Source : Free On-Line Dictionary of Computing

Alternating bit protocol
     
         (ABP) A simple {data link layer} {protocol} that
        retransmits lost or corrupted messages.
     
        Messages are sent from transmitter A to receiver B.  Assume
        that the channel from A to B is initialised and that there are
        no messages in transit.  Each message contains a data part, a
        {checksum}, and a one-bit {sequence number}, i.e. a value that
        is 0 or 1.
     
        When A sends a message, it sends it continuously, with the
        same sequence number, until it receives an acknowledgment
        ({ACK}) from B that contains the same sequence number.  When
        that happens, A complements (flips) the sequence number and
        starts transmitting the next message.
     
        When B receives a message from A, it checks the checksum.  If
        the message is not corrupted B sends back an ACK with the same
        sequence number.  If it is the first message with that
        sequence number then it is sent for processing.  Subsequent
        messages with the same sequence bit are simply acknowledged.
        If the message is corrupted B sends back an negative/error
        acknowledgment ({NAK}).  This is optional, as A will continue
        transmitting until it receives the correct ACK.
     
        A treats corrupted ACK messages, and NAK messages in the same
        way.  The simplest behaviour is to ignore them all and
        continue transmitting.
     
        (2000-10-28)