ABSTRACT
In group communication a sending process can broadcast a message in a single operation, but messages from different senders may reach the receivers in different orderings. This chapter explains two solutions to this uniform ordering problem: the All-Ack Protocol and the Early Delivery Protocol. It explains and proves the correctness of a protocol that ensures the Uniform Delivery Property, namely that the messages are delivered to all the receivers in the same order that extends the causal ordering. A situation in which the Early Delivery Protocol seems to have an advantage over the All-Ack Protocol is one in which the normally operating processes send messages which usually reach all other processes after a short delay, but there is also a small number of processes that are momentarily slowed down with the result that their messages take much longer to reach their destinations.
