The Two Generals’ Problem

Yes, I had help with the graphics for this series. There's no way I'd have animated that myself! On that note, thanks to Dashlane for sponsoring and helping me hire an animator: their free trial link is https://www.dashlane.com/tomscott

Couldn’t the red army reposition?

General A sends the entire army over to B's side by labeling one soldier at a time as a "messenger" and sending them over. The majority of the army gets through. They now have a combined army with roughly twice the strength of the original armies and no longer have the valley in the middle. They can agree on a time to attack and then attack.

Tom, please don't cook yourself. self cannibalism isn't good.

Can’t the messenger come back and say that the other side got the message??

couldn’t the general of army a just wait until his messenger got back with a piece of paper that says “acknowledged”

I know this doesn't work for the case of computer networking, but if General A takes his entire army to General B, surely then they'd all know when to attack ;)

2:11 in and thinking of a solution before it's (presumably) given. Here's the theory:

Group A makes many identical messages, one for each messenger they have. Each message includes three bits of data -- the coordinated attack time, a continuous acknowledgement request, and the information that at some designated time prior to the coordinated attack time, Group A will cease sending messages so long as they've received Group B's acknowledgement, and that should messages be received from Group A after that time, Group B should know the attack is off.

This means Group A periodically sends these messages. And should any messenger come from Group B, their arrival implies an agreement on the attack. At this point both groups need no further information. Because Group A knows that Group B agreed upon the time, and Group B doesn't need any other messages to be sure that Group A received one of their messages. In fact, the only way Group B will know that Group A HASN'T gotten their message is if they receive another message after the previously expressed "halt" time. And neither group is in danger of attacking without the other because for both parties to receive any message at all it means all is going according to plan. And if either group doesn't receive a message, or if they receive a message after the established time, they will definitively know something went wrong.

Isn't it solved in three messengers?

Messenger 1A tells general B about the time to attack.
Messenger B tells general A that the time is agreed upon.
Messenger 2A tells general B his confirmation was received and they're preparing for the agreed upon time.

Then they both know the other has agreed to attack at the stated time.

Both armies surround the castle and starve the castle of resources. No messages needed. The street creepers and 4 chan have won. Just like the alamo I am going to troll until the end.

To me is simple the general's problem

A sends "is 8pm ok? Send "ok" back for yes, if you dont receive an ok back from me, send "response?" Back 2 more times. If you dont get an ok from me, consider it null. I too will send a 2 "response?" If i dont listen back

So gen B agrees,
Sends one Ok
gets Ok back
Both attack.

Gen b sends ok
doesnt hear back, received "response?"
Sends ok
Receives ok
both attack

Gen b sends ok
doesnt hear back, doesnt receive response at all. Asumes A's ok has been intercepted as they would have sent one or more "response?" Back otherwise.
Sends "response?"


And so on. If all 5 messagers get caught after the initisl messsge, the attsck wouldnt go through.
If just 2 Oks do go through, the attsck goes through

I mean you need 2 minimum. A keeps sending troops saying: send one troop telling us what time to attack. We will keep sending troops until the package is received, and if the time of attack has not past we are a go.

Don’t rely on technology too much..nothing is built to last!

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

It's 2 am and I regret nothing

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

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.

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?

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?

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

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

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

£7.74 word.

2:55 starts sweating.....

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

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

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)

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.

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.

😂 these damn apps!

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.

Even your paid promotions are interesting to listen to!

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

2:57
umm...

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

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

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

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.

TCP/IP

You could just send the original guy back.

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

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

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

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

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

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?

Send the entire army as the messenger

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

Also your video is great and informative.