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Data Structures

In this document, we give an introduction to each of the protocol data structures and then explain how to encode these data structures for use in other parts of Filecoin (e.g. network protocols and the blockchain).

Address

An address is an identifier that refers to an actor in the Filecoin state. All actors (miner actors, the storage market actor, account actors) have an address. An address encodes information about:

  • Network this address belongs to
  • Type of data the address contains
  • The data itself
  • Checksum (depending on the type of address)

To learn more, take a look at the address spec. For more detail about the different types of addresses and how they are structured and used, take a look at the address spec.

Block

A block header contains information relevant to a particular point in time over which the network may achieve consensus. The block header contains:

  • The address of the miner that mined the block
  • An array of the tickets that led to this particular miner being selected as the leader for this round (see the Secret Leader Election portion of the Expected Consensus spec for more) and a signature on the winning ticket
  • The set of parent blocks and aggregate chain weight of the parents
  • This block's height
  • Merkle root of the state tree (after applying the messages -- state transitions -- included in this block)
  • Merkle root of the messages (state transitions) in this block
  • Merkle root of the message receipts in this block
  • Timestamp

{{% notice note %}} Note: A block is functionally the same as a block header in the Filecoin protocol. While a block header contains Merkle links to the full system state, messages, and message receipts, a block can be thought of as the full set of this information (not just the Merkle roots, but rather the full data of the state tree, message tree, receipts tree, etc.). Because a full block is quite large, our chain consists of block headers rather than full blocks. We often use the terms block and block header interchangeably. {{% /notice %}}

type BlockHeader struct {
	## Miner is the address of the miner actor that mined this block.
	miner Address

	## Tickets is a chain (possibly singleton) of tickets ending with a winning ticket submitted with this block.
	tickets [Ticket]

	## ElectionProof is generated from a past ticket and proves this miner
	## is a leader at this round
	electionProof ElectionProof

	## Parents is an array of distinct CIDs of parents on which this block was based.
	## Typically one, but can be several in the case where there were multiple winning ticket-
	## holders for a round.
	## The order of parent CIDs is not defined.
	parents [&Block]

	## ParentWeight is the aggregate chain weight of the parent set.
	parentWeight UInt

	## Height is the chain height of this block.
	height UInt

	## StateRoot is a cid pointer to the state tree after application of the
	## transactions state transitions.
	stateRoot &StateTree

	## Messages is the set of messages included in this block. This field is the Cid
	## of the TxMeta object that contains the bls and secpk signed message trees
	messages &TxMeta

	## BLSAggregate is an aggregated BLS signature for all the messages in this block that
	## were signed using BLS signatures
	blsAggregate Signature

	## MessageReceipts is a set of receipts matching to the sending of the `Messages`.
	## This field is the Cid of the root of a sharray of MessageReceipts.
	messageReceipts &[MessageReceipt]

	## The block Timestamp is used to enforce a form of block delay by honest miners.
	## Unix time UTC timestamp (in seconds) stored as an unsigned integer.
	timestamp Timestamp

	## BlockSig is a signature over the hash of the entire block with the miners
	## worker key to ensure that it is not tampered with after creation
	blockSig Signature
} representation tuple

type TxMeta struct {
  blsMessages &[&Message]<Sharray>

	secpkMessages &[&SignedMessage]<Sharray>
} representation tuple

TipSet

For more on TipSets, see the Expected Consensus spec. Implementations may choose not to create a TipSet data structure, instead representing its operations in terms of the underlying blocks.

type TipSet [&BlockHeader]

VRF Personalization

We define VDF personalizations as follow, to enable domain separation across operations that make use of the same VRF (e.g. Ticket and ElectionProof).

Type Prefix
Ticket 0x01
ElectionProof 0x02

Ticket

A ticket is a shared random value stapled to a particular block in the chain. Every miner must produce a new ticket each time they run a leader election attempt. In that sense, every new block produced will have one or more associated tickets (in the case the block took multiple leader election attempts to produce).

We use an EC-VRF per Goldberg et al. with Secp256k1 and sha256 to obtain a deterministic, pseudorandom output.

type Ticket struct {

  ## The VRFProof (pi_string in the RFC) is generated by running our VRF on a past ticket in the ticket chain signed with the miner's keypair.
  ## 97 Bytes long (may be compressible to 80)
  VRFProof Bytes

  ## The VDFResult is derived from the VRFResult of the ticket
  ## It is the value that will be used to generate future tickets or ElectionProofs
  VDFResult Bytes

  ## The VDF proves a delay between tickets generated
  VDFProof Bytes
} representation tuple

Min Ticket/Ticket Comparison

The ticket is a struct. Whenever the protocol draws the ticket or in any way uses ticket values (notably in crafting PoSts or running leader election), what is meant is that the Bytes of the VDFResult are used.

Ticket comparison is done using the VDFResult as an unsigned integer (little endian).

