[cap-talk] "coordinated attack" - exploring possibilities

[Jed at Webstart]

At 04:07 PM 2/2/2006, Valerio Bellizzomi wrote:
>On 02/02/2006, at 15.52, Jed at Webstart wrote:
>...
> >I don't think there is an "alternatively".
>
>I am exploring possibilities,
>Alternative may be a full-duplex communication with ack: Both sides send
>their messenger simultaneaously (by means of a previously agreed schedule),
>and each side on receipt of the messenger sends back the same messenger
>with their ack. Now the messengers meet in middle route, exchange their ack
>and return. Each side receives back the messenger of the other side with
>the ack from the other side. Now it is known to both sides that each one
>has received the message and sent back an ack, it is also known that the
>messengers have met in middle route, and it took only one round-trip and a
>half for both to do their job. With the next message each messenger return
>to their proper side with a full report of ack on the previous message.
>Two messages make one message coordinated.

I'm not sure I quite follow what you are suggesting.  From what I guess
I think the vulnerability would be that one messenger but not the other
would get taken out on the way back from the rendezvous.  In that case
one side would act and the other would not.

Remember that the nature of the problem is that you want the communication
to either result in both sides acting, for sure, or neither side 
acting, for sure.
This cannot be achieved.  One only need look at the last (any last) message
to see why.  If it is received then the one receiving side and only that one
receiving side will act.  That receipt or its lack has no effect on 
the action of
the other side.  Change the result of that last message receipt and the
actions become unbalanced.

Multiple messages and retries can help with probabilities, but they
can't deal with the fact that ultimately there has to be a limit, and in
the limit the result can be unbalanced just by changing the result of
the last message sent.  If one can be assured that one of a set of
repeated messages will get through then you can be assured of
coordination.  Without that assurance, however, coordinated attack
can't be achieved.

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