How do consensus mechanisms (e.g., IBFT, RAFT, PBFT) work in permissioned networks? What trade-offs do they introduce?

1. Consensus mechanisms: RAFT, BFT (PBFT, IBFT, Hotstuff)

RAFT

• Type: Crash Fault Tolerant (CFT).
• How it works: A single leader proposes blocks, followers replicate.
• Strengths:
  • Simple, very fast (low latency).
  • Great for private networks with trusted participants.
  • Only tolerates crashes/failures, not malicious actors.
• Clique (PoA, some BFT but not as high as IBFT, slower than RAFT, legacy)

Byzantine Fault Tolerant (BFT) systems

• PBFT (Practical Byzantine Fault Tolerance)
  • Works for fixed validator set, adding or removing nodes means re running the bootstrap
  • Leader proposes → replicas exchange messages (pre-prepare → prepare → commit)
  • Requires 3f + 1 nodes to tolerate f Byzantine faults.
  • Message complexity: O(n²) per round (lots of all-to-all communication).
  • Weakness: Doesn’t scale well beyond tens of validators due to message overhead.
• IBFT (Istanbul Byzantine Fault Tolerant)
  • Variant of PBFT designed for Ethereum clients (Quorum).
  • How it works:
    • Imagine you have n banks (nodes) running the same ledger.
    • One of them is the leader (primary) for this round.
    • A client sends a payment request → the leader proposes it.
    • The proposal goes through three steps:
      1. Pre-prepare: leader broadcasts its proposal.
      2. Prepare: Validators broadcast a “prepare” message if they think the block is valid.
      3. Commit: (If enough votes are collected (typically 2f+1 out of n), validators broadcast a “commit)
    • Once the threshold of commits is reached, the block is officially added to the chain.) and the transaction is final
  • Message complexity: Also O(n²), but optimized for smaller, permissioned networks.
• Strengths:
  • Provides finality (once a block is committed, it’s irreversible).
  • Resilient to some malicious or misconfigured nodes.
• Limitations:
  • More communication overhead, slower than RAFT.
  • Example - In multi-party consortiums where there’s some distrust among participants, or regulators require Byzantine fault tolerance. Cross-border FX PvP where banks in different jurisdictions may not fully trust each other.

Tendermint (2016)

• Used in Cosmos.
• Strong BFT finality, rotating proposers.
• Message complexity O(n²).
• Mature and widely deployed.

Hotstuff (Espresso Systems)