Editing Alternating bit protocol (section)

== 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.

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 [[Negation|complements]] (flips) 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.  (And vice versa.)  It treats such messages as negative-acknowledge codes (NAKs).  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.

=== Bounded Retransmission Protocol ===
'''Bounded Retransmission Protocol''' (BRP) is a variant of the alternating bit protocol introduced by [[Philips]].<ref>{{cite journal |last=Burnett |first=D.J. |author2=Sethi, H.R. |year=1977 |title=Packet Switching at Philips Research Laboratories |url=http://rogerdmoore.ca/PS/NPLPh/PhilipsA.html |url-status=dead |journal=Computer Networks |volume=1 |issue=6 |pages=341–348 |doi=10.1016/0376-5075(77)90010-1 |archive-url=https://web.archive.org/web/20131020142510/http://rogerdmoore.ca/PS/NPLPh/PhilipsA.html |archive-date=2013-10-20 |access-date=2013-08-30}}</ref> 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. Unlike ABP, BRP deals with sequence numbers of datum in the file and interrupts transfer after fixed number of retransmissions for a datum.<ref>{{cite web|url=https://www.irif.fr/~sighirea//trex/demos/brp.html|title=TreX's Examples -- Bounded Retransmission Protocol|website=www.irif.fr}}</ref>