Alternating bit protocol (ABP) is a simple network protocol operating at the data link layer (OSI layer 2)[citation needed] 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.
The protocol may be initialized by sending bogus messages and acks with sequence number 1.
[2] 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.
[3] Donald Davies' team at the National Physical Laboratory introduced the concept of an alternating bit protocol in 1968 for the NPL network.