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An overview of attestations
A minimum of 16,384 validators is required to start Ethereum 2.0 If we multiply that with the information included in each Attestation per Epoch, it adds up quickly. Therefore, Ethereum 2.0 aggregates all of that information and minimises the data growth.
So what does that mean for Attestations? Each block one or more committees are chosen to attest. A committee has a minimum of 128 validators, of which 16 are randomly selected to become an aggregator. As shown below, the validators broadcast their unaggregated attestation to the aggregators (red arrow). The aggregators then merge the attestations and forward a single aggregated attestation to the block proposer.
Source: ConsenSys Codefi Analysis
(Validator effective balance * 2**6) / SQRT(Effective balance of all active validators)
At the time when the validators voted on the head of the chain (Block 0),
Block 1was not proposed yet. Therefore attestations naturally get included one block later; so all attestations who voted on
Block 0being the chain head got included in
Block 1and, the inclusion delay is 1.
As shown below, an Inclusion delay of 2 causes the the reward to drop by 50%.
Missing Voting Validator
These validators have a maximum of 1 epoch to submit their attestation. If the attestation was missed in epoch 0, they can submit it with an inclusion delay in epoch 1.
There are 16 Aggregators per epoch in total, additionally, random validators from the beacon-chain subscribe to two subnets for 256 Epochs and serve as a backup in case aggregators are missing.
Note that in some cases a lucky aggregator may also become the block proposer. If the attestation was not included because the block proposer has gone missing, the next block proposer would pick the aggregated attestation up and include it into the next block. However, the inclusion delay will increase by one.