Take Aways:

• The tree is mainly used for doing attestation efficiently (i.e. efficiently distributing blocks and aggregating signatures)

• The tip of the tree is where the quorum certificate is created

  • since signature aggregation is not done in a cryptographic manner, the tip is more susceptible to trust failure issues
  • similarly the tip of the tree is a POF for the network, an unlucky network partition in the root committee will lead to a liveness failure in the network

• What is Partial Synchrony?

• Claims made by Carnot

• System Model

  • Blocks have:
    • curView
    • parent
    • QC (of parent block)
    • batch of transactions
  • Leaders are selected randomly
  • Leaders build blocks and gets agreement from validators

• Overlay Tree Formation and Msg Dissemination

  • Definition 2:

    

    • This seems to be saying that Robust Child Committees are those that maintain the BFT assumption, this assumption is applied to every committee and justified by a statistical analysis showing that this assumption holds with high probability (w.h.p)

  • Overlay Formation

    • Shuffle Nodes
    • divide the shuffled list of nodes into K parts of size n, the last r committes will hold one extra node (remainder of nodes who do not divide evenly)
    • map the committees on to the binary tree induced by the list (l_child(p) = 2p, r_child(p) = 2p + 1

• Protocol

  • Happy Path:
    • Leader builds block and proposes to the network
    • each node recv’s block, validates it
    • leaf committes submit votes up to parent committees
    • once a parent committee node receives 2/3rds of expected votes, it compresses them into an aggregated QC and sends it up to it’s parents along with their own vote
    • Once the root nodes receive 2/3rds the votes, it forwards them to the leader along with their vote
    • The leader builds a QC from 2/3rds the votes and proposes the block to the network.
  • Unhappy Path:
    • On timeout, nodes notify the root committee

• Q: when do committees know to forward votes up the tree?

  • Do they make the 2/3rds assumption for every committee?
    • Yes, justified by w.h.p proof
  • What happens when the root committee is faulty? or when one of the intermediate committees fails? I don’t see logic to allow routing around the faulty committees
    • This is assumed not to happen w.h.p.
  • What happens when the leader is faulty?
    • timeout and a new block is proposed

• So it seems the main POF is the root committee

Attacks:

• How long to wait for unlikely events: