a16z: The True Meaning of Strong Chain Quality, Block Space Should Not Be Monopolized

Original Title: Beyond a core blockchain property: "Strong Chain Quality"

Original Authors: ittaia, PGarimidi, jneu_net, a16z

Original Translation: AididiaoJP, Foresight News

Chain Quality (CQ) is a core property of a blockchain. In simple terms, it means:

If you hold 3% of the stake, then on average, you can control 3% of the block space over time.

For early blockchains with lower throughput, Chain Quality has been sufficient. However, modern blockchains have much larger bandwidth, with a single block capable of containing a large number of transactions.

This leads to a stronger and more nuanced concept. It not only focuses on the average block space ratio over time but also looks at the block space allocation within each block. We call it "Strong Chain Quality (SCQ)";

If you hold 3% of the stake, then in each block, you can control 3% of the block space.

Essentially, this property allows stakeholders to have a "virtual lane" within a high-throughput blockchain, ensuring that their transactions can be included.

Chain Quality in Blockchain

One of btc-42">Bitcoin's key innovations - a feature now present in almost every blockchain - is the introduction of a reward mechanism for block proposers within the protocol: successfully attaching a block to the state machine entitles one to newly minted tokens and transaction fees. These rewards are governed by the state transition function and ultimately reflected in the system state.

In a traditional distributed computing model, participants are divided into honest and malicious parties. There is no need to reward honest parties because honest behavior is the default assumption in the model.

However, in the cryptographic economic model, participants are seen as rational actors with unknown utility functions. The goal is to design incentive measures that align these participants' pursuit of profit maximization with the successful operation of the protocol. By combining the protocol's internal reward mechanism, we can derive the following idealized definition of Chain Quality:

Chain Quality (CQ): A consortium holding X% of the total staked weight, after the Global Stabilization Time (GST), has an X% chance of being the proposer of each block entering the chain.

If a chain deviates from the Chain Quality requirement, it may allow certain consortiums to receive a disproportionate share of the rewards, weakening the incentive for honest behavior and threatening the protocol's security.

Many blockchains aim to meet or strive to meet this property through a "Stake-Weighted Random Leader Selection Mechanism."

Typical challenges currently faced include: the "selfish mining" problem in Bitcoin; Monad's tail-fork resistance problem; and issues in the Ethereum LMD GHOST protocol.

Origin of "Strong Chain Quality"

When block space is abundant, it is not necessary to have a single proposer monopolize the entire block's contents. Instead, multiple participants can collectively partition the block space of the same block. The cryptographic-economic concept expressed by "Strong Chain Quality" embodies this idea:

Strong Chain Quality (SCQ): A consortium holding X% of the total staked weight, after the Global Stabilization Time (GST), is able to control X% of the block space in each block.

This idealized attribute implicitly introduces the abstract concept of a "virtual lane." In other words, the consortium can effectively control a certain percentage of dedicated block space in each block.

From an economic perspective, owning a virtual lane is akin to possessing a productive asset that can generate revenue, which may come from transaction fees or MEV (Maximal Extractable Value). External entities compete around staked weight to acquire and maintain these lanes, creating a sustained demand for the underlying L1 token. The greater the economic value a lane can generate, the stronger the incentive for stakeholders to compete for staked weight, and the higher the value that staked weight controlling access to these block spaces can accumulate. Through this abstraction, we can translate stronger censorship resistance into the SCQ validity property within the protocol.

Strong Chain Quality and Censorship Resistance

Recent research indicates that censorship-resistant protocols are crucial. These protocols must ensure not only that honest parties' inputs are eventually included but also that they are included promptly. Strong Chain Quality (SCQ) can be seen as an extension of this property when block capacity is limited.

In a real-world scenario, if the transaction volume to be included exceeds the available block space, no protocol can achieve perfect censorship resistance. SCQ takes a more pragmatic approach to address this limitation: it does not require that all honest transactions always be included but instead allocates a "budget" to each staking node to ensure that transactions within this budget can be included.

The MCP protocol is proposed as a component built on top of existing Practical Byzantine Fault Tolerance (PBFT) consensus protocols to make these protocols censorship-resistant. The protocol also meets the requirements of SCQ—it allocates block space to proposers based on stake proportion. Existing Byzantine Fault Tolerance (BFT) protocols based on Directed Acyclic Graphs (DAGs) provide a way to implement a multi-writer mempool and also offer some level of censorship resistance.

The standard implementations of these protocols often do not strictly meet SCQ because they allow leaders to selectively delay certain subsets of transactions. However, with slight modifications to these protocols, it is possible to reimplement SCQ. One related direction is "forced transaction inclusion" to reduce censorship behavior.

MCP also demonstrates how to achieve stronger privacy attributes. With this attribute, stakeholders can create virtual private lanes where the content is only revealed when the entire block is publicly disclosed. We will elaborate on this further in upcoming articles.

How to Achieve Strong Chain Quality

To achieve strong chain quality after Global Stable Time (GST), it is crucial to ensure that proposers cannot arbitrarily censor stakeholders' input. This can be accomplished through a two-round protocol. On almost all view-based BFT protocol foundations, only two small changes are needed:

First Round: Each participant broadcasts their authenticated input to all other participants.

Second Round: If a participant receives authenticated input from participant i, they add i to their inclusion list. Subsequently, the participant sends their inclusion list to the leader. This operation is akin to a commitment: only accepting blocks that include all inputs from that list.

BFT Proposal: Upon receiving these messages, the leader includes the union of all received inclusion lists in the block.

BFT Voting: A participant only casts an affirmative vote if a block includes all the inputs from their own inclusion list.

It is easy to see that, according to this protocol sketch, a complete protocol can be constructed. The protocol can achieve strong chain quality (SCQ) after Global Stable Time (GST), provide censorship resistance, and maintain liveness when the leader is honest. To achieve SCQ before GST, it is also necessary to wait for a sufficient number (quorum) of values or lists in each round. We will elaborate on this protocol and its extension in future articles.

Recent research has shown that to achieve strong chain quality and censorship resistance, two additional rounds need to be added on top of the regular BFT protocol's voting rounds (as shown in the protocol sketch above). We will also detail this result in future articles.

Although strong chain quality (SCQ) dictates the proportion of the block space controlled by the coalition, it does not fully specify the order of transactions within a block. SCQ can be understood as: space is allocated for each staking node, but no guarantee is provided regarding the order of transactions within this space.

This opens up rich research opportunities for transaction ordering mechanisms. A good ordering mechanism is expected to further enhance fairness and efficiency in the blockchain ecosystem. One promising direction is to prioritize transactions based on transaction fees.

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