[LN Decentralization E] Proof of Collection Finality

A collection guarantee is an assertion about collection c . From the protocol perspective, a collection guarantee must prove two aspects: (i) the cluster block that embodies collection c was finalized and (ii) which collectors promised to provide the collection data to other nodes in the network (they will be slashed if c goes missing).

We currently trust the collectors to only send finalized collections to consensus but for BFT, finalization would need to be verified by all consensus nodes.

Details

Groups of collectors (known as ‘collector clusters’) use a mini-version of HotStuff to agree on which transactions to include in their collections. This ensures robust data availability and mitigates resource exhaustion attacks by preventing individual collectors from including garbage or repetitive transactions. As long as the fraction of Byzantine collectors in each cluster is less than one-third, malicious collections proposed by permissionless collectors will not be finalized by the cluster consensus, and the proposing collector will eventually be slashed.

Important terminology:

• collection is a batch of transactions

  Note that it does not contain any signatures of other evidence for its validity.

• Cluster blocks are produced by each cluster consensus. Essentially, a cluster block’s payload is a collection. In other words, the cluster block header carries the information about who proposed the block, votes for the parent collection, signatures, etc.

• A Collection Guarantee is a cryptographic proof about collection c

  • the cluster block that embodies collection c was finalized
  • which collectors promised to provide the collection data to other nodes in the network (they will be slashed if c goes missing).

The Flow protocol mandates that collectors may only send finalized collections to the consensus nodes for inclusion in blocks. However, we have a shortcut in our implementation assumes only honest collectors and hence never checks whether collections are finalized. However, a single byzantine collector could submit collection guarantees to consensus nodes lying that they are finalized, when intact they are not.

Before admitting permissionless collectors, this shortcut must be removed by adding proofs of collection finality.

Start with model/flow/collection.go, model/cluster/block.go, and model/flow/collectionGuarantee.go to understand the collection, cluster-block, and guarantee structures. Trace where collection guarantees are admitted by consensus and determine how finalized-collection evidence can be verified; done means consensus nodes reject guarantees without valid collection-finality proofs.