Fork ID

Overview

A fork ID is a compact compatibility signal exchanged by peers. In Ethereum’s discovery and handshake conventions, it combines a hash derived from genesis and past fork activations with information about the next expected fork. Peers use it to reject clearly incompatible connections while allowing nodes that are safely before or after known upgrades to communicate. It is not the same as an EVM chain ID and does not authenticate a peer or prove canonical state.

Client implementations must calculate fork IDs from the correct genesis and schedule, including network-specific upgrades. Operators should investigate widespread mismatches as configuration or upgrade failures. Applications still need chain ID, genesis verification, synchronization, and finality checks beyond peer-level fork compatibility. Fork ID should be tied to specific competing block hashes, heights, fork-choice rules, and the point at which the local canonical view changed. A height alone is insufficient because several valid or invalid branches can contain different transactions at the same position.

Credits, confirmations, balances, and callbacks must not be duplicated when the same transaction reappears in a different block. Applications affected by Fork ID should roll dependent state backward idempotently and then replay the new canonical history. The likelihood and impact of Fork ID depend on consensus design, network partitions, client faults, finality, and participant concentration. Finalized states may have stronger guarantees than merely included blocks, but exceptional recovery can still involve governance or software intervention. High-value payment policies should choose thresholds that reflect network-specific reorganization risk. Monitoring should retain old and new block references, affected transactions, confirmation depths, and the trigger for every business-state change.