Two Generals' Problem
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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. It is related to the more general Byzantine Generals' Problem (though published long before that later generalization) and appears often in introductory classes about computer networking (particularly with regards to the Transmission Control Protocol), though it can also apply to other types of communication. Some authors also refer to this as the Two army problem.
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[edit] Definition
Two armies, each led by a general, are preparing to attack a city. The armies are encamped outside of the city, each on its own hill. A valley separates the two hills, and the only way for the two generals to communicate is by sending messengers 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. Note that while the two generals have agreed that they will attack, they haven't agreed upon a time for attack before taking up their positions on their respective hills.
The two generals must attack the city at the same time to succeed. They must thus communicate with each other to decide on a time to attack and to agree to attack at that time. The thought experiment involves considering how they might go about doing this. In its simplest form one general (referred to as the "first general" below) is known to be the leader, decides on the time of attack, and must communicate this time to the other general. The requirement that causes the "problem" is that both generals must attack at the agreed upon time to succeed. Having a single general attack is considered a disastrous failure. 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:
- Yes, we will both attack at the agreed upon time.
Note that it is quite simple for the generals to come to an agreement on the time to attack. One successful message with a successful acknowledgement suffices for that. The subtlety of the Two Generals' Problem is in the impossibility of designing algorithms for the generals to use to safely agree to the above statement.
[edit] Illustrating the problem
The first general may start by sending a message "Let us attack at 9 o'clock in the morning." However, once dispatched, the first general has no idea whether or not the messenger got through. Any amount of uncertainty may lead the first general to hesitate to attack, since if the second general does not also attack at that time, the city will repel the advance, with grievous losses for that attacking general. Knowing this, the second general may send a confirmation back to the first: "I received your message and will attack at 9 o'clock." However, what if the confirmation messenger were captured? The second general, knowing that the first will hesitate without the confirmation, may himself hesitate. A solution might seem to be to have the first general send a second confirmation: "I received your confirmation of the planned attack." However, what if that messenger were captured? It quickly becomes evident that no matter how many rounds of confirmation are made there is no way to guarantee the second requirement that both generals agree the message was delivered.
[edit] Proof
Suppose there is any sequence of messages, some successfully delivered and some not, which suffice to meet the requirement of shared certainty for both generals to attack. In that case there must be some minimal subset of the successfully delivered messages that suffices (at least one message with the time/plan must be delivered). Consider the last such message that was successfully delivered in such a minimal sequence. If that last message had not been successfully delivered then the requirement wouldn't have been met and one general at least (presumably the receiver) would decide not to attack. From the viewpoint of the sender of that last message, however, the sequence of messages sent and delivered is exactly the same as it would have been had that message been delivered. Therefore the general sending that last message will still decide to attack. We've now constructed a circumstance where with the purported solution algorithms one general will attack and the other will not - contradicting the assumption that the algorithms were a solution to the problem.
[edit] Engineering approaches
A pragmatic approach to dealing with the Two Generals' Problem is to use schemes that accept the uncertainty of the communications channel and do not attempt to eliminate it, but rather mediate it to an acceptable degree. For example, the first general could send 100 messengers, anticipating that the probability of all being captured is low. With this approach the first general will attack no matter what and the second general will attack if any message is received. Alternatively the first general could send a stream of messages and the second general could send acknowledgements to each, with each general feeling more comfortable with every message received. As seen in the proof, however, neither can be certain that the attack will be coordinated. There's no algorithm that they can use (e.g. attack if more than 4 messages are received) which will be certain to prevent one from attacking without the other. If the channel can be made to be reliable then one message will suffice and additional messages do not help. The last is as likely to get lost as the first.
[edit] History
The Two Generals' Problem and its impossibility proof was first published by E. A. Akkoyunlu, K. Ekanadham, and R. V. Huber in 1975 in "Some Constraints and Tradeoffs in the Design of Network Communications" [1] where it's described starting on page 73 in the context of communication between two groups of gangsters.
This problem was given the name the Two Generals Paradox by Jim Gray [2] in 1978 in "Notes on Data Base Operating Systems" [3] starting on page 465. This reference is widely given as a source for the definition of the problem and the impossibility proof, though both were published previously as above.
