  We present DAG-Rider, the first asynchronous Byzantine Atomic Broadcast
protocol that achieves optimal resilience, optimal amortized communication
complexity, and optimal time complexity. DAG-Rider is post-quantum safe and
ensures that all messages proposed by correct processes eventually get decided.
We construct DAG-Rider in two layers: In the first layer, processes reliably
broadcast their proposals and build a structured Directed Acyclic Graph (DAG)
of the communication among them. In the second layer, processes locally observe
their DAGs and totally order all proposals with no extra communication.
