Construct a solution to reliable, totally ordered multicast in a synchronous system, using a reliable multicast and a solution to the consensus problem.

What will be an ideal response?


To RTO-multicast (reliable, totally-ordered multicast) a message m, a process attaches a totally-ordered, unique identifier to m and R-multicasts it.
Each process records the set of message it has R-delivered and the set of messages it has RTO-delivered. Thus it knows which messages have not yet been RTO-delivered.

From time to time it proposes its set of not-yet-RTO-delivered messages as those that should be delivered next. A sequence of runs of the consensus algorithm takes place, where the k’th proposals (k = 1, 2, 3, ...) of all the processes are collected and a unique decision set of messages is the result.

When a process receives the k’th consensus decision, it takes the intersection of the decision value and its set of not-yet-RTO-delivered messages and delivers them in the order of their identifiers, moving them to the record of messages it has RTO-delivered.
In this way, every process delivers messages in the order of the concatenation of the sequence of consensus results. Since the consensus results given to different correct processes are identical, we have a RTO multicast.

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