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nomos-specs/carnot/spec.md

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Carnot Specification

This is the pseudocode specification of the Carnot consensus algorithm. In this specification we will omit any cryptographic material, block validity and proof checks. A real implementation is expected to check those before hitting this code. In addition, all types can be expected to have their invariants checked by the type contract, e.g. in an instance of type Qc::Aggregate the high_qc field is always the most recent qc among the aggregate qcs and the code can skip this check.

'Q:' is used to indicate unresolved questions. Notation is loosely based on CDDL.

Messages

A critical piece in the protocol, these are the different kind of messages used by participants during the protocol execution.

  • Block: propose a new block
  • Vote: vote for a block proposal
  • NewView: propose to jump to a new view after a proposal for the current one was not received before a configurable timeout.

Block

(sometimes also called proposal)

view: View
qc: Qc

We assume an unique identifier of the block can be obtained, for example by hashing its contents. We will use the id() function to access the identifier of the current block. We also assume that a unique tree order of blocks can be determined, and in particular each participant can identify the parent of each block. We will use the parent() function to access such parent block.

@dataclass
class Block:
    view: View
    qc: Qc
View
view_n: uint

a monotonically increasing number (considerations about the size?)

View = int
Qc

There are currently two different types of QC:

Qc = Standard / Aggregate

Qc = StandardQc | AggregateQc
Standard
view: View
block: Id

Q: there can only be one block on which consensus in achieved for a view, so maybe the block field is redundant?


@dataclass
class StandardQc:
    view: View
    block: Id
Aggregate
view: View
high_qc: Qc

high_qc is Qc for the most recent view among the aggregated ones. The rest of the qcs are ignored in the rest of this algorithm.

We assume there is a block function available that returns the block for the Qc. In case of a standard qc, this is trivially qc.block, while for aggregate it can be obtained by accessing high_qc. high_qc is guaranteed to be a 'Standard' qc.

@dataclass
class AggregateQc:
    view: View
    qcs: List[Qc]

    def high_qc(self) -> Qc:
        return max(self.qcs, key=lambda qc: qc.view)
Id

undef, will assume a 32-byte opaque string

Id = bytearray

Vote

A vote for block in view

block: Id
view: View 
voter: Id
? qc: Qc

qc is the optional field containing the QC built by root nodes from 2/3 + 1 votes from their child committees and forwarded the the next view leader.

@dataclass
class Vote:
    block: Id
    view: View
    voter: Id
    qc: Option[Qc]

NewView

view: View
high_qc: Qc

@dataclass
class NewView:
    view: View
    high_qc: Qc

Local Variables

Participants in the protocol are expected to mainting the following data in addition to the DAG of received proposal:

  • current_view
  • local_high_qc
  • latest_committed_view
  • collection: TODO rename
CURRENT_VIEW: View
LOCAL_HIGH_QC: Qc
LATEST_COMMITTED_VIEW: View
HIGH_COMMITTED_QC:Qc # This is not needed for consensus but actually helps a lot for any node fallen behind to catchup.
COLLECTION: Q?

Available Functions

The following functions are expected to be available to participants during the execution of the protocol:

  • leader(view): returns the leader of the view.

  • reset_timer(): resets timer. If the timer expires the timeout routine is triggered.

  • extends(block, ancestor): returns true if block is descendant of the ancestor in the chain.

  • download(view): Download missing block for the view. getMaxViewQC(qcs): returns the qc with the highest view number.

  • member_of_leaf_committee(): returns true if the participant executing the function is in the leaf committee of the committee overlay.

  • member_of_root_com(): returns true if the participant executing the function is member of the root committee withing the tree overlay.

  • member_of_internal_com(): returns truee if the participant executing the function is member of internal committees within the committee tree overlay

  • child_committee(participant): returns true if the participant passed as argument is member of the child committee of the participant executing the function.

  • supermajority(votes): the behavior changes with the position of a participant in the overlay:

    • Root committee: returns if the number of distinctive signers of votes for a block in the child committee is equal to the threshold.

  • leader_supermajority(votes): returns if the number of distinct voters for a block is 2/3 + 1 for both children committees of root committee and overall 2/3 + 1

  • morethanSsupermajority(votes): returns if the number of distinctive signers of votes for a block is is more than the threshold: TODO

  • parent_committe: return the parent committee of the participant executing the function withing the committee tree overlay. Result is undefined if called from a participant in the root committee.

Core functions

These are the core functions necessary for the Carnot consensus protocol, to be executed in response of incoming messages, except for timeout which is triggered by a participant configurable timer.

Receive block

def receive_block(block: Block):
    if block.id() is known or block.view <=LATEST_COMMITTED_VIEW:
        return

    # Recursively make sure that we process blocks in order
    if block.parent() is missing:
        parent: Block = download(block.parent())
        receive(parent)
        
    if safe_block(block):
        # This is not in the original spec, but 
        # let's validate I have this clear.
        assert block.view == current_view
        
        update_high_qc(block.qc)

        vote = create_vote()
        if member_of_leaf_committee():
            if member_of_root_committee():
                send(vote, leader(current_view + 1))
            else:
                send(vote, parent_commitee())
            current_view += 1
            reset_timer()
            try_to_commit_grandparent(block)
Auxiliary functions

def safe_block(block: Block):
    match block.qc:
        case StandardQc() as standard:
            # Previous leader did not fail and its proposal was certified
            if standard.view <= LATEST_COMMITED_BLOCK:
                return False
            # this check makes sure block is not old 
            # and the previous leader did not fail
            return block.view >= LATEST_COMMITED_BLOCK and block.view == (standard.view + 1)        
        
        case AggregateQc() as aggregated_qc:
            # Verification of block.aggQC.highQC along 
            # with signature or block.aggQC.signature is sufficient.
            # No need to verify each qc inside block.aggQC
            if aggregated_qc.high_qc().view <= LATEST_COMMITED_BLOCK:
                return False
            return block.view >= CURRENT_VIEW
            # we ensure by construction this extends the block in
            # high_qc since that is by definition the parent of this block

