Ethereum's Proposer-Builder Separation: Promises and Realities

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.