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spec: migrate v0.35 spec to tendermint

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docs/rfc/001-block-retention.md
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# RFC 001: Configurable Block Retention
## Changelog
- 2020-03-23: Initial draft (@erikgrinaker)
- 2020-03-25: Use local config for snapshot interval (@erikgrinaker)
- 2020-03-31: Use ABCI commit response for block retention hint
- 2020-04-02: Resolved open questions
## Author(s)
- Erik Grinaker (@erikgrinaker)
## Context
Currently, all Tendermint nodes contain the complete sequence of blocks from genesis up to some height (typically the latest chain height). This will no longer be true when the following features are released:
- [Block pruning](https://github.com/tendermint/tendermint/issues/3652): removes historical blocks and associated data (e.g. validator sets) up to some height, keeping only the most recent blocks.
- [State sync](https://github.com/tendermint/tendermint/issues/828): bootstraps a new node by syncing state machine snapshots at a given height, but not historical blocks and associated data.
To maintain the integrity of the chain, the use of these features must be coordinated such that necessary historical blocks will not become unavailable or lost forever. In particular:
- Some nodes should have complete block histories, for auditability, querying, and bootstrapping.
- The majority of nodes should retain blocks longer than the Cosmos SDK unbonding period, for light client verification.
- Some nodes must take and serve state sync snapshots with snapshot intervals less than the block retention periods, to allow new nodes to state sync and then replay blocks to catch up.
- Applications may not persist their state on commit, and require block replay on restart.
- Only a minority of nodes can be state synced within the unbonding period, for light client verification and to serve block histories for catch-up.
However, it is unclear if and how we should enforce this. It may not be possible to technically enforce all of these without knowing the state of the entire network, but it may also be unrealistic to expect this to be enforced entirely through social coordination. This is especially unfortunate since the consequences of misconfiguration can be permanent chain-wide data loss.
## Proposal
Add a new field `retain_height` to the ABCI `ResponseCommit` message:
```proto
service ABCIApplication {
rpc Commit(RequestCommit) returns (ResponseCommit);
}
message RequestCommit {}
message ResponseCommit {
// reserve 1
bytes data = 2; // the Merkle root hash
uint64 retain_height = 3; // the oldest block height to retain
}
```
Upon ABCI `Commit`, which finalizes execution of a block in the state machine, Tendermint removes all data for heights lower than `retain_height`. This allows the state machine to control block retention, which is preferable since only it can determine the significance of historical blocks. By default (i.e. with `retain_height=0`) all historical blocks are retained.
Removed data includes not only blocks, but also headers, commit info, consensus params, validator sets, and so on. In the first iteration this will be done synchronously, since the number of heights removed for each run is assumed to be small (often 1) in the typical case. It can be made asynchronous at a later time if this is shown to be necessary.
Since `retain_height` is dynamic, it is possible for it to refer to a height which has already been removed. For example, commit at height 100 may return `retain_height=90` while commit at height 101 may return `retain_height=80`. This is allowed, and will be ignored - it is the application's responsibility to return appropriate values.
State sync will eventually support backfilling heights, via e.g. a snapshot metadata field `backfill_height`, but in the initial version it will have a fully truncated block history.
## Cosmos SDK Example
As an example, we'll consider how the Cosmos SDK might make use of this. The specific details should be discussed in a separate SDK proposal.
The returned `retain_height` would be the lowest height that satisfies:
- Unbonding time: the time interval in which validators can be economically punished for misbehavior. Blocks in this interval must be auditable e.g. by the light client.
- IAVL snapshot interval: the block interval at which the underlying IAVL database is persisted to disk, e.g. every 10000 heights. Blocks since the last IAVL snapshot must be available for replay on application restart.
- State sync snapshots: blocks since the _oldest_ available snapshot must be available for state sync nodes to catch up (oldest because a node may be restoring an old snapshot while a new snapshot was taken).
- Local config: archive nodes may want to retain more or all blocks, e.g. via a local config option `min-retain-blocks`. There may also be a need to vary rentention for other nodes, e.g. sentry nodes which do not need historical blocks.
![Cosmos SDK block retention diagram](images/block-retention.png)
## Status
Accepted
## Consequences
### Positive
- Application-specified block retention allows the application to take all relevant factors into account and prevent necessary blocks from being accidentally removed.