# Commit a grand parent if the grandparent and 
# the parent have been added during two consecutive views.
def try_to_commit_grand_parent(block: Block):
    parent = block.parent()
    grand_parent = parent.parent()
    return (
            parent.view == (grand_parent.view + 1) and
            isinstance(block.qc, (StandardQc, )) and # Q: Is this necessary?
            isinstance(parent.qc, (StandardQc, )) # Q: Is this necessary?
    )
    # Update last_committed_view ?

# Update the latest certification (qc)
def update_high_qc(qc: Qc):
    match qc:
        # Happy case
        case Standard() as qc:
            # TODO: revise
            if qc.view > LOCAL_HIGH_QC.view:
                LOCAL_HIGH_QC = qc
            # Q: The original pseudocde checked for possilbly
            # missing view and downloaded them, but I think
            # we already dealt with this in receive_block
        # Unhappy case
        case Aggregate() as qc:
            high_qc = qc.high_qc()
            if high_qc.view != LOCAL_HIGH_QC.view:
                LOCAL_HIGH_QC = high_qc
                # Q: same thing about missing views

Receive Vote

Q: this whole function needs to be revised


def receive_vote(vote: Vote):
    if vote.block is missing:
        block = download(vote.block)
        receive(block)

    # Q: we should probably return if we already received this vote
    if member_of_internal_com() and not_member_of_root():
        if child_commitee(vote.voter):
            COLLECTION[vote.block].append(vote)
        else:
            # Q: not returning here would mean it's extremely easy to
            # trigger building a new vote in the following branches
            return
        
        if supermajority(COLLECTION[vote.block]):
            # Q: should we send it to everyone in the committee?
            self_vote = build_vote()
            send(self_vote, parent_committee)
            # Q: why here?
            current_view += 1
            reset_timer()
            # Q: why do we do this here? 
            try_to_commit_grand_parent(block)
    
    if member_of_root():
        if child_commitee(vote.voter):
            COLLECTION[vote.block].append(vote)
        else:
            # Q: not returning here would mean it's extremely easy to
            # trigger building a new vote in the following branches
            return

        if supermajority(COLLECTION[vote.block]):
            # Q: The vote to send is not the one received but
            # the one build by this participant, right?
            self_vote = build_vote();
            qc = build_qc(collection[vote.block])
            self_vote.qc=qc
            send(self_vote, leader(current_view + 1))
            # Q: why here?
            current_view += 1
            reset_timer()
            # Q: why here?
            try_to_commit_grandparent(block)


        # Q: this means that we send a message for every incoming
        # message after the threshold has been reached, i.e. a vote
        # from a node in the leaf committee can trigger
        # at least height(tree) messages.
        if morethanSsupermajority(collection[vote.block]):
            # just forward the vote to the leader
            # Q: But then childcommitte(vote.voter) would return false
            # in the leader, as it's a granchild, not a child
            send(vote, leader(current_view + 1))

    if leader(view): # Q? Which view? CURRENT_VIEW or vote.view?
        if vote.view < CURRENT_VIEW - 1:
            return

        # Q: No filtering? I can just create a key and  vote?
        COLLECTION[vote.block].append(vote)
        if supermajority(collection[vote.block]):
            qc = build_qc(collection[vote.block])
            block = build_block(qc)
            broadcast(block)

Receive NewView

# Failure Case
Func receive(newView) {
    # download the missing block 
    if newview.highQC.block missing {
        let block = download(new_view.high_qc.block)
        receive(block)
    }

    # It's an old message. Ignore it.
    if newView.view < current_view {
        return 
    }

    # Q: this was update_high_qc_and_view(new_view.high_qc, Null)
    update_high_qc(new_view.high_qc)

    if member_of_internal_com() {
        collection[newView.view].append(newView)
        if supermajority[newView.view]{
            newViewQC=buildQC(collection[newView.view])
            if member_of_root(){
                send(newViewQC, leader(view+1))
                curView++
            } else {
                send(newViewQC, parent_committee())
            }
        }
    }
}

Timeout

def timeout():
    if (member_of_internal_com() AND not member_of_root()) OR member_of_leaf:
                        let timeoutMsg = create_newView(CURRENT_VIEW,HIGH_QC,HIGH_COMMITTED_QC, TIMEOUT_QC)
                        send(timeoutMsg, parent_committee())
                 
                
    if member_of_root():
                        let timeoutMsg = create_newView(CURRENT_VIEW,HIGH_QC,HIGH_COMMITTED_QC, TIMEOUT_QC)
                        send(timeoutMsg,root_committee()) # Need to be discussed. It can only be sent to the next leader but since the RB needs agreement to generate the seed for the leader+overlay, therefore newView is sent to the root_committee().

We need to make sure that qcs can't be removed from aggQc when going up the tree