Alternating bit protocol
Alternating bit protocol is a simple network protocol operating at the data link layer that retransmits lost or corrupted messages using FIFO semantics. It is the foundational form of a stop-and-wait ARQ, which is the simplest possible implementation 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.
History
team at the National Physical Laboratory introduced the concept of an alternating bit protocol in 1967 for the proposed NPL network and published their work in 1968 and 1969. An ABP was used by the ARPANET and by the European Informatics Network.Bounded Retransmission Protocol is a variant of the alternating bit protocol introduced by Philips in the mid-1970s.
Design
s are sent from transmitter A to receiver B. Assume that the channel from A to B is initialized and that there are no messages in transit. Each message from A to B contains a data part and a one-bit sequence number, i.e., a value that is 0 or 1. B has two acknowledge codes that it can send to A: ACK0 and ACK1.When A sends a message, it resends it continuously, with the same sequence number, until it receives an acknowledgment from B that contains the same sequence number. When that happens, A complements the sequence number and starts transmitting the next message.
When B receives a message that is not corrupted and has sequence number 0, it starts sending ACK0 and keeps doing so until it receives a valid message with number 1. Then it starts sending ACK1, etc.
This means that A may still receive ACK0 when it is already transmitting messages with sequence number one. It treats such messages as negative-acknowledge codes. The simplest behaviour is to ignore them all and continue transmitting.
The protocol may be initialized by sending bogus messages and acks with sequence number 1. The first message with sequence number 0 is a real message.