ElectionProof

An election proof is generated from a past ticket (chosen based on public network parameters) by a miner during the leader election process. Its output value determines whether the miner is elected leader and may produce a block. Its inclusion in the block allows other network participants to verify that the block was mined by a valid leader.

type ElectionProof struct {
  ## The VRFProof is generated by running our VRF on a past ticket in the ticket chain signed with the miner's keypair.
  ## 97 Bytes long (may be compressible to 80)
  VRFProof Bytes
}

Message

type Message union {
    | UnsignedMessage 0
    | SignedMessage 1
} representation keyed
type UnsignedMessage struct {
	to   Address
	from Address

	## When receiving a message from a user account the nonce in the message must match the expected
	## nonce in the from actor. This prevents replay attacks.
	nonce UInt

	value UInt

	gasPrice UInt
	gasLimit UInt

	method Uint

	## Serialized parameters to the method
	params Bytes
} representation tuple
type SignedMessage struct {
	message   UnsignedMessage
	signature Signature
} representation tuple

The signature is a serialized signature over the serialized base message. For more details on how the signature itself is done, see the signatures spec.

State Tree

The state tree keeps track of all state in Filecoin. It is a map of addresses to actors in the system. The ActorState is defined in the actors spec.

type StateTree map {ID:Actor}<Hamt>

Message Receipt

type MessageReceipt struct {
	exitCode UInt
	return Bytes
	gasUsed UInt
} representation tuple

Actor

type Actor struct {
	## Cid of the code object for this actor.
	code Cid

	## Reference to the root of this actors state.
	head &ActorState

	## Counter of the number of messages this actor has sent.
	nonce UInt

	## Current balance of filecoin of this actor.
	balance UInt
}

Signature

All signatures in Filecoin come with a type that signifies which key type was used to create the signature.

For more details on signature creation, see the signatures spec.

type Signature union {
	| Secp256k1Signature 0
	| Bls12_381Signature 1
} representation byteprefix

type Secp256k1Signature Bytes
type Bls12_381Signature Bytes

FaultSet

FaultSets are used to denote which sectors failed at which block height.

type FaultSet struct {
	index    UInt
	bitField BitField
}

The index field is a block height offset from the start of the miners proving period (in order to make it more compact).

Basic Types

CID

For most objects referenced by Filecoin, a Content Identifier (CID for short) is used. This is effectively a hash value, prefixed with its hash function (multihash) prepended with a extra labels to inform applications about how to deserialize the given data. CID Spec contains the detailed spec.

Timestamp

type Timestamp UInt

PublicKey

The public key type is simply an array of bytes.

type PublicKey Bytes

BytesAmount

BytesAmount is just a re-typed Integer.

type BytesAmount UInt

PeerId

The serialized bytes of a libp2p peer ID.

Spec incomplete, take a look at this PR: https://github.com/libp2p/specs/pull/100

type PeerId Bytes

Bitfield

Bitfields are a set encoded using a custom run length encoding: RLE+.

type Bitfield Bytes

SectorSet

A sector set stores a mapping of sector IDs to the respective commRs.

type SectorSet map{SectorID:Bytes}

{{% notice todo %}} Improve on this, see https://github.com/filecoin-project/specs/issues/116 {{% /notice %}}

SealProof

SealProof is an opaque, dynamically-sized array of bytes.

PoStProof

PoStProof is an opaque, dynamically-sized array of bytes.

TokenAmount

A type to represent an amount of filecoin tokens.

type TokenAmount UInt

SectorID

Uniquely identifies a miner's sector.

type SectorID uint64

RLE+ Bitset Encoding

RLE+ is a lossless compression format based on RLE. It's primary goal is to reduce the size in the case of many individual bits, where RLE breaks down quickly, while keeping the same level of compression for large sets of contiugous bits.

In tests it has shown to be more compact than RLE iteself, as well as Concise and Roaring.

Format

The format consists of a header, followed by a series of blocks, of which there are three different types.

The format can be expressed as the following BNF grammar.

    <encoding> ::= <header> <blocks>
      <header> ::= <version> <bit>
     <version> ::= "00"
      <blocks> ::= <block> <blocks> | ""
       <block> ::= <block_single> | <block_short> | <block_long>
<block_single> ::= "1"
 <block_short> ::= "01" <bit> <bit> <bit> <bit>
  <block_long> ::= "00" <unsigned_varint>
         <bit> ::= "0" | "1"

An <unsigned_varint> is defined as specified here.

Header

The header indiciates the very first bit of the bit vector to encode. This means the first bit is always the same for the encoded and non encoded form.

Blocks

The blocks represent how many bits, of the current bit type there are. As 0 and 1 alternate in a bit vector the inital bit, which is stored in the header, is enough to determine if a length is currently referencing a set of 0s, or 1s.

Block Single

If the running length of the current bit is only 1, it is encoded as a single set bit.

Block Short

If the running length is less than 16, it can be encoded into up to four bits, which a short block represents. The length is encoded into a 4 bits, and prefixed with 01, to indicate a short block.

Block Long

If the running length is 16 or larger, it is encoded into a varint, and then prefixed with 00 to indicate a long block.

Note: The encoding is unique, so no matter which algorithm for encoding is used, it should produce the same encoding, given the same input.

Bit Numbering

For Filecoin, byte arrays representing RLE+ bitstreams are encoded using LSB 0 bit numbering.

Other Considerations

  • The maximum size of an Object should be 1MB (2^20 bytes). Objects larger than this are invalid.
  • Hashes should use a blake2b-256 multihash.
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