Proposer Builder Separation

Essence

The architectural shift known as Proposer Builder Separation (PBS) fundamentally reconfigures the block production process in Proof-of-Stake systems. It separates the responsibility of creating a valid block (the builder) from the right to propose that block to the network (the proposer or validator). This mechanism directly addresses the systemic risks inherent in Maximal Extractable Value (MEV) by restructuring the incentive landscape.

Before PBS, the validator possessed a high degree of discretionary power to manipulate transaction ordering within a block, allowing them to extract value by frontrunning, backrunning, or sandwiching transactions. PBS introduces a specialized market for block construction, transforming MEV from a validator-specific, non-transparent profit source into a competitive auction where builders bid for the right to have their block proposed. This separation is a direct response to the market microstructure challenges of decentralized finance.

It creates a more efficient and transparent market for blockspace. The core financial impact of PBS lies in its ability to smooth out validator revenue and reduce the systemic risk associated with validator-specific MEV extraction. By forcing builders to compete, the value extracted from users is redistributed to the proposers in a more standardized manner, rather than being concentrated in the hands of a few powerful validators with superior technical capabilities.

The introduction of this new market layer directly impacts how financial products, including crypto options and derivatives, can be structured and priced, as it changes the underlying volatility characteristics of the settlement layer.

Proposer Builder Separation rearchitects block production to create a competitive market for blockspace, mitigating the risks associated with discretionary transaction ordering.

The design of PBS creates a new class of participants: the block builders. These entities specialize in optimizing block construction to maximize MEV extraction and proposer payments. The proposer’s role is simplified to a capital-intensive, low-latency task: selecting the highest-paying block header from a set of options presented by various builders.

This separation creates a necessary abstraction layer between the transaction ordering logic and the consensus mechanism, which is vital for building a robust and resilient financial infrastructure. The design attempts to minimize the potential for proposers to engage in malicious behavior by limiting their ability to see the contents of the block before committing to it.

Origin

The concept of PBS arose directly from the practical challenges and theoretical insights surrounding MEV. The initial iterations of Proof-of-Stake protocols, particularly Ethereum post-merge, faced a significant problem: validators could observe the mempool, which is a public repository of pending transactions.

This visibility allowed validators to identify profitable arbitrage opportunities and liquidations. The ability to extract this value, known as MEV, led to several critical issues. First, it created a race condition where validators with lower latency and more sophisticated algorithms could capture disproportionately large amounts of value.

This created an unfair playing field and incentivized the centralization of staking power among entities capable of investing heavily in MEV infrastructure. The pre-PBS system created an adversarial environment where a validator’s primary goal was to maximize their MEV extraction. This led to a situation where a validator might intentionally reorder transactions or censor certain transactions to maximize their own profit.

This behavior introduced systemic risk into the network’s financial layer. The community recognized that this concentration of power was antithetical to the goals of decentralization and financial fairness. Research from organizations like Flashbots demonstrated the extent of MEV extraction and proposed solutions.

The primary solution developed to address this problem was the concept of PBS. The initial implementations of PBS, such as the Flashbots auction mechanism, introduced a sidecar system where proposers could receive blocks from a set of competing builders without revealing the contents of those blocks beforehand. This “out-of-protocol” solution paved the way for the eventual integration of PBS into the core protocol design.

The goal was to remove the ability for proposers to frontrun and ensure that the value generated from MEV was distributed more broadly across the network. The implementation of PBS transformed MEV from a hidden, non-transparent source of profit into a transparent, competitive auction. This evolution from a single-actor model to a two-actor model (proposer and builder) was necessary to secure the network against internal economic attacks and provide a stable foundation for advanced financial strategies.

Theory

The theoretical foundation of PBS rests on game theory and market microstructure principles.

It introduces a separation of concerns that changes the incentives for all network participants. The primary mechanism involves a private order flow auction where builders compete to create the most valuable block for the proposer. This process fundamentally alters the dynamics of transaction ordering and price discovery.

