Ethereum reorgs after the merge
What is a fork choice rule?
- A function, evaluated by the client
- Input = the set of blocks and other messages that have been seen
- Output = the "canonical chain"
Example: Nakamoto PoW (highest-total-difficulty)
There are three major types of fork choice rules
| Algorithm | Finality | Reorgs | Fork choice |
|---|---|---|---|
| Nakamoto | (eg. PoW Ethereum / Bitcoin) | None | Frequent |
| GASPER (eg. PoS ethereum) | Every 2-epochs (~12 min) | Very occasional | Chain with strongest support after last finalized block |
| Tendermint | Single block (~1-10s) | Never | Only finalized blocks |
Important: the fork choice question is separate from PoW vs PoS.
| PoW | PoS | |
|---|---|---|
| Nakamoto-style | Bitcoin, PoW Ethereum | Peercoin, NXT, Tezos |
| Parallelized | GHOST | Gasper |
| Instant-finality | - | Tendermint |
So… how do reorgs work in each case?
Reorgs in Nakamoto PoW
- PoW fork choice uses the "longest total difficulty" rule
- One consequence of this is that a block can often be reverted by a single block.
Reorgs in Gasper
- There is a large committee (~6,300 in Ethereum today) attesting at each slot
- To revert a block, you need to overcome the attesters, not just a single proposer
- Hence, reorging is extremely hard unless you have a large portion of total stake.
One caveat…
- Fork choice is currently done on blocks, not (block, slot) pairs.
- This means that in the above case, the top-right attestations do NOT impact the B vs no B choice
- The attacker can wait for
Cto come out and then publishBwith the attestations, revertingC - This reduces the threshold for attacks to significantly below 50%
Reorgs in Tendermint
- Impossible (single slot finality)
Q: How do these theoretical differences translate into practice?
One way to look at the incentives:
| Consensus algorithm | Game chart |
|---|---|
| Nakamoto |  |
| Gasper |  |
| Tendermint |  |
In other words:
- In Nakamoto, small groups can profitably reorg
- In Gasper, need a large group (>30%) to profitably reorg
- In Tendermint, cannot profitably reorg
Conclusions for Ethereum
- Short term, two options:
- Do not worry: miners have no incentive to attack because that will accelerate the merge
- Tweak Nakamoto fork choice to favor earlier blocks and make reorgs a little harder
- Medium term:
- Prioritize the merge, to make reorgs much more difficult
- Adjust beacon chain fork choice rule to harden against <50% attacks
- Long term:
- Explore more strongly penalizing even single-slot reversions
Ethereum reorgs after the merge
{"metaMigratedAt":"2023-06-16T06:20:35.788Z","metaMigratedFrom":"YAML","title":"Ethereum reorgs after the merge","breaks":true,"slideOptions":"{\"transition\":\"slide\",\"parallaxBackgroundImage\":\"https://i.imgur.com/kBNEMDJ.png\",\"parallaxBackgroundSize\":\"100% 100%\",\"parallaxBackgroundHorizontal\":0}","contributors":"[{\"id\":\"1d678dc3-c84d-4629-8c9b-69b6187e7a0b\",\"add\":3599,\"del\":229}]"}