tendermint/docs/architecture/adr-074-timeout-params.md

ADR 74: Migrate Timeout Parameters to Consensus Parameters

Changelog

• 03-Jan-2022: Initial draft (@williambanfield)

Status

Proposed

Context

Background

Tendermint's consensus timeout parameters are currently configured locally by each validator in the validator's config.toml. This means that the validators on a Tendermint network may have different timeouts from each other. There is no reason for validators on the same network to configure different timeout values. Proper functioning of the Tendermint consensus algorithm relies on these parameters being uniform across validators.

The configurable values are as follows:

• TimeoutPropose
  • How long the consensus algorithm waits for a proposal block before issuing a prevote.
  • If no prevote arrives by TimeoutPropose, then the consensus algorithm will issue a nil prevote.
• TimeoutProposeDelta
  • How much the TimeoutPropose grows each round.
• TimeoutPrevote
  • How long the consensus algorithm waits after receiving +2/3 prevotes with no quorum for a value before issuing a precommit for nil. (See the arXiv paper, Algorithm 1, Line 34)
• TimeoutPrevoteDelta
  • How much the TimeoutPrevote increases with each round.
• TimeoutPrecommit
  • How long the consensus algorithm waits after receiving +2/3 precommits that do not have a quorum for a value before entering the next round. (See the arXiv paper, Algorithm 1, Line 47)
• TimeoutPrecommitDelta
  • How much the TimeoutPrecommit increases with each round.
• TimeoutCommit
  • How long the consensus algorithm waits after committing a block but before starting the new height.
  • This gives a validator a chance to receive slow precommits.
• SkipTimeoutCommit
  • Make progress as soon as the node has 100% of the precommits.

Overview of Change

We will consolidate the timeout parameters and migrate them from the node-local config.toml file into the network-global consensus parameters.

The 8 timeout parameters will be consolidated down to 6. These will be as follows:

• TimeoutPropose
  • Same as current TimeoutPropose.
• TimeoutProposeDelta
  • Same as current TimeoutProposeDelta.
• TimeoutVote
  • How long validators wait for votes in both the prevote and precommit phase of the consensus algorithm. This parameter subsumes the current TimeoutPrevote and TimeoutPrecommit parameters.
• TimeoutVoteDelta
  • How much the TimeoutVote will grow each successive round. This parameter subsumes the current TimeoutPrevoteDelta and TimeoutPrecommitDelta parameters.
• TimeoutCommit
  • Same as current TimeoutCommit.
• TimeoutCommitBypassEnabled
  • Same as current SkipTimeoutCommit, renamed for clarity.

A safe default will be provided by Tendermint for each of these parameters and networks will be able to update the parameters as they see fit. Local updates to these parameters will no longer be possible; instead, the application will control updating the parameters. Applications using the Cosmos SDK will be automatically be able to change the values of these consensus parameters via a governance proposal.

This change is low-risk. While parameters are locally configurable, many running chains do not change them from their default values. For example, initializing a node on Osmosis, Terra, and the Cosmos Hub using the their init command produces a config.toml with Tendermint's default values for these parameters.

Why this parameter consolidation?

Reducing the number of parameters is good for UX. Fewer superfluous parameters makes running and operating a Tendermint network less confusing.

The Prevote and Precommit messages are both similar sizes, require similar amounts of processing so there is no strong need for them to be configured separately.

The TimeoutPropose parameter governs how long Tendermint will wait for the proposed block to be gossiped. Blocks are much larger than votes and therefore tend to be gossiped much more slowly. It therefore makes sense to keep TimeoutPropose and the TimeoutProposeDelta as parameters separate from the vote timeouts.

TimeoutCommit is used by chains to ensure that the network waits for the votes from slower validators before proceeding to the next height. Without this timeout, the votes from slower validators would consistently not be included in blocks and those validators would not be counted as 'up' from the chain's perspective. Being down damages a validator's reputation and causes potential stakers to think twice before delegating to that validator.

