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eip title description author discussions-to status type category created requires
4844
Shard Blob Transactions
Shard Blob Transactions scale data-availability of Ethereum in a simple, forwards-compatible manner.
Vitalik Buterin (@vbuterin), Dankrad Feist (@dankrad), Diederik Loerakker (@protolambda), George Kadianakis (@asn-d6), Matt Garnett (@lightclient), Mofi Taiwo (@Inphi), Ansgar Dietrichs (@adietrichs)
Review
Standards Track
Core
2022-02-25
1559, 2718, 2930, 4895

Abstract

Introduce a new transaction format for "blob-carrying transactions" which contain a large amount of data that cannot be accessed by EVM execution, but whose commitment can be accessed. The format is intended to be fully compatible with the format that will be used in full sharding.

Motivation

Rollups are in the short and medium term, and possibly in the long term, the only trustless scaling solution for Ethereum. Transaction fees on L1 have been very high for months and there is greater urgency in doing anything required to help facilitate an ecosystem-wide move to rollups. Rollups are significantly reducing fees for many Ethereum users: Optimism and Arbitrum frequently provide fees that are ~3-8x lower than the Ethereum base layer itself, and ZK rollups, which have better data compression and can avoid including signatures, have fees ~40-100x lower than the base layer.

However, even these fees are too expensive for many users. The long-term solution to the long-term inadequacy of rollups by themselves has always been data sharding, which would add ~16 MB per block of dedicated data space to the chain that rollups could use. However, data sharding will still take a considerable amount of time to finish implementing and deploying.

This EIP provides a stop-gap solution until that point by implementing the transaction format that would be used in sharding, but not actually sharding those transactions. Instead, the data from this transaction format is simply part of the beacon chain and is fully downloaded by all consensus nodes (but can be deleted after only a relatively short delay). Compared to full data sharding, this EIP has a reduced cap on the number of these transactions that can be included, corresponding to a target of ~0.25 MB per block and a limit of ~0.5 MB.

Specification

Parameters

Constant Value
BLOB_TX_TYPE Bytes1(0x05)
FIELD_ELEMENTS_PER_BLOB 4096
BLS_MODULUS 52435875175126190479447740508185965837690552500527637822603658699938581184513
BLOB_COMMITMENT_VERSION_KZG Bytes1(0x01)
POINT_EVALUATION_PRECOMPILE_ADDRESS Bytes20(0x14)
POINT_EVALUATION_PRECOMPILE_GAS 50000
MAX_DATA_GAS_PER_BLOCK 2**19
TARGET_DATA_GAS_PER_BLOCK 2**18
MIN_DATA_GASPRICE 1
DATA_GASPRICE_UPDATE_FRACTION 2225652
MAX_VERSIONED_HASHES_LIST_SIZE 2**24
MAX_CALLDATA_SIZE 2**24
MAX_ACCESS_LIST_SIZE 2**24
MAX_ACCESS_LIST_STORAGE_KEYS 2**24
MAX_TX_WRAP_KZG_COMMITMENTS 2**12
LIMIT_BLOBS_PER_TX 2**12
DATA_GAS_PER_BLOB 2**17
HASH_OPCODE_BYTE Bytes1(0x49)
HASH_OPCODE_GAS 3

Type aliases

Type Base type Additional checks
BLSFieldElement uint256 x < BLS_MODULUS
Blob Vector[BLSFieldElement, FIELD_ELEMENTS_PER_BLOB]
VersionedHash Bytes32
KZGCommitment Bytes48 Same as BLS standard "is valid pubkey" check but also allows 0x00..00 for point-at-infinity
KZGProof Bytes48 Same as for KZGCommitment

Cryptographic Helpers

Throughout this proposal we use cryptographic methods and classes defined in the corresponding consensus 4844 specs.

Specifically, we use the following methods from polynomial-commitments.md:

Helpers

def kzg_to_versioned_hash(kzg: KZGCommitment) -> VersionedHash:
    return BLOB_COMMITMENT_VERSION_KZG + sha256(kzg)[1:]

Approximates factor * e ** (numerator / denominator) using Taylor expansion:

def fake_exponential(factor: int, numerator: int, denominator: int) -> int:
    i = 1
    output = 0
    numerator_accum = factor * denominator
    while numerator_accum > 0:
        output += numerator_accum
        numerator_accum = (numerator_accum * numerator) // (denominator * i)
        i += 1
    return output // denominator

