ISAAC-draft.md

Abstract

Status of This Draft

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Table of Contents

Introduction

ISAAC Protocol

The ISAAC is a consensus protocol based on PBFT. In the ISAAC, all nodes will vote two times to determine which transactions to include in this block. For each voting unit, the nodes have the role proposer or validator. The proposer gathers the transactions for this vote and puts them in a ballot and passes it to the validators. The validator checks that the transactions in the ballot are valid and then votes. If the voting determines that the ballot is valid, all nodes confirm the transactions in the ballot.

State Diagram

                                      'Height + 1'
  ┌──────────────────────────────────────────────────────────────────────────────────┐
  ╎                                                                                  ╎
  ╎                                                                                  ╎
  ╎    'get ballot'               'get 67% of YES'           'get 67% of YES'        ╎
  ▼                                                                                  ╎
 INIT ────────────────────> SIGN ──────────────────> ACCEPT ──────────────────> ALL_CONFIRM
  ▲                          ╎                         ╎
  ╎                          ╎                         ╎
  ╎  'get 34% of NO or EXP'  ╎  'get 34% of NO or EXP' ╎
  ╎                          ╎                         ╎
  └────────────────────────────────────────────────────┘
                       'Round + 1'

ISAAC States

1. INIT - Proposed or received a ballot with transactions.

2. SIGN - Checked that the ballot is valid by itself.

3. ACCEPT - Checked that the ballot is valid by validators.

4. ALL_CONFIRM - Confirmed block with the transactions in the ballot.

Voting Hole

1. YES - Agreed

2. NO - Disagreed

3. EXP - Expired

Terms

• TIMEOUT_INIT - The timeout for INIT state. The default value is 2 sec.

• TIMEOUT_SIGN - The timeout for SIGN state. The default value is 2 sec.

• TIMEOUT_ACCEPT - The timeout for ACCEPT state. The default value is 2 sec.

• B(ISAAC State, Voting Hole) - A ballot with ISAAC state and voting hole.

Examples

• B(INIT, YES) - A ballot with ISAAC state INIT and voting hole YES.

• B(SIGN, NO) - A ballot with ISAAC state INIT and voting hole NO.

• B(ACCEPT, EXP) - A ballot with ISAAC state INIT and voting hole EXP.

Voting Process

Network Start

• At the beginning of the network, the genesis block is saved with block height 1 and round 0.
• The node start with INIT state, height 2 and round 0.

INIT

1. The timer is set to TIMEOUT_INIT.
2. The steps to propose transactions are as follows.
   • If the node is proposer of this round, it broadcasts a B(INIT, YES),
     • The ballot includes valid transactions(only hashes) or empty(If the transaction pool is empty).
     • When the node broadcasts the ballot, the node goes to the next state.
   • If the node is not proposer, it waits until receiving the B(INIT, YES).
     • When it receives the ballot, the node goes to the next state.
   • When the timer expires, the node goes to the next state.

SIGN

1. The timer is reset to TIMEOUT_SIGN.
2. The node checks the proposed ballot is valid.
   • If the proposed ballot is valid(or empty), the node broadcasts B(SIGN, YES).
   • If the proposed ballot is invalid, the node broadcasts B(SIGN, NO).
   • When the timer expires, the node hasn't sent a ballot in SIGN state yet, it broadcasts B(SIGN, EXP).
3. Each node receives ballots and goes to the next state after,
   • the number of B(SIGN, YES) is greater than or equal to 67% of validators, the node broadcasts B(ACCEPT, YES).
   • the number of B(SIGN, NO) or B(SIGN, EXP) is greater than 33% of validators, the node goes back to INIT state with same height and round + 1.

ACCEPT

1. The timer is reset to TIMEOUT_ACCEPT.
2. When the timer expires, the node hasn't sent a ballot in ACCEPT state yet, it broadcasts B(ACCEPT, EXP).
3. Each node receives ballots and,
   • if the number of B(ACCEPT, YES) is greater than or equal to 67% of validators, the node goes to the next state.
   • if the number of B(ACCEPT, NO) or B(ACCEPT, EXP) is greater than 33% of validators, the node goes back to INIT state with same height and round + 1.

ALL-CONFIRM

1. the node confirms and saves the block with proposed transactions even though it is empty.
2. the node filters invalid transactions in the transaction pool.
3. It waits for block time buffer.
4. It goes to INIT state with height + 1 and round 0.

Types

Ballot

// Before being transmitted, the ballot is converted to a []byte for efficiency by the json.Marshal (v interface {}) method of the golang built-in package encoding/json.
type Ballot struct {
	H BallotHeader
	B BallotBody
}

type BallotHeader struct {
	Version           string // version of `BallotBody`
	Hash              string // hash of `BallotBody`
	Signature         string // signed by source node of <networkID> + `Hash`
	ProposerSignature string // signed by proposer of <networkID> + `Hash` of `BallotBodyProposed`
}

type BallotBody struct {
	Confirmed string            // created time, ISO8601
	Proposed  BallotBodyProposed
	Source    string            // Sender Address
	State     State             // ISAAC States
	Vote      voting.Hole       // Voting Result
}

type BallotBodyProposed struct {
	Confirmed           string              // created time, ISO8601
	Proposer            string
	VotingBasis         Basis
	Transactions        []string            // Transaction Hashes
	ProposerTransaction ProposerTransaction // Transaction always included in the block
}

type Basis struct {
	Round     uint64 // round sequence number
	Height    uint64 // last block height
	BlockHash string // hash of last block
	TotalTxs  uint64
	TotalOps  uint64
}

Signing

Purpose

All ballots and transactions are signed for verifying sender with the KP.Sign() method of the "github.com/stellar/go/keypair" package.

Sign

func MakeSignature(kp stellar.KP, networkID []byte, hash string) ([]byte, error)

It returns signature which is used for verifying contents.

Verify

func (kp stellar.KP) Verify(input []byte, signature []byte) error

Usually, input includes NetworkID and Hash. Therefore, we can verify that this signature is valid with keypair's publicKey, networkID, and hash.