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 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 low throughput, chain quality is already sufficiently applicable. But modern blockchains have much greater bandwidth, allowing a single block to contain many transactions.
This introduces a stronger and more refined concept. It not only focuses on the average block space ratio 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 that their transactions can be included.
Chain Quality in Blockchain
One of the key innovations of Bitcoin — which now nearly every blockchain possesses — 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 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 here because honest behavior itself is the default assumption in the model.
In the cryptoeconomic model, participants are seen as rational actors, and their utility functions may be unknown. The goal is to design incentives that align these participants' pursuit of profit maximization with the successful operation of the protocol. Combining the reward mechanism within 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 Stabilization Time (GST).
If a chain deviates from the requirements of chain quality, it could allow certain alliances to gain a reward share beyond the normal proportion, thus weakening the incentive for honest behavior and threatening the security of the protocol.
Many blockchains utilize a "staking-weighted random leader rotation mechanism" to meet or strive to meet this property.
Typical current challenges include: Bitcoin's "selfish mining" problem; Monad’s tail fork resistance issue; and issues in Ethereum's LMD GHOST protocol.
The Origin of "Strong Chain Quality"
When block space is sufficiently abundant, we do not need to allow a single proposer to monopolize the entire contents of a block. Conversely, the block space within the same block can be collectively divided by multiple participants. The cryptoeconomic definition of strong chain quality conveys 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 Stabilization Time (GST).
This idealized property implicitly introduces the abstract concept of "virtual lanes." In other words, alliances can effectively 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 income, which could derive from transaction fees or MEV (Maximum Extractable Value). External entities will compete around the staked equity to gain and maintain these lanes, creating sustained demand for the underlying L1 tokens. The greater the economic value generated by a lane, the stronger the incentive for parties to compete for staked equity, and the higher the value that the L1 staked equity controlling access to these block spaces can accumulate. Through this abstraction, we can convert stronger censorship resistance into the effective attribute of SCQ in the protocol.
Strong Chain Quality and Censorship Resistance
Recent studies indicate that censorship-resistant protocols are very important. Such protocols must not only ensure that the inputs of honest parties are ultimately included but also that they are immediately included. Strong Chain Quality (SCQ) can be viewed as an extension of this property under conditions of limited block capacity.
In practical scenarios, if the volume of pending transactions exceeds the available block space, then no protocol can guarantee ideal censorship resistance. SCQ addresses this limitation with a more pragmatic approach: it does not require all honest transactions to always be included but instead allocates a "budget" for each staking node, ensuring their transactions can be included within this budget.
The MCP protocol was proposed as a component built on existing Practical Byzantine Fault Tolerance (PBFT) style consensus protocols, with the aim of providing these protocols with censorship resistance. This protocol also meets the requirements of SCQ — it allocates corresponding block space to proposers based on their staked equity ratio. Existing DAG-based BFT protocols provide a way to realize a multi-writer memory pool, also possessing a certain degree of censorship resistance.
Standard implementations of these protocols often fail 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 "mandatory transaction inclusion" to reduce censorship behavior.
MCP also demonstrates how to achieve a stronger hiding property. With this property, stakeholders can create virtual private lanes, where the contents are only revealed when the entire block is publicly disclosed. We will elaborate further on this point in subsequent articles.
How to Achieve Strong Chain Quality
To achieve strong chain quality after the Global Stabilization 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. Based on 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 have received authenticated inputs from participant i, will add i to their inclusion list. Then, this participant sends their inclusion list to the leader. This operation effectively commits them to only accept blocks that include all inputs on this 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 easy to see, according to this protocol sketch, that a complete protocol can be constructed. This protocol can satisfy strong chain quality after the Global Stabilization Time (GST), provide censorship resistance, and remain active when the leader is an honest party. If we want to achieve SCQ before GST, we will also need to wait for a sufficient number (quorum) of values or lists in each round. We will elaborate on this protocol and its extended forms in subsequent articles.
Recent studies have shown that to achieve strong chain quality and censorship resistance, two additional rounds must be added on top of the voting rounds of conventional BFT protocols (as outlined in the protocol sketch above). We will detail this result in future articles.
Although Strong Chain Quality (SCQ) specifies the proportion of block space that an alliance can control, it does not fully define the order of transactions within the block. SCQ can be understood as reserving space for each staking node but does not guarantee the order of transactions within these spaces.
This opens up rich research opportunities for the design of transaction ordering mechanisms. A good ordering mechanism has the potential to further enhance fairness and efficiency in the blockchain ecosystem. One noteworthy direction is to prioritize transaction ordering based on fees.
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