The Free On-line Dictionary of Computing (30 December 2018):

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)