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Ethereum 2.0 Altair fork

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

Introduction
Configuration
Fork to Altair
- Fork trigger
- Upgrading the state

Introduction

This document describes the process of the first upgrade of Ethereum 2.0: the Altair hard fork, introducing light client support and other improvements.

Configuration

Warning: this configuration is not definitive.

Name	Value
`ALTAIR_FORK_VERSION`	`Version('0x01000000')`
`ALTAIR_FORK_EPOCH`	`Epoch(18446744073709551615)` TBD

Fork to Altair

Fork trigger

TBD. Social consensus, along with state conditions such as epoch boundary, finality, deposits, active validator count, etc. may be part of the decision process to trigger the fork. For now we assume the condition will be triggered at epoch ALTAIR_FORK_EPOCH.

Note that for the pure Altair networks, we don't apply upgrade_to_altair since it starts with Altair version logic.

Upgrading the state

After process_slots of Phase 0 finishes, if state.slot % SLOTS_PER_EPOCH == 0 and compute_epoch_at_slot(state.slot) == ALTAIR_FORK_EPOCH, an irregular state change is made to upgrade to Altair.

def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
    epoch = phase0.get_current_epoch(pre)
    post = BeaconState(
        # Versioning
        genesis_time=pre.genesis_time,
        genesis_validators_root=pre.genesis_validators_root,
        slot=pre.slot,
        fork=Fork(
            previous_version=pre.fork.current_version,
            current_version=ALTAIR_FORK_VERSION,
            epoch=epoch,
        ),
        # History
        latest_block_header=pre.latest_block_header,
        block_roots=pre.block_roots,
        state_roots=pre.state_roots,
        historical_roots=pre.historical_roots,
        # Eth1
        eth1_data=pre.eth1_data,
        eth1_data_votes=pre.eth1_data_votes,
        eth1_deposit_index=pre.eth1_deposit_index,
        # Registry
        validators=pre.validators,
        balances=pre.balances,
        # Randomness
        randao_mixes=pre.randao_mixes,
        # Slashings
        slashings=pre.slashings,
        # Participation
        previous_epoch_participation=[ParticipationFlags(0b0000_0000) for _ in range(len(pre.validators))],
        current_epoch_participation=[ParticipationFlags(0b0000_0000) for _ in range(len(pre.validators))],
        # Finality
        justification_bits=pre.justification_bits,
        previous_justified_checkpoint=pre.previous_justified_checkpoint,
        current_justified_checkpoint=pre.current_justified_checkpoint,
        finalized_checkpoint=pre.finalized_checkpoint,
        # Inactivity
        inactivity_scores=[uint64(0) for _ in range(len(pre.validators))],
    )
    # Fill in sync committees
    post.current_sync_committee = get_sync_committee(post, get_current_epoch(post))
    post.next_sync_committee = get_sync_committee(post, get_current_epoch(post) + EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
    return post

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