a16z: “Strong Chain Quality” Grants Each Staker Dedicated Space Within a Block
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a16z: “Strong Chain Quality” Grants Each Staker Dedicated Space Within a Block
From Chain Quality to Strong Chain Quality: How to Truly Achieve “My Transaction, My Choice” in the High-Throughput Era
Navigating Web3 tides with focused insightsAuthors: @ittaia, @PGarimidi, and @jneu_net
Translation: AididiaoJP, Foresight News
Chain Quality (CQ) is a core property of blockchains. Informally, it means:
If you hold 3% of the staked stake, then, on average over time, you control 3% of the block space.
For early low-throughput blockchains, chain quality has been sufficient. However, modern blockchains offer significantly higher bandwidth, enabling each block to contain a large number of transactions.
This gives rise to a stronger and more refined concept—not only concerned with the average proportion of block space over time, but also focused on how block space is allocated within each individual block. We call this “Strong Chain Quality” (SCQ):
If you hold 3% of the staked stake, then in every block, you control 3% of the block space.
Essentially, this property grants stakeholders “virtual lanes” inside a high-throughput blockchain, guaranteeing inclusion of their transactions.
Chain Quality in Blockchains
One of Bitcoin’s key innovations—now adopted by nearly every blockchain—is the introduction of an internal reward mechanism for block proposers: parties who successfully append a block to the state machine receive 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 and malicious parties. There is no need to reward honest behavior, since honesty is assumed by default in the model.
In contrast, cryptoeconomic models treat participants as rational agents whose utility functions may be unknown. The goal is to design incentives such that, in pursuing their own profit maximization, participants naturally align with the protocol’s successful operation. Combining this with the protocol’s internal reward mechanism yields the following idealized definition of Chain Quality:
Chain Quality (CQ): A coalition holding X% of the total staked stake has an X% probability of being the proposer of each block entering the chain after Global Stabilization Time (GST).
If a chain deviates from CQ requirements, certain coalitions may receive disproportionately large reward shares, thereby weakening incentives for honest behavior and threatening protocol security.
Many blockchains satisfy—or strive to satisfy—this property via “stake-weighted random leader rotation.” Typical challenges include Bitcoin’s “selfish mining” problem; Monad’s tail-fork resistance issue; and problems arising in Ethereum’s LMD GHOST protocol.
Origins of Strong Chain Quality
When block space is sufficiently abundant, there is no need to assign the entire contents of a block to a single proposer. Instead, block space within the same block can be jointly allocated among multiple participants. Strong Chain Quality expresses precisely this idea as a cryptoeconomic definition:
Strong Chain Quality (SCQ): A coalition holding X% of the total staked stake controls X% of the block space in every block after Global Stabilization Time (GST).
This idealized property implicitly introduces the abstraction of “virtual lanes”: i.e., coalitions effectively control a dedicated portion of block space in every block.
Economically, owning a virtual lane is equivalent to holding a productive, revenue-generating asset—revenue that may come from transaction fees or MEV (Maximal Extractable Value). External entities compete for and maintain these lanes by acquiring staked stake, thereby creating sustained demand for the underlying L1 token. The greater the economic value generated by a lane, the stronger the incentive to acquire staked stake—and thus the higher the value accumulated by L1 staked stake controlling access to this block space. Through this abstraction, stronger censorship resistance can be translated into the SCQ effectiveness property of the protocol.
Strong Chain Quality and Censorship Resistance
Recent research shows that censorship-resistant protocols are critically important. Such protocols must not only guarantee eventual inclusion of honest parties’ inputs, but also ensure immediate inclusion. Strong Chain Quality (SCQ) can be viewed as an extension of this property under finite block capacity.
In practice, if the volume of transactions awaiting inclusion exceeds available block space, no protocol can satisfy ideal censorship resistance. SCQ addresses this limitation with a more pragmatic approach: rather than demanding all honest transactions always be included, it allocates a “budget” to each staking node, ensuring inclusion of its transactions up to that budget.
The MCP protocol was proposed as a component layered atop existing practical Byzantine Fault Tolerance (PBFT)-style consensus protocols to endow them with censorship resistance. It simultaneously satisfies SCQ—allocating block space to proposers proportionally to their staked stake. Existing Directed Acyclic Graph (DAG)-based BFT protocols provide a way to implement multi-writer mempools and also possess some degree of censorship resistance.
Standard implementations of these protocols typically fail to strictly satisfy SCQ because they allow leaders to selectively delay certain subsets of transactions. However, minor modifications to these protocols could restore SCQ compliance. A related direction is “forced transaction inclusion,” aimed at reducing censorship.
MCP further demonstrates how to realize a stronger hidden property: using it, stakeholders can create virtual private lanes whose contents remain concealed until the entire block is publicly revealed. We will elaborate on this in a subsequent article.
How to Achieve Strong Chain Quality
To achieve Strong Chain Quality after Global Stabilization Time (GST), the key is ensuring proposers cannot arbitrarily censor stakeholders’ inputs. This can be accomplished via a two-round protocol. Starting from almost any view-based BFT protocol, only two small modifications are needed:
- Round 1: Each participant broadcasts its authenticated input to all other participants.
- Round 2: Each participant adds participant i to its inclusion list upon receiving i’s authenticated input, then sends its inclusion list to the leader. This acts as a commitment: the participant will only accept blocks containing all inputs in its inclusion list.
- BFT Proposal: Upon receiving these messages, the leader includes the union of all received inclusion lists in the block.
- BFT Voting: A participant votes “yes” only if the block contains all inputs in its own inclusion list.
It is straightforward to see that this protocol sketch can be fleshed out into a full protocol satisfying Strong Chain Quality after GST, providing censorship resistance and maintaining liveness when the leader is honest. To achieve SCQ before GST, each round would additionally require waiting for a quorum of values or lists. We will detail this protocol and its extensions in a subsequent article.
Recent research indicates that achieving Strong Chain Quality and censorship resistance requires adding two extra rounds (as outlined above) atop standard BFT voting rounds. We will also explain this result in detail in a subsequent article.
While Strong Chain Quality (SCQ) specifies the proportion of block space a coalition controls, it does not fully constrain the ordering of transactions within that space. SCQ can be understood as reserving space for each staking node—but makes no guarantees about the order of transactions within that reserved space.
This opens up rich research avenues for transaction ordering mechanisms. A well-designed ordering mechanism holds promise to further improve fairness and efficiency across the blockchain ecosystem. One promising direction is ordering transactions by priority fee.
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