tendermint/docs/specification/new-spec/bft-time.md

BFT time in Tendermint

Tendermint provides a deterministic, Byzantine fault-tolerant, source of time. Time in Tendermint is defined with the Time field of the block header.

It satisfies the following properties:

• Time Monotonicity: Time is monotonically increasing, i.e., given a header H1 for height h1 and a header H2 for height h2 = h1 + 1, H1.Time < H2.Time.
• Time Validity: Given a set of Commit votes that forms the block.LastCommit field, a range of valid values for the Time field of the block header is defined only by
  Precommit messages (from the LastCommit field) sent by correct processes, i.e., a faulty process cannot arbitrarily increase the Time value.

In the context of Tendermint, time is of type int64 and denotes UNIX time in milliseconds, i.e., corresponds to the number of milliseconds since January 1, 1970. Before defining rules that need to be enforced by the Tendermint consensus protocol, so the properties above holds, we introduce the following definition:

• median of a set of Vote messages is equal to the median of Vote.Time fields of the corresponding Vote messages, where the value of Vote.Time is counted number of times proportional to the process voting power. As in Tendermint the voting power is not uniform (one process one vote), a vote message is actually an aggregator of the same votes whose number is equal to the voting power of the process that has casted the corresponding votes message.

Let's consider the following example:

• we have four processes p1, p2, p3 and p4, with the following voting power distribution (p1, 23), (p2, 27), (p3, 10) and (p4, 10). The total voting power is 70 (N = 3f+1, where N is the total voting power, and f is the maximum voting power of the faulty processes), so we assume that the faulty processes have at most 23 of voting power. Furthermore, we have the following vote messages in some LastCommit field (we ignore all fields except Time field):
  • (p1, 100), (p2, 98), (p3, 1000), (p4, 500). We assume that p3 and p4 are faulty processes. Let's assume that the block.LastCommit message contains votes of processes p2, p3 and p4. Median is then chosen the following way: the value 98 is counted 27 times, the value 1000 is counted 10 times and the value 500 is counted also 10 times. So the median value will be the value 98. No matter what set of messages with at least 2f+1 voting power we choose, the median value will always be between the values sent by correct processes.

We ensure Time Monotonicity and Time Validity properties by the following rules:

• let rs denotes RoundState (consensus internal state) of some process. Then rs.ProposalBlock.Header.Time == median(rs.LastCommit) && rs.Proposal.Timestamp == rs.ProposalBlock.Header.Time.

• Furthermore, when creating the vote message, the following rules for determining vote.Time field should hold:

  • if rs.Proposal is defined then vote.Time = max(rs.Proposal.Timestamp + 1, time.Now()), where time.Now() denotes local Unix time in milliseconds.

  • if rs.Proposal is not defined and rs.Votes contains +2/3 of the corresponding vote messages (votes for the current height and round, and with the corresponding type (Prevote or Precommit)), then

  vote.Time = max(median(getVotes(rs.Votes, vote.Height, vote.Round, vote.Type)), time.Now()),

  where getVotes function returns the votes for particular Height, Round and Type.
  The second rule is relevant for the case when a process jumps to a higher round upon receiving +2/3 votes for a higher round, but the corresponding Proposal message for the higher round hasn't been received yet.