Describe light client sync process

Provides documentation about the process that a light client may use to
sync a local `LightClientStore`.

README.md

@@ -18,7 +18,7 @@ Features are researched and developed in parallel, and then consolidated into se
 | Seq. | Code Name | Fork Epoch | Specs |
 | - | - | - | - |
 | 0 | **Phase0** |`0` | <ul><li>Core</li><ul><li>[The beacon chain](specs/phase0/beacon-chain.md)</li><li>[Deposit contract](specs/phase0/deposit-contract.md)</li><li>[Beacon chain fork choice](specs/phase0/fork-choice.md)</li></ul><li>Additions</li><ul><li>[Honest validator guide](specs/phase0/validator.md)</li><li>[P2P networking](specs/phase0/p2p-interface.md)</li><li>[Weak subjectivity](specs/phase0/weak-subjectivity.md)</li></ul></ul> |
-| 1 |  **Altair** | `74240` | <ul><li>Core</li><ul><li>[Beacon chain changes](specs/altair/beacon-chain.md)</li><li>[Altair fork](specs/altair/fork.md)</li></ul><li>Additions</li><ul><li>[Light client sync protocol](specs/altair/light-client/sync-protocol.md) ([full node](specs/altair/light-client/full-node.md))</li><li>[Honest validator guide changes](specs/altair/validator.md)</li><li>[P2P networking](specs/altair/p2p-interface.md)</li></ul></ul> |
+| 1 |  **Altair** | `74240` | <ul><li>Core</li><ul><li>[Beacon chain changes](specs/altair/beacon-chain.md)</li><li>[Altair fork](specs/altair/fork.md)</li></ul><li>Additions</li><ul><li>[Light client sync protocol](specs/altair/light-client/sync-protocol.md) ([full node](specs/altair/light-client/full-node.md), [light client](specs/altair/light-client/light-client.md))</li><li>[Honest validator guide changes](specs/altair/validator.md)</li><li>[P2P networking](specs/altair/p2p-interface.md)</li></ul></ul> |
 | 2 | **Bellatrix** <br/> (["The Merge"](https://ethereum.org/en/upgrades/merge/)) | TBD | <ul><li>Core</li><ul><li>[Beacon Chain changes](specs/bellatrix/beacon-chain.md)</li><li>[Bellatrix fork](specs/bellatrix/fork.md)</li><li>[Fork choice changes](specs/bellatrix/fork-choice.md)</li></ul><li>Additions</li><ul><li>[Honest validator guide changes](specs/bellatrix/validator.md)</li><li>[P2P networking](specs/bellatrix/p2p-interface.md)</li></ul></ul> |
 
 ### In-development Specifications

setup.py

@@ -934,6 +934,7 @@ class PySpecCommand(Command):
             if self.spec_fork in (ALTAIR, BELLATRIX, CAPELLA, EIP4844):
                 self.md_doc_paths += """
                     specs/altair/light-client/full-node.md
+                    specs/altair/light-client/light-client.md
                     specs/altair/light-client/sync-protocol.md
                     specs/altair/beacon-chain.md
                     specs/altair/bls.md

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

@@ -0,0 +1,37 @@
+# Altair Light Client -- Light Client
+
+**Notice**: This document is a work-in-progress for researchers and implementers.
+
+## Table of contents
+
+<!-- TOC -->
+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+
+- [Introduction](#introduction)
+- [Light client sync process](#light-client-sync-process)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+<!-- /TOC -->
+
+## 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`](./sync-protocol.md#lightclientbootstrap) object for the configured trusted block root. The `bootstrap` object is passed to [`initialize_light_client_store`](./sync-protocol.md#initialize_light_client_store) to obtain a local [`LightClientStore`](./sync-protocol.md#lightclientstore).
+
+4. The light client tracks the sync committee periods `finalized_period` from `store.finalized_header.slot`, `optimistic_period` from `store.optimistic_header.slot`, and `current_period` from `current_slot` based on the local clock.
+
+   1. When `finalized_period == optimistic_period` and [`is_next_sync_committee_known`](./sync-protocol.md#is_next_sync_committee_known) indicates `False`, the light client fetches a [`LightClientUpdate`](./sync-protocol.md#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`](./sync-protocol.md#lightclientfinalityupdate) and [`LightClientOptimisticUpdate`](./sync-protocol.md#lightclientoptimisticupdate). Received objects are passed to [`process_light_client_finality_update`](./sync-protocol.md#process_light_client_finality_update) and [`process_light_client_optimistic_update`](./sync-protocol.md#process_light_client_optimistic_update). This ensures that `finalized_header` and `optimistic_header` reflect the latest blocks.
+
+5. [`process_light_client_store_force_update`](./sync-protocol.md#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.

specs/altair/light-client/sync-protocol.md

@@ -51,6 +51,7 @@ uses sync committees introduced in [this beacon chain extension](./beacon-chain.
 
 Additional documents describe how the light client sync protocol can be used:
 - [Full node](./full-node.md)
+- [Light client](./light-client.md)
 
 ## Constants