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- ---
- order: 5
- ---
-
- # State Sync
-
- ## Channels
-
- State sync has four distinct channels. The channel identifiers are listed below.
-
- | Name | Number |
- |-------------------|--------|
- | SnapshotChannel | 96 |
- | ChunkChannel | 97 |
- | LightBlockChannel | 98 |
- | ParamsChannel | 99 |
-
- ## Message Types
-
- ### SnapshotRequest
-
- When a new node begin state syncing, it will ask all peers it encounters if it has any
- available snapshots:
-
- | Name | Type | Description | Field Number |
- |----------|--------|-------------|--------------|
-
- ### SnapShotResponse
-
- The receiver will query the local ABCI application via `ListSnapshots`, and send a message
- containing snapshot metadata (limited to 4 MB) for each of the 10 most recent snapshots: and stored at the application layer. When a peer is starting it will request snapshots.
-
- | Name | Type | Description | Field Number |
- |----------|--------|-----------------------------------------------------------|--------------|
- | height | uint64 | Height at which the snapshot was taken | 1 |
- | format | uint32 | Format of the snapshot. | 2 |
- | chunks | uint32 | How many chunks make up the snapshot | 3 |
- | hash | bytes | Arbitrary snapshot hash | 4 |
- | metadata | bytes | Arbitrary application data. **May be non-deterministic.** | 5 |
-
- ### ChunkRequest
-
- The node running state sync will offer these snapshots to the local ABCI application via
- `OfferSnapshot` ABCI calls, and keep track of which peers contain which snapshots. Once a snapshot
- is accepted, the state syncer will request snapshot chunks from appropriate peers:
-
- | Name | Type | Description | Field Number |
- |--------|--------|-------------------------------------------------------------|--------------|
- | height | uint64 | Height at which the chunk was created | 1 |
- | format | uint32 | Format chosen for the chunk. **May be non-deterministic.** | 2 |
- | index | uint32 | Index of the chunk within the snapshot. | 3 |
-
- ### ChunkResponse
-
- The receiver will load the requested chunk from its local application via `LoadSnapshotChunk`,
- and respond with it (limited to 16 MB):
-
- | Name | Type | Description | Field Number |
- |---------|--------|-------------------------------------------------------------|--------------|
- | height | uint64 | Height at which the chunk was created | 1 |
- | format | uint32 | Format chosen for the chunk. **May be non-deterministic.** | 2 |
- | index | uint32 | Index of the chunk within the snapshot. | 3 |
- | hash | bytes | Arbitrary snapshot hash | 4 |
- | missing | bool | Arbitrary application data. **May be non-deterministic.** | 5 |
-
- Here, `Missing` is used to signify that the chunk was not found on the peer, since an empty
- chunk is a valid (although unlikely) response.
-
- The returned chunk is given to the ABCI application via `ApplySnapshotChunk` until the snapshot
- is restored. If a chunk response is not returned within some time, it will be re-requested,
- possibly from a different peer.
-
- The ABCI application is able to request peer bans and chunk refetching as part of the ABCI protocol.
-
- ### LightBlockRequest
-
- To verify state and to provide state relevant information for consensus, the node will ask peers for
- light blocks at specified heights.
-
- | Name | Type | Description | Field Number |
- |----------|--------|----------------------------|--------------|
- | height | uint64 | Height of the light block | 1 |
-
- ### LightBlockResponse
-
- The receiver will retrieve and construct the light block from both the block and state stores. The
- receiver will verify the data by comparing the hashes and store the header, commit and validator set
- if necessary. The light block at the height of the snapshot will be used to verify the `AppHash`.
-
- | Name | Type | Description | Field Number |
- |---------------|---------------------------------------------------------|--------------------------------------|--------------|
- | light_block | [LightBlock](../../core/data_structures.md#lightblock) | Light block at the height requested | 1 |
-
- State sync will use [light client
- verification](https://github.com/tendermint/tendermint/blob/master/docs/tendermint-core/light-client.md)
- to verify the light blocks.
-
-
- If no state sync is in progress (i.e. during normal operation), any unsolicited response messages
- are discarded.
-
- ### ParamsRequest
-
- In order to build tendermint state, the state provider will request the params at the height of the snapshot and use the header to verify it.
-
- | Name | Type | Description | Field Number |
- |----------|--------|----------------------------|--------------|
- | height | uint64 | Height of the consensus params | 1 |
-
-
- ### ParamsResponse
-
- A reciever to the request will use the state store to fetch the consensus params at that height and return it to the sender.
-
- | Name | Type | Description | Field Number |
- |----------|--------|---------------------------------|--------------|
- | height | uint64 | Height of the consensus params | 1 |
- | consensus_params | [ConsensusParams](../../core/data_structures.md#ConsensusParams) | Consensus params at the height requested | 2 |
-
-
- ### Message
-
- Message is a [`oneof` protobuf type](https://developers.google.com/protocol-buffers/docs/proto#oneof). The `oneof` consists of eight messages.
-
- | Name | Type | Description | Field Number |
- |----------------------|--------------------------------------------|----------------------------------------------|--------------|
- | snapshots_request | [SnapshotRequest](#snapshotrequest) | Request a recent snapshot from a peer | 1 |
- | snapshots_response | [SnapshotResponse](#snapshotresponse) | Respond with the most recent snapshot stored | 2 |
- | chunk_request | [ChunkRequest](#chunkrequest) | Request chunks of the snapshot. | 3 |
- | chunk_response | [ChunkRequest](#chunkresponse) | Response of chunks used to recreate state. | 4 |
- | light_block_request | [LightBlockRequest](#lightblockrequest) | Request a light block. | 5 |
- | light_block_response | [LightBlockResponse](#lightblockresponse) | Respond with a light block | 6 |
- | params_request | [ParamsRequest](#paramsrequest) | Request the consensus params at a height. | 7 |
- | params_response | [ParamsResponse](#paramsresponse) | Respond with the consensus params | 8 |
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