The reliable multicast protocol of Atomic Group employs sequence numbers to achieve reliable message delivery. The protocol uses a receiver-initiated strategy, which provides low latency in the case of message loss. Positive acknowledgments piggybacked on messages allow buffers to be managed effectively. To keep the overhead due to acknowledgments as small as possible, the protocol uses timestamp acknowledgments. The ordered mechanism of Atomic Group depends on timestamps derived from physical clocks, and messages are timestamped and delivered according to their timestamps.
Clock synchronization is achieved by either software or hardware.
For such a system, the maximum latency equals the diameter of the system plus the maximum time between the sending of two messages. To keep the latency small even if no messages are sent, the protocol creates null messages. In case the application can dispense with total order, Atomic Group provides delivery of messages in causal order to process groups without the use of vector timestamps.