docs: add an overview of the proposer-based timestamps algorithm (#8058)

This change adds an overview of the proposer-based timestamps algorithm. The goal of this documentation is to give a plain enough explanation of the algorithm so that application developers and validators can understand both the utility of the algorithm and understand how the new constrains may affect their network and topology. 

I'm blanking on the scheme we decided on for docs linking, so if anyone could remind me what link format we decided on, I'll go clean that up ASAP.

Once this is merged, I intend to create a runbook for chains that see slower block-times or higher nil prevotes and link that runbook to this document to provide a higher-level overview.

closes: #8046

docs/tendermint-core/consensus/proposer-based-timestamps.md

@@ -0,0 +1,94 @@
+--- order: 3 ---
+
+# PBTS
+
+ 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 
+
+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 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`
+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.
+
+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
+
+### Block timestamps
+
+Each block produced by the Tendermint consensus engine contains a timestamp.
+The timestamp produced in each block is a meaningful representation of time that is
+useful for the protocols and applications built on top of Tendermint.
+
+The following protocols and application features require a reliable source of time:
+
+* 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)
+
+### 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.
+
+### 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.
+
+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 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 PBTS specification](https://github.com/tendermint/tendermint/blob/master/spec/consensus/proposer-based-timestamp/README.md)
+ contains all of the details of the algorithm.
+

spec/abci/apps.md

@@ -346,6 +346,19 @@ a block minus it's overhead ( ~ `MaxBytes`).
 
 Must have `MaxNum > 0`.
 
+### SynchronyParams.Precision
+
+`SynchronyParams.Precision` is a parameter of the Proposer-Based Timestamps algorithm.
+that 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`.
+
+### SynchronyParams.MessageDelay
+
+`SynchronyParams.MessageDelay` is a parameter of the Proposer-Based Timestamps 
+algorithm that configures the acceptable upper-bound for transmitting a `Proposal`
+message from the proposer to all of the validators on the network.
+
 ### Updates
 
 The application may set the ConsensusParams during InitChain, and update them during