Briefing
The core research problem in Layer 2 rollups is the critical reliance on a single, centralized sequencer for transaction ordering and batch submission, which introduces systemic risks of censorship and maximal extractable value (MEV) exploitation. This paper introduces the Arranger service, a formally defined, fully decentralized mechanism that merges the sequencer and the Data Availability Committee (DAC) functions. The foundational breakthrough is the use of Set Byzantine Consensus (SBC) , a novel extension of BFT where participants agree on a subset of proposed values rather than a single value. This new primitive allows the Arranger to achieve consensus on a batch of transactions and its corresponding data availability proof, fundamentally mitigating the single point of failure and ensuring the L2 system inherits the censorship resistance and liveness properties of the underlying Layer 1.
Context
Before this research, the prevailing architecture of Layer 2 rollups was characterized by a fundamental compromise → achieving high throughput and low cost through off-chain computation while maintaining security via the Layer 1 chain, but sacrificing the core principle of decentralization by using a single, trusted sequencer. This centralized model created a single point of failure, enabling the sequencer to arbitrarily censor transactions, front-run users, and extract MEV, directly challenging the decentralized ethos and economic fairness of the entire rollup ecosystem. The academic challenge was designing a robust, performant consensus protocol specifically tailored to the unique requirements of L2 sequencing and data availability without relying on a single leader.
Analysis
The core mechanism is the Set Byzantine Consensus (SBC) protocol, which is specifically engineered to enable the decentralized Arranger. SBC allows a set of sequencers to propose sets of transaction batches. The protocol then formally reaches consensus on a subset of the union of all proposed batches.
This mechanism fundamentally differs from prior BFT approaches by decoupling the final transaction set from a single leader’s proposal, thereby distributing the power of block construction across the entire committee. This ensures that as long as the Byzantine fault tolerance threshold is not exceeded, the service remains live and resistant to censorship, as the final agreed-upon set of transactions is a collective outcome, not a single-party decision.
Parameters
- Fault Tolerance Threshold → The protocol maintains safety and liveness with up to one-third of the Arranger nodes being malicious, consistent with classical Byzantine Fault Tolerance protocols.
- Consensus Target → Agreement is reached on a subset of proposed transaction batches, not a single, atomic block proposal, which is the defining feature of Set Byzantine Consensus.
- Arranger Function → Combines the Sequencer and Data Availability Committee (DAC) roles into a single, unified, and fully decentralized service.
Outlook
The introduction of Set Byzantine Consensus and the Arranger primitive opens up a new avenue for L2 architecture, shifting the focus from simply scaling computation to truly decentralizing the control plane. In the next 3-5 years, this research will likely drive the design of shared sequencing layers, enabling atomic cross-rollup composability and eliminating the need for trust assumptions in L2 operators. Future research will concentrate on optimizing the communication complexity of SBC and integrating it with verifiable delay functions to enforce fair transaction ordering, ultimately leading to a modular blockchain ecosystem where all layers are cryptographically and economically decentralized.
Verdict
The Set Byzantine Consensus primitive provides the foundational theoretical mechanism required to finally resolve the critical centralization risk inherent in current Layer 2 rollup architectures.
