[cap-talk] impossibility of solving the "coordinated attack problem" ("generals problem")

Jed at Webstart donnelley1 at webstart.com
Thu Feb 2 18:52:02 EST 2006


At 02:36 PM 2/2/2006, Valerio Bellizzomi wrote:
>On 02/02/2006, at 14.14, Jed at Webstart wrote:
>...What it really means is
> >that it is NEVER possible to come to agreement, to do the
> >"coordination" and to have both entities agree.  I think perhaps
> >an algorithmic approach is the best way to clarify the fundamental
> >problem.  You really have to put yourself in the position of the
> >boss or the lieutenant.  Let's try laying out possible algorithms:
>
>At some point someone makes a decision of trust: he trusts that the
>messenger will arrive, and so he executes the action anyway.

That's it.

>Alternatively,...

I don't think there is an "alternatively".

>if the communication is simultaneous in both directions,

It is, or at least it can be, of course, but the decision is made on 
one side only.

>it just takes one round-trip in both directions.

To do what?  Not come to agreement/coordination.

>In this case it is a question
>of timing. If one messenger arrives out of time, the algorithm must
>restart.

I don't understand what you're getting at above.  In any case
the last one to send a message is in an ambiguous situation
and can't be confident/sure that the attack is "coordinated".
That last message might not arrive.  There is no way out
of this "paradox"/problem.

--Jed http://www.webstart.com/jed/ 



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