CIP: Coordinated prevote times

cips/cip-prevote.md

@@ -0,0 +1,72 @@
+---
+title: Coordinated prevote times
+description: Scheduled prevote times for consistent blocks
+author: Callum Waters (@cmwaters)
+discussions-to: https://forum.celestia.org/t/coordinated-start-time-intervals/1768
+status: Draft
+type: Standards Track
+category: Core
+created: 2024-07-12
+---
+
+## Abstract
+
+Block propagation speed is dependent on block size, thus larger blocks have later block times. It is desirable that regardless of block size that blocks are finalized with a consistent cadence. This CIP proposes a solution to this problem, namely, correct processes that receive a valid proposal withthold their prevote until the `ProposalTimeout` has elapsed.
+
+## Motivation
+
+The current network has a cap of 2MB. We have observed in testnets that larger blocks (~8MB) shift the block time by up to 7 seconds (18 seconds in total with the default 11 second `TimeoutCommit`). This range of 11 - 18 seconds for blocks 0-8MB in size is too inconsistent for users. We want to provide block finality with a consistent cadence.
+
+## Specification
+
+The current system relied on a fixed timeout (known as `TimeoutCommit`) after finality to dictate the interval between block times. The proposed modification targets the proposal timeout. Currently, if a correct process does not receive a proposal within a set time `TimeoutPropose` after the round's start time, the process will prevote nil. This mechanism ensures liveness in the event that the proposer fails. This CIP makes the following modification:
+
+- Upon receiving a correct proposal, a process will not immediately PREVOTE but wait until `TimeoutPropose` before sending their vote.
+- Upon receiing an invalid proposal, the process will immediately PREVOTE nil.
+- If a process is locked on a block, they will send PREVOTE votes immediately (this situaiton applies after the first round).
+
+This mechanism can be switched on and off and is controlled by the application via the `ConsensusParams` in the `EndBlock` of the ABCI interface.
+
+In addition, `TimeoutCommit` will also move from a local variable to one controlled by the application, as outlined in [ADR115](https://github.com/cometbft/cometbft/blob/main/docs/references/architecture/adr-115-predictable-block-times.md).
+
+It is expected that enabling this mechanism would work alongside reducing the `TimeoutCommit`.
+
+### Parameters
+
+The proposal adds two variables that can be controlled by the application but will not be exposed by governance:
+
+1. `EnableCoordinatedPrevotes`
+2. `TimeoutCommit` (If 0, a node's local config time will be used - for backwards compatibility)
+3. `TimeoutPropose` (If 0, a node's local config time will be used - for backwards compatibility)
+
+Given a target block rate of 12 seconds, enabling this mechanism would coincide with changes to the following timeouts:
+
+- `TimeoutPropose` remains at 10 seconds
+- `TimeoutCommit` goes from 11 seconds to 1 second
+- `EnableCoordinatedPrevotes` is set to `true`.
+
+NOTE: These numbers are subject to benchmarking
+
+### Rationale
+
+The variables `TimeoutCommit` and `TimeoutPropose` were previously part of a node's local configuration. Switching these variables to be coordinated by consensus itself is critical.
+
+## Backwards Compatibility
+
+Enabling of this mechanism must be coordinated amongst all consensus nodes in the network. It should tie in with a major upgrade. The changes themselves can be made in a backwards compatible manner to `celestia-core` by having them be disabled by default.
+
+## Test Cases
+
+Testing must be designed around validating that this approach does achieve more consistent block times (i.e. a 1 second standard deviation). As this modifies the voting logic, testing should also verify that correct nodes always vote.
+
+## Reference Implementation
+
+TBC
+
+## Security Considerations
+
+This modification is small in scope and logically shouldn't imapact the properties of consensus, however this still makes modifications to the consensus algorithm and thus there is implementation risk.
+
+## Copyright
+
+Copyright and related rights waived via [CC0](../LICENSE).