consensus-specs/specs/altair/light-client/light-client.md

Altair Light Client -- Light Client

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

Table of contents

• Introduction
• Light client sync process

Introduction

This document explains how light clients MAY obtain light client data to sync with the network.

Light client sync process

1. The light client MUST be configured out-of-band with a spec/preset (including fork schedule), with genesis_state (including genesis_time and genesis_validators_root), and with a trusted block root. The trusted block SHOULD be within the weak subjectivity period, and its root SHOULD be from a finalized Checkpoint.
2. The local clock is initialized based on the configured genesis_time, and the current fork digest is determined to browse for and connect to relevant light client data providers.
3. The light client fetches a LightClientBootstrap object for the configured trusted block root. The bootstrap object is passed to initialize_light_client_store to obtain a local LightClientStore.
4. The light client tracks the sync committee periods finalized_period from store.finalized_header.beacon.slot, optimistic_period from store.optimistic_header.beacon.slot, and current_period from current_slot based on the local clock.
   1. When finalized_period == optimistic_period and is_next_sync_committee_known indicates False, the light client fetches a LightClientUpdate for finalized_period. If finalized_period == current_period, this fetch SHOULD be scheduled at a random time before current_period advances.
   2. When finalized_period + 1 < current_period, the light client fetches a LightClientUpdate for each sync committee period in range [finalized_period + 1, current_period) (current period excluded)
   3. When finalized_period + 1 >= current_period, the light client keeps observing LightClientFinalityUpdate and LightClientOptimisticUpdate. Received objects are passed to process_light_client_finality_update and process_light_client_optimistic_update. This ensures that finalized_header and optimistic_header reflect the latest blocks.
5. process_light_client_store_force_update MAY be called based on use case dependent heuristics if light client sync appears stuck. If available, falling back to an alternative syncing mechanism to cover the affected sync committee period is preferred.