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49 lines
2.6 KiB
Markdown
49 lines
2.6 KiB
Markdown
# BABE (Blind Assignment for Blockchain Extension)
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BABE is a slot-based block production mechanism which uses a VRF PRNG to
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randomly perform the slot allocation. On every slot, all the authorities
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generate a new random number with the VRF function and if it is lower than a
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given threshold (which is proportional to their weight/stake) they have a
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right to produce a block. The proof of the VRF function execution will be
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used by other peer to validate the legitimacy of the slot claim.
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The engine is also responsible for collecting entropy on-chain which will be
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used to seed the given VRF PRNG. An epoch is a contiguous number of slots
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under which we will be using the same authority set. During an epoch all VRF
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outputs produced as a result of block production will be collected on an
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on-chain randomness pool. Epoch changes are announced one epoch in advance,
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i.e. when ending epoch N, we announce the parameters (randomness,
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authorities, etc.) for epoch N+2.
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Since the slot assignment is randomized, it is possible that a slot is
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assigned to multiple validators in which case we will have a temporary fork,
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or that a slot is assigned to no validator in which case no block is
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produced. Which means that block times are not deterministic.
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The protocol has a parameter `c` [0, 1] for which `1 - c` is the probability
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of a slot being empty. The choice of this parameter affects the security of
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the protocol relating to maximum tolerable network delays.
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In addition to the VRF-based slot assignment described above, which we will
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call primary slots, the engine also supports a deterministic secondary slot
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assignment. Primary slots take precedence over secondary slots, when
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authoring the node starts by trying to claim a primary slot and falls back
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to a secondary slot claim attempt. The secondary slot assignment is done
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by picking the authority at index:
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`blake2_256(epoch_randomness ++ slot_number) % authorities_len`.
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The secondary slots supports either a `SecondaryPlain` or `SecondaryVRF`
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variant. Comparing with `SecondaryPlain` variant, the `SecondaryVRF` variant
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generates an additional VRF output. The output is not included in beacon
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randomness, but can be consumed by parachains.
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The fork choice rule is weight-based, where weight equals the number of
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primary blocks in the chain. We will pick the heaviest chain (more primary
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blocks) and will go with the longest one in case of a tie.
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An in-depth description and analysis of the protocol can be found here:
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<https://research.web3.foundation/en/latest/polkadot/block-production/Babe.html>
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License: GPL-3.0-or-later WITH Classpath-exception-2.0
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