- Node operators can independently decide whether they want to provide complete block histories (if local configuration for this is provided) and snapshots.
### Negative
- Social coordination is required to run archival nodes, failure to do so may lead to permanent loss of historical blocks.
- Social coordination is required to run snapshot nodes, failure to do so may lead to inability to run state sync, and inability to bootstrap new nodes at all if no archival nodes are online.
### Neutral
- Reduced block retention requires application changes, and cannot be controlled directly in Tendermint.
- Application-specified block retention may set a lower bound on disk space requirements for all nodes.
## References
- State sync ADR: <https://github.com/tendermint/tendermint/blob/master/docs/architecture/adr-053-state-sync-prototype.md>
- State sync issue: <https://github.com/tendermint/tendermint/issues/828>
- Block pruning issue: <https://github.com/tendermint/tendermint/issues/3652>
docs/rfc/002-nonzero-genesis.md
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# RFC 002: Non-Zero Genesis
## Changelog
- 2020-07-26: Initial draft (@erikgrinaker)
- 2020-07-28: Use weak chain linking, i.e. `predecessor` field (@erikgrinaker)
- 2020-07-31: Drop chain linking (@erikgrinaker)
- 2020-08-03: Add `State.InitialHeight` (@erikgrinaker)
## Author(s)
- Erik Grinaker (@erikgrinaker)
## Context
The recommended upgrade path for block protocol-breaking upgrades is currently to hard fork the
chain (see e.g. [`cosmoshub-3` upgrade](https://blog.cosmos.network/cosmos-hub-3-upgrade-announcement-39c9da941aee)).
This is done by halting all validators at a predetermined height, exporting the application
state via application-specific tooling, and creating an entirely new chain using the exported
application state.
As far as Tendermint is concerned, the upgraded chain is a completely separate chain, with e.g.
a new chain ID and genesis file. Notably, the new chain starts at height 1, and has none of the
old chain's block history. This causes problems for integrators, e.g. coin exchanges and
wallets, that assume a monotonically increasing height for a given blockchain. Users also find
it confusing that a given height can now refer to distinct states depending on the chain
version.
An ideal solution would be to always retain block backwards compatibility in such a way that chain
history is never lost on upgrades. However, this may require a significant amount of engineering
work that is not viable for the planned Stargate release (Tendermint 0.34), and may prove too
restrictive for future development.
As a first step, allowing the new chain to start from an initial height specified in the genesis
file would at least provide monotonically increasing heights. There was a proposal to include the
last block header of the previous chain as well, but since the genesis file is not verified and
hashed (only specific fields are) this would not be trustworthy.
External tooling will be required to map historical heights onto e.g. archive nodes that contain
blocks from previous chain version. Tendermint will not include any such functionality.
## Proposal
Tendermint will allow chains to start from an arbitrary initial height:
- A new field `initial_height` is added to the genesis file, defaulting to `1`. It can be set to any
non-negative integer, and `0` is considered equivalent to `1`.
- A new field `InitialHeight` is added to the ABCI `RequestInitChain` message, with the same value
and semantics as the genesis field.
- A new field `InitialHeight` is added to the `state.State` struct, where `0` is considered invalid.
Including the field here simplifies implementation, since the genesis value does not have to be
propagated throughout the code base separately, but it is not strictly necessary.
ABCI applications may have to be updated to handle arbitrary initial heights, otherwise the initial
block may fail.
## Status
Accepted
## Consequences
### Positive
- Heights can be unique throughout the history of a "logical" chain, across hard fork upgrades.
### Negative
- Upgrades still cause loss of block history.
- Integrators will have to map height ranges to specific archive nodes/networks to query history.
### Neutral
- There is no explicit link to the last block of the previous chain.
## References
- [#2543: Allow genesis file to start from non-zero height w/ prev block header](https://github.com/tendermint/tendermint/issues/2543)
docs/rfc/003-ed25519-verification.md
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# RFC 003: Ed25519 Verification
## Changelog
- August 21, 2020: initialized
## Author(s)
- Marko (@marbar3778)
## Context
Ed25519 keys are the only supported key types for Tendermint validators currently. Tendermint-Go wraps the ed25519 key implementation from the go standard library. As more clients are implemented to communicate with the canonical Tendermint implementation (Tendermint-Go) different implementations of ed25519 will be used. Due to [RFC 8032](https://www.rfc-editor.org/rfc/rfc8032.html) not guaranteeing implementation compatibility, Tendermint clients must to come to an agreement of how to guarantee implementation compatibility. [Zcash](https://z.cash/) has multiple implementations of their client and have identified this as a problem as well. The team at Zcash has made a proposal to address this issue, [Zcash improvement proposal 215](https://zips.z.cash/zip-0215).
