Two Generals' Problem: Difference between revisions

Revision as of 04:46, 9 January 2022

Revision as of 20:21, 25 February 2022

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Two [[army|armies]], each led by a different [[general]], are preparing to attack a fortified city. The armies are encamped near the city, each in its own valley. A third valley separates the two hills, and the only way for the two generals to communicate is by sending [[Runner (war)|messenger]]s through the valley. Unfortunately, the valley is occupied by the city's defenders and there's a chance that any given messenger sent through the valley will be captured.

While the two generals have agreed that they will attack, they haven't agreed upon a time for an attack. It is required that the two generals have their armies attack the city simultaneously to succeed, lest the lone attacker army will die trying. They must thus communicate with each other to decide on a time to attack and to agree to attack at that time, and each general must know that the other general knows that they have agreed to the attack plan. Because [[Acknowledgement (data networks)|acknowledgement of message receipt]] can be lost as easily as the original message, a potentially infinite series of messages is required to come to [[Consensus (computer science)|consensus]].

The thought experiment involves considering how they might go about coming to a consensus. In its simplest form, one general is known to be the leader, decides on the time of the attack, and must communicate this time to the other general. The problem is to come up with algorithms that the generals can use, including sending messages and processing received messages, that can allow them to correctly conclude: