The Node That Keeps Answering, and Lies

Subtitle: Silence is easy to spot. A convincing lie is not.

• Panel 1 - Name the fault model:
• It defines how a node may fail
• Every consensus design assumes one
• Fail-stop means crash, then silence
• A missing heartbeat exposes it
• Survivors agree the node is gone

• Panel 2 - The live liar:
• Byzantine means up, but untrustworthy
• Replica A hears X; replica B hears Y
• Health checks can remain green
• One observer sees liar or slow link
• Absence detects nothing

• Panel 3 - Count, with the condition:
• Crash faults f need 2f+1 replicas
• Oral-message BFT needs 3f+1
• For f=1: three nodes versus four
• Signatures can change the bound
• Never quote 3f+1 unconditionally

• Panel 4 - Falsify your assumption:
• Can one live node send conflicting data?
• Can bad bytes still look plausible?
• If yes, fail-stop consensus is not enough
• Raft-family clusters assume fail-stop
• Pay for BFT when peers cannot trust