What is Loop Fault Tolerance?

Loop Fault Tolerance (LFT) is the consensus algorithm developed by ICON Foundation for the ICON blockchain network. LFT is a BFT-based protocol that modifies the traditional PBFT approach to improve efficiency for blockchain use cases. The protocol operates with a set of validators (called P-Reps or Public Representatives) elected through delegated proof-of-stake, where ICX token holders vote for their preferred representatives. In each round, a leader proposes a block, and validators go through voting phases to reach agreement. LFT 2.0, an upgraded version, introduced an optimized two-phase voting process that reduces the number of message rounds needed to reach consensus compared to the original protocol.

LFT provides deterministic finality — once a block is confirmed, it cannot be reversed. The protocol tolerates up to one-third of the validator set being Byzantine. Block production occurs approximately every 2 seconds, enabling reasonable throughput for the network's interoperability-focused use cases. LFT 2.0 improved upon the original by simplifying the leader election process and making the consensus more resilient to network asynchrony. The protocol is designed to be lightweight and efficient for the ICON network's specific architecture, which emphasizes cross-chain interoperability through its Blockchain Transmission Protocol (BTP) rather than raw transaction throughput.

LFT powers the ICON network, which launched its mainnet in January 2018. ICON was developed by ICONLOOP (formerly theloop), a South Korean blockchain company, and aims to connect different blockchains and real-world institutions through its interoperability protocol. The network has been adopted in South Korea for various enterprise and government applications, including digital identity, decentralized finance, and cross-chain communication. ICON uses a delegation system where ICX holders vote for P-Reps who operate validator nodes and participate in network governance, combining the consensus mechanism with on-chain governance for protocol upgrades and fund allocation.