TimeoutCommit also prevents the network from producing the next height as soon as validators on the fastest hardware with a summed voting power of +2/3 of the network's total have completed execution of the block. Allowing the network to proceed as soon as the fastest +2/3 completed execution would have a cumulative effect over heights, eventually leaving slower validators unable to participate in consensus at all. TimeoutCommit therefore allows networks to have greater variability in hardware. Additional discussion of this can be found in tendermint issue 5911 and [spec issue 359][spec-359].

Alternative Approaches

Hardcode the parameters

Many Tendermint networks run on similar cloud-hosted infrastructure. Therefore, they have similar bandwidth and machine resources. The timings for propagating votes and blocks are likely to be reasonably similar across networks. As a result, the timeout parameters are good candidates for being hardcoded. Hardcoding the timeouts in Tendermint would mean entirely removing these parameters from any configuration that could be altered by either an application or a node operator. Instead, Tendermint would ship with a set of timeouts and all applications using Tendermint would use this exact same set of values.

While Tendermint nodes often run with similar bandwidth and on similar cloud-hosted machines, there are enough points of variability to make configuring consensus timeouts meaningful. Namely, Tendermint network topologies are likely to be very different from chain to chain. Additionally, applications may vary greatly in how long the Commit phase may take. Applications that perform more work during Commit require a longer TimeoutCommit to allow the application to complete its work and be prepared for the next height.

• From https://github.com/tendermint/spec/issues/359 it looks like TimeoutCommit really only exists for the purpose of allowing precommits to be collected after the +2/3 has initially been seen. Not much more value to it than that.

• allowing long timeout commit discussed here: https://github.com/tendermint/tendermint/issues/5911#issuecomment-973560381

Decision

None

Detailed Design

New Consensus Parameters

A new TimeoutParams message will be added to the [params.proto file][consensus-params-proto]. This message will have the following form:

message TimeoutParams {
 google.protobuf.Duration propose = 1;
 google.protobuf.Duration propose_delta = 2;
 google.protobuf.Duration vote = 3;
 google.protobuf.Duration vote_delta = 4;
 google.protobuf.Duration commit = 5;
 bool commit_timeout_bypass_enabled = 6;
}

This new message will be added as a field into the [ConsensusParams message][consensus-params-proto]. The same default values that are currently set for these parameters in the local configuration file will be used as the defaults for these new consensus parameters in the consensus parameter defaults.

The new consensus parameters will be subject to the same validity rules as the current configuration values, namely, each value must be non-negative.

Migration

Migration to using these new parameters will occur across two releases. During the v0.36 release, Tendermint will add these as consensus parameters. The old parameters config-local parameters will continue to exist as well in the v0.36 release. If the parameters are set as consensus parameters, the config-local parameters will be ignored. If the consensus parameters are not set, Tendermint will fall back to using the values contained in the config.toml file.

During the v0.36 release cycle, if non-default values are set in the config.toml, the node will log a warning alerting operators that the parameters will soon be removed in the upcoming release.

In the v0.37 release, the config-local parameters will be removed. If a node continues to set the parameters in the configuration file, an error will be logged that indicates how to upgrade or remove them.

Add New Consensus Parameters to HashedParams

Tendermint currently only verifies that a subset of the consensus parameters are equal across all validators. These parameters are the BlockMaxBytes and the BlockMaxGas. A hash of these parameters is included in the block. Validators ensure their values of the parameters match by hashing their value of the parameters and checking that their hashed value matches the hash included in the block.

Including the new parameters in this hash could break verification of old blocks. To add the parameters in a hash-compatible way, we will only include these consensus parameters in the hash if they have ever been updated to be non-default values. This is safe for validation of historic blocks because the parameters could not have been set during heights where they did not exist. Eliding them from the hash them will therefore produce the same hash that would have been produced as before they existed.

For backwards compatibility, Tendermint will keep track of these initial default values for each of these parameters. If the defaults ever change, our hash-compatible upgrade scheme requires remembering the initial defaults so as to ignore the parameters when that initial default is being used.

Consequences

Positive

• Timeout parameters will be equal across all of the validators in a Tendermint network.
• Remove superfluous timeout parameters.

Negative

Neutral

• Timeout parameters require consensus to change.

References