Part 4: Upgrades

Altair

The Altair upgrade took place at 10:56:23 UTC on October the 27th, 2021.

The main goals of the Altair upgrade were to

1. introduce sync committees for supporting light clients,
2. significantly rework the beacon chain reward and penalty accounting, and
3. begin increasing some penalty parameters towards their final values.

The following changes were made for sync committee support.

• New cryptographic domains for the sync committee functions.
• New data structures to support sync committees. Namely, SyncAggregate and SyncCommittee.
• Functions for managing sync committees:
  • get_next_sync_committee_indices();
  • get_next_sync_committee();
  • process_sync_aggregate(), which takes care of reward and penalty accounting for sync committee participation; and
  • process_sync_committee_updates().
• Gossip topics were added to the P2P specification to support sync committee activity

The reforms to the accounting were extensive, and I won't list them all here as they are thoroughly covered elsewhere in the annotated spec and book. But in summary,

• There was a move away from doing all the accounting for attestation inclusion at epoch boundaries to performing much of the work on an ongoing basis during epochs. The epoch transition is quite heavy in any case; this spreads the workload and is simpler overall.
• Incentives were tweaked for different behaviours, such as late attestations and block proposal rewards. We also took the opportunity to simplify the rewards and penalties calculations.
• The inactivity leak was changed so that it is now applied on a per-validator basis rather than globally.

As for the penalty parameters, the following parameters were updated. These had been softened at genesis as we got used to running the beacon chain:

• INACTIVITY_PENALTY_QUOTIENT decreased from $2^{26}$ to $3 \times 2^{24}$. This reduces stakes more quickly during an inactivity leak.
• MIN_SLASHING_PENALTY_QUOTIENT decreased from 128 to 64. This sets the initial slashing penalty to 0.5 ETH for a validator with a full stake rather than the 0.25 ETH of Phase 0.
• PROPORTIONAL_SLASHING_MULTIPLIER increased from 1 to 2 so that, in the event of over one-third of validators being slashed together, the full penalty would be the removal of two-thirds of their stakes rather than the one-third of Phase 0.

The full description of the changes between Phase 0 and Altair is in the Altair specs.