tendermint/docs/architecture/adr-071-proposer-based-timestamps.md

ADR 71: Proposer-Based Timestamps

Changelog

• July 15 2021: Created by @williambanfield
• Aug 4 2021: Draft completed by @williambanfield
• Aug 5 2021: Draft updated to include data structure changes by @williambanfield
• Aug 20 2021: Language edits completed by @williambanfield
• Oct 25 2021: Update the ADR to match updated spec from @cason by @williambanfield
• Nov 10 2021: Additional language updates by @williambanfield per feedback from @cason

Status

Accepted

Context

Tendermint currently provides a monotonically increasing source of time known as BFTTime. This mechanism for producing a source of time is reasonably simple. Each correct validator adds a timestamp to each Precommit message it sends. The timestamp it sends is either the validator's current known Unix time or one millisecond greater than the previous block time, depending on which value is greater. When a block is produced, the proposer chooses the block timestamp as the weighted median of the times in all of the Precommit messages the proposer received. The weighting is proportional to the amount of voting power, or stake, a validator has on the network. This mechanism for producing timestamps is both deterministic and byzantine fault tolerant.

This current mechanism for producing timestamps has a few drawbacks. Validators do not have to agree at all on how close the selected block timestamp is to their own currently known Unix time. Additionally, any amount of voting power >1/3 may directly control the block timestamp. As a result, it is quite possible that the timestamp is not particularly meaningful.

These drawbacks present issues in the Tendermint protocol. Timestamps are used by light clients to verify blocks. Light clients rely on correspondence between their own currently known Unix time and the block timestamp to verify blocks they see; However, their currently known Unix time may be greatly divergent from the block timestamp as a result of the limitations of BFTTime.

The proposer-based timestamps specification suggests an alternative approach for producing block timestamps that remedies these issues. Proposer-based timestamps alter the current mechanism for producing block timestamps in two main ways:

1. The block proposer is amended to offer up its currently known Unix time as the timestamp for the next block instead of the BFTTime.
2. Correct validators only approve the proposed block timestamp if it is close enough to their own currently known Unix time.

The result of these changes is a more meaningful timestamp that cannot be controlled by <= 2/3 of the validator voting power. This document outlines the necessary code changes in Tendermint to implement the corresponding proposer-based timestamps specification.

Alternative Approaches

Remove timestamps altogether

Computer clocks are bound to skew for a variety of reasons. Using timestamps in our protocol means either accepting the timestamps as not reliable or impacting the protocol’s liveness guarantees. This design requires impacting the protocol’s liveness in order to make the timestamps more reliable. An alternate approach is to remove timestamps altogether from the block protocol. BFTTime is deterministic but may be arbitrarily inaccurate. However, having a reliable source of time is quite useful for applications and protocols built on top of a blockchain.

We therefore decided not to remove the timestamp. Applications often wish for some transactions to occur on a certain day, on a regular period, or after some time following a different event. All of these require some meaningful representation of agreed upon time. The following protocols and application features require a reliable source of time:

• Tendermint Light Clients rely on correspondence between their known time and the block time for block verification.
• Tendermint Evidence validity is determined either in terms of heights or in terms of time.
• Unbonding of staked assets in the Cosmos Hub occurs after a period of 21 days.
• IBC packets can use either a timestamp or a height to timeout packet delivery.

Finally, inflation distribution in the Cosmos Hub uses an approximation of time to calculate an annual percentage rate. This approximation of time is calculated using block heights with an estimated number of blocks produced in a year. Proposer-based timestamps will allow this inflation calculation to use a more meaningful and accurate source of time.

Decision

Implement proposer-based timestamps and remove BFTTime.

Detailed Design

Overview

Implementing proposer-based timestamps will require a few changes to Tendermint’s code. These changes will be to the following components:

• The internal/consensus/ package.
• The state/ package.
• The Vote, CommitSig and Header types.
• The consensus parameters.

Changes to CommitSig

The CommitSig struct currently contains a timestamp. This timestamp is the current Unix time known to the validator when it issued a Precommit for the block. This timestamp is no longer used and will be removed in this change.

