Section 3.2.2 Behind Closed Doors
With a stable system and a fixed quorum, the president can optimize message passing to reduce network chatter while maintaining safety and progress. This can be streamlined to 3 messages passed between the president and the quorum
(3) The president sent a BeginBallot message to each legislator in a quorum.LAMPORT, P. 17 — §3.2.2
(4) Each legislator in the quorum sent a Voted message to the president.
(5) The president sent a Success message to every legislator.
In a large system, the total number of messages can grow quickly. By piggybacking messages, the protocol reduces the total message count to about 2N messages per decree, where N is the number of legislators in the parliament.
This is a total of three message delays and about 3N messages, assuming a parliament of N legislators and a quorum of about N/2. Moreover, if Parliament was busy, the president would combine the BeginBallot message for one decree with the Success message for a previous one, for a total of only 2N messages per decree.LAMPORT, P. 17 — §3.2.2
The key optimization is batching. A single message can carry both the BeginBallot for a new decree and the Success notifications for decrees that have already reached consensus. When the leader handles multiple decrees simultaneously, it can batch several BeginBallot and Success messages together, reducing overall message traffic while maintaining correctness.
These optimizations rely on the assumption of a stable quorum — no legislators enter or leave the Chamber during these exchanges. This is why the process is described as “behind closed doors.”