Arrow Glacier is the 3rd network upgrade planned for the proof of work chain of the Ethereum Network in 2021. Earlier this year, the Ethereum mainnet successfully deployed the Berlin upgrade in April and the London upgrade in August with 4 EIPs & 5 EIPs respectively.

Arrow Glacier is tentatively expected on December 8th, 2021.

What changes are going into the Arrow Glacier upgrade?

Arrow Glacier is another network upgrade similar to Muir Glacier. This will include only one improvement proposal to push the difficulty bomb until summer next year to keep the chain safe from freezing a.k.a the “ice age”.

EIP-4345: Difficulty Bomb Delay to June 2022

The EIP proposes to push the bomb back for a total of 10,700,000 blocks, an increase of 1,000,000 blocks since the last pushback, in London. The bomb will likely show up in June 2022.

Initially proposed by Tim Beiko on October 5th, the EIP suggested pushing the bomb to 10.5 mil blocks, until May 2022. However, further discussion in following AllCoreDevs resulted in the decision to move the bomb till summer next year.

The reasonable confidence of core developers in having the Merge upgrade before the summer of next year was definitely a factor for consideration. In addition to that, Thomas Jay Rush’s post Adventures in Difficulty Bombing provided a better rationale on how far back to set the bomb and helped justify the delay to move it further from what was originally planned. The Ethereum clients’ team wanted to avoid having to organize another pushback while working on the next big upgrade.

History of the “Difficulty Bomb”

The difficulty bomb is a set of codes added to the Ethereum blockchain to exponentially raise the difficulty level, making it difficult for miners to keep up with the increased difficulty. Block time will be increased and will lead to a blockchain freeze. It’s a very “steep” exponential curve; it is flat for a very long time, and then kicks in very quickly when the bomb “explodes”.

Why difficulty bomb was added in the first place?

The difficulty adjustment process was put into place originally to facilitate the transition to proof of stake. Over time, it has gained the additional property of forcing those who want to keep running Ethereum to upgrade their nodes. This means that staying on the old chain is not an option.

Timeline and related EIPs

Nov 2015: EIP-2: Homestead Hard-fork Changes introduced the change to the pre-homestead difficulty adjustment algorithm.

April 2016: EIP-100: Change difficulty adjustment to target mean block time including uncles updated the difficulty adjustment algorithm to target a constant average rate of blocks produced including uncles, and so ensures a highly predictable issuance rate that cannot be manipulated upward by manipulating the uncle rate.

June 2016: EIP-649: Metropolis Difficulty Bomb Delay and Block Reward Reduction was introduced to delay the difficulty bomb for approximately one and a half years and to reduce the block rewards with the Byzantium fork, the first part of the Metropolis fork.

July 2018: EIP-1234: Constantinople Difficulty Bomb Delay and Block Reward Adjustment delayed the difficulty bomb for approximately 12 months and reduced the block rewards with the Constantinople fork, the second part of the Metropolis fork.

Nov 2019: EIP-2384: Muir Glacier Difficulty Bomb Delay delayed the difficulty bomb for the next 4,000,000 blocks (~611 days).

May 2021: EIP-3554: Difficulty Bomb Delay to December 2021. This EIP was deployed with the London upgrade in August to delay the difficulty bomb to show the effect in the first week of December 2021.

Oct 2021: EIP-4345: Difficulty Bomb Delay to June 2022. This EIP is proposed to push the difficulty bomb 10,700,000 blocks later than the actual block number. Most likely, it will be activated on the Ethereum mainnet on December 8th, 2021.

What do I need to do?

If you’re running any number of Ethereum PoW nodes, it is highly recommended to upgrade nodes with the latest client release before the Arrow Glacier upgrade activation date. Stay tuned for more information coming up in the Ethereum Foundation’s announcement blog.

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