<networking> (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)

Alternating bit protocol (ABP) is a simple network protocol operating at the data link layer (OSI layer 2) that retransmits lost or corrupted messages using FIFO semantics. It can be seen as a special case of a sliding window protocol where a simple timer restricts the order of messages to ensure receivers send messages in turn while using a window of 1 bit.

Bounded Retransmission Protocol is a variant of the alternating bit protocol introduced by Philips. The service it delivers is to transfer in a reliable manner, if possible, large files (sequence of data of arbitrary length) from a sender to a receiver.