The Two Generals’ Problem


Comments • 2 680

  • Tom Scott
    Tom Scott   4 months back

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    • Akin Turhan
      Akin Turhan  1 months back

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    • The Superginge
      The Superginge  2 months back

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    • Matti Lüdke
      Matti Lüdke  2 months back

      Why not send a person from each army to the middle of the valley. Then both discuss a time and go back to their army. That way they will know if the message was received by the other army. As the Danger of the route lies in the middle of the valley they will get back safely.

    • صالح
      صالح  2 months back

      Hey there is a way to solve the 2 genrals problem and it's a horne or war drums because they are loud enugh to hear

    • Dark Thoughts
      Dark Thoughts  3 months back

      Now you know the issues protocol standards committees have to 'foresee' when designing a communication protocol.

  • Name
    Name  12 hours back

    Just make an agreement that 2 or more acknowledgements means you can attack

    • Jo 1337
      Jo 1337  2 days back

      It's 2 am and I regret nothing

      • Vysair
        Vysair  4 days back

        Or maybe both general could send dozen more message until it hit certain threshold and that will confirm it.

        • 10k12
          10k12  6 days back

          2:35 why not let them meet in the middle, so if they perish, they perish together and no message is received, but if they manage to meet, they can return and there are no threats on the way back to the generals.

          • Shaurya Pant
            Shaurya Pant  6 days back

            Well, after both the teams have received confirmation of the other party that they know that you know and will go at 8PM, I don't see any further need of sending messengers. Can someone explain, please?

            • Andre L
              Andre L  6 days back

              How come this problem exists at all, if the proposed time is 8 pm, and both of them acknowledged the acknowledgments, then why would A and B need to keep sending acknowledgments? They both acknowledged the proposal and acknowledgment, so it's already planned? They both already know that it made it to the other side at least twice, so what's the problem?

              This just feels like it's not really a problem.... How is this line of reasoning incorrect?

              • Markus B
                Markus B  1 weeks back

                I should be asleep but general B didn’t give me a confirmation

                • john wick
                  john wick  1 weeks back

                  Simply. General A sends a messenger which returns after delivery of the message. If the messenger does not return after a suitable time span. It is safe to assume something happend and another messenger needs to be sent

                  • Space Griffin
                    Space Griffin  1 weeks back

                    but wouldn't a and b know they've received the message by the third message?

                    • DukeOfEarle88
                      DukeOfEarle88  1 weeks back

                      £7.74 word.

                      • UberKrassMann
                        UberKrassMann  1 weeks back

                        2:55 starts sweating.....

                        • Skepty
                          Skepty  1 weeks back

                          Just tell them to make a fire signal after they do, as confirmation. Not that hard.

                          • Ashor
                            Ashor  1 weeks back

                            for the two generals, couldnt one side send a messenger with the message telling them to attack at a certain time (sundown) but to ensure they both know they both have to shoot a cannon ball to the west of the other general at sunrise) if they both observe the cannon ball west of them then they know they will attack at sundown

                            • Midhunraj R Pillai
                              Midhunraj R Pillai  1 weeks back

                              I wonder how the generals problem is unsolvable. Please correct me whats the mistake of the solution below,

                              Step1: General A sends message saying attack at 6pm
                              (Here, A establishes the time)

                              Step 2: General B gets the message, sends back an acknowledgement 1 for the message 'we go the time'

                              (B gets the time)

                              Step 3: General A gets the ACK 1
                              (now, A knows that B got the time of attack. But B won't attack because B don't know that ACK 1 is reached at A. so..)
                              A sends a confirmation ACK 2 back

                              Step 4: General B gets the ACK 2
                              (now, B knows that both A and B establishes the time. Also B got the ACK 2, meaning B knows that ACK 1 reached A. But B knows that A won't attack until A confirm that B got the ACK 2. so...)

