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

[Ben Laurie]

Ian G wrote:
> David Wagner wrote:
>> Ian G <iang at systemics.com> writes:
>>
>>> I would say that the usefulness of the result is to show that there are
>>> no reliable channels.  Full stop.
>>
>>
>> Nope, that's wrong.  The argument says that *if* you use an unreliable
>> channel, then such-and-such can happen.  Note the difference between
>> a premise and a conclusion.  The question of whether reliable channels
>> exist or not is a question of physics, not of algorithms, and this proof
>> does not purport to show that reliable channels cannot exist.
> 
> OK, so I need to add in some hidden assumptions?
> 
> The proof purports to show that we cannot build a
> reliable channel ("achieve coordination between
> two remote parties") out of an unreliable physical
> channel and an algorithm.
> 
> Which is fine ... but:
> 
> If we add that to a (physical) physics claim that
> there are only unreliable channels (certainly between
> nodes over a network, and arguably elsewhere), then
> we can conclude that there are no reliable channels.
> 
> And I reach my conclusion that there are no reliable
> channels.
> 
> It seems an odd thing.  If we could make this algorithm
> work, and solve the problem, then would we then say
> that physics provides *only* unreliable channels, but
> we can fix the physical limitations with some algorithms?
> 
> If we talk about the statics of beams in a building,
> we would generally include all the algorithms needed
> to calculate those loads.  I don't understand why we
> would not call that physics - it seems like hair-splitting
> to me?

Clearly if we have a face-to-face conversation we can agree about
something. So, the argument falls apart in the face of (massive?) scaling.

In the end, it's probabilistic, as Paul Syverson has pointed out (is
there a link for that?). When you have a vast number of channels (as in
a face-to-face meeting) the probabilities get so close to 100% you can
neglect the difference.

Cheers,

Ben.

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