Fork choice rule

When two valid blocks appear almost simultaneously, the network forks temporarily and each node needs a shared criterion to pick a branch. In Proof of Work, Bitcoin uses the most-accumulated-work rule, loosely called the longest chain: the branch with the most total difficulty wins. On Ethereum, after the move to Proof of Stake, consensus is Gasper, which combines two pieces. LMD-GHOST is the fork choice rule proper: at each fork it selects the subtree with the most validator vote weight (attestations), counting only each validator's latest message. Casper FFG is the finality gadget: over the chosen chain, it marks certain blocks at epoch boundaries as justified and then finalized, giving them economic security.

Without a common choice rule, different nodes would diverge permanently. The rule determines which transactions count, when a transaction is final, the safe confirmation depth and the possibility of reorganizations. It is the operational backbone of consensus and what makes the rest of the guarantees applicable in practice.

Gasper's formal specification was published by Vitalik Buterin and coauthors in 2020 in the paper Combining GHOST and Casper. The literature has studied attacks on LMD-GHOST, such as balancing attacks and ex-ante reorgs, which prompted patches like proposer boost, and there is an active research line toward Single Slot Finality. In enterprise and permissioned deployments, by contrast, BFT-style schemes with immediate finality simplify this choice, which fits the traceability requirements of institutional settings.