A PPP frame starts and ends with a Flag with value 0x7E. The Flag (start) is followed by Address and Control fields, with are not needed for PPP, but are present for compatibility with High Level Data Link Control Protocol (HDLC). Default values are 0xFF for Address (broadcast) and 3 for Control. The fields are suppressed when using Field compression. (0xFF03 is not a valid Protocol field, and thus the difference is made).
The Protocol field ranges from 0x0000 to 0xFFFF, each used for a particular protocol. Some values are not used (see above). The first octet is even and the second octet is odd. The value when encapsulating an IP packet is 0x0021. When using IPCP, the value is 0x8021. (Add 8 to first digit for getting corresponding NCP value).
The Information Field has the Network Layer data encapsulated. It may be followed by Padding.
This is followed by FSC (Frame Check Sequence), which is a CRC code of the data. It is 2(default) or 4 bytes in size.
The last byte is, ofcourse, the Flag, 0x7E.
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