MEV Relay

The relay operates a sealed-bid auction where block builders submit their proposed blocks and associated bids. This process ensures bid privacy, preventing front-running of the builder's transaction ordering strategy. The relay's primary function is to collect these bids and present the most profitable, valid block to the validator.

• Sealed-bid: Builders cannot see competing bids.
• Deadline: Bids must be submitted before the relay's cutoff time for a given slot.

Before forwarding a block, the relay performs cryptographic and economic validation. This includes verifying the block's execution payload is valid, the builder's signature is correct, and the promised payment to the validator is included and sufficient.

• Payload Validity: Checks state transitions and gas usage.
• Payment Guarantee: Ensures the validator payment (e.g., via coinbase transaction) is executable.

The relay provides a trust-minimized delivery channel between builders and validators. It cryptographically commits to delivering the winning block header to the validator, who then signs it. This creates a commit-reveal scheme where the validator's signature is contingent on the full block being revealed. This mechanism is crucial for censorship resistance, as validators can choose relays with favorable policies.

Relays enforce the secure transfer of MEV rewards from builders to validators. The payment, often called the validator payment or priority fee, is embedded in the block's coinbase transaction. The relay validates this transaction is present and that its value matches the builder's bid, ensuring the validator is paid for selecting their block.

• Payment Enforcement: The block is invalid without the correct payment.

Many relays maintain a reputation system for block builders. Builders who consistently submit valid, high-value blocks and honor their payments gain a positive reputation. This can lead to preferential treatment, such as being whitelisted or having lower collateral requirements. Conversely, builders who submit invalid blocks may be penalized or banned.

A relay's primary role is to enforce the Builder-Proposer Separation (BPS) model. It sits between block builders (who construct blocks) and validators/proposers (who propose them). The relay receives sealed bids from builders, selects the highest-value block, and delivers it to the winning proposer, ensuring the proposer cannot see or steal the block's contents.

A key function of a neutral relay is to prevent transaction censorship. It does this by accepting blocks from any builder and forwarding the highest-paying one to the proposer, regardless of the transactions it contains. This mitigates risks like OFAC compliance pressures, where a validator might otherwise be compelled to exclude certain transactions.

The Ethereum ecosystem is supported by several major, permissionless relays that validators can connect to. Prominent examples include:

• Flashbots Relay: The original and historically dominant relay.
• bloXroute Relay: Known for its high-performance global network.
• Eden Network Relay: Focuses on fair ordering and fast block delivery.
• Ultrasound Money Relay: Aims for maximal credibly neutral and open-source infrastructure. Validators often connect to multiple relays to maximize their rewards.

While relays are designed to be trust-minimized, they are not trustless. Users and builders must trust that the relay:

• Does not censor transactions or builders.
• Correctly selects the highest valid bid.
• Does not front-run or steal block contents.
• Maintains high uptime. The PBS landscape is evolving with protocols like PBS-CR (Censorship Resistance) to further reduce these trust assumptions.

Relays expose standard APIs for builders and validators. Builders submit blocks via builder_submitBlindedBlock, and validators fetch the winning block via validator_getPayload. This standardization, part of the Builder API specification, allows validators to easily connect their consensus client and execution client to multiple relays through their validator client software.

Relays are central to the MEV supply chain, facilitating billions in value transfer. Key observable metrics include:

• Relay Market Share: The percentage of Ethereum blocks sourced through a specific relay.
• Inclusion Rate: The rate at which a relay's winning bids are successfully proposed.
• Top Builder Distribution: Shows which builders are most successful via a given relay. These metrics are tracked by sites like mevboost.pics and relayscan.io.

A primary security concern is a relay's ability to censor transactions. A relay can filter transactions based on origin (e.g., from a sanctioned address) or content (e.g., a specific DeFi arbitrage). This creates a centralized point of failure, undermining the permissionless nature of the network. The relay's inclusion list policy is a key determinant of its censorship posture.

Validators must trust the relay's builder selection algorithm. The relay acts as an intermediary, presenting what it claims is the most profitable block from its connected builders. A malicious relay could:

• Present a less valuable block, skimming the difference (theft of MEV).
• Collude with a specific builder to exclude competitors.
• Present an invalid block, causing the validator to miss their slot.

Relays handle sensitive data, creating two key risks:

• Data withholding: A relay could receive a valuable block from a builder but not deliver it to the validator, executing the arbitrage itself in a later block (time-bandit attack).
• Transaction privacy: Builders send full block contents to the relay. A malicious relay could front-run the transactions it sees before the block is published, though cryptographic commitments like commit-reveal schemes mitigate this.

The ecosystem often consolidates around a few dominant relays. This creates systemic risk:

• Single point of failure: An outage or compromise of a major relay can significantly impact block production.
• Coordinated censorship: Relays could align on filtering policies.
• Barrier to entry: The technical and reputational requirements to run a trusted relay are high, limiting decentralization. The relay diversity metric is crucial for health.

Using a relay impacts a validator's economic incentives. The proposer-builder separation (PBS) model, facilitated by relays, aims to democratize MEV. However, risks include:

• Extractable Value (EV) leakage: Inefficient relay algorithms may not capture all available MEV, reducing validator rewards.
• Slashing risk: While relays validate block correctness, a validator is ultimately responsible for signing and proposing a block, even if sourced from a relay.

The maximum value that can be extracted from block production beyond standard block rewards and gas fees, often through reordering, inserting, or censoring transactions. MEV is the fundamental economic force that relays are designed to manage and democratize.

• Examples: Arbitrage, liquidations, frontrunning.
• Impact: Can lead to network congestion and increased costs for users.

A specialized node operator that constructs full block proposals by selecting and ordering transactions from the mempool and private order flows. Builders compete in relay auctions to have their block accepted by a validator.

• Key Function: Aggregates transactions to create the most profitable or desirable block.
• Separation of Duties: Decouples block construction from block proposal, a core principle of Proposer-Builder Separation (PBS).

A design paradigm that separates the roles of the block proposer (validator) and block builder to mitigate centralization risks from MEV. Relays are the critical infrastructure enabling PBS.

• Validator Role: Chooses the highest-value block from a relay.
• Builder Role: Competes to create valuable blocks.
• Goal: Prevents validators from being forced to run complex, centralized MEV extraction operations.

A critical property of decentralized networks where validators cannot be compelled to exclude certain transactions from blocks. Relays play a complex role; while they can filter transactions, the validator ultimately chooses which relay's block to propose.

• Relay Policy: Some relays enforce regulatory compliance (e.g., OFAC lists), creating centralization vectors.
• Network-Level Solutions: Techniques like inclusion lists are proposed to enforce censorship resistance at the protocol level.

A secure, isolated hardware environment (like Intel SGX) used by some relays to provide strong commitments in the block auction process. It allows builders to submit encrypted block bids without revealing their contents until after the validator commits to accepting the block.

• Function: Prevents bid stealing, where a malicious validator could steal a profitable block template from a builder.
• Example: The Flashbots SUAVE initiative aims to decentralize block building using TEEs and a cross-chain mempool.