New transaction type

We introduce a new EIP-2718 transaction type, with the format being the single byte BLOB_TX_TYPE followed by an SSZ encoding of the SignedBlobTransaction container comprising the transaction contents:

class SignedBlobTransaction(Container):
    message: BlobTransaction
    signature: ECDSASignature

class BlobTransaction(Container):
    chain_id: uint256
    nonce: uint64
    max_priority_fee_per_gas: uint256
    max_fee_per_gas: uint256
    gas: uint64
    to: Union[None, Address] # Address = Bytes20
    value: uint256
    data: ByteList[MAX_CALLDATA_SIZE]
    access_list: List[AccessTuple, MAX_ACCESS_LIST_SIZE]
    max_fee_per_data_gas: uint256
    blob_versioned_hashes: List[VersionedHash, MAX_VERSIONED_HASHES_LIST_SIZE]

class AccessTuple(Container):
    address: Address # Bytes20
    storage_keys: List[Hash, MAX_ACCESS_LIST_STORAGE_KEYS]

class ECDSASignature(Container):
    y_parity: boolean
    r: uint256
    s: uint256

The max_priority_fee_per_gas and max_fee_per_gas fields follow EIP-1559 semantics, and access_list as in EIP-2930.

EIP-2718 is extended with a "wrapper data", the typed transaction can be encoded in two forms, dependent on the context:

  • Network (default): TransactionType || TransactionNetworkPayload, or LegacyTransaction
  • Minimal (as in execution payload): TransactionType || TransactionPayload, or LegacyTransaction

Execution-payloads / blocks use the minimal encoding of transactions. In the transaction-pool and local transaction-journal the network encoding is used.

For previous types of transactions the network encoding is no different, i.e. TransactionNetworkPayload == TransactionPayload.

The TransactionNetworkPayload wraps a TransactionPayload with additional data: When a blob transaction is passed through the network, the TransactionNetworkPayload version of the transaction also includes blobs and kzgs (commitments list). This wrapping data SHOULD be verified directly before or after signature verification (see the Networking section below).

There are also new validity conditions for blocks with respect to this transaction type, both on the consensus-layer and the execution-layer side. See the Consensus-layer validation and the Execution-layer validation sections below for details.

The signature is verified and tx.origin is calculated as follows:

def unsigned_tx_hash(tx: SignedBlobTransaction) -> Bytes32:
    # The pre-image is prefixed with the transaction-type to avoid hash collisions with other tx hashers and types
    return keccak256(BLOB_TX_TYPE + ssz.serialize(tx.message))

def get_origin(tx: SignedBlobTransaction) -> Address:
    sig = tx.signature
    # v = int(y_parity) + 27, same as EIP-1559
    return ecrecover(unsigned_tx_hash(tx), int(sig.y_parity)+27, sig.r, sig.s)

The hash of a signed blob transaction should be computed as:

def signed_tx_hash(tx: SignedBlobTransaction) -> Bytes32:
    return keccak256(BLOB_TX_TYPE + ssz.serialize(tx))

Header extension

The current header encoding is extended with a new 256-bit unsigned integer field excess_data_gas. This is the running total of excess data gas consumed on chain since this EIP was activated. If the total amount of data gas is below the target, excess_data_gas is capped at zero.

The resulting RLP encoding of the header is therefore:

rlp([
    parent_hash,
    0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347, # ommers hash
    coinbase,
    state_root,
    txs_root,
    receipts_root,
    logs_bloom,
    0, # difficulty
    number,
    gas_limit,
    gas_used,
    timestamp,
    extradata,
    prev_randao,
    0x0000000000000000, # nonce
    base_fee_per_gas,
    withdrawals_root,
    excess_data_gas
])

The value of excess_data_gas can be calculated using the parent header and number of blobs in the block.

def calc_excess_data_gas(parent: Header, new_blobs: int) -> int:
    consumed_data_gas = new_blobs * DATA_GAS_PER_BLOB
    if parent.excess_data_gas + consumed_data_gas < TARGET_DATA_GAS_PER_BLOCK:
        return 0
    else:
        return parent.excess_data_gas + consumed_data_gas - TARGET_DATA_GAS_PER_BLOCK

For the first post-fork block, parent.excess_data_gas is evaluated as 0.

Opcode to get versioned hashes

We add an opcode DATAHASH (with byte value HASH_OPCODE_BYTE) which reads index from the top of the stack as big-endian uint256, and replaces it on the stack with tx.message.blob_versioned_hashes[index] if index < len(tx.message.blob_versioned_hashes), and otherwise with a zeroed bytes32 value. The opcode has a gas cost of HASH_OPCODE_GAS.

