[messaging] Group messaging consistency under resource constraints

Trevor Perrin trevp at trevp.net
Fri Oct 10 15:09:45 PDT 2014

On Fri, Oct 10, 2014 at 1:21 PM, Ximin Luo <infinity0 at pwned.gg> wrote:
> On 10/10/14 21:06, Trevor Perrin wrote:
>> It's true that the asynchronous setting requires a message to be
>> stored somewhere, awaiting delivery.
>> But I disagree that it's "not much effort" for this somewhere to
>> retransmit messages to anyone in a group who asks.  That's not how
>> transports like (SMS, Google Cloud Messaging, email, Pond, or mix
>> networks) work.
> Why do you think it's a lot of effort? Those transports don't try to achieve group consistency; why is it unreasonable to suppose that consistency requires changes to (or, enhancements on top of) how the transport works?

Improving reliability is *already* a huge focus for any messaging system.

Nonetheless, projects like TextSecure find themselves dealing with
unreliable message delivery.  And even if it could be improved in some
transports, the protocol might be adapted to other transports where
it's harder.

>> [1] https://moderncrypto.org/mail-archive/messaging/2014/000372.html
> This [1] doesn't achieve consistency. I tried to explain why both in its "next message in thread" and in the first post of this thread, but it looks like my warnings are falling on deaf ears; here is a more concrete example:
> A: (1) Who wants ice cream? (last-message-seen: 0)
> A: (2) Who wants to kill the president? (last-message-seen: 1) (sent to everyone, *except B*)
> B: (3) No thanks... (last-message-seen: 2)
> C: (4) Me! (last-message-seen: 3)

Thanks for the concrete example.

It would be great to have a list of cases like this so we could
compare how different proposals handle them.

In this case, with Moxie's proposal, C is warned about the missing
message before saying "Yes!".  And anyone reading the (obviously
ambiguous) transcript could long-click on C's "Yes!" and see what it's
responding to.

Maybe that's good enough, maybe it's not.  A better taxonomy of
possible issues and proposals would help make these comparisons.


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