Control field formats: (codes) 8 7 6 5 4 3 2 1 +--------------+-------+-------+ octet | | | | S - format 4 |X X X X | S S | 0 1 | supervisory frame +--------------+-------+---+---+ octet | | | 5 | N(R) |P/F| +--------------------------+---+ +--+---+---+---+---+---+---+---+ S |0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | RR -receive ready +--+---+---+---+---+---+---+---+ | N(R) |P/F| +--------------------------+---+ +--+---+---+---+---+---+---+---+ S |0 | 0 | 0 | 0 | 0 | 1 | 0 | 1 | RNR -receive not ready +--+---+---+---+---+---+---+---+ | N(R) |P/F| +--------------------------+---+ +--+---+---+---+---+---+---+---+ S |0 | 0 | 0 | 0 | 1 | 0 | 0 | 1 | REJ -reject +--+---+---+---+---+---+---+---+ request retransmission from N(R) | N(R) |P/F| +--------------------------+---+ N(R) Receive sequence number P/F Poll/final bit