Point evaluation precompile

Add a precompile at POINT_EVALUATION_PRECOMPILE_ADDRESS that verifies a KZG proof which claims that a blob (represented by a commitment) evaluates to a given value at a given point.

The precompile costs POINT_EVALUATION_PRECOMPILE_GAS and executes the following logic:

def point_evaluation_precompile(input: Bytes) -> Bytes:
    """
    Verify p(z) = y given commitment that corresponds to the polynomial p(x) and a KZG proof.
    Also verify that the provided commitment matches the provided versioned_hash.
    """
    # The data is encoded as follows: versioned_hash | z | y | commitment | proof |
    versioned_hash = input[:32]
    z = input[32:64]
    y = input[64:96]
    commitment = input[96:144]
    kzg_proof = input[144:192]

    # Verify commitment matches versioned_hash
    assert kzg_to_versioned_hash(commitment) == versioned_hash

    # Verify KZG proof
    assert verify_kzg_proof(commitment, z, y, kzg_proof)

    # Return FIELD_ELEMENTS_PER_BLOB and BLS_MODULUS as padded 32 byte big endian values
    return Bytes(U256(FIELD_ELEMENTS_PER_BLOB).to_be_bytes32() + U256(BLS_MODULUS).to_be_bytes32())

The precompile MUST reject non-canonical field elements (i.e. provided field elements MUST be strictly less than BLS_MODULUS).

Gas accounting

We introduce data gas as a new type of gas. It is independent of normal gas and follows its own targeting rule, similar to EIP-1559. We use the excess_data_gas header field to store persistent data needed to compute the data gas price. For now, only blobs are priced in data gas.

def calc_data_fee(tx: SignedBlobTransaction, parent: Header) -> int:
    return get_total_data_gas(tx) * get_data_gasprice(header)

def get_total_data_gas(tx: SignedBlobTransaction) -> int:
    return DATA_GAS_PER_BLOB * len(tx.message.blob_versioned_hashes)

def get_data_gasprice(header: Header) -> int:
    return fake_exponential(
        MIN_DATA_GASPRICE,
        header.excess_data_gas,
        DATA_GASPRICE_UPDATE_FRACTION
    )

The block validity conditions are modified to include data gas checks (see the Execution-layer validation section below).

The actual data_fee as calculated via calc_data_fee is deducted from the sender balance before transaction execution and burned, and is not refunded in case of transaction failure.

Consensus-layer validation

On the consensus-layer the blobs are referenced, but not fully encoded, in the beacon block body. Instead of embedding the full contents in the body, the contents of the blobs are propagated separately, as "sidecars".

This "sidecar" design provides forward compatibility for further data increases by black-boxing is_data_available(): with full sharding is_data_available() can be replaced by data-availability-sampling (DAS) thus avoiding all blobs being downloaded by all beacon nodes on the network.

Note that the consensus-layer is tasked with persisting the blobs for data availability, the execution-layer is not.

The ethereum/consensus-specs repository defines the following consensus-layer changes involved in this EIP:

  • Beacon chain: process updated beacon blocks and ensure blobs are available.
  • P2P network: gossip and sync updated beacon block types and new blob sidecars.
  • Honest validator: produce beacon blocks with blobs; sign and publish the associated blob sidecars.

Execution-layer validation

On the execution-layer, the block validity conditions are extended as follows:

def validate_block(block: Block) -> None:
    ...

    block_data_gas = 0

    for tx in block.transactions:
        ...

        if type(tx) is SignedBlobTransaction:

            # there must be at least one blob
            assert len(tx.message.blob_versioned_hashes) > 0

            # the signer must be able to afford the transaction
            assert signer(tx).balance >= tx.message.gas * tx.message.max_fee_per_gas + get_total_data_gas(tx) * tx.message.max_fee_per_data_gas

            # ensure that the user was willing to at least pay the current data gasprice
            assert tx.message.max_fee_per_data_gas >= get_data_gasprice(parent(block).header)

            # keep track of total data gas spent in the block
            block_data_gas += get_total_data_gas(tx)

    # ensure the total data gas spent is at most equal to the limit
    assert block_data_gas <= MAX_DATA_GAS_PER_BLOCK

Networking

Nodes must not automatically broadcast blob transactions to their peers. Instead, those transactions are only announced using NewPooledTransactionHashes messages, and can then be manually requested via GetPooledTransactions.

