Gasper

Gasper is the full consensus protocol securing Ethereum's beacon chain, named for its two halves: GASfer combines the GHOST fork choice with the Casper finality gadget (the formal paper is titled 'Combining GHOST and Casper'). It is the answer to a practical problem: Casper FFG only finalizes checkpoints once per epoch, but the network must agree on a single chain head every twelve-second slot to keep producing blocks. Gasper layers a live fork-choice rule for the head over the slow-but-certain finality of checkpoints.

The division of labor is clean. LMD-GHOST is the fork-choice rule that, slot by slot, picks the canonical head by weighing validators' latest attestations, so honest validators agree on where to build even before anything is finalized. Casper FFG runs underneath, using attestations' source-and-target votes to justify and finalize checkpoints roughly every two epochs, providing the irreversible backbone. Crucially, the fork choice is constrained to never revert a finalized checkpoint, so liveness from GHOST and safety from Casper coexist.

Gasper is what makes Ethereum a chain-based BFT system rather than a round-based one like Tendermint: it keeps making progress and only forks transiently, then finalizes in arrears, whereas Tendermint finalizes each block instantly but halts under heavy fault. Gasper also pairs naturally with mechanisms like the inactivity leak (to recover finality when too many validators are offline) and proposer boost (to harden the fork choice against reorg games).