London hard fork
Mainnet 2021-08-05
Fees were split into a base fee and a tip, and the base fee began to be burned. Fees became easier to predict and ETH issuance began to be offset.
What changed
- Users
- Fees are now split into a base fee, which is set per block, and a priority fee, which is a tip paid to the block producer. The base fee rises or falls automatically depending on how full the previous block was, so wallets can more easily predict how much a transaction needs to pay to be included. Whatever remains of the maximum amount you set after the actual base fee and tip are subtracted is refunded to you. The base fee is burned, which began to offset some of the newly issued ETH.
- Wallet & app developers
- The transaction format changed so that it carries two values, a maximum fee (maxFeePerGas) and a maximum tip (maxPriorityFeePerGas), instead of a single gas price. The old format is still accepted, but fee estimation and signing code had to be updated for the new format. Contracts can read the current block's base fee with the BASEFEE opcode. The gas refund for clearing storage was cut sharply, so the gas token approach of stockpiling gas in advance to claim refunds became useless. Deployment of bytecode that starts with 0xEF is rejected.
- Validators
- At the time of London, the party producing blocks was the miner. The base fee was burned instead of going to miners, who received only the block reward and tips, so their fee income fell. The difficulty bomb had its activation pushed back, which prevented mining from slowing down before the transition to proof of stake. Today's validators inherit this structure as is and receive only the tip out of the transaction fees.
- Node operators
- Clients had to be upgraded to a version that supports London. The block gas limit changed to an elastic structure that allows a block to grow to twice the target, so at moments of heavy demand a single block can be larger than before. In response the base fee rises, so the next block returns toward the target, and the average block size stays similar to what it was before.