Fork Choice Rule

The rule must enforce safety: once a block is finalized (agreed upon), it must never be reverted, preventing double-spends and ensuring transaction finality. This property protects against conflicting histories. A rule that allows a finalized block to be orphaned in favor of a competing chain breaks safety. Mechanisms like Casper FFG's checkpoint finality or Tendermint's instant finality explicitly encode safety. In longest-chain protocols, safety is probabilistic and increases with block depth (confirmations).

This property defines what information the rule requires.

• Objective Rules: Rely solely on information intrinsic to the blockchain data structure (e.g., block hash, parent pointer, timestamp). Nakamoto's longest-chain rule is objective; a node only needs the blocks themselves.
• Subjective Rules: Depend on external messages or votes from validators (e.g., attestations, prepares, commits). Casper FFG and Tendermint are subjective; a node must see a quorum of validator votes to decide on the chain. Subjective rules enable faster finality but require active participation and message propagation.

A stronger form of liveness, plausible liveness means that from any state of the network—even one caused by arbitrary failures or attacks—it is always possible for honest validators to produce new blocks and continue the chain, provided they follow the protocol. This prevents a single past failure from permanently halting the system. Protocols like Casper FFG are designed with plausible liveness, ensuring that honest validators can always finalize a new chain regardless of past equivocation or partitions.

A security property where, if safety is violated (e.g., two conflicting blocks are finalized), the protocol can cryptographically identify the set of validators responsible for the fault. This enables slashing, where the malicious validators' staked funds are destroyed. Casper FFG is a prime example of accountable safety. Its checkpointing system creates slashable conditions for validators who vote for two conflicting checkpoints. This property strongly disincentivizes attacks, moving security from purely economic (Proof of Work) to cryptographic and economic.

The irreversible guarantee that a block and its transactions will never be reverted. Fork Choice Rules determine how and when finality is achieved.

• Probabilistic Finality: Used in longest-chain protocols (e.g., Bitcoin). The probability a block will be reorganized decreases exponentially as more blocks are built on top.
• Provable (Instant) Finality: Used in BFT-style protocols (e.g., Tendermint, Casper FFG). A block is finalized after a supermajority of validators cryptographically commits to it, making reversion impossible barring a catastrophic failure.

The two fundamental guarantees of any consensus protocol, which a Fork Choice Rule must help uphold.

• Safety: The guarantee that all honest nodes agree on the same chain and no two honest nodes decide on conflicting values. A faulty Fork Choice Rule can break safety.
• Liveness: The guarantee that the network eventually makes progress and produces new blocks. A Fork Choice Rule that cannot resolve forks can harm liveness.

These properties are often in tension, as formalized by the CAP theorem.

The Greedy Heaviest Observed SubTree protocol is an alternative Fork Choice Rule designed for fast block times. Instead of choosing the longest chain, it selects the chain with the heaviest subtree (including orphaned blocks).

• Purpose: Mitigates the high orphan rate (wasted work) in high-latency networks with fast block production.
• Implementation: Used as the basis for Ethereum's original Fork Choice Rule. It incentivizes miners to publish blocks quickly, even if they are not on the main chain, as their work still contributes weight.

A participant in a consensus network (e.g., Proof of Stake) responsible for proposing and attesting to blocks. Validators are the actors who execute the Fork Choice Rule.

• Role: They run node software that continuously evaluates the block tree using the protocol's defined Fork Choice Rule to identify the canonical head.
• Incentives: Validators are typically rewarded for honest participation (following the Fork Choice Rule) and slashed for provable violations like equivocation (creating blocks that could cause forks).