The hierarchical daisy architecture for causal delivery

We propose the hierarchical daisy architecture, which provides causal delivery of messages sent to any subset of processes. The architecture provides fault tolerance and maintains the amount of control information within a reasonable size. It divides processes into logical groups. Messages inside a logical group are sent directly, while messages that need to cross logical group boundaries are forwarded by servers. We prove the correctness of the daisy architecture and discuss possible optimizations.