The Heartbeat of Cardano.

Ouroboros Leios: A New Era in Blockchain Performance

I read the Ouroboros Leios paper, so you don’t have to. Here’s what I learnt:

Explain like I’m 5

Imagine the world of blockchain as a city, and we’re trying to make the traffic flow faster and more efficiently. Ouroboros Leios is like a new traffic system that helps with this goal. It makes sure the cars (transactions) can move around better and not get stuck in traffic jams.

Leios does this by creating a more complex road system (blockchain structure) that lets cars move in different directions at the same time, so they don’t have to wait for one another. This new system also has some cool features like different lanes (tiered transaction fees) for faster or slower service and quicker traffic updates (faster chain synchronization).

However, before we can use this new traffic system in our city (Cardano), we need to make sure it’s safe and works well. We have to plan, test, and make adjustments to fit it into our existing city layout. This process has many steps, like finalizing the design, understanding how it will work with our current city, and making sure it’s secure and efficient.

Once all these steps are done, we can use this new traffic system (Ouroboros Leios) in our city (Cardano), and everyone can enjoy faster and smoother rides in the world of blockchain.


As the world of blockchain technology evolves, new developments emerge to address existing limitations and pave the way for more efficient and secure systems. One such innovation is Ouroboros Leios, a groundbreaking variant of the Ouroboros blockchain algorithm. 

Designed to increase throughput while maintaining robust security properties, Ouroboros Leios aims to overcome the challenges faced by its predecessors and push the boundaries of blockchain technology.

The limitations of current Ouroboros variants stem from data and communication dependencies within the distributed algorithm, which restrict throughput. Ouroboros Leios tackles these limitations with an innovative blockchain structure featuring concurrent data dependencies.

 This approach enables increased parallelism and resource utilization, resulting in higher throughput and enhanced overall performance.

In addition to addressing throughput concerns, Ouroboros Leios incorporates modern features to enhance practicality and user experience. These features include tiered transaction fees with corresponding levels of service priority and faster chain synchronization achieved by eliminating the need to execute every smart contract.

It is important to acknowledge that the implementation of Ouroboros Leios within Cardano is not without its challenges. The process will require careful preparation, prototyping, analysis, and formalization before it can be successfully integrated into the existing codebase.

As the development of Ouroboros Leios progresses, its potential impact on the blockchain industry will undoubtedly attract attention and generate discussion. 

With its innovative approach to overcoming the limitations of previous Ouroboros variants, Ouroboros Leios represents a significant step forward in the ongoing evolution of blockchain technology.

Design Goals of Ouroboros Leios:

  • Increase throughput while maintaining excellent security properties: The primary design goal of Ouroboros Leios is to enhance throughput without compromising security. This is achieved through an innovative blockchain structure with concurrent data dependencies, enabling increased parallelism and resource utilization. Concurrent data dependencies are leveraged using a concurrent parallel distributed algorithm for constructing the blockchain. The rigorous security analysis process ensures the reliability and security of the Ouroboros Leios algorithm.
  • Compatibility with existing blockchain infrastructure: Ouroboros Leios aims to be compatible with current blockchain infrastructure, allowing it to integrate with existing networks without significant changes to the underlying infrastructure. This compatibility facilitates adoption by developers and users, allowing them to benefit from increased throughput and modern features.
  • Incorporate modern features such as:

 — Tiered transaction fees with corresponding levels of service priority: Ouroboros Leios introduces tiered transaction fees based on service priority levels. Users can select the service level according to their needs and pay a corresponding fee. This approach makes Ouroboros Leios more practical and user-friendly, moving away from a one-size-fits-all model.

 — Faster chain synchronisation by removing the need to execute every smart contract: Ouroboros Leios enables faster chain synchronization by eliminating the requirement to execute all smart contracts. Nodes can synchronize with the network without executing every smart contract, reducing synchronization time. This improvement makes Ouroboros Leios more practical and user-friendly, enhancing network performance and synchronization efficiency.

How it works

The core innovation behind Ouroboros Leios lies in its novel blockchain structure with concurrent data dependencies. Unlike traditional linear blockchain structures, Ouroboros Leios features a more complex structure that allows for concurrent data dependencies, ultimately enabling increased parallelism and resource utilization.

To exploit these concurrent data dependencies, Ouroboros Leios employs a concurrent parallel distributed algorithm for constructing the blockchain. The algorithm revolves around several objects, including input blocks, endorsement blocks, endorsement reports, endorsement certificates, and ranking blocks.

Input blocks are created by users looking to add transactions to the blockchain, while endorsement blocks are generated by validators who validate the transactions in input blocks and endorse them. 

Endorsement reports are then produced by other validators who endorse the endorsement block, and endorsement certificates are created by yet another set of validators who endorse the endorsement report. 

Ranking blocks serve to rank validators based on their performance in endorsing transactions, with higher-ranking validators wielding more influence in the consensus process.

The seamless interaction of these objects ensures that the blockchain is constructed in a manner that maximizes parallelism and resource utilization while maintaining outstanding security properties. 

The concurrent data dependencies facilitate simultaneous processing of multiple transactions, leading to increased throughput and reduced transaction confirmation times.