## Proposal
- Tendermint-Go would adopt [hdevalence/ed25519consensus](https://github.com/hdevalence/ed25519consensus).
- This library is implements `ed25519.Verify()` in accordance to zip-215. Tendermint-go will continue to use `crypto/ed25519` for signing and key generation.
- Tendermint-rs would adopt [ed25519-zebra](https://github.com/ZcashFoundation/ed25519-zebra)
- related [issue](https://github.com/informalsystems/tendermint-rs/issues/355)
Signature verification is one of the major bottlenecks of Tendermint-go, batch verification can not be used unless it has the same consensus rules, ZIP 215 makes verification safe in consensus critical areas.
This change constitutes a breaking changes, therefore must be done in a major release. No changes to validator keys or operations will be needed for this change to be enabled.
This change has no impact on signature aggregation. To enable this signature aggregation Tendermint will have to use different signature schema (Schnorr, BLS, ...). Secondly, this change will enable safe batch verification for the Tendermint-Go client. Batch verification for the rust client is already supported in the library being used.
As part of the acceptance of this proposal it would be best to contract or discuss with a third party the process of conducting a security review of the go library.
## Status
Proposed
## Consequences
### Positive
- Consistent signature verification across implementations
- Enable safe batch verification
### Negative
#### Tendermint-Go
- Third_party dependency
- library has not gone through a security review.
- unclear maintenance schedule
- Fragmentation of the ed25519 key for the go implementation, verification is done using a third party library while the rest
uses the go standard library
### Neutral
## References
[It’s 255:19AM. Do you know what your validation criteria are?](https://hdevalence.ca/blog/2020-10-04-its-25519am)
docs/rfc/004-abci++.md
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# RFC 004: ABCI++
## Changelog
- January 11, 2020: initialized
## Author(s)
- Dev (@valardragon)
- Sunny (@sunnya97)
## Context
ABCI is the interface between the consensus engine and the application.
It defines when the application can talk to consensus during the execution of a blockchain.
At the moment, the application can only act at one phase in consensus, immediately after a block has been finalized.
This restriction on the application prohibits numerous features for the application, including many scalability improvements that are now better understood than when ABCI was first written.
For example, many of the scalability proposals can be boiled down to "Make the miner / block proposers / validators do work, so the network does not have to".
This includes optimizations such as tx-level signature aggregation, state transition proofs, etc.
Furthermore, many new security properties cannot be achieved in the current paradigm, as the application cannot enforce validators do more than just finalize txs.
This includes features such as threshold cryptography, and guaranteed IBC connection attempts.
We propose introducing three new phases to ABCI to enable these new features, and renaming the existing methods for block execution.
#### Prepare Proposal phase
This phase aims to allow the block proposer to perform more computation, to reduce load on all other full nodes, and light clients in the network.
It is intended to enable features such as batch optimizations on the transaction data (e.g. signature aggregation, zk rollup style validity proofs, etc.), enabling stateless blockchains with validator provided authentication paths, etc.
This new phase will only be executed by the block proposer. The application will take in the block header and raw transaction data output by the consensus engine's mempool. It will then return block data that is prepared for gossip on the network, and additional fields to include into the block header.
#### Process Proposal Phase
This phase aims to allow applications to determine validity of a new block proposal, and execute computation on the block data, prior to the blocks finalization.
It is intended to enable applications to reject block proposals with invalid data, and to enable alternate pipelined execution models. (Such as Ethereum-style immediate execution)
This phase will be executed by all full nodes upon receiving a block, though on the application side it can do more work in the even that the current node is a validator.
#### Vote Extension Phase
This phase aims to allow applications to require their validators do more than just validate blocks.
Example usecases of this include validator determined price oracles, validator guaranteed IBC connection attempts, and validator based threshold crypto.
This adds an app-determined data field that every validator must include with their vote, and these will thus appear in the header.
