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eth2.0-specs/specs/electra/light-client/sync-protocol.md

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Update light client specifications for Electra Electra introduces two changes that affect light client data handling: 1. The `ExecutionPayloadHeader` is extended with new fields. This is handled similarly as before with the Deneb fork. 2. The `BeaconState` generalized indices change due to lack of EIP-6493. This is handled by making the generalized index be fork dependent via a helper function that computes it dynamically. Furthermore, the case where pre-Electra light client data is consumed by an Electra based `LightClientStore` requires normalizing the shorter proof of the pre-Electra data to fit into the Electra data structure by prepending a zero hash.
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# Electra Light Client -- Sync Protocol
**Notice**: This document is a work-in-progress for researchers and implementers.
## Table of contents
<!-- TOC -->
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- [Introduction](#introduction)
Freeze Altair constants at Deneb
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- [Custom types](#custom-types)
Update light client specifications for Electra Electra introduces two changes that affect light client data handling: 1. The `ExecutionPayloadHeader` is extended with new fields. This is handled similarly as before with the Deneb fork. 2. The `BeaconState` generalized indices change due to lack of EIP-6493. This is handled by making the generalized index be fork dependent via a helper function that computes it dynamically. Furthermore, the case where pre-Electra light client data is consumed by an Electra based `LightClientStore` requires normalizing the shorter proof of the pre-Electra data to fit into the Electra data structure by prepending a zero hash.
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- [Constants](#constants)
Freeze Altair constants at Deneb
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- [Frozen constants](#frozen-constants)
- [New constants](#new-constants)
Update light client specifications for Electra Electra introduces two changes that affect light client data handling: 1. The `ExecutionPayloadHeader` is extended with new fields. This is handled similarly as before with the Deneb fork. 2. The `BeaconState` generalized indices change due to lack of EIP-6493. This is handled by making the generalized index be fork dependent via a helper function that computes it dynamically. Furthermore, the case where pre-Electra light client data is consumed by an Electra based `LightClientStore` requires normalizing the shorter proof of the pre-Electra data to fit into the Electra data structure by prepending a zero hash.
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- [Helper functions](#helper-functions)
- [Modified `finalized_root_gindex_at_slot`](#modified-finalized_root_gindex_at_slot)
- [Modified `current_sync_committee_gindex_at_slot`](#modified-current_sync_committee_gindex_at_slot)
- [Modified `next_sync_committee_gindex_at_slot`](#modified-next_sync_committee_gindex_at_slot)
- [Modified `get_lc_execution_root`](#modified-get_lc_execution_root)
- [Modified `is_valid_light_client_header`](#modified-is_valid_light_client_header)
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## Introduction
This upgrade updates light client data to include the Electra changes to the [`ExecutionPayload`](../beacon-chain.md) structure and to the generalized indices of surrounding containers. It extends the [Deneb Light Client specifications](../../deneb/light-client/sync-protocol.md). The [fork document](./fork.md) explains how to upgrade existing Deneb based deployments to Electra.
Additional documents describes the impact of the upgrade on certain roles:
- [Full node](./full-node.md)
- [Networking](./p2p-interface.md)
Use separate constant names for post-Electra gindices
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## Custom types
| Name | SSZ equivalent | Description |
| - | - | - |
| `FinalityBranch` | `Vector[Bytes32, floorlog2(FINALIZED_ROOT_GINDEX_ELECTRA)]` | Merkle branch of `finalized_checkpoint.root` within `BeaconState` |
| `CurrentSyncCommitteeBranch` | `Vector[Bytes32, floorlog2(CURRENT_SYNC_COMMITTEE_GINDEX_ELECTRA)]` | Merkle branch of `current_sync_committee` within `BeaconState` |
| `NextSyncCommitteeBranch` | `Vector[Bytes32, floorlog2(NEXT_SYNC_COMMITTEE_GINDEX_ELECTRA)]` | Merkle branch of `next_sync_committee` within `BeaconState` |
Update light client specifications for Electra Electra introduces two changes that affect light client data handling: 1. The `ExecutionPayloadHeader` is extended with new fields. This is handled similarly as before with the Deneb fork. 2. The `BeaconState` generalized indices change due to lack of EIP-6493. This is handled by making the generalized index be fork dependent via a helper function that computes it dynamically. Furthermore, the case where pre-Electra light client data is consumed by an Electra based `LightClientStore` requires normalizing the shorter proof of the pre-Electra data to fit into the Electra data structure by prepending a zero hash.
