# EIP-649: Metropolis Difficulty Bomb Delay and Block Reward Reduction Source

Author Afri Schoedon, Vitalik Buterin https://github.com/ethereum/EIPs/issues/649 Final Standards Track Core 2017-06-21

## Simple Summary

The average block times are increasing due to the difficulty bomb (also known as the “ice age”) slowly accelerating. This EIP proposes to delay the difficulty bomb for approximately one and a half year and to reduce the block rewards with the Byzantium fork, the first part of the Metropolis fork.

## Abstract

Starting with BYZANTIUM_FORK_BLKNUM the client will calculate the difficulty based on a fake block number suggesting the client that the difficulty bomb is adjusting around 3 million blocks later than previously specified with the Homestead fork. Furthermore, block rewards will be adjusted to a base of 3 ETH, uncle and nephew rewards will be adjusted accordingly.

## Motivation

The Casper development and switch to proof-of-stake is delayed, the Ethash proof-of-work should be feasible for miners and allow sealing new blocks every 15 seconds on average for another one and a half years. With the delay of the ice age, there is a desire to not suddenly also increase miner rewards. The difficulty bomb has been known about for a long time and now it’s going to stop from happening. In order to maintain stability of the system, a block reward reduction that offsets the ice age delay would leave the system in the same general state as before. Reducing the reward also decreases the likelihood of a miner driven chain split as Ethereum approaches proof-of-stake.

## Specification

#### Relax Difficulty with Fake Block Number

For the purposes of calc_difficulty, simply replace the use of block.number, as used in the exponential ice age component, with the formula:

fake_block_number = max(0, block.number - 3_000_000) if block.number >= BYZANTIUM_FORK_BLKNUM else block.number


#### Adjust Block, Uncle, and Nephew rewards

To ensure a constant Ether issuance, adjust the block reward to new_block_reward, where

new_block_reward = 3_000_000_000_000_000_000 if block.number >= BYZANTIUM_FORK_BLKNUM else block.reward


(3E18 wei, or 3,000,000,000,000,000,000 wei, or 3 ETH).

Analogue, if an uncle is included in a block for block.number >= BYZANTIUM_FORK_BLKNUM such that block.number - uncle.number = k, the uncle reward is

new_uncle_reward = (8 - k) * new_block_reward / 8


This is the existing pre-Metropolis formula for uncle rewards, simply adjusted with new_block_reward.

The nephew reward for block.number >= BYZANTIUM_FORK_BLKNUM is

new_nephew_reward = new_block_reward / 32


This is the existing pre-Metropolis formula for nephew rewards, simply adjusted with new_block_reward.

## Rationale

This will delay the ice age by 42 million seconds (approximately 1.4 years), so the chain would be back at 30 second block times at the end of 2018. An alternate proposal was to add special rules to the difficulty calculation to effectively pause the difficulty between different blocks. This would lead to similar results.

This was previously discussed at All Core Devs Meeting #09, #12, #13, and #14. Consensus on the specification was achieved in All Core Devs Meeting #19 and specification drafted in EIP issue #649. It was decided to replace EIP #186 and include the block reward reduction along with the difficulty bomb delay in All Core Devs Meeting #20 and #21; accepted in #22.

## Backwards Compatibility

This EIP is not forward compatible and introduces backwards incompatibilities in the difficulty calculation, as well as the block, uncle and nephew reward structure. Therefore, it should be included in a scheduled hardfork at a certain block number. It’s suggested to include this EIP in the first of the two Metropolis hard-forks, the Byzantium fork.

## Test Cases

Test cases exist in ethereum/tests #269.

## Implementation

The following clients implemented EIP-649:

The Yellow Paper implements EIP-649 in #333.

Other notable implementations:

Copyright and related rights waived via CC0.

## Citation

Please cite this document as:

Afri Schoedon, Vitalik Buterin, "EIP-649: Metropolis Difficulty Bomb Delay and Block Reward Reduction," Ethereum Improvement Proposals, no. 649, June 2017. [Online serial]. Available: https://eips.ethereum.org/EIPS/eip-649.