#### Rename {BeginBlock, [DeliverTx], EndBlock} to FinalizeBlock
The prior phases gives the application more flexibility in their execution model for a block, and they obsolete the current methods for how the consensus engine relates the block data to the state machine. Thus we refactor the existing methods to better reflect what is happening in the new ABCI model.
This rename doesn't on its own enable anything new, but instead improves naming to clarify the expectations from the application in this new communication model. The existing ABCI methods `BeginBlock, [DeliverTx], EndBlock` are renamed to a single method called `FinalizeBlock`.
#### Summary
We include a more detailed list of features / scaling improvements that are blocked, and which new phases resolve them at the end of this document.
<image src="images/abci.png" style="float: left; width: 40%;" /> <image src="images/abci++.png" style="float: right; width: 40%;" />
On the top is the existing definition of ABCI, and on the bottom is the proposed ABCI++.
## Proposal
Below we suggest an API to add these three new phases.
In this document, sometimes the final round of voting is referred to as precommit for clarity in how it acts in the Tendermint case.
### Prepare Proposal
*Note, APIs in this section will change after Vote Extensions, we list the adjusted APIs further in the proposal.*
The Prepare Proposal phase allows the block proposer to perform application-dependent work in a block, to lower the amount of work the rest of the network must do. This enables batch optimizations to a block, which has been empirically demonstrated to be a key component for scaling. This phase introduces the following ABCI method
```rust
fn PrepareProposal(Block) -> BlockData
```
where `BlockData` is a type alias for however data is internally stored within the consensus engine. In Tendermint Core today, this is `[]Tx`.
The application may read the entire block proposal, and mutate the block data fields. Mutated transactions will still get removed from the mempool later on, as the mempool rechecks all transactions after a block is executed.
The `PrepareProposal` API will be modified in the vote extensions section, for allowing the application to modify the header.
### Process Proposal
The Process Proposal phase sends the block data to the state machine, prior to running the last round of votes on the state machine. This enables features such as allowing validators to reject a block according to whether state machine deems it valid, and changing block execution pipeline.
We introduce three new methods,
```rust
fn VerifyHeader(header: Header, isValidator: bool) -> ResponseVerifyHeader {...}
fn ProcessProposal(block: Block) -> ResponseProcessProposal {...}
fn RevertProposal(height: usize, round: usize) {...}
```
where
```rust
struct ResponseVerifyHeader {
accept_header: bool,
evidence: Vec<Evidence>
}
struct ResponseProcessProposal {
accept_block: bool,
evidence: Vec<Evidence>
}
```
Upon receiving a block header, every validator runs `VerifyHeader(header, isValidator)`. The reason for why `VerifyHeader` is split from `ProcessProposal` is due to the later sections for Preprocess Proposal and Vote Extensions, where there may be application dependent data in the header that must be verified before accepting the header.
If the returned `ResponseVerifyHeader.accept_header` is false, then the validator must precommit nil on this block, and reject all other precommits on this block. `ResponseVerifyHeader.evidence` is appended to the validators local `EvidencePool`.
Upon receiving an entire block proposal (in the current implementation, all "block parts"), every validator runs `ProcessProposal(block)`. If the returned `ResponseProcessProposal.accept_block` is false, then the validator must precommit nil on this block, and reject all other precommits on this block. `ResponseProcessProposal.evidence` is appended to the validators local `EvidencePool`.
Once a validator knows that consensus has failed to be achieved for a given block, it must run `RevertProposal(block.height, block.round)`, in order to signal to the application to revert any potentially mutative state changes it may have made. In Tendermint, this occurs when incrementing rounds.
**RFC**: How do we handle the scenario where honest node A finalized on round x, and honest node B finalized on round x + 1? (e.g. when 2f precommits are publicly known, and a validator precommits themself but doesn't broadcast, but they increment rounds) Is this a real concern? The state root derived could change if everyone finalizes on round x+1, not round x, as the state machine can depend non-uniformly on timestamp.
The application is expected to cache the block data for later execution.
The `isValidator` flag is set according to whether the current node is a validator or a full node. This is intended to allow for beginning validator-dependent computation that will be included later in vote extensions. (An example of this is threshold decryptions of ciphertexts.)