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## Constants
Freeze Altair constants at Deneb
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### Frozen constants
Fix link
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Existing `GeneralizedIndex` constants are frozen at their [Altair](../../altair/light-client/sync-protocol.md#constants) values.
Freeze Altair constants at Deneb
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| Name | Value |
| - | - |
| `FINALIZED_ROOT_GINDEX` | `get_generalized_index(altair.BeaconState, 'finalized_checkpoint', 'root')` (= 105) |
| `CURRENT_SYNC_COMMITTEE_GINDEX` | `get_generalized_index(altair.BeaconState, 'current_sync_committee')` (= 54) |
| `NEXT_SYNC_COMMITTEE_GINDEX` | `get_generalized_index(altair.BeaconState, 'next_sync_committee')` (= 55) |
### New constants
Update light client specifications for Electra Electra introduces two changes that affect light client data handling: 1. The `ExecutionPayloadHeader` is extended with new fields. This is handled similarly as before with the Deneb fork. 2. The `BeaconState` generalized indices change due to lack of EIP-6493. This is handled by making the generalized index be fork dependent via a helper function that computes it dynamically. Furthermore, the case where pre-Electra light client data is consumed by an Electra based `LightClientStore` requires normalizing the shorter proof of the pre-Electra data to fit into the Electra data structure by prepending a zero hash.
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| Name | Value |
| - | - |
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| `FINALIZED_ROOT_GINDEX_ELECTRA` | `get_generalized_index(BeaconState, 'finalized_checkpoint', 'root')` (= 169) |
| `CURRENT_SYNC_COMMITTEE_GINDEX_ELECTRA` | `get_generalized_index(BeaconState, 'current_sync_committee')` (= 86) |
| `NEXT_SYNC_COMMITTEE_GINDEX_ELECTRA` | `get_generalized_index(BeaconState, 'next_sync_committee')` (= 87) |
Update light client specifications for Electra Electra introduces two changes that affect light client data handling: 1. The `ExecutionPayloadHeader` is extended with new fields. This is handled similarly as before with the Deneb fork. 2. The `BeaconState` generalized indices change due to lack of EIP-6493. This is handled by making the generalized index be fork dependent via a helper function that computes it dynamically. Furthermore, the case where pre-Electra light client data is consumed by an Electra based `LightClientStore` requires normalizing the shorter proof of the pre-Electra data to fit into the Electra data structure by prepending a zero hash.
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## Helper functions
### Modified `finalized_root_gindex_at_slot`
```python
def finalized_root_gindex_at_slot(slot: Slot) -> GeneralizedIndex:
epoch = compute_epoch_at_slot(slot)
# [Modified in Electra]
if epoch >= ELECTRA_FORK_EPOCH:
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return FINALIZED_ROOT_GINDEX_ELECTRA
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return FINALIZED_ROOT_GINDEX
Update light client specifications for Electra Electra introduces two changes that affect light client data handling: 1. The `ExecutionPayloadHeader` is extended with new fields. This is handled similarly as before with the Deneb fork. 2. The `BeaconState` generalized indices change due to lack of EIP-6493. This is handled by making the generalized index be fork dependent via a helper function that computes it dynamically. Furthermore, the case where pre-Electra light client data is consumed by an Electra based `LightClientStore` requires normalizing the shorter proof of the pre-Electra data to fit into the Electra data structure by prepending a zero hash.
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```
### Modified `current_sync_committee_gindex_at_slot`
```python
def current_sync_committee_gindex_at_slot(slot: Slot) -> GeneralizedIndex:
epoch = compute_epoch_at_slot(slot)
# [Modified in Electra]
if epoch >= ELECTRA_FORK_EPOCH:
Use separate constant names for post-Electra gindices
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return CURRENT_SYNC_COMMITTEE_GINDEX_ELECTRA
Remove unnecessary cast
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return CURRENT_SYNC_COMMITTEE_GINDEX
Update light client specifications for Electra Electra introduces two changes that affect light client data handling: 1. The `ExecutionPayloadHeader` is extended with new fields. This is handled similarly as before with the Deneb fork. 2. The `BeaconState` generalized indices change due to lack of EIP-6493. This is handled by making the generalized index be fork dependent via a helper function that computes it dynamically. Furthermore, the case where pre-Electra light client data is consumed by an Electra based `LightClientStore` requires normalizing the shorter proof of the pre-Electra data to fit into the Electra data structure by prepending a zero hash.
