London hard fork

London hard fork. The London hard fork was a major, consensus-layer upgrade to the Ethereum blockchain, activated at block height 12,965,000 on the Ethereum mainnet in August 2021. It introduced a set of five Ethereum Improvement Proposals (EIPs) designed to improve the network's transaction fee market, enhance security, and optimize gas costs for certain operations. The most prominent and transformative change was the implementation of EIP-1559, which fundamentally altered how transaction fees are handled and initiated a deflationary mechanism for Ether.

Background and context

The upgrade was developed in response to long-standing issues with Ethereum's first-price auction model for transaction fees, which led to unpredictable costs and poor user experience, especially during periods of high network congestion like the 2021 cryptocurrency boom. Preceding network upgrades such as the Berlin hard fork had laid groundwork for efficiency improvements, but the core economic model remained unchanged. The broader roadmap for Ethereum, including the transition to a proof-of-stake consensus mechanism via the Ethereum 2.0 upgrades, created a pressing need to reform the fee market before the merger with the Beacon Chain. Extensive research and community discussions, led by core developers like Vitalik Buterin and teams at the Ethereum Foundation, culminated in the formalization of the proposals bundled into this upgrade.

Key features and EIPs

The fork bundled five distinct Ethereum Improvement Proposals. EIP-1559 overhauled the fee market by introducing a base fee that is burned and a priority fee for miners, making gas prices more predictable. EIP-3198 added the `BASEFEE` opcode, allowing smart contracts to access the current block's base fee. EIP-3529 reduced gas refunds for operations like `SELFDESTRUCT`, mitigating certain Denial-of-service attack vectors and reducing state bloat. EIP-3541 laid the groundwork for future EVM object format changes by reserving a new namespace. Finally, EIP-3554 delayed the Difficulty Bomb, a mechanism designed to incentivize the transition to proof-of-stake, by pushing it back several months to ensure a smooth migration.

Implementation and activation

The upgrade was implemented on several Ethereum testnets prior to mainnet deployment, including successful activations on the Ropsten, Goerli, and Rinkeby networks in mid-2021 to ensure stability. Client teams such as Geth, OpenEthereum, Nethermind, and Besu released compatible versions. It was activated on the Ethereum mainnet at block 12,965,000 on August 5, 2021, requiring all node operators to upgrade their clients to remain in consensus. The smooth activation was coordinated by core developers like Tim Beiko and James Hancock, with broad support from mining pools and infrastructure providers despite some initial opposition from parts of the mining community concerned about revenue changes.

Impact on Ethereum network

The most immediate impact was the introduction of EIP-1559's fee-burning mechanism, which began permanently removing Ether from circulation, leading to periods where the net issuance of ETH became negative. This created a new economic dynamic, often referred to as "ultrasound money" by proponents. While the upgrade made fee estimation more reliable, it did not inherently lower gas fees, which remained high due to underlying demand for block space from applications like DeFi protocols and NFT marketplaces. The changes from EIP-3529 and EIP-3541 improved network security and paved the way for future upgrades like the Shanghai hard fork.

Reactions and community response

The upgrade received widespread support from Ethereum users, developers, and many investors, who viewed the burning mechanism as a positive value accrual method for ETH. However, it faced notable opposition from some miners and mining pools, who saw their revenue from transaction fees potentially decrease, leading to discussions of a potential chain split which ultimately did not materialize. Figures like Vitalik Buterin and Anthony Sassano were vocal advocates, while critics from other blockchain communities debated its efficacy. The successful deployment was seen as a critical test of Ethereum's governance and its ability to execute complex, economically significant upgrades on a live network.