Advancing Ouroboros: Leios as the next leap in scalability

As Cardano evolves under full community governance in 2025, scalability remains one of the top priorities.

Ouroboros Leios is a major redesign of Cardano’s Ouroboros consensus, designed to achieve significant scalability and throughput – pushing Cardano well beyond its current limits. As Cardano’s long-term scaling engine, Leios is a transformative upgrade that paves the way for mass adoption, advanced decentralized finance (DeFi), and global decentralized application (DApp) infrastructure – all while preserving the security and decentralization that define Cardano.

Advancing Ouroboros

The Ouroboros family of protocols has continually expanded the capabilities of the Cardano blockchain. Each iteration – from Ouroboros Classic to Praos, Genesis, and beyond – has built upon rigorous, peer-reviewed research to enhance security, decentralization, and efficiency.

While these advancements have positioned Cardano as a scientifically grounded blockchain design leader, further throughput improvements are essential to accommodate the expected growth of the user base. The current version, Ouroboros Praos, is not primarily constrained by node resources such as network capacity or CPU performance. Rather, its throughput is limited by the consensus algorithm's underlying data and communication dependencies. Addressing this requires a fundamentally new design – one that can unlock additional network capacity while preserving Cardano’s strong security properties.

Ouroboros Leios, the latest evolution in the Ouroboros family, is designed to achieve precisely this. 

The purpose of Ouroboros Leios

Leios will introduce a novel concurrent blockchain structure and parallel processing approach by leveraging underutilized computational and network resources. 

Ouroboros Praos faces throughput limitations not because of hardware constraints but due to inherent inefficiencies in how the blockchain algorithm works:

• Block diffusion: in Praos, block diffusion only occupies a fraction of the overall block time. On average, on Cardano's mainnet, blocks are produced every 20 seconds – though actual timing can vary due to randomness, with two blocks sometimes created in the same slot or just seconds apart. Since block diffusion typically takes about five seconds, the system is idle roughly 75% of the time. The 20-second interval provides enough time for propagation, reducing forking risks while maintaining reasonable confirmation speeds.
• Resource utilization during diffusion: even during block diffusion, most nodes remain largely idle. Only nodes actively processing a block use their CPU and network resources, and this happens sequentially as blocks propagate through the network.

Ouroboros Leios addresses these inefficiencies by redesigning the blockchain structure and consensus algorithm. Leios’s primary goals are to:

1. Increase data throughput: current Cardano throughput is about 4.4kB/s, but a typical server could handle around 900kB/s per peer.
2. Increase script throughput: measured in CPU milliseconds per wall-clock second (ms/s). Current Cardano script throughput is about 2ms/s, while a single CPU core offers 1,000ms/s.
3. Improve transaction throughput: this is a derived measure based on data and script throughput. Simulation tools show that by leveraging node parallelism, Leios could offer targets of over 1,000 transactions per second (TPS).
4. Maintain strong security: achieve high throughput without compromising the security properties of previous Ouroboros versions, which can resist adversaries controlling up to 50% of stake.
5. Maintain decentralization: ensure that the protocol is efficient enough without compromising on decentralization or protocol security properties. 

Preserving security and decentralization

Leios preserves the core security guarantees of Ouroboros Praos while significantly increasing throughput. It retains the same ranking block interval – approximately 20 seconds – and uses carefully timed protocol stages within that interval. This design ensures that Praos’s security proofs carry over to Leios, maintaining resistance to adversaries. Additional safeguards, such as endorsement thresholds, reinforce the protocol’s integrity by ensuring only valid input blocks contribute to the final ledger.

Leios also upholds Cardano’s decentralization model. It increases node resource usage, allowing current Praos operators to transition smoothly. The stake-weighted VRF mechanism remains in place, keeping participation accessible, while the protocol continues to follow a Nakamoto-style consensus – preserving Cardano’s open and inclusive network structure.

Concurrent blockchain design

While traditional blockchains are inherently sequential (meaning they have a linear structure where each block depends directly on the previous one), a network of nodes is a parallel system. This leads to resource underutilization, and the sequentiality creates data dependencies:

• Block B depends on Block A
• Block C depends on Block B
• And so on…

Leios introduces a concurrent blockchain structure where multiple blocks can exist at the same level without depending on each other. This creates a more complex dependency pattern that allows parallel processing.

In this concurrent structure:

• Some blocks can depend on multiple previous blocks
• Some blocks can be processed simultaneously
• The overall structure allows for much higher parallelism.

Figure 1. Linear and concurrent blockchain structures

Handling transactions in a concurrent blockchain

A concurrent blockchain creates new challenges for transaction processing. Two key issues emerge:

1. What happens when concurrent blocks contain conflicting transactions?
2. How do we interpret the ledger when joining results from concurrent blocks?

Leios adopts an optimistic approach to answer these questions based on the use of sharding to ensure that most in-flight transactions are independent of each other:

1. Multiple blocks of transactions are processed concurrently
2. Within each block, transactions are processed in order
3. When results are joined, blocks are put into a linear order
4. This creates a linear order for all transactions
5. Any conflicting transactions are detected and handled. The first transaction is kept, and later conflicting ones are discarded.

This approach is particularly well suited for Cardano’s extended UTXO (EUTXO) ledger. The EUTXO model extends the basic UTXO model with smart contract capabilities. Unlike account-based models, EUTXO ensures deterministic transactions, meaning their outcomes can be precisely predicted before execution. This makes it ideal for scalability and parallel processing, as transactions can be validated independently as long as their dependencies are met.

With Leios:

• EUTXO transactions explicitly identify all inputs and outputs upfront
• Dependencies between transactions are clear and complete
• Conflicts can be easily detected
• Transactions can be reordered without changing results (provided dependencies are respected).

How it works

Ouroboros Leios introduces a three-tier blockchain structure that enables parallel transaction processing while maintaining security. Instead of a single type of block handling all tasks, Leios divides responsibilities across three specialized block types: 

• Input blocks for transactions
• Endorsement blocks for validation
• Ranking blocks for ordering. 

This separation allows multiple input blocks to be created and validated concurrently, significantly increasing throughput.

The process begins with input blocks, which are created frequently and carry transactions referencing a recent ledger state for validation. Endorsement blocks then bundle these input blocks and get verification from nodes selected through stake-weighted lotteries. Once enough endorsements are gathered, endorsement certificates confirm the validity of the referenced input blocks. Ranking blocks, produced at a lower frequency, establish the final transaction order by indirectly referencing multiple endorsement blocks. This approach enables nodes to adopt the blockchain state without needing to process every transaction immediately, ensuring scalability while preserving security guarantees.

By decoupling transaction processing from ordering and validation, Leios maximizes resource utilization. Transactions are continuously processed, reducing idle time for network nodes, while parallel validation distributes the workload efficiently. 

Learn more 

The Ouroboros Leios project is currently in the research and development stages. Ongoing research focuses on formal validation, Haskell and Rust simulations, network performance modeling, and infrastructure improvements to ensure the protocol’s feasibility and effectiveness.

The roadmap includes refining models to better reflect real-world conditions in the short term, integrating findings into broader Cardano tools in the midterm, and ultimately informing future consensus upgrades to enhance efficiency and scalability.

To learn more, see:

• Leios documentation
• Research and development overview
• Weekly updates
• GitHub repository and discussions.

This blog post was written with the input and support of William Wolff, Giorgos Panagiotakos, Nicolas Biri, and Christian Badertscher.