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ADR 74: Migrate Timeout Parameters to Consensus Parameters

Changelog

  • 03-Jan-2022: Initial draft (@williambanfield)
  • 13-Jan-2022: Updated to indicate work on upgrade path needed (@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.
  • BypassCommitTimeout
    • 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.

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.

Decision

The decision has been made to implement this work, with the caveat that the specific mechanism for introducing the new parameters to chains is still ongoing.

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 bypass_commit_timeout = 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

The new ConsensusParameters will be added during an upcoming release. In this release, the old config.toml parameters will cease to control the timeouts and an error will be logged on nodes that continue to specify these values. The specific mechanism by which these parameters will added to a chain is being discussed in RFC-009 and will be decided ahead of the next release.

The specific mechanism for adding these parameters depends on work related to soft upgrades, which is still ongoing.

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