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

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# Capella Light Client -- Sync Protocol
**Notice**: This document is a work-in-progress for researchers and implementers.
## Table of contents
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- [Introduction](#introduction)
- [Custom types](#custom-types)
- [Constants](#constants)
- [Containers](#containers)
- [Modified `LightClientHeader`](#modified-lightclientheader)
- [Helper functions](#helper-functions)
- [`get_lc_execution_root`](#get_lc_execution_root)
- [Modified `is_valid_light_client_header`](#modified-is_valid_light_client_header)
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## Introduction
This upgrade adds information about the execution payload to light client data as part of the Capella upgrade. It extends the [Altair Light Client specifications](../../altair/light-client/sync-protocol.md). The [fork document](./fork.md) explains how to upgrade existing Altair based deployments to Capella.
Additional documents describes the impact of the upgrade on certain roles:
- [Full node](./full-node.md)
- [Networking](./p2p-interface.md)
## Custom types
| Name | SSZ equivalent | Description |
| - | - | - |
| `ExecutionBranch` | `Vector[Bytes32, floorlog2(EXECUTION_PAYLOAD_GINDEX)]` | Merkle branch of `execution_payload` within `BeaconBlockBody` |
## Constants
| Name | Value |
| - | - |
| `EXECUTION_PAYLOAD_GINDEX` | `get_generalized_index(BeaconBlockBody, 'execution_payload')` (= 25) |
## Containers
### Modified `LightClientHeader`
```python
class LightClientHeader(Container):
# Beacon block header
beacon: BeaconBlockHeader
# Execution payload header corresponding to `beacon.body_root` (from Capella onward)
execution: ExecutionPayloadHeader
execution_branch: ExecutionBranch
```
## Helper functions
### `get_lc_execution_root`
```python
def get_lc_execution_root(header: LightClientHeader) -> Root:
epoch = compute_epoch_at_slot(header.beacon.slot)
if epoch >= CAPELLA_FORK_EPOCH:
return hash_tree_root(header.execution)
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 < 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,
)
```