Vitalik Buterin Proposes Big FOCIL to Tackle Builder Centralization

TLDR

• Big FOCIL could let attesters include all block transactions
• ePBS separates proposers from builders in Glamsterdam
• Encrypted mempools aim to curb sandwich and frontrun attacks
• Network-layer privacy tools may protect transaction routing

Ethereum co-founder Vitalik Buterin has proposed “Big FOCIL” to reduce block builder centralization. The proposal comes ahead of Ethereum’s planned Glamsterdam upgrade. It expands on earlier efforts to secure the block building pipeline.

Buterin said that enshrined Proposer-Builder Separation, known as ePBS, will allow proposers to outsource block construction to a permissionless market. He warned that this approach does not fully solve builder concentration. He presented Big FOCIL as a protocol-level response.

Big FOCIL and the Block Building Pipeline

Ethereum developers plan to introduce ePBS in the Glamsterdam upgrade. The mechanism separates block proposers from block builders. Builders compete in an open market, while proposers select the best bid. Buterin wrote that ePBS prevents builder dominance from spilling into staking. 

However, he asked what should be done about builder centralization itself. He noted that a few advanced actors can optimize transaction ordering and capture more value. Developers are introducing Forward Obligatory Commitment to Inclusion Lists, or FOCIL, where 16 randomly selected attesters choose transactions that must be included in a block. If excluded, the block is rejected. 

Buterin said even a hostile builder controlling all block production cannot prevent transaction inclusion, ensuring censorship resistance. Big FOCIL expands the concept by allowing larger inclusion lists that could cover all block transactions. Participants would select transactions based on sender address rules and pending status to limit duplication and maintain fairness. Builders would then focus on MEV-related transactions and state execution, increasing distributed involvement.

Encrypted Mempools and Toxic MEV

Buterin also addressed toxic MEV practices such as sandwiching and frontrunning. These strategies allow traders to extract value from users by reordering trades. Encrypted mempools are being explored as a technical solution. 

Transactions would remain encrypted until block inclusion. This prevents other actors from seeing details in advance. Buterin wrote, “If a transaction is encrypted until it’s included, no one gets the opportunity to ‘wrap’ it in a hostile way.” 

The main challenge is to ensure that encrypted transactions remain valid and are decrypted at the correct time. Developers are studying how to guarantee efficient validation and timely decryption. The goal is to maintain compatibility with existing mempool systems while improving user protection.

Transaction Ingress and Long Term Distribution

Buterin examined the transaction ingress layer. This layer covers the path between user submission and block inclusion. He noted that transactions sent in the clear can be observed and exploited. He listed risks such as sandwich attacks and transaction griefing. He also pointed out that RPC providers or public nodes can observe private activity. Work is underway on network-layer anonymization.

Proposals include Tor routing, Ethereum-focused mixnets, and bandwidth-heavy but low-latency systems like Flashnet. Buterin referenced the Kohaku initiative and said it may integrate support for such tools. In the longer term, he discussed distributed block building. Ethereum relies on synchronous shared state, which can centralize execution. Big FOCIL partially addresses this issue.

He suggested that many transactions do not require full global state. New transaction categories could support distributed processing and lower costs. Existing transaction types would remain but may cost more. The proposal frames Big FOCIL as a step toward limiting builder control while preserving Ethereum’s core structure. Developers continue to review these ideas as the Glamsterdam upgrade approaches.

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