Transactions are presented as TransactionType || TransactionNetworkPayload on the execution layer network, the payload is a SSZ encoded container:

class BlobTransactionNetworkWrapper(Container):
    tx: SignedBlobTransaction
    # KZGCommitment = Bytes48
    blob_kzgs: List[KZGCommitment, MAX_TX_WRAP_KZG_COMMITMENTS]
    # BLSFieldElement = uint256
    blobs: List[Vector[BLSFieldElement, FIELD_ELEMENTS_PER_BLOB], LIMIT_BLOBS_PER_TX]
    # KZGProof = Bytes48
    kzg_aggregated_proof: KZGProof

We do network-level validation of BlobTransactionNetworkWrapper objects as follows:

def validate_blob_transaction_wrapper(wrapper: BlobTransactionNetworkWrapper):
    versioned_hashes = wrapper.tx.message.blob_versioned_hashes
    commitments = wrapper.blob_kzgs
    blobs = wrapper.blobs
    # note: assert blobs are not malformatted
    assert 0 < len(versioned_hashes) == len(commitments) == len(blobs)

    # ensure the blobs can fit within the current block data gas limit
    assert get_total_data_gas(wrapper.tx) <= MAX_DATA_GAS_PER_BLOCK

    # Verify that commitments match the blobs by checking the KZG proof
    assert verify_aggregate_kzg_proof(blobs, commitments, wrapper.kzg_aggregated_proof)

    # Now that all commitments have been verified, check that versioned_hashes matches the commitments
    for versioned_hash, commitment in zip(versioned_hashes, commitments):
        assert versioned_hash == kzg_to_versioned_hash(commitment)

Rationale

On the path to sharding

This EIP introduces blob transactions in the same format in which they are expected to exist in the final sharding specification. This provides a temporary but significant scaling relief for rollups by allowing them to initially scale to 0.25 MB per slot, with a separate fee market allowing fees to be very low while usage of this system is limited.

The core goal of rollup scaling stopgaps is to provide temporary scaling relief, without imposing extra development burdens on rollups to take advantage of this relief. Today, rollups use calldata. In the future, rollups will have no choice but to use sharded data (also called "blobs") because sharded data will be much cheaper. Hence, rollups cannot avoid making a large upgrade to how they process data at least once along the way. But what we can do is ensure that rollups need to only upgrade once. This immediately implies that there are exactly two possibilities for a stopgap: (i) reducing the gas costs of existing calldata, and (ii) bringing forward the format that will be used for sharded data, but not yet actually sharding it. Previous EIPs were all a solution of category (i); this EIP is a solution of category (ii).

The main tradeoff in designing this EIP is that of implementing more now versus having to implement more later: do we implement 25% of the work on the way to full sharding, or 50%, or 75%?

The work that is already done in this EIP includes:

  • A new transaction type, of the exact same format that will need to exist in "full sharding"
  • All of the execution-layer logic required for full sharding
  • All of the execution / consensus cross-verification logic required for full sharding
  • Layer separation between BeaconBlock verification and data availability sampling blobs
  • Most of the BeaconBlock logic required for full sharding
  • A self-adjusting independent gasprice for blobs

The work that remains to be done to get to full sharding includes:

  • A low-degree extension of the blob_kzgs in the consensus layer to allow 2D sampling
  • An actual implementation of data availability sampling
  • PBS (proposer/builder separation), to avoid requiring individual validators to process 32 MB of data in one slot
  • Proof of custody or similar in-protocol requirement for each validator to verify a particular part of the sharded data in each block

This EIP also sets the stage for longer-term protocol cleanups:

  • It adds an SSZ transaction type, and paves the precedent that all new transaction types should be SSZ
  • It defines TransactionNetworkPayload to separate network and block encodings of a transaction type
  • Its (cleaner) gas price update rule could be applied to the primary basefee

How rollups would function

Instead of putting rollup block data in transaction calldata, rollups would expect rollup block submitters to put the data into blobs. This guarantees availability (which is what rollups need) but would be much cheaper than calldata. Rollups need data to be available once, long enough to ensure honest actors can construct the rollup state, but not forever.

Optimistic rollups only need to actually provide the underlying data when fraud proofs are being submitted. The fraud proof can verify the transition in smaller steps, loading at most a few values of the blob at a time through calldata. For each value it would provide a KZG proof and use the point evaluation precompile to verify the value against the versioned hash that was submitted before, and then perform the fraud proof verification on that data as is done today.

ZK rollups would provide two commitments to their transaction or state delta data: the kzg in the blob and some commitment using whatever proof system the ZK rollup uses internally. They would use a commitment proof of equivalence protocol, using the point evaluation precompile, to prove that the kzg (which the protocol ensures points to available data) and the ZK rollup's own commitment refer to the same data.