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```
### Modified `next_sync_committee_gindex_at_slot`
```python
def next_sync_committee_gindex_at_slot(slot: Slot) -> GeneralizedIndex:
epoch = compute_epoch_at_slot(slot)
# [Modified in Electra]
if epoch >= ELECTRA_FORK_EPOCH:
Use separate constant names for post-Electra gindices
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return NEXT_SYNC_COMMITTEE_GINDEX_ELECTRA
Remove unnecessary cast
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return NEXT_SYNC_COMMITTEE_GINDEX
Update light client specifications for Electra Electra introduces two changes that affect light client data handling: 1. The `ExecutionPayloadHeader` is extended with new fields. This is handled similarly as before with the Deneb fork. 2. The `BeaconState` generalized indices change due to lack of EIP-6493. This is handled by making the generalized index be fork dependent via a helper function that computes it dynamically. Furthermore, the case where pre-Electra light client data is consumed by an Electra based `LightClientStore` requires normalizing the shorter proof of the pre-Electra data to fit into the Electra data structure by prepending a zero hash.
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```
### Modified `get_lc_execution_root`
```python
def get_lc_execution_root(header: LightClientHeader) -> Root:
epoch = compute_epoch_at_slot(header.beacon.slot)
# [New in Electra]
if epoch >= ELECTRA_FORK_EPOCH:
return hash_tree_root(header.execution)
# [Modified in Electra]
if epoch >= DENEB_FORK_EPOCH:
execution_header = deneb.ExecutionPayloadHeader(
parent_hash=header.execution.parent_hash,
fee_recipient=header.execution.fee_recipient,
state_root=header.execution.state_root,
receipts_root=header.execution.receipts_root,
logs_bloom=header.execution.logs_bloom,
prev_randao=header.execution.prev_randao,
block_number=header.execution.block_number,
gas_limit=header.execution.gas_limit,
gas_used=header.execution.gas_used,
timestamp=header.execution.timestamp,
extra_data=header.execution.extra_data,
base_fee_per_gas=header.execution.base_fee_per_gas,
block_hash=header.execution.block_hash,
transactions_root=header.execution.transactions_root,
withdrawals_root=header.execution.withdrawals_root,
blob_gas_used=header.execution.blob_gas_used,
excess_blob_gas=header.execution.excess_blob_gas,
)
return hash_tree_root(execution_header)
if epoch >= CAPELLA_FORK_EPOCH:
execution_header = capella.ExecutionPayloadHeader(
parent_hash=header.execution.parent_hash,
fee_recipient=header.execution.fee_recipient,
state_root=header.execution.state_root,
receipts_root=header.execution.receipts_root,
logs_bloom=header.execution.logs_bloom,
prev_randao=header.execution.prev_randao,
block_number=header.execution.block_number,
gas_limit=header.execution.gas_limit,
gas_used=header.execution.gas_used,
timestamp=header.execution.timestamp,
extra_data=header.execution.extra_data,
base_fee_per_gas=header.execution.base_fee_per_gas,
block_hash=header.execution.block_hash,
transactions_root=header.execution.transactions_root,
withdrawals_root=header.execution.withdrawals_root,
)
return hash_tree_root(execution_header)
return Root()
```
### Modified `is_valid_light_client_header`
```python
def is_valid_light_client_header(header: LightClientHeader) -> bool:
epoch = compute_epoch_at_slot(header.beacon.slot)
if epoch < DENEB_FORK_EPOCH:
if header.execution.blob_gas_used != uint64(0) or header.execution.excess_blob_gas != uint64(0):
return False
if epoch < CAPELLA_FORK_EPOCH:
return (
header.execution == ExecutionPayloadHeader()
and header.execution_branch == ExecutionBranch()
)
return is_valid_merkle_branch(
leaf=get_lc_execution_root(header),
branch=header.execution_branch,
depth=floorlog2(EXECUTION_PAYLOAD_GINDEX),
index=get_subtree_index(EXECUTION_PAYLOAD_GINDEX),
root=header.beacon.body_root,
)
```