### DeliverTx rename to FinalizeBlock
After implementing `ProcessProposal`, txs no longer need to be delivered during the block execution phase. Instead, they are already in the state machine. Thus `BeginBlock, DeliverTx, EndBlock` can all be replaced with a single ABCI method for `ExecuteBlock`. Internally the application may still structure its method for executing the block as `BeginBlock, DeliverTx, EndBlock`. However, it is overly restrictive to enforce that the block be executed after it is finalized. There are multiple other, very reasonable pipelined execution models one can go for. So instead we suggest calling this succession of methods `FinalizeBlock`. We propose the following API
Replace the `BeginBlock, DeliverTx, EndBlock` ABCI methods with the following method
```rust
fn FinalizeBlock() -> ResponseFinalizeBlock
```
where `ResponseFinalizeBlock` has the following API, in terms of what already exists
```rust
struct ResponseFinalizeBlock {
updates: ResponseEndBlock,
tx_results: Vec<ResponseDeliverTx>
}
```
`ResponseEndBlock` should then be renamed to `ConsensusUpdates` and `ResponseDeliverTx` should be renamed to `ResponseTx`.
### Vote Extensions
The Vote Extensions phase allow applications to force their validators to do more than just validate within consensus. This is done by allowing the application to add more data to their votes, in the final round of voting. (Namely the precommit)
This additional application data will then appear in the block header.
First we discuss the API changes to the vote struct directly
```rust
fn ExtendVote(height: u64, round: u64) -> (UnsignedAppVoteData, SelfAuthenticatingAppData)
fn VerifyVoteExtension(signed_app_vote_data: Vec<u8>, self_authenticating_app_vote_data: Vec<u8>) -> bool
```
There are two types of data that the application can enforce validators to include with their vote.
There is data that the app needs the validator to sign over in their vote, and there can be self-authenticating vote data. Self-authenticating here means that the application upon seeing these bytes, knows its valid, came from the validator and is non-malleable. We give an example of each type of vote data here, to make their roles clearer.
- Unsigned app vote data: A use case of this is if you wanted validator backed oracles, where each validator independently signs some oracle data in their vote, and the median of these values is used on chain. Thus we leverage consensus' signing process for convenience, and use that same key to sign the oracle data.
- Self-authenticating vote data: A use case of this is in threshold random beacons. Every validator produces a threshold beacon share. This threshold beacon share can be verified by any node in the network, given the share and the validators public key (which is not the same as its consensus public key). However, this decryption share will not make it into the subsequent block's header. They will be aggregated by the subsequent block proposer to get a single random beacon value that will appear in the subsequent block's header. Everyone can then verify that this aggregated value came from the requisite threshold of the validator set, without increasing the bandwidth for full nodes or light clients. To achieve this goal, the self-authenticating vote data cannot be signed over by the consensus key along with the rest of the vote, as that would require all full nodes & light clients to know this data in order to verify the vote.
The `CanonicalVote` struct will acommodate the `UnsignedAppVoteData` field by adding another string to its encoding, after the `chain-id`. This should not interfere with existing hardware signing integrations, as it does not affect the constant offset for the `height` and `round`, and the vote size does not have an explicit upper bound. (So adding this unsigned app vote data field is equivalent from the HSM's perspective as having a superlong chain-ID)
**RFC**: Please comment if you think it will be fine to have elongate the message the HSM signs, or if we need to explore pre-hashing the app vote data.
The flow of these methods is that when a validator has to precommit, Tendermint will first produce a precommit canonical vote without the application vote data. It will then pass it to the application, which will return unsigned application vote data, and self authenticating application vote data. It will bundle the `unsigned_application_vote_data` into the canonical vote, and pass it to the HSM to sign. Finally it will package the self-authenticating app vote data, and the `signed_vote_data` together, into one final Vote struct to be passed around the network.
#### Changes to Prepare Proposal Phase
There are many use cases where the additional data from vote extensions can be batch optimized.
This is mainly of interest when the votes include self-authenticating app vote data that be batched together, or the unsigned app vote data is the same across all votes.
To allow for this, we change the PrepareProposal API to the following
```rust
fn PrepareProposal(Block, UnbatchedHeader) -> (BlockData, Header)
```
where `UnbatchedHeader` essentially contains a "RawCommit", the `Header` contains a batch-optimized `commit` and an additional "Application Data" field in its root. This will involve a number of changes to core data structures, which will be gone over in the ADR.
