No finite exchange of messages over an unreliable channel can guarantee mutual certainty of agreement.
Akkoyunlu, Ekanadham and Huber's 1975 result. Each acknowledgement needs its own acknowledgement, and the regress does not terminate — which is why real protocols settle for probable agreement.
In practice
- Two people fixing a meeting by text each want confirmation that their confirmation arrived, and at some point one of them simply turns up.
- Two teams coordinating a cutover by message can never both be certain the other is ready, which is why real protocols settle for probable agreement and a rollback plan.
- Further reading
- Wikipedia
- Subject
- Technology & Software
- Essay
- How technologies spread, scale, decay and frighten us