MEV-Boost Relay

A MEV-Boost relay is a critical piece of infrastructure in Ethereum's proposer-builder separation (PBS) model. Its primary function is to receive blocks from competitive builders—who construct blocks packed with Maximal Extractable Value (MEV) opportunities—and present them to validators (proposers) for selection. The relay acts as a trusted intermediary that does not see the block contents, ensuring builders cannot front-run or steal the MEV from the proposer. Validators choose the block offering the highest bid, which is paid directly to them, thereby democratizing access to MEV profits.

The relay's architecture is designed for censorship resistance and credible neutrality. It receives encrypted block bodies from builders and unencrypted block headers, which include the validator's fee (the bid). The validator selects a header based on the highest bid, then requests the corresponding full block from the relay. This separation ensures the proposer cannot see the transaction details before commitment, preventing theft of the MEV bundle. Major relays are operated by entities like Flashbots, BloXroute, and Eden Network, each competing on reliability and latency.

Relays enforce strict validity criteria on the blocks they receive, checking for conditions like correct execution, sufficient fee payments, and compliance with specific rules (e.g., Flashbots' no front-running rule). This validation protects validators from publishing invalid blocks, which would result in slashing penalties. By aggregating builders and providing a standardized auction platform, relays increase competition, which generally leads to higher validator rewards and more efficient MEV extraction that benefits the overall network through increased staking yields.

The security and decentralization of the relay network are ongoing concerns. While relays are permissionless to use, their operation is currently permissioned, creating potential centralization risks. The Ethereum roadmap addresses this with in-protocol PBS, which would embed relay functionality directly into the consensus layer, eliminating the need for these external trusted parties. Until then, MEV-Boost relays remain an essential, albeit temporary, piece of infrastructure that enables efficient and fair MEV distribution post-The Merge.

A MEV-Boost Relay is a trusted, neutral intermediary service that receives blocks from builders and forwards them to validators (proposers) within the MEV-Boost ecosystem. Its primary function is to act as a commit-reveal scheme, preventing validators from stealing block content. The relay receives encrypted block bids, holds them in escrow, and only reveals the full block data to the winning validator after they have cryptographically committed to it. This architecture is essential for enabling a competitive, permissionless market for block production separate from block proposal.

The relay's operation follows a strict, timed sequence. First, block builders submit their bids—consisting of an execution payload header and an associated fee—to one or more relays. The relay validates the bid's correctness, including the proposed block's hash and the fee's attestation. When a validator is selected to propose a block, it connects to the relay network via its MEV-Boost software, requesting the highest-paying header. The validator signs a commitment to this header, which the relay verifies before finally delivering the full, unencrypted block body for the validator to publish to the network.

Relays enforce critical validity and data availability rules to protect the network. They verify that the builder's execution payload is valid according to Ethereum's consensus rules and that the full block data is available for download. This prevents proposers from committing to invalid or unavailable blocks, which would result in a missed slot and a penalty. Major relays often implement additional filtering, such as rejecting blocks containing censorship transactions or those from non-compliant builders, making them a point of policy enforcement within the MEV supply chain.

The economic and security model of a relay relies on its reputation for neutrality and reliability. Builders and validators are free to connect to any public relay, creating a competitive landscape. A relay that acts maliciously—by censoring, front-running bids, or failing to deliver data—would quickly lose users. This decentralized network of relays prevents any single entity from controlling block flow. Prominent examples include the Flashbots Relay, Ultrasound Money Relay, and Agnostic Relay, each operating with publicly stated policies and open-source software.

The journey toward Proposer-Builder Separation (PBS) began as a pragmatic response to the rise of Maximal Extractable Value (MEV) and centralization risks in Ethereum block production. The off-chain MEV-Boost middleware, introduced after The Merge, was the first widespread implementation of PBS. It allows validators (proposers) to outsource block construction to specialized builders via a competitive marketplace facilitated by relays. This separation lets proposers earn higher rewards while builders compete to create the most valuable blocks, but it introduced new dependencies on these off-chain, trusted intermediaries.

MEV-Boost operates as a 'soft' or opt-in PBS, meaning validators can choose to use it. Its architecture relies on a trusted relay network to receive blocks from builders and forward them to proposers. The relay performs critical duties: it attests to the builder's payment, validates the block's contents, and prevents certain attacks like frontrunning. However, this model has drawbacks, including relay centralization, potential censorship, and the added complexity of an extra layer outside the core consensus protocol. These limitations highlighted the need for a more robust, protocol-native solution.

The logical next step is enshrined PBS, where the separation of roles and the auction mechanism are baked directly into the Ethereum protocol. Enshrined PBS aims to eliminate trust in external relays by moving their critical functions—such as payment verification and block validity checks—into the consensus layer itself. This would be achieved through new cryptographic primitives like builder signatures and possibly a cr-list (censorship resistance list) mechanism, making the system more secure, decentralized, and resistant to censorship by design.

The path to enshrined PBS involves significant research and protocol upgrades. Key proposals, such as the two-slot PBS design, outline how a builder's block and the proposer's commitment could be integrated across multiple slots in the consensus process. The transition must carefully balance complexity, efficiency, and the core principles of decentralization. Successfully enshrining PBS would represent a major evolution in Ethereum's design, solidifying a fair and efficient block production market as a fundamental property of the chain.