The Finality Calculator: Analyzing and Quantifying Filecoin's Finality Guarantees

TL;DR

The finality of the Filecoin network is analyzed, focusing on dynamic probabilistic guarantees of tipset permanence in the canonical chain, to provide a practical algorithm that only requires visibility into the blocks produced by honest participants, and lays the foundation for further analysis of other DAG-structured blockchains.

Abstract

In this paper, we analyze the finality of the Filecoin network, focusing on dynamic probabilistic guarantees of tipset permanence in the canonical chain. Our approach differs from static analyses that consider only the worst-case scenario; instead, we dynamically compute the error probability at each round using the live chain history, providing a more accurate and efficient assessment. We provide a practical algorithm that only requires visibility into the blocks produced by honest participants, which can be implemented by clients or off-chain applications without any change to Filecoin's consensus mechanisms. We demonstrate that, under typical operating conditions, the sought-after error probability of $2^{-30}$ is achievable in approximately 30 rounds, a 30x improvement over the 900 rounds that the network currently encodes as a fixed threshold. This finding promises to expedite transactions and enhance network efficiency, and lays the foundation for further analysis of other DAG-structured blockchains.

Figures (1)

• Figure 1: Example of a possible tipset chain in Filecoin's block DAG. Block $G$ is the genesis block. In round 1, five blocks are produced, all pointing to $G$. Round 2 sees five more blocks, each pointing to all blocks from the first round (Tipset T1). In round 3, five blocks are again produced; four point to T2, which contains four blocks, while one (block $3e$) points to T2', which contains all five blocks. This situation can arise if block $2e$ is kept private by its producer and shared only with the producer of block $3e$. Despite block $3e$ pointing to all blocks of round 2, blocks in round 4 cannot combine it with the rest of round 3 blocks due to different parent tipsets. Additionally, although T2' is heavier than T2, the chain T1 $\leftarrow$ T2' $\leftarrow$ T3' is lighter (11 blocks) than the chain T1 $\leftarrow$ T2 $\leftarrow$ T3 (14 blocks), leading correct nodes to extend the latter.

Theorems & Definitions (2)

• Definition 1: Consistent broadcast
• Definition 2: Finality