Original Title: Beyond a core blockchain property: “Strong Chain Quality”

Original Authors: ittaia, PGarimidi, jneu_net, a16z

Original Compilation: AididiaoJP, Foresight News

Chain Quality (CQ) is a core property of blockchains. Simply put, it means:

If you hold 3% of the staked equity, then on average, you will control 3% of the block space.

For early blockchains with low throughput, chain quality was sufficient. But modern blockchains have much greater bandwidth, capable of including a large number of transactions within a single block.

This leads us to a stronger and more refined concept. It not only focuses on the average block space proportion over time but also considers the allocation of block space within each block. We call this “Strong Chain Quality (SCQ)”:

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

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

Chain Quality in Blockchains

One of Bitcoin’s key innovations—now present in nearly every blockchain—is the introduction of a reward mechanism for block proposers within the protocol: the party that successfully adds a block to the state machine can receive newly minted tokens and transaction fees. These rewards are specified by the state transition function and ultimately reflected in the system state.

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

In cryptoeconomic models, participants are viewed as rational actors whose utility functions may be unknown. The goal is to design incentives that align these participants’ self-interest with the successful operation of the protocol. Combining the internal reward mechanisms of the protocol, we can derive the following idealized definition of chain quality:

Chain Quality (CQ): An alliance holding X% of the total staked equity has an X% probability of becoming the proposer of each incoming block after the Global Stability Time (GST).

If a chain deviates from the requirements of chain quality, it may allow certain alliances to gain an excessive share of rewards beyond the normal proportion, weakening the motivation for honest behavior and threatening the security of the protocol.

Many blockchains strive to meet this property through a “randomized leader rotation mechanism based on stake weight.”

Typical challenges currently faced include: Bitcoin’s “selfish mining” problem; Monad’s tail fork resistance issue; and problems in the Ethereum LMD GHOST protocol.

Origins of “Strong Chain Quality”

When block space is sufficiently abundant, we do not have to allow a single proposer to monopolize the entire content of a block. Instead, the block space of the same block can be jointly partitioned by multiple participants. The cryptoeconomic definition of strong chain quality expresses this idea:

Strong Chain Quality (SCQ): An alliance holding X% of the total staked equity can control X% of the block space in each block after the Global Stability Time (GST).

This idealized property implicitly introduces the abstract concept of “virtual lanes.” In other words, alliances can actually control a certain proportion of dedicated block space in each block.

From an economic perspective, owning a virtual lane is equivalent to holding a productive asset that generates revenue, which may come from transaction fees or MEV (Maximum Extractable Value). External entities will compete around the staked equity to acquire and maintain these lanes, creating sustained demand for the underlying L1 tokens. The greater the economic value a lane can produce, the stronger the motivation for parties to compete for staked equity, and the higher the value that L1 staked equity can accumulate for controlling access to this block space. Through this abstraction, we can translate stronger censorship resistance into the SCQ effectiveness property within the protocol.

Strong Chain Quality and Censorship Resistance

Recent research suggests that censorship-resistant protocols are very important. Such protocols must not only ensure that honest inputs are eventually included but also guarantee that they can be included immediately. Strong Chain Quality (SCQ) can be seen as an extension of this property under limited block capacity.

In real scenarios, if the volume of transactions waiting to be included exceeds the available block space, no protocol can satisfy the ideal notion of censorship resistance. SCQ addresses this limitation with a more pragmatic approach: it does not insist that all honest transactions are always included, but rather allocates a “budget” for each staked 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, aiming to enable these protocols with censorship resistance. This protocol also satisfies the requirements of SCQ—it allocates corresponding block space to proposers based on the proportion of staked equity. Existing directed acyclic graph (DAG)-based BFT protocols provide a way to implement multi-writer memory pools, also offering a degree of censorship resistance.

The standard implementation of these protocols often fails to strictly meet SCQ because they allow leaders to selectively delay certain subsets of transactions. However, slight modifications to these protocols could potentially re-implement SCQ. One related direction is “forced transaction inclusion,” aimed at reducing censorship behavior.

MCP also demonstrates how to achieve a stronger hidden property. With this property, stakeholders can create virtual private lanes, the contents of which will only be revealed when the entire block is made public. We will elaborate on this in future articles.

How to Achieve Strong Chain Quality

To achieve strong chain quality after the Global Stability Time (GST), it is crucial to ensure that proposers cannot arbitrarily censor the inputs of stakeholders. This can be implemented through a two-round protocol. On the basis of almost all view-based BFT protocols, only two small modifications are needed:

First Round: Each participant sends their authenticated inputs to all other participants.

Second Round: Each participant, if they received authenticated inputs from participant i, adds i to their inclusion list. Subsequently, that participant sends their inclusion list to the leader. This action serves as a commitment: they will only accept blocks that include all inputs from that list.

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

BFT Voting: A participant will only vote in favor if a block contains all inputs from their own inclusion list.

It is clear that according to this protocol sketch, a complete protocol can be constructed. This protocol can satisfy strong chain quality after the Global Stability Time (GST), provide censorship resistance, and maintain liveness when the leader is an honest party. To achieve SCQ before GST, it will require waiting for a sufficient number (quorum) of values or lists in each round. We will elaborate on this protocol and its extended forms in future articles.

Recent research indicates that achieving strong chain quality and censorship resistance requires adding two more rounds of voting on top of conventional BFT protocols (as indicated in the above protocol sketch). We will also detail this result in subsequent articles.

While strong chain quality (SCQ) specifies the proportion of block space that an alliance can control, it does not fully constrain the ordering of transactions within the block. SCQ can be understood as reserving space for each staked node, but it does not guarantee the order of transactions within those spaces.

This opens up rich research opportunities for the design of transaction ordering mechanisms. A good ordering mechanism is expected to further enhance fairness and efficiency within the blockchain ecosystem. One direction worth paying attention to is prioritizing transactions based on fees.

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