CommitSig will be updated as follows:

type CommitSig struct {
	BlockIDFlag      BlockIDFlag `json:"block_id_flag"`
	ValidatorAddress Address     `json:"validator_address"`
--	Timestamp        time.Time   `json:"timestamp"`
	Signature        []byte      `json:"signature"`
}

Changes to Vote messages

Precommit and Prevote messages use a common Vote struct. This struct currently contains a timestamp. This timestamp is set using the voteTime function and therefore vote times correspond to the current Unix time known to the validator. For precommits, this timestamp is used to construct the CommitSig that is included in the block in the LastCommit field. For prevotes, this field is currently unused. Proposer-based timestamps will use the timestamp that the proposer sets into the block and will therefore no longer require that a timestamp be included in the vote messages. This timestamp is therefore no longer useful and will be dropped.

Vote will be updated as follows:

type Vote struct {
	Type             tmproto.SignedMsgType `json:"type"`
	Height           int64                 `json:"height"`
	Round            int32                 `json:"round"`    
	BlockID          BlockID               `json:"block_id"` // zero if vote is nil.
--	Timestamp        time.Time             `json:"timestamp"`
	ValidatorAddress Address               `json:"validator_address"`
	ValidatorIndex   int32                 `json:"validator_index"`
	Signature        []byte                `json:"signature"`
}

New consensus parameters

The proposer-based timestamp specification includes multiple new parameters that must be the same among all validators. These parameters are PRECISION, MSGDELAY, and ACCURACY.

The PRECISION and MSGDELAY parameters are used to determine if the proposed timestamp is acceptable. A validator will only Prevote a proposal if the proposal timestamp is considered timely. A proposal timestamp is considered timely if it is within PRECISION and MSGDELAY of the Unix time known to the validator. More specifically, a proposal timestamp is timely if validatorLocalTime - PRECISION < proposalTime < validatorLocalTime + PRECISION + MSGDELAY.

Because the PRECISION and MSGDELAY parameters must be the same across all validators, they will be added to the consensus parameters as durations.

The proposer-based timestamp specification also includes a new ACCURACY parameter. Intuitively, ACCURACY represents the difference between the ‘real’ time and the currently known time of correct validators. The currently known Unix time of any validator is always somewhat different from real time. ACCURACY is the largest such difference between each validator's time and real time taken as an absolute value. This is not something a computer can determine on its own and must be specified as an estimate by community running a Tendermint-based chain. It is used in the new algorithm to calculate a timeout for the propose step. ACCURACY is assumed to be the same across all validators and therefore should be included as a consensus parameter.

The consensus will be updated to include this Timestamp field as follows:

type ConsensusParams struct {
	Block     BlockParams     `json:"block"`
	Evidence  EvidenceParams  `json:"evidence"`
	Validator ValidatorParams `json:"validator"`
	Version   VersionParams   `json:"version"`
++	Timestamp TimestampParams `json:"timestamp"`
}

type TimestampParams struct {
	Accuracy  time.Duration `json:"accuracy"`
	Precision time.Duration `json:"precision"`
	MsgDelay  time.Duration `json:"msg_delay"`
}

Changes to the block proposal step

Proposer selects block timestamp

Tendermint currently uses the BFTTime algorithm to produce the block's Header.Timestamp. The proposal logic sets the weighted median of the times in the LastCommit.CommitSigs as the proposed block's Header.Timestamp.

In proposer-based timestamps, the proposer will still set a timestamp into the Header.Timestamp. The timestamp the proposer sets into the Header will change depending on if the block has previously received a polka or not.

Proposal of a block that has not previously received a polka

If a proposer is proposing a new block, then it will set the Unix time currently known to the proposer into the Header.Timestamp field. The proposer will also set this same timestamp into the Timestamp field of the Proposal message that it issues.

Re-proposal of a block that has previously received a polka

If a proposer is re-proposing a block that has previously received a polka on the network, then the proposer does not update the Header.Timestamp of that block. Instead, the proposer simply re-proposes the exact same block. This way, the proposed block has the exact same block ID as the previously proposed block and the validators that have already received that block do not need to attempt to receive it again.

