A whatsapp conversation is not dependent on the parties coming to a consensus on the timing for a common event. Also, an army can only send so many messengers

A whatsapp conversation is not dependent on the parties coming to a consensus on the timing for a common event. Also, an army can only send so many messengers before you run out of soldiers (or operational readiness). Packets are free and plentiful. If you're on a network link so bad that you can't even occasionally receive a TCP ack, then you're very much an outlier.