In computing, the '''Two Generals' Problem''' is a [[thought experiment]] meant to illustrate the pitfalls and design challenges of attempting to coordinate an action by communicating over an unreliable link. In the experiment, two generals are only able to communicate with one another by sending a messenger through enemy territory. The experiment asks how they might reach an agreement on the time to launch an attack, while knowing that any messenger they send could be captured.
The Two Generals' Problem appears often as an introduction to the more general [[Byzantine Generals]] problem in introductory classes about [[computer networking]] (particularly with regard to the [[Transmission Control Protocol]], where it shows that TCP can'tcannot guarantee state consistency between endpoints and why this is the case), though it applies to any type of two-party communication where failures of communication are possible. A key concept in [[epistemic logic]], this problem highlights the importance of [[common knowledge (logic)|common knowledge]]. Some authors also refer to this as the '''Two Generals' Paradox''', the '''Two Armies Problem''', or the '''Coordinated Attack Problem'''.<ref>{{cite book|last=Gmytrasiewicz|first=Piotr J.|author2=Edmund H. Durfee |chapter=Decision-Theoretic Recursive Modeling and the Coordinated Attack Problem |title=Artificial Intelligence Planning Systems|journal=Proceedings of the First International Conference on Artificial Intelligence Planning Systems|year=1992|pages=88–95|chapter-url=http://dl.acm.org/citation.cfm?id=139492.139503|accessdate=27 December 2013|publisher=Morgan Kaufmann Publishers|___location=San Francisco|doi=10.1016/B978-0-08-049944-4.50016-1|isbn=9780080499444}}</ref><ref>[http://www.dsi.uniroma1.it/~asd3/dispense/attack+amazons.pdf The coordinated attack and the jealous amazons] Alessandro Panconesi. Retrieved 2011-05-17.</ref> The Two Generals' Problem was the first computer communication problem to be proven to be unsolvable.<ref>
Leslie Lamport.
[https://lamport.azurewebsites.net/pubs/solved-and-unsolved.pdf "Solved Problems, Unsolved Problems and Non-Problems in Concurrency"].
