Round Robin Proposer

The proposer schedule is known in advance, calculated from the validator set and the current block height. This eliminates randomness and ensures each validator gets a predictable turn, enhancing protocol predictability and simplifying validator resource planning.

By guaranteeing each validator an equal opportunity to propose, the mechanism promotes fairness in block space access and transaction inclusion. It reduces the risk of a single entity dominating block production, a key component of censorship resistance.

The pre-defined order reduces the complexity of the consensus process. Validators do not need to run complex leader election algorithms for each block, which can lower communication overhead and increase finality speed in networks like Tendermint-based chains.

A key trade-off is between liveness and predictability. If a scheduled proposer is offline, the network must wait for its timeout period before skipping to the next, potentially causing delays. This contrasts with random selection, which can immediately choose an online validator.

The Tendermint consensus engine uses a round robin proposer selection within each consensus round. The proposer for a round is determined by the formula: ProposerIndex = (Round + Height) % ValidatorSetSize. This provides deterministic liveness.

Unlike Ethereum's RANDAO-based proposer selection, which uses verifiable randomness, round robin is non-random and deterministic. RANDAO aims for unpredictability to prevent manipulation, while round robin prioritizes simplicity and guaranteed turns.

Within the Avalanche ecosystem, custom subnets can implement a Round Robin proposer selection for their Primary Network validation. This is a common choice for private or permissioned subnets where a known, fixed set of validators require a simple, fair, and leaderless mechanism for block proposal.

Round Robin is the standard choice for private and consortium blockchains (e.g., those built with Hyperledger Besu or Fabric). Its key advantages in these settings are:

• Predictability: Known order of operation aids in auditing.
• Simplicity: Eliminates the complexity of cryptographic leader election.
• Fairness: Guarantees each authorized participant an equal turn.

The original Binance Smart Chain (BSC) employed a variant of Round Robin among its 21 validators. Each validator was assigned a specific time slot to propose blocks, creating a highly predictable 3-second block time. This design was a key factor in achieving high throughput while maintaining decentralization among a permissioned set.

Round Robin is distinctly different from the probabilistic leader election used by chains like Ethereum (PoS) or Solana. Key contrasts:

• Predictability vs. Randomness: Round Robin is deterministic; others use RANDAO or VRF for randomness.
• Fairness vs. Weighted Chance: Round Robin gives equal turns; probabilistic systems weight chances by stake.
• Use Case: Round Robin suits known validator sets, while probabilistic is essential for large, permissionless networks.

Validators take turns in a pre-determined, cyclical order, creating a predictable schedule for block creation. This eliminates the randomness and computational lottery of Proof-of-Work (PoW).

• Enables reliable block time estimation.
• Simplifies network monitoring and coordination for validators.

The deterministic rotation ensures every active validator gets an equal opportunity to propose blocks, proportional to their stake or committee position. This prevents a single entity from dominating block production.

• Mitigates temporal centralization risks.
• Reduces the potential for transaction censorship by a single proposer.

Unlike PoW's competitive hashing, round robin selection requires minimal computational overhead for leader election. Validators know precisely when their turn is, conserving energy and resources.

• Eliminates wasteful computation between blocks.
• Allows validators to optimize hardware for specific tasks.

When combined with a BFT voting mechanism (e.g., Tendermint), the known proposer schedule streamlines the commit process. Validators can prepare votes in advance for the expected proposer, leading to faster finality.

• Reduces communication complexity in the consensus round.
• Enables faster block finalization times.

The predictability allows the protocol to implement strict slashing conditions for liveness failures. A validator that misses its assigned slot can be penalized, as there is no ambiguity about whose responsibility it was.

• Enforces validator accountability.
• Deters downtime and improves network liveness guarantees.

Contrasts with alternative leader selection mechanisms:

• PoW / VRF-based: Random, probabilistic selection (e.g., Ethereum beacon chain).
• Stake-Weighted: Probability proportional to stake size.
• DPoS Voting: Leaders elected by stakeholder votes. Round robin is favored for its simplicity and strong fairness in permissioned or consortium chains.

The deterministic schedule of a round-robin system makes the next block proposer known in advance. This predictability can enable targeted attacks, such as Denial-of-Service (DoS) attacks aimed at the upcoming proposer to prevent them from proposing a block, thereby disrupting network liveness. Attackers can focus resources on a single known target rather than a random, unpredictable one.

A known proposer schedule can facilitate transaction censorship. Malicious actors, including the proposer themselves or external parties, can identify which validator will propose the next block and attempt to bribe or coerce them to exclude certain transactions. This is a more straightforward attack than in a random selection system where the next proposer is unknown until the last moment.

If a validator in the rotation is offline or maliciously withholds its block, the protocol must have a mechanism to skip the slot and proceed to the next proposer. This ensures liveness but temporarily breaks the strict fairness guarantee. The network must define timeout conditions and penalties for missed proposals to maintain overall health.

In permissioned or stake-weighted round-robin systems, an attacker could create multiple validator identities (Sybils) to gain a disproportionate share of proposal rights within the rotation cycle. This undermines the intended fairness. Mitigation requires robust identity verification or a staking mechanism with significant economic cost per validator seat.

The predictable order allows for advanced coordination of Maximal Extractable Value (MEV). Entities can plan complex multi-block MEV strategies, knowing which validator will propose a sequence of future blocks. This can lead to centralized MEV supply chains and exacerbate the negative externalities of MEV, such as increased transaction costs for regular users.

Many modern blockchains use hybrid models to balance fairness and security. Common approaches include:

• Randomized Round Robin: The order is randomly permuted each epoch.
• Weighted Random Selection: Proposer chance is proportional to stake, with algorithms like RANDAO+VDF providing unbiased randomness.
• Leader Election Cryptography: Using verifiable random functions (VRFs) to select the next proposer privately and unpredictably.

The specific group of nodes authorized to participate in block proposal and validation for a given epoch or period. The Round Robin algorithm sequentially iterates through this ordered list. The set's size and security (e.g., minimum stake requirements) are critical to network decentralization.

The profit a block proposer can make by including, excluding, or reordering transactions. In a Round Robin system, the predictable order of proposers allows for advanced MEV strategies, as entities can know when they or specific others will propose.

Node software (e.g., Prysm, Lighthouse) responsible for running the Proof-of-Stake consensus logic. It manages the validator's duties, including executing the Round Robin algorithm to determine if it is the current block proposer and participating in attestations.

An alternative to deterministic Round Robin, where the next proposer is chosen via a verifiable random function (VRF). This increases unpredictability, enhancing security against targeted attacks or MEV exploitation, but adds complexity compared to simple rotation.