Market Microstructure and Order Flow

In a traditional exchange, order flow is critical for market makers. In decentralized finance, order flow takes the form of pending transactions in the mempool. Pre-PBS, validators had direct access to this order flow and could act as market makers, extracting value directly from users.

PBS changes this dynamic by creating a competitive bidding process for the right to order these transactions. Builders aggregate order flow from various sources and then construct the most profitable block possible. The proposer’s role is reduced to selecting the highest bid.

This system effectively creates a secondary market for blockspace where builders bid against each other. The competition among builders ensures that the value extracted from users (MEV) is returned to the proposers in the form of a bid payment, rather than being captured by the builder as profit. The equilibrium of this market is achieved when the bid price from the builder approaches the total value of the MEV contained within the block.

Protocol Physics and Consensus

The core challenge in implementing PBS within a consensus protocol is ensuring censorship resistance and liveness. The protocol must ensure that proposers cannot simply reject blocks that do not meet their specific criteria (e.g. blocks that include transactions from sanctioned addresses). The current implementation relies on a trusted relay that acts as an intermediary between builders and proposers.

The relay receives blocks from builders, verifies their validity, and then provides a blinded block header to the proposer. The proposer signs this header and proposes it to the network. The system relies on a two-step commitment process:

1. Builder Commitment: The builder commits to a specific block structure and a payment to the proposer.
2. Proposer Commitment: The proposer commits to proposing the block header without knowing its contents.

This separation of information prevents the proposer from frontrunning or censoring transactions. However, this model introduces new centralization risks related to the relays. If a few relays dominate the market, they could collectively censor transactions or manipulate the auction process.

Quantitative Finance and Greeks

From a quantitative finance perspective, PBS impacts the underlying volatility of validator revenue. Before PBS, validator revenue was highly variable and depended on specific MEV opportunities. This volatility made it difficult to model and price derivatives based on validator earnings.

PBS introduces a smoothing effect by transforming variable MEV extraction into a more predictable auction payment.

The competitive bidding process of PBS creates a more stable revenue stream for validators, which can be modeled more effectively. This stability is a necessary prerequisite for developing advanced financial instruments like options and futures contracts on staking yield. The reduced volatility of validator revenue also reduces the overall systemic risk of the network, as validators are less incentivized to engage in risky or malicious behavior.

Approach

The implementation of PBS, specifically on Ethereum, currently relies on a hybrid architecture that balances decentralization with efficiency.

The current approach involves several key components that work together to execute the block auction and proposal process.

Block Construction Process

The block construction process begins with a set of specialized actors called builders. Builders receive transaction bundles from various sources, including searchers (who identify MEV opportunities) and private transaction relays. Builders compete to assemble the most profitable block.

This competition is crucial to the design; it ensures that the majority of the MEV value is passed back to the proposer. The builder’s goal is to maximize the difference between the MEV captured and the payment offered to the proposer. The builder submits a block to a relay.

The relay acts as a trusted intermediary, verifying the validity of the block and ensuring that the builder’s payment to the proposer is correct. The relay then passes a blinded header of the block to the proposer. The proposer’s software selects the block header with the highest payment.

The proposer then signs this header and proposes it to the network.

Order Flow Auctions and Financial Strategy

The financial strategy of a builder revolves around optimizing block construction and participating in the auction. Builders must:

• Acquire order flow, often by offering private transaction relays to users to avoid the public mempool.
• Identify and execute MEV strategies, such as arbitrage, liquidations, and sandwich attacks.
• Model the competitive landscape to determine the optimal bid to offer the proposer.

The proposer’s strategy is simpler: maximize profit by selecting the highest bid. This creates a clear separation of financial roles. Proposers are essentially passive recipients of auction revenue, while builders are active participants in a highly competitive market.

The current PBS implementation uses relays as trusted intermediaries, creating a new layer of centralization risk that must be managed to preserve network integrity.

The system’s current implementation, while effective at mitigating frontrunning by individual proposers, introduces a new point of centralization: the relays themselves. If a few relays become dominant, they can potentially collude to censor specific transactions or manipulate the auction. This structural weakness in the current approach is a major area of focus for future protocol upgrades.

