ITL - Internet Teaching Laboratory

Point to Point Protocol (PPP) Frame Structure

A PPP frame contains the following fields:

field	size (bytes)	value
flag	1	7E
addr	1	FF
control	1	03
protocol	2	see below
information	<= 1500	see below
CRC	2
flag	1	7E

protocol	information
0021	IP datagram
C021	link control data
8021	network control data

Byte value 0x7E is the flag character, so when 0x7E appears in the information field it must be escaped to distinguish it from a flag field. The escape character is 0x7D. A character that is escaped has its sixth bit complemented.

• The byte with the value 0x7E is transmitted as the 2-byte sequence 0x7D, 0x5E.
• The byte 0x7D is transmitted as the 2-byte sequence 0x7D, 0x5D.
• A byte with a value less than 0x20 (i.e. ASCII control characters) is escaped. For example, 0x01 is transmitted as the 2-byte sequence 0x7D, 0x21.