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docs/tendermint-core/consensus/proposer-based-timestamps.md
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# Proposer-Based Timestamps
# PBTS
This document provides an overview of the Proposer-Based Timestamp algorithm
added to Tendermint in the v0.36 release. This document outlines the core functionality
as well as the parameters and constraints of the this algorithm.
This document provides an overview of the Proposer-Based Timestamp (PBTS)
algorithm added to Tendermint in the v0.36 release. It outlines the core
functionality as well as the parameters and constraints of the this algorithm.
# Algorithm Overview
## Algorithm Overview
The proposer-based timestamps algorithm defines a way for a Tendermint blockchain
to create block timestamps that are within a reasonable bound of the clocks of the
validators on the network.
The PBTS algorithm defines a way for a Tendermint blockchain to create block
timestamps that are within a reasonable bound of the clocks of the validators on
the network. This replaces the original BFTTime algorithm for timestamp
assignment that relied on the timestamps included in precommit messages.
## Algorithm Parameters
The functionality of the Proposer-Based Timestamps algorithm is governed by two
parameters within Tendermint. These two parameters are [consensus parameters](https://github.com/tendermint/tendermint/blob/c42c6d06d2986791ad1cb8742824818edd9b6323/spec/abci/apps.md#L291),
The functionality of the PBTS algorithm is governed by two parameters within
Tendermint. These two parameters are [consensus
parameters](https://github.com/tendermint/tendermint/blob/master/spec/abci/apps.md#L291),
meaning they are configured by the ABCI application and are expected to be the
same across all nodes on the network.
### `Precision`
The `Precision` parameter configures the acceptable upper-bound of clock drift among
all of the nodes on a Tendermint network. Any two nodes on a Tendermint network
are expected to have clocks that differ by at most `Precision` seconds? Ms? at
any given instant.
The `Precision` parameter configures the acceptable upper-bound of clock drift
among all of the nodes on a Tendermint network. Any two nodes on a Tendermint
network are expected to have clocks that differ by at most `Precision`
milliseconds any given instant.
### `MessageDelay`
The `MessageDelay` parameter configures the acceptable upper-bound for transmitting
a `Proposal` message from the proposer to _all_ of the validators on the network.
The `MessageDelay` parameter configures the acceptable upper-bound for
transmitting a `Proposal` message from the proposer to _all_ of the validators
on the network.
Networks that want very tight bounds on their timestamps should set a smaller value
for `MessageDelay`. Highly distributed networks or networks with generally slow
rates of transmission should set a larger value for `MessageDelay`.
Networks should choose as small a value for `MessageDelay` as is practical,
provided it is large enough that messages can reach all participants with high
probability given the number of participants and latency of their connections.
## Algorithm Concepts
@@ -49,41 +50,45 @@ The following protocols and application features require a reliable source of ti
* Tendermint Light Clients [rely on correspondence between their known time](https://github.com/tendermint/tendermint/blob/master/spec/light-client/verification/README.md#definitions-1) and the block time for block verification.
* Tendermint Evidence validity is determined [either in terms of heights or in terms of time](https://github.com/tendermint/tendermint/blob/master/spec/consensus/evidence.md#verification).
* Unbonding of staked assets in the Cosmos Hub [occurs after a period of 21 days](https://github.com/cosmos/governance/blob/master/params-change/Staking.md#unbondingtime).
* IBC packets can use either a [timestamp or a height to timeout packet delivery](https://docs.cosmos.network/v0.44/ibc/overview.html#acknowledgements)
* Unbonding of staked assets in the Cosmos Hub [occurs after a period of 21
days](https://github.com/cosmos/governance/blob/master/params-change/Staking.md#unbondingtime).
* IBC packets can use either a [timestamp or a height to timeout packet
delivery](https://docs.cosmos.network/v0.44/ibc/overview.html#acknowledgements)
### Proposer Selects a Block Timestamp
When the proposer node creates a new block proposal, the node reads the time from
its local clock and uses this reading as the timestamp for the proposed block.
When the proposer node creates a new block proposal, the node reads the time
from its local clock and uses this reading as the timestamp for the proposed
block.
### Timely-ness
### Timeliness
When each validator on a Tendermint network receives a proposed block, it performs
a series of checks to ensure that the block can be considered valid as a candidate
to be the next block in the chain.
When each validator on a Tendermint network receives a proposed block, it
performs a series of checks to ensure that the block can be considered valid as
a candidate to be the next block in the chain.
The Proposer-Based Timestamps algorithm performs a validity check on the timestamp of proposed blocks.
When a validator receives a proposal it ensures that the timestamp in the proposal
is within a bound of the validator's local clock. Specifically, the algorithm
checks that the timestamp is no more than `Precision` greater than the node's local clock
and no less than `Precision` + `MessageDelay` behind than the node's local clock.
This creates range of acceptable timestamps around the node's local time. If
the timestamp is within this range, the Proposer-Based Timestamps algorithm considers
the block `timely`. If a block is not `timely`, the node will issue a `nil` `prevote`
for this block, signaling to the rest of the network that the node does not consider
the block to be valid.
The PBTS algorithm performs a validity check on the timestamp of proposed
blocks. When a validator receives a proposal it ensures that the timestamp in
the proposal is within a bound of the validator's local clock. Specifically, the
algorithm checks that the timestamp is no more than `Precision` greater than the
node's local clock and no less than `Precision` + `MessageDelay` behind than the
node's local clock. This creates range of acceptable timestamps around the
node's local time. If the timestamp is within this range, the PBTS algorithm
considers the block **timely**. If a block is not **timely**, the node will
issue a `nil` `prevote` for this block, signaling to the rest of the network
that the node does not consider the block to be valid.
### Clock Synchronization
The Proposer-Based Timestamps algorithm requires the clocks of the validators on a
Tendermint network are within `Precision` of each other. In practice, this means
that validators should periodically synchronize to a reliable NTP server. Validators
that drift too far away from the rest of the network will no longer propose
blocks with valid timestamps. Additionally they will not view the timestamps of blocks proposed
by their peers to be valid either.
The PBTS algorithm requires the clocks of the validators on a Tendermint network
are within `Precision` of each other. In practice, this means that validators
should periodically synchronize to a reliable NTP server. Validators that drift
too far away from the rest of the network will no longer propose blocks with
valid timestamps. Additionally they will not view the timestamps of blocks
proposed by their peers to be valid either.
## See Also
* [The Proposer-Based Timestamps specification][https://github.com/tendermint/tendermint/blob/master/spec/consensus/proposer-based-timestamp/README.md] contains all of the details of
the algorithm.
* [The PBTS specification](https://github.com/tendermint/tendermint/blob/master/spec/consensus/proposer-based-timestamp/README.md)
contains all of the details of the algorithm.