                              B sends a confirmation ACK 3 back

                              Ahhhh this trend continues and no one will attack since they cannot make sure that the previous ACK sent by them is reached at the other end!!! I am sorry (T_T)

                              • Ivan farlenkov
                                Ivan farlenkov  2 weeks back

                                Have generals send messagers at set periods of time even when there is nothing to send. If there is no messager, ask about it in the next message.

                                • Gareth H
                                  Gareth H  1 weeks back

                                  @Ivan farlenkov You really do not get it. Oh well, nevermind. Stick to flipping burgers.

                                • Ivan farlenkov
                                  Ivan farlenkov  1 weeks back

                                  @Gareth H Why not? If messagee is lost, request to resend will be eventualy get to the other side.

                                • Gareth H
                                  Gareth H  2 weeks back

                                  That is the ping approximation that is used. But it is not a solution to the problem, there is no solution.

                              • Monsieur P.
                                Monsieur P.  2 weeks back

                                My bank card would’ve texted me to confirm charge after being pinged to debit my account identical amount of money. It’s kind of stupid that nobody selling the food didn’t notice.

                                • Monsieur P.
                                  Monsieur P.  2 weeks back

                                  😂 these damn apps!

                                  • novicetheaf
                                    novicetheaf  2 weeks back

                                    This is a classic lack of dev testing prior to sending it for testing prior to deploying it to the end user, and QA testing.

                                    • Kevi Kiru
                                      Kevi Kiru  2 weeks back

                                      Even your paid promotions are interesting to listen to!

                                      • Oasia
                                        Oasia  2 weeks back

                                        Why dont the say just to fire a cannon if they recieve the message

                                        • Julian Danzer
                                          Julian Danzer  2 weeks back


                                          • Floofzy Kitty
                                            Floofzy Kitty  2 weeks back

                                            1:38 What are you talking about? Of course they made it to the other side. Just not the side they intended.

                                            • Muhammad Adam
                                              Muhammad Adam  3 weeks back

                                              Surely after the 4 messengers they wouldn't need confirmation any further confirmation because both would have already known that it's agreed upon

                                              • Terrain
                                                Terrain  3 weeks back

                                                You didn’t solve the two generals problem, you solved another problem created by it being unsolvable, which arguably is more dangerous than the two generals problem

                                                • BeforeReform
                                                  BeforeReform  3 weeks back

                                                  per your ad... there is *always* the possibility of brute force getting through. It could take years, or decades on average, but there is always a chance someone gets lucky.

                                                  • Angelmarauder
                                                    Angelmarauder  3 weeks back


                                                    • its_a_trab
                                                      its_a_trab  3 weeks back

                                                      You could just send the original guy back.

                                                      • Emerald Block Boat
                                                        Emerald Block Boat  3 weeks back

                                                        I'd say that if B received two verifications then do it

                                                        • TARINunit9
                                                          TARINunit9  3 weeks back

                                                          Proposition for the two generals problem: instead of sending one messenger, send a pair of messengers. One messenger stays at camp B and the other returns to camp A. If no messengers return to camp A, then it's obvious that the message was intercepted on one of the two trips, so you send another pair

                                                          • Lloyd Thorpe
                                                            Lloyd Thorpe  3 weeks back

                                                            What they should have done is just starve the castle out like they most commonly did

                                                            • The Nintendo Fan
                                                              The Nintendo Fan  3 weeks back

                                                              Honestly, the red army would automatically assume the blue army got the message and would attack on that time

                                                              • Eric ON
                                                                Eric ON  3 weeks back

                                                                Atleast once invocation solves this issue as well. I.e. let the client keep spamming the same request until one confirm has been recieved.

                                                                • Emil Sørensen
                                                                  Emil Sørensen  4 weeks back

                                                                  I'd definitely not like having a master password that, if lost, renders my entire online life unrecoverable.

                                                                  How can you possibly make your one password unforgettable? Chisel it into an immovable rock?

