fields for PPP are as follows:

Flag--A single byte that indicates the beginning or end of a frame. The flag field consists
of the binary sequence 01111110.

Address--A single byte that contains the binary sequence 11111111, the standard
broadcast address. PPP does not assign individual station addresses.

Control--A single byte that contains the binary sequence 00000011, which calls for
transmission of user data in an unsequenced frame.

Protocol--Two bytes that identify the protocol encapsulated in the information field of the
frame. The most up-to-date values of the protocol field are specified in the most recent
Assigned Numbers RFC. At press time, this was RFC 1700. For more information, see
www.isi.edu/in-notes/rfc1700.txt.

Data--Zero or more bytes that contain the datagram for the protocol specified in the
Protocol field. The end of the Data 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 1500 bytes. By prior agreement, consenting PPP implementations can use other values
for the maximum Data field length.

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, are always clearly distinguishable from standard frames.

Variable

2 or 4

Flag

Address

Control

Protocol

Data

FCS