The proposer will set the re-proposed block's Header.Timestamp as the Proposal message's Timestamp.

Proposer waits

Block timestamps must be monotonically increasing. In BFTTime, if a validator’s clock was behind, the validator added 1 millisecond to the previous block’s time and used that in its vote messages. A goal of adding proposer-based timestamps is to enforce some degree of clock synchronization, so having a mechanism that completely ignores the Unix time of the validator time no longer works.

Validator clocks will not be perfectly in sync. Therefore, the proposer’s current known Unix time may be less than the previous block's Header.Time. If the proposer’s current known Unix time is less than the previous block's Header.Time, the proposer will sleep until its known Unix time exceeds it.

This change will require amending the defaultDecideProposal method. This method should now schedule a timeout that fires when the proposer’s time is greater than the previous block's Header.Time. When the timeout fires, the proposer will finally issue the Proposal message.

Changes to the propose step timeout

Currently, a validator waiting for a proposal will proceed past the propose step if the configured propose timeout is reached and no proposal is seen. Proposer-based timestamps requires changing this timeout logic.

The proposer will now wait until its current known Unix time exceeds the previous block's Header.Time to propose a block. The validators must now take this and some other factors into account when deciding when to timeout the propose step. Specifically, the propose step timeout must also take into account potential inaccuracy in the validator’s clock and in the clock of the proposer. Additionally, there may be a delay communicating the proposal message from the proposer to the other validators.

Therefore, validators waiting for a proposal must wait until after the previous block's Header.Time before timing out. To account for possible inaccuracy in its own clock, inaccuracy in the proposer’s clock, and message delay, validators waiting for a proposal will wait until the previous block's Header.Time + 2*ACCURACY + MSGDELAY. The spec defines this as waitingTime.

The propose step’s timeout is set in enterPropose in state.go. enterPropose will be changed to calculate waiting time using the new consensus parameters. The timeout in enterPropose will then be set as the maximum of waitingTime and the configured proposal step timeout.

Changes to proposal validation rules

The rules for validating a proposed block will be modification to implement proposer-based timestamps. We will change the validation logic to ensure that a proposal is timely.

Per the proposer-based timestamps spec, timely only needs to be checked if a block has not received a +2/3 majority of Prevotes in a round. If a block previously received a +2/3 majority of prevotes in a previous round, then +2/3 of the voting power considered the block's timestamp near enough to their own currently known Unix time in that round.

The validation logic will be updated to check timely for blocks that did not previously receive +2/3 prevotes in a round. Receiving +2/3 prevotes in a round is frequently referred to as a 'polka' and we will use this term for simplicity.

Current timestamp validation logic

To provide a better understanding of the changes needed to timestamp validation, we will first detail how timestamp validation works currently in Tendermint.

The validBlock function currently validates the proposed block timestamp in three ways. First, the validation logic checks that this timestamp is greater than the previous block’s timestamp.

Second, it validates that the block timestamp is correctly calculated as the weighted median of the timestamps in the block’s LastCommit.

Finally, the validation logic authenticates the timestamps in the LastCommit.CommitSig. The cryptographic signature in each CommitSig is created by signing a hash of fields in the block with the voting validator’s private key. One of the items in this signedBytes hash is the timestamp in the CommitSig. To authenticate the CommitSig timestamp, the validator authenticating votes builds a hash of fields that includes the CommitSig timestamp and checks this hash against the signature. This takes place in the VerifyCommit function.

Remove unused timestamp validation logic

BFTTime validation is no longer applicable and will be removed. This means that validators will no longer check that the block timestamp is a weighted median of LastCommit timestamps. Specifically, we will remove the call to MedianTime in the validateBlock function. The MedianTime function can be completely removed.

Since CommitSigs will no longer contain a timestamp, the validator authenticating a commit will no longer include the CommitSig timestamp in the hash of fields it builds to check against the cryptographic signature.

