Written by: @ittaia, @PGarimidi, and @jneu_net

Translated by: AididiaoJP, Foresight News

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

If you hold 3% of the staking rights, then on average, you will be able to control 3% of the block space.

For early blockchains with lower throughput, chain quality was sufficient. However, modern blockchains have vastly greater bandwidth and can include many transactions within a single block.

This leads to a stronger and more nuanced concept. It not only focuses on the average block space ratio over time but also considers the division of block space within each block. We call this "Strong Chain Quality" (SCQ):

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

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

Chain Quality in Blockchain

One of the key innovations of Bitcoin—which is now present in nearly every blockchain—is the introduction of a reward mechanism for block proposers within the protocol: the party that successfully attaches a block to the state machine can receive newly minted tokens and transaction fees. These rewards are stipulated 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 these participants naturally align with the successful operation of the protocol while pursuing their own profit maximization. Combining this reward mechanism within the protocol, we can derive the following idealized definition of chain quality:

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

If a chain strays from the requirements of chain quality, it may allow certain alliances to receive an excessive share of rewards beyond the normal ratio, thereby undermining the motivation for honest behavior and threatening the security of the protocol.

Many blockchains attempt to meet or strive to meet this attribute through "staking-weighted random leader rotation mechanisms." Typical challenges currently faced include Bitcoin's "selfish mining" issue; the tail fork resistance issue in Monad; and issues within the Ethereum LMD GHOST protocol.

The Origin of "Strong Chain Quality"

When block space is sufficiently abundant, there is no need for the entire content of a block to be monopolized by a single proposer. Instead, the block space of the same block can be jointly divided by multiple participants. The definition of strong chain quality in cryptoeconomics expresses this idea:

Strong Chain Quality (SCQ): An alliance holding X% of the total staking rights can control X% of the block space in every block after the global stable time (GST).

This idealized property implicitly introduces the abstract concept of "virtual lanes". This means that alliances can actually control a certain proportion of dedicated block space in every block.

From an economic perspective, having a virtual lane is equivalent to holding a productive asset that can generate income, which may come from transaction fees or MEV (Maximum Extractable Value). External entities will compete around staking rights to gain and maintain these lanes, creating a continuous demand for the underlying L1 tokens. The greater the economic value generated by a lane, the stronger the motivation for parties to compete for staking rights, and the higher the value that L1 staking rights can accumulate to control access to this block space. Through this abstraction, we can convert stronger anti-censorship capabilities into the validity attribute of SCQ within the protocol.

Strong Chain Quality and Anti-Censorship

Recent research indicates that anti-censorship protocols are very important. Such protocols must not only ensure that honest parties' inputs are ultimately included, but also guarantee that they can be immediately included. Strong Chain Quality (SCQ) can be seen as an extension of this attribute in cases of 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 anti-censorship. SCQ addresses this limitation with a more pragmatic approach: it does not require that all honest transactions are always included, but rather allocates a "budget" for each staking node, ensuring that their transactions can be included within that budget.

The MCP protocol is proposed as a component on top of existing practical Byzantine Fault Tolerance (PBFT) consensus protocols to endow these protocols with anti-censorship capabilities. This protocol also satisfies the requirements of SCQ—it allocates corresponding block space to proposers based on the proportion of staking rights. Existing BFT protocols based on directed acyclic graphs (DAG) offer a way to achieve multi-writer memory pools and also possess a degree of anti-censorship capabilities.

Standard implementations of these protocols often fail to strictly meet SCQ because they allow leaders to selectively delay certain subsets of transactions. However, with slight modifications to these protocols, it may be possible to re-establish SCQ. One direction related to this is "mandatory transaction inclusion", which is used 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 subsequent articles.

How to Achieve Strong Chain Quality

To achieve strong chain quality after global stable time (GST), the key is to ensure that proposers cannot arbitrarily censor stakeholders' inputs. This can be achieved through a two-round protocol. On the basis of almost all view-based BFT protocols, only two minor changes are needed:

• First Round: Each participant sends their authenticated inputs to all other participants.
• Second Round: Each participant, if they receive authenticated input from participant i, adds i to their inclusion list. Subsequently, that participant sends their inclusion list to the leader. This operation effectively commits to only accepting blocks that include all the 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 includes all inputs from that participant's 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 global stable time (GST), provide anti-censorship capabilities, and remain active 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 subsequent articles.

Recent research indicates that achieving strong chain quality and anti-censorship requires two additional rounds of voting beyond conventional BFT protocols (as shown in the above protocol sketch). We will also elaborate on this result in subsequent articles.

Although 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 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 in the blockchain ecosystem. One promising direction is to order transactions based on priority fees.