[messaging] Minimal requirements for group chat

[Michael Rogers]

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On 16/04/14 19:34, Michael Rogers wrote:
> This variant doesn't require retransmissions or nacks. A message
> may be acked by some but not all of the members in one or more
> rounds before it's finally acked by everyone in the same round and
> committed.

After sleeping on the ideas I posted yesterday I've realised they
don't work. There are at least two serious problems:

* A member can send inconsistent acks to other members, in order to
make them believe that different messages have been acked by everyone

* There's a race condition between everyone receiving everyone else's
acks for a message, and everyone receiving a better message for the
same slot

Sorry for posting before sleeping. :-) However, I still think this
cut-down version of the problem is worth pursuing.

Cheers,
Michael
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