BLOCKS

Blocks are batches of transactions with a hash of the previous block in the chain. This links blocks together (in a chain) because hashes are cryptographically derived from the block data. This prevents fraud, because one change in any block in history would invalidate all the following blocks as all subsequent hashes would change and everyone running the blockchain would notice.

WHY BLOCKS?

To ensure that all participants on the BTC20 Smart Chain network maintain a synchronized state and agree on the precise history of transactions, we batch transactions into blocks. This means dozens (or hundreds) of transactions are committed, agreed on, and synchronized all at once.

By spacing out commits, we give all network participants enough time to come to consensus: even though transaction requests occur dozens of times per second, blocks are only created and committed on BTC20 Smart Chain once every twelve seconds.

HOW BLOCKS WORK

To preserve the transaction history, blocks are strictly ordered (every new block created contains a reference to its parent block), and transactions within blocks are strictly ordered as well. Except in rare cases, at any given time, all participants on the network are in agreement on the exact number and history of blocks, and are working to batch the current live transaction requests into the next block.

Once a block is put together by a randomly selected validator on the network, it is propagated to the rest of the network; all nodes add this block to the end of their blockchain, and a new validator is selected to create the next block. The exact block-assembly process and commitment/consensus process is currently specified by BTC20 Smart Chain's “proof-of-stake” protocol.

PROOF-OF-STAKE PROTOCOL

Proof-of-stake means the following:

• Validating nodes have to stake 1000 BTCC into a deposit contract as collateral against bad behavior. This helps protect the network because provably dishonest activity leads to some or all of that stake being destroyed.

• In every slot (spaced twelve seconds apart) a validator is randomly selected to be the block proposer. They bundle transactions together, execute them and determine a new 'state'. They wrap this information into a block and pass it around to other validators.

• Other validators who hear about the new block re-execute the transactions to ensure they agree with the proposed change to the global state. Assuming the block is valid, they add it to their own database.

• If a validator hears about two conflicting blocks for the same slot they use their fork-choice algorithm to pick the one supported by the most staked BTCC.

WHAT'S IN A BLOCK?

There is a lot of information contained within a block. At the highest level a block contains the following fields:

Field	Description
slot	the slot the block belongs to
proposer_index	the ID of the validator proposing the block
parent_root	the hash of the preceding block
state_root	the root hash of the state object
body	an object containing several fields, as defined below

The block body contains several fields of its own:

Field	Description
randao_reveal	a value used to select the next block proposer
btcc1_data	information about the deposit contract
graffiti	arbitrary data used to tag blocks
proposer_slashings	list of validators to be slashed
attester_slashings	list of validators to be slashed
attestations	list of attestations in favor of the current block
deposits	list of new deposits to the deposit contract
voluntary_exits	list of validators exiting the network
sync_aggregate	subset of validators used to serve light clients
execution_payload	transactions passed from the execution client

The attestations field contains a list of all the attestations in the block. Attestations have their own data type that contains several pieces of data. Each attestation contains:

Field	Description
aggregation_bits	a list of which validators participated in this attestation
data	a container with multiple subfields
signature	aggregate signature of all attesting validators

The data field in the attestation contains the following:

Field	Description
slot	the slot the attestation relates to
index	indices for attesting validators
beacon_block_root	the root hash of the Beacon block containing this object
source	the last justified checkpoint
target	the latest epoch boundary block

Executing the transactions in the execution_payload updates the global state. All clients re-execute the transactions in the execution_payload to ensure the new state matches that in the new block state_root field. This is how clients can tell that a new block is valid and safe to add to their blockchain. The execution payload itself is an object with several fields. There is also an execution_payload_header that contains important summary information about the execution data. These data structures are organized as follows:

The execution_payload_header contains the following fields:

Field	Description
parent_hash	hash of the parent block
fee_recipient	account address for paying transaction fees to
state_root	root hash for the global state after applying changes in this block
receipts_root	hash of the transaction receipts trie
logs_bloom	data structure containing event logs
prev_randao	value used in random validator selection
block_number	the number of the current block
gas_limit	maximum gas allowed in this block
gas_used	the actual amount of gas used in this block
timestamp	the block time
extra_data	arbitrary additional data as raw bytes
base_fee_per_gas	the base fee value
block_hash	Hash of execution block
transactions_root	root hash of the transactions in the payload
withdrawal_root	root hash of the withdrawals in the payload

The execution_payload itself contains the following (notice this is identical to the header except that instead of the root hash of the transactions it includes the actual list of transactions and withdrawal information) :

Field	Description
parent_hash	hash of the parent block
fee_recipient	account address for paying transaction fees to
state_root	root hash for the global state after applying changes in this block
receipts_root	hash of the transaction receipts trie
logs_bloom	data structure containing event logs
prev_randao	value used in random validator selection
block_number	the number of the current block
gas_limit	maximum gas allowed in this block
gas_used	the actual amount of gas used in this block
timestamp	the block time
extra_data	arbitrary additional data as raw bytes
base_fee_per_gas	the base fee value
block_hash	Hash of execution block
transactions	list of transactions to be executed
withdrawals	list of withdrawal objects

The withdrawals list contains withdrawal objects structured in the following way:

Field	Description
address	account address that has withdrawn
amount	withdrawal amount
index	withdrawal index value
validatorIndex	validator index value

BLOCK TIME

Block time refers to the time separating blocks. In BTC20 Smart Chain, time is divided up into twelve second units called 'slots'. In each slot a single validator is selected to propose a block. Assuming all validators are online and fully functional there will be a block in every slot, meaning the block time is 12s. However, occasionally validators might be offline when called to propose a block, meaning slots can sometimes go empty.

This implementation differs from proof-of-work based systems where block times are probabilistic and tuned by the protocol's target mining difficulty. BTC20 Smart Chain's average block time is a perfect example of this whereby the transition from proof-of-work to proof-of-stake can be clearly inferred based on the consistency of the new 12s block time.

BLOCK SIZE

A final important note is that blocks themselves are bounded in size. Each block has a target size of 15 million gas but the size of blocks will increase or decrease in accordance with network demands, up until the block limit of 30 million gas (2x target block size). The total amount of gas expended by all transactions in the block must be less than the block gas limit. This is important because it ensures that blocks can’t be arbitrarily large. If blocks could be arbitrarily large, then less performant full nodes would gradually stop being able to keep up with the network due to space and speed requirements. The larger the block, the greater the computing power required to process them in time for the next slot. This is a centralizing force, which is resisted by capping block sizes.

FURTHER READING

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