The `Unbatched` header and `rawcommit` will never be broadcasted, they will be completely internal to consensus.
#### Inter-process communication (IPC) effects
For brevity in exposition above, we did not discuss the trade-offs that may occur in interprocess communication delays that these changs will introduce.
These new ABCI methods add more locations where the application must communicate with the consensus engine.
In most configurations, we expect that the consensus engine and the application will be either statically or dynamically linked, so all communication is a matter of at most adjusting the memory model the data is layed out within.
This memory model conversion is typically considered negligible, as delay here is measured on the order of microseconds at most, whereas we face milisecond delays due to cryptography and network overheads.
Thus we ignore the overhead in the case of linked libraries.
In the case where the consensus engine and the application are ran in separate processes, and thus communicate with a form of Inter-process communication (IPC), the delays can easily become on the order of miliseconds based upon the data sent. Thus its important to consider whats happening here.
We go through this phase by phase.
##### Prepare proposal IPC overhead
This requires a round of IPC communication, where both directions are quite large. Namely the proposer communicating an entire block to the application.
However, this can be mitigated by splitting up `PrepareProposal` into two distinct, async methods, one for the block IPC communication, and one for the Header IPC communication.
Then for chains where the block data does not depend on the header data, the block data IPC communication can proceed in parallel to the prior block's voting phase. (As a node can know whether or not its the leader in the next round)
Furthermore, this IPC communication is expected to be quite low relative to the amount of p2p gossip time it takes to send the block data around the network, so this is perhaps a premature concern until more sophisticated block gossip protocols are implemented.
##### Process Proposal IPC overhead
This phase changes the amount of time available for the consensus engine to deliver a block's data to the state machine.
Before, the block data for block N would be delivered to the state machine upon receiving a commit for block N and then be executed.
The state machine would respond after executing the txs and before prevoting.
The time for block delivery from the consensus engine to the state machine after this change is the time of receiving block proposal N to the to time precommit on proposal N.
It is expected that this difference is unimportant in practice, as this time is in parallel to one round of p2p communication for prevoting, which is expected to be significantly less than the time for the consensus engine to deliver a block to the state machine.
##### Vote Extension IPC overhead
This has a small amount of data, but does incur an IPC round trip delay. This IPC round trip delay is pretty negligible as compared the variance in vote gossip time. (the IPC delay is typically on the order of 10 microseconds)
## Status
Proposed
## Consequences
### Positive
- Enables a large number of new features for applications
- Supports both immediate and delayed execution models
- Allows application specific data from each validator
- Allows for batch optimizations across txs, and votes
### Negative
- This is a breaking change to all existing ABCI clients, however the application should be able to have a thin wrapper to replicate existing ABCI behavior.
- PrepareProposal - can be a no-op
- Process Proposal - has to cache the block, but can otherwise be a no-op
- Vote Extensions - can be a no-op
- Finalize Block - Can black-box call BeginBlock, DeliverTx, EndBlock given the cached block data
- Vote Extensions adds more complexity to core Tendermint Data Structures
- Allowing alternate alternate execution models will lead to a proliferation of new ways for applications to violate expected guarantees.
### Neutral
- IPC overhead considerations change, but mostly for the better
## References
Reference for IPC delay constants: <http://pages.cs.wisc.edu/~adityav/Evaluation_of_Inter_Process_Communication_Mechanisms.pdf>
### Short list of blocked features / scaling improvements with required ABCI++ Phases
| Feature | PrepareProposal | ProcessProposal | Vote Extensions |
| :--- | :---: | :---: | :---: |
| Tx based signature aggregation | X | | |
| SNARK proof of valid state transition | X | | |
| Validator provided authentication paths in stateless blockchains | X | | |
| Immediate Execution | | X | |
| Simple soft forks | | X | |
| Validator guaranteed IBC connection attempts | | | X |
| Validator based price oracles | | | X |
| Immediate Execution with increased time for block execution | X | X | X |
| Threshold Encrypted txs | X | X | X |
docs/rfc/005-reverse-sync.md
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# RFC 004: ReverseSync - fetching historical data
## Changelog
- 2021-04-19: Use P2P to gossip necessary data for reverse sync.
- 2021-03-03: Simplify proposal to the state sync case.
- 2021-02-17: Add notes on asynchronicity of processes.