The current approach relies on social consensus and reputation to ensure relays act honestly.

Evolution

The evolution of PBS from its initial concept to its current implementation on Ethereum highlights the tension between efficiency and decentralization. The initial design of PBS relied on external, non-protocol solutions like Flashbots. This approach, while effective, introduced a reliance on trusted third parties (relays) to coordinate the block auction process.

The Role of Relays and Centralization Risk

The current state of PBS has led to a concentration of block construction and relay services. A small number of relays handle a significant majority of blocks. This concentration poses a potential risk to censorship resistance.

If a dominant relay decides to filter specific transactions (e.g. based on regulatory pressure or malicious intent), it can effectively censor those transactions from being included in blocks. This issue is particularly relevant for options and derivatives protocols that rely on consistent and unbiased transaction processing. To mitigate this, the ecosystem has developed several strategies:

• Multiple Relays: The use of multiple, competing relays reduces reliance on any single entity. Proposers can connect to multiple relays and select the highest-paying block, regardless of which relay provides it.
• Decentralized Relays: Research into decentralized relay architectures aims to remove the need for a single trusted party. These designs often use cryptographic techniques to ensure that relays cannot censor transactions or collude with builders.

MEV Smoothing and Validator Economics

The most significant change introduced by PBS is the smoothing of validator revenue. Before PBS, a validator might earn large, infrequent profits from specific MEV opportunities. After PBS, the validator receives a consistent stream of payments from the builder auction.

This shift has significant implications for validator economics and the financial stability of staking pools.

The evolution of PBS has created a more stable financial environment for validators. This stability reduces the risk profile of staking itself, making it a more attractive asset for institutional investors and paving the way for more complex financial products based on staking yield. The challenge remains to transition from the current trusted relay model to a truly trustless, enshrined PBS system.

Horizon

The future of PBS centers on a transition from the current external, trusted relay model to an “enshrined PBS” (ePBS) where the separation logic is built directly into the core protocol.

This move would eliminate the centralization risks associated with relays and fully realize the potential of PBS for creating a robust financial foundation.

Enshrined PBS (ePBS) and Protocol Physics

ePBS aims to move the block auction logic into the consensus layer itself. The protocol would enforce the separation of roles cryptographically, ensuring that proposers cannot see the contents of the block before committing to it. This design removes the need for trusted third-party relays, which are currently the primary point of failure for censorship resistance.

The implementation of ePBS would provide a higher degree of security and decentralization, creating a more stable and predictable environment for financial applications. The implications for options and derivatives markets are significant. By reducing the reliance on external relays, ePBS would decrease counterparty risk and increase the overall trust in the network’s settlement layer.

This stability allows for the development of more sophisticated financial products, such as options on MEV itself, or complex derivatives that hedge against specific types of protocol risk.

MEV Smoothing and Derivatives

The concept of MEV smoothing, where MEV revenue is distributed across all validators over time, will likely become standard practice. This process transforms the highly volatile and unpredictable MEV revenue stream into a stable, yield-bearing asset. This stability creates new opportunities for financial engineering:

• Yield-Based Options: Derivatives can be created on the smoothed MEV yield, allowing stakers to hedge against fluctuations in network activity.
• Risk-Adjusted Staking: The predictable revenue stream from PBS allows stakers to accurately model their risk and return, enabling the creation of new financial products that offer guaranteed minimum returns or leveraged staking positions.
• Censorship Resistance Premiums: The market may develop a premium for blocks that are provably censorship-resistant, creating a new financial incentive for builders and relays to adhere to specific ethical standards.

The transition to ePBS and the standardization of MEV smoothing will create a new set of financial primitives. The current market for crypto options often struggles with the high volatility and unpredictable nature of the underlying assets. PBS provides a structural solution to this problem by creating a more stable foundation for the network’s economic activity. This allows for a shift from purely speculative trading to more sophisticated, risk-managed financial strategies. The ultimate goal is to build a financial operating system where the underlying mechanics are transparent and predictable, allowing for a new generation of derivatives to accurately price risk.