Versioned hashes & precompile return data

We use versioned hashes (rather than kzgs) as references to blobs in the execution layer to ensure forward compatibility with future changes. For example, if we need to switch to Merkle trees + STARKs for quantum-safety reasons, then we would add a new version, allowing the point evaluation precompile to work with the new format. Rollups would not have to make any EVM-level changes to how they work; sequencers would simply have to switch over to using a new transaction type at the appropriate time.

However, the point evaluation happens inside a finite field, and it is only well defined if the field modulus is known. Smart contracts could contain a table mapping the commitment version to a modulus, but this would not allow smart contract to take into account future upgrades to a modulus that is not known yet. By allowing access to the modulus inside the EVM, the smart contract can be built so that it can use future commitments and proofs, without ever needing an upgrade.

In the interest of not adding another precompile, we return the modulus and the polynomial degree directly from the point evaluation precompile. It can then be used by the caller. It is also "free" in that the caller can just ignore this part of the return value without incurring an extra cost -- systems that remain upgradable for the foreseeable future will likely use this route for now.

Data gasprice update rule

The data gasprice update rule is intended to approximate the formula data_gasprice = MIN_DATA_GASPRICE * e**(excess_data_gas / DATA_GASPRICE_UPDATE_FRACTION), where excess_data_gas is the total "extra" amount of data gas that the chain has consumed relative to the "targeted" number (TARGET_DATA_GAS_PER_BLOCK per block). Like EIP-1559, it's a self-correcting formula: as the excess goes higher, the data_gasprice increases exponentially, reducing usage and eventually forcing the excess back down.

The block-by-block behavior is roughly as follows. If block N consumes X data gas, then in block N+1 excess_data_gas increases by X - TARGET_DATA_GAS_PER_BLOCK, and so the data_gasprice of block N+1 increases by a factor of e**((X - TARGET_DATA_GAS_PER_BLOCK) / DATA_GASPRICE_UPDATE_FRACTION). Hence, it has a similar effect to the existing EIP-1559, but is more "stable" in the sense that it responds in the same way to the same total usage regardless of how it's distributed.

The parameter DATA_GASPRICE_UPDATE_FRACTION controls the maximum rate of change of the blob gas price. It is chosen to target a maximum change rate of e(TARGET_DATA_GAS_PER_BLOCK / DATA_GASPRICE_UPDATE_FRACTION) ≈ 1.125 per block.

Throughput

The values for TARGET_DATA_GAS_PER_BLOCK and MAX_DATA_GAS_PER_BLOCK are chosen to correspond to a target of 2 blobs (0.25 MB) and maximum of 4 blobs (0.5 MB) per block. These small initial limits are intended to minimize the strain on the network created by this EIP and are expected to be increased in future upgrades as the network demonstrates reliability under larger blocks.

Backwards Compatibility

Blob non-accessibility

This EIP introduces a transaction type that has a distinct mempool version (BlobTransactionNetworkWrapper) and execution-payload version (SignedBlobTransaction), with only one-way convertibility between the two. The blobs are in the BlobTransactionNetworkWrapper and not in the SignedBlobTransaction; instead, they go into the BeaconBlockBody. This means that there is now a part of a transaction that will not be accessible from the web3 API.

Mempool issues

Blob transactions have a large data size at the mempool layer, which poses a mempool DoS risk, though not an unprecedented one as this also applies to transactions with large amounts of calldata.

By only broadcasting announcements for blob transactions, receiving nodes will have control over which and how many transactions to receive, allowing them to throttle throughput to an acceptable level. EIP-5793 will give further fine-grained control to nodes by extending the NewPooledTransactionHashes announcement messages to include the transaction type and size.

In addition, we recommend including a 1.1x data gasprice bump requirement to the mempool transaction replacement rules.

Test Cases

TBD

Security Considerations

This EIP increases the storage requirements per Beacon block by a maximum of ~0.5 MB. This is 4x larger than the theoretical maximum size of a block today (30M gas / 16 gas per calldata byte = 1.875M bytes), and so it will not greatly increase worst-case bandwidth. Post-merge, block times are expected to be static rather than an unpredictable Poisson distribution, giving a guaranteed period of time for large blocks to propagate.

The sustained load of this EIP is much lower than alternatives that reduce calldata costs, even if the calldata is limited, because there is no existing software that stores the blobs indefinitely and there is no expectation that they need to be stored for as long as an execution payload. This makes it easier to implement a policy that these blobs should be deleted after e.g. 30-60 days, a much shorter delay compared to proposed (but yet to be implemented) one-year rotation times for execution payload history.

Copyright

Copyright and related rights waived via CC0.