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'''Alternating bit protocol''' ('''ABP''') is a simple [[network protocol]] operating at the [[data link layer]] ([[OSI model|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.<ref>{{cite book|last=Tel|first=Gerard|title=Introduction to distributed algorithms|year=2000|publisher=Cambridge|isbn=0521794838|pages=85}}</ref>
It was used by the [[ARPANET]] and the [[Packet switching#EIN|European Informatics Network]].<ref>{{Cite book|last=Davies|first=Donald Watts|url=http://archive.org/details/computernetworks00davi|title=Computer networks and their protocols|date=1979|publisher=Chichester, [Eng.] ; New York : Wiley|others=Internet Archive|year=|isbn=|___location=|pages=206}}</ref>
== Design ==
[[Message]]s are sent from [[transmitter]] A to [[Receiver (information theory)|receiver]] B. Assume that the [[Communication channel|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.
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