sync-protocol.md

Capella Light Client -- Sync Protocol

Notice: This document is a work-in-progress for researchers and implementers.

Table of contents

• Introduction
• Custom types
• Constants
• Containers
  • Modified LightClientHeader
• Helper functions
  • get_lc_execution_root
  • Modified is_valid_light_client_header

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. The fork document explains how to upgrade existing Altair based deployments to Capella.

Additional documents describes the impact of the upgrade on certain roles:

• Full node
• Networking

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

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

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

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,
    )