Phase0 - Beacon Chain

Question 1

The BeaconState encapsulates all of the information relating to:

Answer 1

• Who the validators are
• Which state the validators are in
• Which chain in the block tree this state belongs to

Question 2

What is the number of shard supported in Ethereum POS

Answer 2

64

Question 3

The ethereum beacon chain supports how many shards per slot?

Answer 3

64

Question 4

To enable state transitions, the beacon chain has a state_transition function which takes as input a _________ and a _______ and returns a new ___________

Answer 4

To enable state transitions, the beacon chain has a state_transition function which takes as input a BeaconState (pre state) and a BeaconBlock and returns a new beacon state (what we call a post state)

source: https://notes.ethereum.org/@djrtwo/Bkn3zpwxB

Question 5

What is the pre state

Answer 5

The beacon state before a state transition

Question 6

What is a post state

Answer 6

The beacon state after a state transition has been applied

Question 7

What are needed for state transition in beacon chain?

Answer 7

• A pre state (Beacon state)
• A beacon block
• State transition function

Question 8

When is the post state of a beacon block considered valid?

Answer 8

> Beginning with the genesis state, the post state of a block is considered valid if it passes all of the guards within the state_transition function

https://notes.ethereum.org/@djrtwo/Bkn3zpwxB

Question 9

What does the fork choice rule do?

Answer 9

> Given a block tree, the fork choice rule provides a single chain (the canonical chain) and resulting state based upon recent messages from validators

> The fork choice rule consumes the block-tree along with the set of most recent attestations from active validators, and identifies a block as the current head of the chain

https://notes.ethereum.org/@djrtwo/Bkn3zpwxB

Question 10

What does LMD GHOST stand for?

Answer 10

Largest Message Driven Greedy Heaviest Observed Sub Tree

Question 11

What is the fork choice rule of the Beacon Chain?

Answer 11

LMD GHOST (“Latest Message Driven Greedy Heaviest-Observed Sub-Tree”),

Question 12

How many attestation can a validator make within a slot within an epoch?

Answer 12

1

Question 13

How long is a slot?

Answer 13

12 seconds

Question 14

How many slots make an epoch?

Answer 14

32 slots

Question 15

How long is an epoch?

Answer 15

12 seconds * 32 = 384 seconds = 6,4 minutes

Question 16

A validator attests to what via their vote?

Answer 16

A beacon block

Question 17

In addition to voting on the canonical chain (via the LMD GHOST algorithm), validators also contribute to deciding what? for blocks.

Answer 17

> In addition to voting on the canonical chain, validators also contribute to deciding finality of blocks. a process that tells us when a Beacon block can be considered final and non-revertible.

source: https://notes.ethereum.org/@djrtwo/Bkn3zpwxB

Question 18

What is the difference between fork choice rule and finality?

Answer 18

> In other words, whereas the fork choice rule allows us to choose a single canonical blockchain through the block tree, finality provides guarantees that certain blocks will remain within the canonical chain.

source: https://notes.ethereum.org/@djrtwo/Bkn3zpwxB

Question 19

What is the algorithm used by the beacon chain for finality called?

Answer 19

A modified version of Casper FFG.

Question 20

What does Casper FFG stand for?

Answer 20

Casper the Friendly Finality Gadget

Question 21

What is a finality gadget

Answer 21

> “Finality gadget” is a component that overlays any Nakamoto-inspired block proposal mechanism and provides deterministic finality with safety robust against network failure (asynchronous safety, Footnote 1). This idea is initially proposed by Casper FFG and now adopted by Polkadot (GRANDPA 5), NEAR (Nightshade Finality Gadget 6), and Concordium (Afgjort 4).

Source: https://ethresear.ch/t/hierarchical-finality-gadget/6829

Question 22

What does Casper FFG provides?

Answer 22

> Casper provides “accountable safety” that certain blocks will always remain in the canonical chain unless some threshold of validators burn their staked capital. This is a “crypto-economic” claim of safety rather than a more traditional safety found in traditional distributed systems consensus algorithms.

source: https://notes.ethereum.org/@djrtwo/Bkn3zpwxB

Question 23

What are the 3 primary responsibility of a validator?

Answer 23

1. Attesting to the beacon chain (once per epoch).
2. Aggregating attestations from validators in the same committee (occasionally).
3. Creating beacon blocks when selected (infrequently).

Question 24

How are validators distributed across committees, slots and epochs?

Answer 24

Validators are grouped into a single committee, per slot per epoch. Each validator attests to a block in a slot. If validator is not block proposer or aggregator, then each validator in the committee attests to the head of the beacon chain (and the recent data of their assigned shard) at their assigned slot.