                                                                  • ͔ ͔
                                                                    ͔ ͔  4 weeks back

                                                                    Send the entire army as the messenger

                                                                    • Beowulf Macbethson
                                                                      Beowulf Macbethson  4 weeks back

                                                                      I was watching a video about Morgoth from Lord of the Rings, why am I here.

                                                                      Also your video is great and informative.

                                                                      • TIMΞ СнΛИGΣ
                                                                        TIMΞ СнΛИGΣ  4 weeks back

                                                                        But the generals would know after a few acknowledgements? What am I missing here

                                                                        • andrew allen
                                                                          andrew allen  4 weeks back

                                                                          Good grief, a Hewlett-Packard computer I can do component level repairs and maintenance on!

                                                                          • man0z
                                                                            man0z  4 weeks back

                                                                            I may have double paid for Nandos last night

                                                                            • IlluminatiBG
                                                                              IlluminatiBG  4 weeks back

                                                                              We solved this problem for banking application at work. The app is designed to work offline and synchronize when online. The acknowledgment problem is that you do not know if a message did not reach the server or it reached the server, but the confirmation was lost. As a result each message is uniquely identified in the client and safe to repeat. You can resend a message to the server a million times, but it will result in one transaction only for both client and the server.

                                                                              • Curtis Moore
                                                                                Curtis Moore  4 weeks back

                                                                                This sort of thing happened to me 3 years ago through Amazon. The progress wheel after clicking "Buy Now" kept freezing when I hit it. I foolishly kept refreshing and hitting "Buy Now" again until getting to the confirmation screen. This was why, two weeks later, I had to ship back 15 identical USB phone chargers.

                                                                                • Joshua Anderson
                                                                                  Joshua Anderson  4 weeks back

                                                                                  So how does general A know that general B got the message?

                                                                                  • Joshua Anderson
                                                                                    Joshua Anderson  4 weeks back

                                                                                    @lorvincent Thank you but I'm still not getting it for some reason. Are you saying that general A and B have 1000 other generals to get messages from? If general B relays the message to 1000 other generals how does general B know the other generals got the message? I'm stuck on something here...

                                                                                  • lorvincent
                                                                                    lorvincent  4 weeks back

                                                                                    Because 1000 other generals also got the message, and general B relayed to all 1001 of them, which caused the other 1000 to relay to general A that B got the message. It isn't the method described in the video, but this is the basic idea behind distributed ledgers with byzantine fault tolerance.

                                                                                • Asma Hasmalaria
                                                                                  Asma Hasmalaria  4 weeks back

                                                                                  I have the solution: Instead of sending messengers to general B, general A picks up his phone, dials the number of the favorite pizzeria and says "I want a valley castle, with cheese dip and onions" and Deliveroo gets nothing while the pizzeria gets the money they deserve.

                                                                                  • iHunterKiller
                                                                                    iHunterKiller  4 weeks back

                                                                                    We’re looking at this as a 2d plane. In 3d, don’t go through the castle, go around it, meet up, attack together. Done.

                                                                                    • MrCocktaiI
                                                                                      MrCocktaiI  3 weeks back

                                                                                      Few things are as boring as giving 'clever' answers to puzzles that totally disregard the actual problem

                                                                                  • Banned From Existance
                                                                                    Banned From Existance  4 weeks back

                                                                                    So what if we get two messengers, one tries to go through the valley meanwhile the other observes the messenger at a safe distance. If the message gets killed, the other person can come back to let them know they failed! Thus they keep attempting until the get it right.

                                                                                    • BillyViBritannia
                                                                                      BillyViBritannia  1 months back

                                                                                      So the actual question is 'can you make sure a message arrives if you can't ensure the message' s arrival?'
                                                                                      Well, doesn't take a genius to see why that's impossible.

                                                                                      • BillyViBritannia
                                                                                        BillyViBritannia  1 months back

                                                                                        How is the comment section of such a technical/educational video so God damn funny.

                                                                                        • Jonny boy
                                                                                          Jonny boy  1 months back

                                                                                          Can quantum computing utilizing quantum entanglement solve this issue?