- 2020-12-10: Rename backfill blocks to reverse sync.
- 2020-11-25: Initial draft.
## Author(s)
- Callum Waters (@cmwaters)
## Context
Two new features: [Block pruning](https://github.com/tendermint/tendermint/issues/3652)
and [State sync](https://github.com/tendermint/tendermint/blob/master/docs/architecture/adr-042-state-sync.md)
meant nodes no longer needed a complete history of the blockchain. This
introduced some challenges of its own which were covered and subsequently
tackled with [RFC-001](https://github.com/tendermint/spec/blob/master/rfc/001-block-retention.md).
The RFC allowed applications to set a block retention height; an upper bound on
what blocks would be pruned. However nodes who state sync past this upper bound
(which is necessary as snapshots must be saved within the trusting period for
the assisting light client to verify) have no means of backfilling the blocks
to meet the retention limit. This could be a problem as nodes who state sync and
then eventually switch to consensus (or fast sync) may not have the block and
validator history to verify evidence causing them to panic if they see 2/3
commit on what the node believes to be an invalid block.
Thus, this RFC sets out to instil a minimum block history invariant amongst
honest nodes.
## Proposal
A backfill mechanism can simply be defined as an algorithm for fetching,
verifying and storing, headers and validator sets of a height prior to the
current base of the node's blockchain. In matching the terminology used for
other data retrieving protocols (i.e. fast sync and state sync), we
call this method **ReverseSync**.
We will define the mechanism in four sections:
- Usage
- Design
- Verification
- Termination
### Usage
For now, we focus purely on the case of a state syncing node, whom after
syncing to a height will need to verify historical data in order to be capable
of processing new blocks. We can denote the earliest height that the node will
need to verify and store in order to be able to verify any evidence that might
arise as the `max_historical_height`/`time`. Both height and time are necessary
as this maps to the BFT time used for evidence expiration. After acquiring
`State`, we calculate these parameters as:
```go
max_historical_height = max(state.InitialHeight, state.LastBlockHeight - state.ConsensusParams.EvidenceAgeHeight)
max_historical_time = max(GenesisTime, state.LastBlockTime.Sub(state.ConsensusParams.EvidenceAgeTime))
```
Before starting either fast sync or consensus, we then run the following
synchronous process:
```go
func ReverseSync(max_historical_height int64, max_historical_time time.Time) error
```
Where we fetch and verify blocks until a block `A` where
`A.Height <= max_historical_height` and `A.Time <= max_historical_time`.
Upon successfully reverse syncing, a node can now safely continue. As this
feature is only used as part of state sync, one can think of this as merely an
extension to it.
In the future we may want to extend this functionality to allow nodes to fetch
historical blocks for reasons of accountability or data accessibility.
### Design
This section will provide a high level overview of some of the more important
characteristics of the design, saving the more tedious details as an ADR.
#### P2P
Implementation of this RFC will require the addition of a new channel and two
new messages.
```proto
message LightBlockRequest {
uint64 height = 1;
}
```
```proto
message LightBlockResponse {
Header header = 1;
Commit commit = 2;
ValidatorSet validator_set = 3;
}
```
The P2P path may also enable P2P networked light clients and a state sync that
also doesn't need to rely on RPC.
### Verification
ReverseSync is used to fetch the following data structures:
- `Header`
- `Commit`
- `ValidatorSet`
Nodes will also need to be able to verify these. This can be achieved by first
retrieving the header at the base height from the block store. From this trusted
header, the node hashes each of the three data structures and checks that they are correct.
1. The trusted header's last block ID matches the hash of the new header
```go
header[height].LastBlockID == hash(header[height-1])
```
2. The trusted header's last commit hash matches the hash of the new commit
```go
header[height].LastCommitHash == hash(commit[height-1])
```
3. Given that the node now trusts the new header, check that the header's validator set
hash matches the hash of the validator set
```go
header[height-1].ValidatorsHash == hash(validatorSet[height-1])
```
### Termination
ReverseSync draws a lot of parallels with fast sync. An important consideration
for fast sync that also extends to ReverseSync is termination. ReverseSync will
finish it's task when one of the following conditions have been met:
1. It reaches a block `A` where `A.Height <= max_historical_height` and
`A.Time <= max_historical_time`.
2. None of it's peers reports to have the block at the height below the
processes current block.