More Detail on Ouroboros Leios Blockchain Structure

The Ouroboros Leios blockchain structure is composed of four distinct block types: Ranking blocks (RBs), Endorsement blocks (EBs), Endorsement reports (ERs), and Input blocks (IBs). Each block type serves a specific purpose and maintains a particular relationship with other block types.

  1. Ranking blocks (RBs): RBs are responsible for achieving consensus and establishing an overall ordering. Each RB references the previous RB and zero or more EBs, including their corresponding endorsement certificates. RBs are created using a private leader schedule based on Verifiable Random Functions (VRFs) and weighted by stake.
  2. Endorsement blocks (EBs): EBs help agree on the existence and validity of input blocks by bundling multiple input blocks to allow reporting on them as a single unit. EBs are created using a private leader schedule based on VRFs and weighted by stake.
  3. Endorsement reports (ERs): ERs demonstrate the existence and validity of the bundle of input blocks referenced by the EB. When aggregated, with enough ERs to statistically represent over 50% of the stake, they are referred to as an endorsement certificate. ERs are created by nodes elected as reporters in a separate private stake-weighted VRF lottery.
  4. Input blocks (IBs): IBs carry the blockchain payload, which consists of transactions. Each IB contains a sequence of transactions and references a recent RB. IBs are created using a private leader schedule based on VRFs.

The relationships between these block types are as follows:

  • RBs reference the previous RB and zero or more EBs, including their corresponding endorsement certificates.
  • EBs reference multiple IBs and zero or more other EBs.
  • IBs reference a recent RB.
  • ERs reference the EB they are reporting on.

The frequency of creation for each block type varies. RBs are created approximately every 15–30 seconds, while EBs are created more frequently than RBs. IBs are generated at a high rate by tuning the VRF lottery threshold. ERs are created on a fixed number of slots after the creation of the EB they are reporting on.

Further Details on Ouroboros Leios Features

  1. Mempool sharding: Transactions are divided into separate mempools, enabling parallel processing of transactions and improved throughput.
  2. Minimum and maximum times for inclusion: Transactions are included in a block only if they fall within a specified time range, which helps prevent delays in block creation and ensures timely processing.
  3. Double signing protection: This feature prevents block producers from signing multiple blocks simultaneously, enhancing the security and integrity of the blockchain.
  4. Network resource prioritization: Network resources are prioritized to avoid congestion and ensure efficient block propagation, contributing to a stable and high-performing network.
  5. Ledger state lags behind ranking blocks: The ledger state is updated after the creation of ranking blocks, which allows for efficient transaction processing and a more streamlined consensus process.
  6. Pessimistic mode: This mode enables the processing of transactions in a pessimistic manner, where transactions are only included if they are guaranteed to be valid. This feature enhances the security and reliability of the blockchain.
  7. The time slot of a transaction: Transactions are assigned a time slot, which allows for efficient processing and helps prevent conflicts that could otherwise arise due to simultaneous transactions. This feature contributes to the overall performance and stability of the network.

Requirements and Constraints for Ouroboros Leios Integration

Ouroboros Leios introduces new constraints on the ledger layer due to its concurrent blockchain structure. To support this innovative consensus algorithm, the ledger layer and the specific set of ‘mini protocols’ must be extended. While the existing Cardano ledger is well-suited to meet the requirements of Ouroboros Leios, some preparatory work will be necessary.

The existing ledger rules must satisfy the determinism property, which is crucial for the correct operation of the Ouroboros Leios algorithm. This property must be formalized and used to prove the correctness of the reassembly operation.

Ouroboros Leios design trades off increased latency for improved throughput. One area of research focuses on investigating modifications to the Leios design that would reduce the minimum latency between dependent transactions.

The Ouroboros Leios design offers vertical scalability. Future research could explore if variations or extensions of the Leios design could provide a degree of horizontal scalability.

If Ouroboros Leios is successfully implemented and deployed on the Cardano mainnet, the community will need to address a new challenge: balancing the trade-off between system performance and the resource costs of participating. 

In a very high throughput configuration, Ouroboros Leios would have resource requirements that would be too high for most end users, necessitating the majority of users to switch to light wallets. Additionally, it would likely be necessary for most end-user full nodes to not store the entire chain history, to minimize resource consumption.

Road to Deployment

The deployment of Ouroboros Leios involves several crucial steps to ensure its success, reliability, and security. The following stages outline the process:

  1. Completing the research design: The first step involves finalizing the research design for Ouroboros Leios, including the security analysis, and submitting it for peer review. This ensures that the design is sound and secure.
  2. Analyzing protocol parameters: Once the security analysis is complete, the various protocol parameters need to be examined based on the security results and calibrated accordingly. This stage may involve additional design work if certain parameters pose practical challenges.
  3. Describing the algorithm in a formal language: To effectively communicate the design to the engineering team and facilitate testing, the Leios algorithm must be described in a suitable computer science formal language, which provides a precise description of its functional behavior.
  4. Implementing and testing: After the algorithm has been described in a formal language, it needs to be implemented and rigorously tested to ensure that it operates as intended and meets all security requirements.
  5. Deployment: Following successful implementation and testing, Ouroboros Leios can be deployed on a blockchain network for public use, enabling users to benefit from its increased throughput and modern features.
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