A brief meditation on two-party message exchange

When two parties communicate, what are the possible patterns of message exchange? Here’s what I’ve come up with so far: (Updated June 22, 2013 with causality arrows, failure modes, sequences and reordering, streams, non-deterministic communication.) Send Also known as: procedure call, one-way, unidirectional, post, fire and forget, In-Only, Out-Only, request (R), trigger, event, notification, command, read more »