a16z: "Strong chain quality" ensures that every staker has a dedicated space within the block.

By @ittaia, @PGarimidi, and @jneu_net

Compiled by AididiaoJP, Foresight News

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

If you hold 3% of the staked stake, then within the average time window, you can control 3% of the block space.

For early blockchains with relatively low throughput, chain quality is already sufficient. But modern blockchains have far more bandwidth—so a single block can include a large number of transactions.

This leads to a stronger, more fine-grained concept. Instead of only caring about the average proportion of block space over time, it focuses on how block space is partitioned within each individual block. We call this “Strong Chain Quality” (Strong Chain Quality, SCQ):

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

In essence, this property enables stakeholders to have “virtual lanes” inside a high-throughput blockchain, ensuring that their transactions are included.

“Chain Quality” in a blockchain

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

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

In contrast, in cryptoeconomic models, participants are viewed as rational actors, whose utility functions may be unknown. The goal is to design incentives so that, as these participants pursue their own profit maximization, they naturally align with the successful operation of the protocol. Combining the protocol’s internal reward mechanism, we can arrive at the following idealized definition of chain quality:

Chain Quality (CQ): A coalition holding X% of the total staked stake will, after the Global Stable Time (GST), have an X% probability of being the proposer of every block that enters the chain.

If a chain deviates from the requirements of chain quality, it could allow certain coalitions to receive reward shares beyond the normal proportion, undermining the motivation for honest behavior and threatening the protocol’s security.

Many blockchains meet or attempt to meet this property through a “stake-weighted random leader rotation” mechanism. Typical challenges currently include Bitcoin’s “selfish mining” issue, Monad’s tail-partition fork resistance issue, and issues in the Ethereum LMD GHOST protocol.

The origin of “Strong Chain Quality”

When block space is plentiful, we don’t need to hand the entire contents of a block to a single proposer monopolistically. Instead, the block space of the same block can be jointly partitioned by multiple participants. The cryptoeconomic definition of strong chain quality expresses exactly this idea:

Strong Chain Quality (SCQ): A coalition holding X% of the total staked stake, after the Global Stable Time (GST), can control X% of the block space in every block.

This idealized property implicitly introduces the abstract concept of “virtual lanes.” That is, the coalition can actually control a certain proportion of dedicated block space in every block.

From an economic perspective, having a virtual lane is akin to owning a productive asset that generates yield—yields that may come from transaction fees or from MEV (maximum extractable value). External entities compete to obtain and maintain these lanes, which creates sustained demand for the underlying L1 token. The greater the economic value a lane can generate, the stronger the incentives for all parties to compete by staking stake. And the more value that can be accumulated by the L1 staked stake that controls access rights to these block spaces. Through this abstraction, we can translate stronger censorship resistance into the protocol’s SCQ effectiveness property.

Strong Chain Quality and Censorship Resistance

Recent research indicates that censorship-resistant protocols are extremely important. Such protocols must not only ensure that inputs from honest parties are eventually included, but also that they are included immediately. Strong Chain Quality (SCQ) can be viewed as an extension of this property under conditions of limited block capacity.

In real-world scenarios, if the volume of transactions to be included exceeds the available block space, then no protocol can satisfy ideal censorship resistance. SCQ addresses this limitation with a more pragmatic approach: it does not insist that all honest transactions are always included; instead, it assigns a “budget” to each staked node, ensuring that within that budget range, its transactions can be included.

The MCP protocol is proposed as a component built on top of existing practical Byzantine Fault Tolerance (PBFT)-style consensus protocols, with the goal of making these protocols censorship-resistant. It satisfies the requirements of SCQ as well—it allocates proposers corresponding block space based on the proportion of staked stake. Existing DAG-based BFT protocols also provide a way to implement a multi-writer mempool, and they have some degree of censorship resistance.

The standard implementations of these protocols typically fail to strictly meet SCQ, usually because they allow leaders to selectively delay certain subsets of transactions. However, with small modifications to these protocols, it may be possible to re-achieve SCQ. A related direction is “forced transaction inclusion,” intended to reduce censorship behavior.

MCP also demonstrates how to implement an even stronger hidden property. With this property, stakeholders can create virtual private lanes, where the contents of those lanes are only revealed when an entire block is published to the outside. We will expand on this further in subsequent articles.

How to Achieve Strong Chain Quality

To obtain strong chain quality after the Global Stable Time (GST), the key is to ensure that proposers cannot freely censor stakeholders’ inputs. This can be achieved with a two-round protocol. Based on almost all view-based BFT protocols, only two small changes are needed:

First round: Each participant sends its authenticated inputs to every other participant.

Second round: If a participant receives authenticated inputs from participant i, it adds i to its own inclusion list. Then, the participant sends its inclusion list to the leader. This effectively commits to accepting only blocks that include all inputs in that list.

BFT proposal: After receiving these messages, the leader records in the block the union of all inclusion lists it received.

BFT voting: A participant votes in favor only if a block contains all inputs in that participant’s own inclusion list.

It’s not hard to see that, following this protocol sketch, you can construct a complete protocol. This protocol is able to satisfy strong chain quality after the Global Stable Time (GST), provide censorship resistance, and remain live when the leader is honest. If you also want to achieve SCQ before GST, you would need to wait for a sufficient number of (quorum) values or lists in each round. We will explain this protocol and its variants in detail in subsequent articles.

Recent research indicates that to achieve strong chain quality and censorship resistance, you need to add two additional rounds (as shown in the protocol sketch above) on top of the voting rounds of a regular BFT protocol. We will also explain this result in detail in later articles.

While Strong Chain Quality (SCQ) specifies the proportion of block space that coalitions can control, it does not fully constrain how transactions are ordered within blocks. SCQ can be understood as reserving space for each staked node, but providing no guarantees about the order of transactions within those spaces.

This opens up a rich research space for the design of transaction ordering mechanisms. A good ordering mechanism is expected to further improve fairness and efficiency in the blockchain ecosystem. One worth paying attention to is ordering transactions based on priority fees.