3. A global timeout.
This implies that we can't guarantee adequate history and thus the term
"invariant" can't be used in the strictest sense. In the case that the first
condition isn't met, the node will log an error and optimistically attempt
to continue with either fast sync or consensus.
## Alternative Solutions
The need for a minimum block history invariant stems purely from the need to
validate evidence (although there may be some application relevant needs as
well). Because of this, an alternative, could be to simply trust whatever the
2/3+ majority has agreed upon and in the case where a node is at the head of the
blockchain, you simply abstain from voting.
As it stands, if 2/3+ vote on evidence you can't verify, in the same manner if
2/3+ vote on a header that a node sees as invalid (perhaps due to a different
app hash), the node will halt.
Another alternative is the method with which the relevant data is retrieved.
Instead of introducing new messages to the P2P layer, RPC could have been used
instead.
The aforementioned data is already available via the following RPC endpoints:
`/commit` for `Header`'s' and `/validators` for `ValidatorSet`'s'. It was
decided predominantly due to the instability of the current RPC infrastructure
that P2P be used instead.
## Status
Proposed
## Consequences
### Positive
- Ensures a minimum block history invariant for honest nodes. This will allow
nodes to verify evidence.
### Negative
- Statesync will be slower as more processing is required.
### Neutral
- By having validator sets served through p2p, this would make it easier to
extend p2p support to light clients and state sync.
- In the future, it may also be possible to extend this feature to allow for
nodes to freely fetch and verify prior blocks
## References
- [RFC-001: Block retention](https://github.com/tendermint/spec/blob/master/rfc/001-block-retention.md)
- [Original issue](https://github.com/tendermint/tendermint/issues/4629)
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---
order: 1
parent:
order: false
---
# Requests for Comments
A Request for Comments (RFC) is a record of discussion on an open-ended topic
related to the design and implementation of Tendermint Core, for which no
immediate decision is required.
The purpose of an RFC is to serve as a historical record of a high-level
discussion that might otherwise only be recorded in an ad hoc way (for example,
via gists or Google docs) that are difficult to discover for someone after the
fact. An RFC _may_ give rise to more specific architectural _decisions_ for
Tendermint, but those decisions must be recorded separately in [Architecture
Decision Records (ADR)](./../architecture).
As a rule of thumb, if you can articulate a specific question that needs to be
answered, write an ADR. If you need to explore the topic and get input from
others to know what questions need to be answered, an RFC may be appropriate.
## RFC Content
An RFC should provide:
- A **changelog**, documenting when and how the RFC has changed.
- An **abstract**, briefly summarizing the topic so the reader can quickly tell
whether it is relevant to their interest.
- Any **background** a reader will need to understand and participate in the
substance of the discussion (links to other documents are fine here).
- The **discussion**, the primary content of the document.
The [rfc-template.md](./rfc-template.md) file includes placeholders for these
sections.
## Table of Contents
- [001-block-retention](./001-block-retention.md)
- [002-nonzero-genesis](./002-nonzero-genesis.md)
- [003-ed25519-verification](./003-ed25519-verification.md)
- [004-abci++](./004-abci++.md)
- [005-reverse-sync](./005-reverse-sync.md)
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# RFC {RFC-NUMBER}: {TITLE}
## Changelog
- {date}: {changelog}
## Author(s)
- {First Name} {github handle}
## Context
> This section contains all the context one needs to understand the current state, and why there is a problem. It should be as succinct as possible and introduce the high level idea behind the solution.
## Proposal
> It should contain a detailed breakdown of how the problem should be resolved including diagrams and other supporting materials needed to present the case and implementation roadmap for the proposed changes. The reader should be able to fully understand the proposal. This section should be broken up using ## subsections as needed.
## Status
> A decision may be "proposed" if it hasn't been agreed upon yet, or "accepted" once it is agreed upon. If a later RFC changes or reverses a decision, it may be marked as "deprecated" or "superseded" with a reference to its replacement.
{Deprecated|Proposed|Accepted}
## Consequences
> This section describes the consequences, after applying the decision. All consequences should be summarized here, not just the "positive" ones.
### Positive
### Negative
### Neutral
## References
> Are there any relevant PR comments, issues that led up to this, or articles referenced for why we made the given design choice? If so link them here!
- {reference link}