Narwhal and Tusk: A DAG-based Mempool and Efficient BFT Consensus
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We propose separating the task of transaction dissemination from transaction ordering, to enable high-performance Byzantine fault-tolerant consensus in a permissioned setting. To this end, we design and evaluate a mempool protocol, Narwhal, specializing in high-throughput reliable dissemination and storage of causal histories of transactions. Narwhal tolerates an asynchronous network and maintains its performance despite failures. We demonstrate that composing with a partially synchronous consensus protocol (HotStuff) yields significantly better throughput even in the presence of faults. However, loss of liveness during view-changes can result in high latency. To achieve overall good performance when faults occur we propose Tusk, a zero-message overhead asynchronous consensus protocol embedded within Narwhal. We demonstrate its high performance under a variety of configurations and faults. Further, Narwhal is designed to easily scale-out using multiple workers at each validator, and we demonstrate that there is no foreseeable limit to the throughput we can achieve for consensus, with a few seconds latency. As a summary of results, on a Wide Area Network (WAN), Hotstuff over Narwhal achieves 170,000 tx/sec with a 2.5-sec latency instead of 1,800 tx/sec with 1-sec latency of Hotstuff. Additional workers increase throughput linearly to 600,000 tx/sec without any latency increase. Tusk achieves 140,000 tx/sec with 4 seconds latency or 20x better than the state-of-the-art asynchronous protocol. Under faults, both Narwhal based protocols maintain high throughput, but the HotStuff variant suffers from slightly higher latency.
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