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 blockchain. In simple terms, it means:

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

For early blockchains with lower throughput, chain quality has been sufficient. But modern blockchains have much larger bandwidth, allowing a single block to contain a large number of transactions.

This leads to a stronger and more refined concept. It not only focuses on the average proportion of block space over time but also looks at 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 their transactions can be included.

Chain Quality in Blockchain

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

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

In the crypto-economic model, participants are viewed as rational actors whose utility functions may be unknown. The goal is to design incentives that naturally align these participants’ pursuit of profit maximization with the successful operation of the protocol. Combined with the internal reward mechanisms of the protocol, we can derive the following idealized definition of chain quality:

Chain Quality (CQ): An alliance holding a total staked equity of X% has an X% probability of becoming the proposer of each block entering the chain after the global stable time (GST).

If a chain deviates from the requirements of chain quality, it may allow some alliances to receive a reward share that exceeds the normal proportion, thereby weakening the incentive for honest behavior and threatening the security of the protocol.

Many blockchains meet or strive to meet this property through a “staking-weighted random leader rotation mechanism.”

Current typical challenges include: Bitcoin’s “selfish mining” problem; Monad’s tail fork resistance problem; and issues within Ethereum’s LMD GHOST protocol.

Origins of “Strong Chain Quality”

When block space is 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 allocated by multiple participants. The crypto-economic definition of strong chain quality expresses this idea:

Strong Chain Quality (SCQ): An alliance holding a total staked equity of X% can control X% of the block space in each block after the global stable time (GST).

This idealized property implicitly introduces the abstract concept of “virtual lanes.” In other words, the alliance can effectively control a dedicated portion of block space in each block.

From an economic perspective, owning a virtual lane is akin to holding a productive asset that can generate returns, which may come from transaction fees or MEV (Maximum Extractable Value). External entities will compete around staking equity to acquire and maintain these lanes, creating continuous demand for the underlying L1 tokens. The greater the economic value a lane can generate, the stronger the motivation for parties to compete for staking equity, and the higher the value that can be accumulated from controlling access to this block space. Through this abstraction, we can convert stronger censorship resistance into the effective property of SCQ within the protocol.

Strong Chain Quality and Censorship Resistance

Recent studies indicate that censorship-resistant protocols are critically important. Such protocols must not only ensure that the inputs of honest parties are ultimately included but also guarantee they can be included immediately. Strong Chain Quality (SCQ) can be viewed as an extension of this property in scenarios with limited block capacity.

In practical scenarios, if the volume of transactions to be included exceeds the available block space, then no protocol can achieve ideal 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 staking node, ensuring that within this budget, their transactions can be included.

The MCP protocol was proposed as a component on top of existing practical Byzantine Fault Tolerance (PBFT)-style consensus protocols to enable these protocols to achieve censorship resistance. This protocol simultaneously meets the requirements of SCQ—allocating appropriate block space to proposers based on their staking equity. Existing DAG-based BFT protocols provide a way to implement multi-writer memory pools, also offering a degree of censorship resistance.

The standard implementations of these protocols often fail to strictly meet SCQ due to allowing leaders to selectively delay certain subsets of transactions. However, with slight modifications, it is possible to re-implement SCQ. One related direction is “mandatory transaction inclusion” to reduce 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 stable time (GST), the key is to ensure that proposers cannot arbitrarily censor the inputs of stakeholders. This can be achieved through a two-round protocol. On the basis of almost all view-based BFT protocols, only two minor modifications are needed:

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

Second Round: Each participant, upon receiving authenticated input from participant i, adds i to their inclusion list. Subsequently, that participant sends their inclusion list to the leader. This operation acts 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: Only when a block includes all inputs from that participant’s own inclusion list will that participant vote in favor.

It is evident that a complete protocol can be constructed based on this protocol sketch. This protocol can satisfy Strong Chain Quality after the global stable time (GST), provide censorship resistance, and maintain liveness when the leader is an honest party. 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 detail this protocol and its extended forms in future articles.

Recent research indicates that to achieve Strong Chain Quality and censorship resistance, it is necessary to add two additional rounds on top of the voting rounds of conventional BFT protocols (as illustrated in the above protocol sketch). We will also elaborate on this result in future articles.

While Strong Chain Quality (SCQ) specifies the proportion of block space 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 staking node, but it does not guarantee the order of transactions within that space.

This opens up rich research space 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 focusing on is to order transactions based on priority fees.

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