With Ethereum's transition from Proof-of-Work to Proof-of-Stake in September 2022 came another paradigm shift, the Proposer-Builder Separation (PBS) scheme. PBS was introduced to decouple the roles of selecting and ordering transactions in a block (i.e., the builder), from those validating its contents and proposing the block to the network as the new head of the blockchain (i.e., the proposer). In this landscape, proposers are the validators in the Proof-of-Stake consensus protocol who validate and secure the network, while now relying on specialized block builders for creating blocks with the most value (e.g., transaction fees) for the proposer. Additionally, relays play a crucial new role in this ecosystem, acting as mediators between builders and proposers, being entrusted with the responsibility of transmitting the most lucrative blocks from the builders to the proposers. PBS is currently an opt-in protocol (i.e., a proposer can still opt-out and build their own blocks). In this work, we study it's adoption and show that the current PBS landscape exhibits significant centralization amongst the builders and relays. We further explore whether PBS effectively achieves its intended objectives of enabling hobbyist validators to maximize block profitability and preventing censorship. Our findings reveal that although PBS grants all validators the same opportunity to access optimized and competitive blocks, it tends to stimulate censorship rather than reduce it. Additionally, our analysis demonstrates that relays do not consistently uphold their commitments and may prove unreliable. Specifically, there are instances where proposers do not receive the complete value as initially promised, and the censorship or filtering capabilities pledged by the relay exhibit significant gaps.

翻译：2022年9月，以太坊从工作量证明共识机制过渡到权益证明共识机制的同时，也引发了另一项范式转变——“提议者-构建者分离”（PBS）方案。PBS的引入旨在将区块中交易的选择和排序职责（即构建者角色）与验证区块内容、并将该区块作为区块链最新区块头向网络提交的职责（即提议者角色）相分离。在此架构中，提议者即权益证明共识协议中的验证者，负责验证并保障网络安全；而现在，他们依赖专门的区块构建者来创建价值最高（例如，交易费收益最多）的区块供自身选择。此外，中继层在这一生态系统中扮演了关键的新角色，作为构建者与提议者之间的中介，负责将构建者生成的最有利可图的区块可靠地传输给提议者。目前，PBS是一个可选协议（即提议者仍可选择退出，自行构建区块）。本研究分析了PBS的采用情况，表明当前的PBS格局在构建者和中继层之间呈现出显著的集中化趋势。我们进一步探讨了PBS是否有效实现了其预期目标，即让业余验证者能够实现区块利润最大化并防止审查。研究结果显示，尽管PBS赋予了所有验证者同等获得经过优化的、具有竞争力的区块的机会，但它更倾向于刺激审查而非减少审查。此外，我们的分析表明，中继层并未始终履行其承诺，可能并不可靠。具体而言，存在提议者未能获得最初承诺的完整价值的情况，且中继层所承诺的审查或过滤能力也表现出显著缺失。