Timestamp validation when a block has not received a polka

The POLRound in the Proposal message indicates which round the block received a polka. A negative value in the POLRound field indicates that the block has not previously been proposed on the network. Therefore the validation logic will check for timely when POLRound < 0.

When a validator receives a Proposal message, the validator will check that the Proposal.Timestamp is at most PRECISION greater than the current Unix time known to the validator, and at minimum PRECISION + MSGDELAY less than the current Unix time known to the validator. If the timestamp is not within these bounds, the proposed block will not be considered timely.

Once a full block matching the Proposal message is received, the validator will also check that the timestamp in the Header.Timestamp of the block matches this Proposal.Timestamp. Using the Proposal.Timestamp to check timely allows for the MSGDELAY parameter to be more finely tuned since Proposal messages do not change sizes and are therefore faster to gossip than full blocks across the network.

A validator will also check that the proposed timestamp is greater than the timestamp of the block for the previous height. If the timestamp is not greater than the previous block's timestamp, the block will not be considered valid, which is the same as the current logic.

Timestamp validation when a block has received a polka

When a block is re-proposed that has already received a +2/3 majority of Prevotes on the network, the Proposal message for the re-proposed block is created with a POLRound that is >= 0. A validator will not check that the Proposal is timely if the propose message has a non-negative POLRound. If the POLRound is non-negative, each validator will simply ensure that it received the Prevote messages for the proposed block in the round indicated by POLRound.

If the validator did not receive Prevote messages for the proposed block in POLRound, then it will prevote nil. Validators already check that +2/3 prevotes were seen in POLRound, so this does not represent a change to the prevote logic.

A validator will also check that the proposed timestamp is greater than the timestamp of the block for the previous height. If the timestamp is not greater than the previous block's timestamp, the block will not be considered valid, which is the same as the current logic.

Additionally, this validation logic can be updated to check that the Proposal.Timestamp matches the Header.Timestamp of the proposed block, but it is less relevant since checking that votes were received is sufficient to ensure the block timestamp is correct.

Changes to the prevote step

Currently, a validator will prevote a proposal in one of three cases:

• Case 1: Validator has no locked block and receives a valid proposal.
• Case 2: Validator has a locked block and receives a valid proposal matching its locked block.
• Case 3: Validator has a locked block, sees a valid proposal not matching its locked block but sees +⅔ prevotes for the proposal’s block, either in the current round or in a round greater than or equal to the round in which it locked its locked block.

The only change we will make to the prevote step is to what a validator considers a valid proposal as detailed above.

Changes to the precommit step

The precommit step will not require much modification. Its proposal validation rules will change in the same ways that validation will change in the prevote step with the exception of the timely check: precommit validation will never check that the timestamp is timely.

Remove voteTime Completely

voteTime is a mechanism for calculating the next BFTTime given both the validator's current known Unix time and the previous block timestamp. If the previous block timestamp is greater than the validator's current known Unix time, then voteTime returns a value one millisecond greater than the previous block timestamp. This logic is used in multiple places and is no longer needed for proposer-based timestamps. It should therefore be removed completely.

Future Improvements

• Implement BLS signature aggregation. By removing fields from the Precommit messages, we are able to aggregate signatures.

Consequences

Positive

• <2/3 of validators can no longer influence block timestamps.
• Block timestamp will have stronger correspondence to real time.
• Improves the reliability of light client block verification.
• Enables BLS signature aggregation.
• Enables evidence handling to use time instead of height for evidence validity.

Neutral

• Alters Tendermint’s liveness properties. Liveness now requires that all correct validators have synchronized clocks within a bound. Liveness will now also require that validators’ clocks move forward, which was not required under BFTTime.

Negative

• May increase the length of the propose step if there is a large skew between the previous proposer and the current proposer’s local Unix time. This skew will be bound by the PRECISION value, so it is unlikely to be too large.

• Current chains with block timestamps far in the future will either need to pause consensus until after the erroneous block timestamp or must maintain synchronized but very inaccurate clocks.

References

• PBTS Spec
• BFTTime spec