Question 25

At what points are proposal assignments known?

Answer 25

> At each slot, a single beacon block proposer is selected (via get_beacon_proposer_index) from one of the active validators. Based on the Beacon RNG, committees are randomly shuffled, and proposers are randomly picked. Proposal assignments are only known within the current epoch, but even then, any amount of public lookahead on block production is cause for DoS concern Souce: https://notes.ethereum.org/@djrtwo/Bkn3zpwxB

Question 26

What are the slash-able offence?

Answer 26

- A double proposal - In a double proposal, a selected proposer tries to propose more than one block for their assigned slot - An FFG surround vote - In an FFG surround vote you are casting a vote for a target and source (current and prior checkpoint blocks) that is surrounded by a previous FFG vote that you made. This basically discounts the validity of your previous vote, and is disallowed. For example you voted for slots 32 as a source and 64 as a target in your previous vote, but in the current epoch vote for Slot 0 and Slot 96. Your current vote would surround the previous one - An FFG double vote - In a double vote, you are casting two FFG votes in the same epoch, instead of the allowed one

Question 27

What are the sections of the Beacon chain spec?

Answer 27

- Constants - Configuration - Presets - Data structures - Helper Functions

Question 28

What are Beacon operations?

Answer 28

Beacon operations are data structures that can be added to a BeaconBlock by a block proposer. They form what constitutes the beacon block that is then operated on by the state transition process to create the post beacon state. > They are the means by which new information is introduced into the beacon chain allowing it to update its internal state

Question 29

What are the Beacon Operations that can be found in a Beacon Block?

Answer 29

- ProposerSlashing: For slashing double proposals - AttesterSlashing: For slashing double attestation or surround attestation - Attestation: - Deposit - VoluntryExit

Question 30

What are the components of the BeaconBlock?

Answer 30

> * slot - the slot for which this block is created. Must be greater than the slot of the block defined by parent_root * parent_root - the block root of the parent block * state_root - the hash root of the post state of running the state transition through this block * body - a BeaconBlockBody containing fields for each of the beacon operation objects as well as a few proposer input fields

Question 31

What are the components of the BeaconBlockBody?

Answer 31

BeaconBlockBody contains fields specific to the action of the block on the current state It has eight fields: * randao_reveal - the BLSSignature of the current epoch (by the current block proposer). Constitutes the seed for randomness when mixed in with the other proposers’ reveals. * eth1_data - a vote on recent data from the Eth1 chain. It consists of the following fields: * deposit_root - the SSZ List hash_tree_root of all of the deposits in the deposit contract * deposit_count - the number of successful validator deposits that have been made into the deposit contract so far * block_hash - the eth1 block hash that contains the deposit_root. This block_hash is intended to be used for finalization of the Eth1 chain in the future * graffiti - 32 bytes for validators to decorate as they choose with no defined in-protocol use * proposer_slashings, attester_slashings, attestations, deposits, voluntary_exits are all lists (with max lengths) – one for each operation type – that can be included into the BeaconBlockBody

Question 32

How often does a validator perform its duties? A - Once every epoch B - Once every slot

Answer 32

During each epoch, each validator makes an attestation. Technically once within a slot, every epoch Answer is A

Question 33

What does an attestation contain?

Answer 33

- A hash of what the validator thinks is the "head" of the beacon chain - A hash of what the validator thinks is the correct shard block to include (must be the shard it is assigned to) - Some other hashes (the "source" and "target" in Casper FFG) Used for finality - A signature from the validator, proving that the validator endorses all of the above

Question 34

What fraction of validators are needed to sign a head?

Answer 34

The chain comes to a consensus as a result of these attestations. Roughly speaking, if 2/3 of active validators sign an attestation supporting a block, that block becomes finalized

Question 35

In the forkchoice rule, how are the nodes in the blocktree weighted?

Answer 35

LMD GHOST (“Latest Message Driven Greedy Heaviest-Observed Sub-Tree”), the fork choice rule of Eth2.0, considers which block the latest attestation from each validator points to and uses this to calculate the total balance that recursively attests to each block in the tree. This is done by setting the weight of a node in the tree to be the sum of the balances of the validators whose latest attestations were for that node or any descendent of that node. The GHOST algorithm then starts at the base of the tree, selecting the child of the current node with the heaviest weight until reaching a leaf node. This leaf node is the head of the chain and recursively defines the canonical chain

Question 36

Through which mechanism does new information get added to the beacon state?

Answer 36

Beacon operations are datastructures that can be added to a BeaconBlock by a block proposer. They are the means by which new information is introduced into the beacon chain allowing it to update its internal state.