Overview
In normal conditions, Ethereum takes between 13 and 19 minutes to finalize a block. Finality comes with strong economic guarantees (at least 1/3 of the stake can be slashed if two conflicting blocks are finalized) and remains safe under arbitrary network delays.
Some users and applications prefer a much quicker signal over such strong guarantees. In roughly 13 seconds, the Fast Confirmation Rule can indicate that a block will always be part of the canonical chain. This relies on the following two assumptions:
- Messages sent by honest validators are received by all honest validators by the end of the slot, which translates to assuming the network latency is less than 8 seconds.
- At least 75% of the stake acts honestly.
Why it matters
Some users and applications fill the gap between inclusion and finalization with unsafe heuristics, like waiting until a block is "deep enough" in the chain.
The Fast Confirmation Rule provides a signal based on the actual measured weight supporting a block. As long as the assumptions stated above hold, that signal can be trusted.
This is a much safer and faster signal than block depth.