The Byron reboot is a series of updates to multiple components of the Cardano network. Namely the Cardano node, but also the Cardano explorer, the wallet backend, and the Daedalus wallet itself. The first part of the Byron reboot – a totally new node implementation – has already been deployed to some relay nodes on the network, and the next few weeks will see core network nodes and more relays being incrementally upgraded to the new system. Soon, users will be able to experience the node improvements directly via a new version of the Daedalus wallet.

Why do we need a reboot?

To prepare for the future, and for Shelley. The node that launched at the start of the Byron era was only ever going to take us so far. The new node implementation has been designed from the ground up to support not only imminent Shelley features, such as delegation and decentralization, but anything else that the future has in store. The improved design is modular, separating the ledger, consensus, and network components of the node, allowing any one of them to be changed, tweaked, and upgraded without affecting the others.

The reboot has also been an opportunity to apply evidence-based formal methods and testing to every single aspect of the node. Rather than try and make these substantial improvements to the existing code, it was more effective to work from scratch. All critical elements of the new node have been formally specified, and the final implementation tested against those specifications. Basic code quality and performance are now significantly higher and more robust across the board, as well as being easier to test and verify going forward.

What’s involved?

Improvements to the Cardano node, of course, but work has also been underway to improve the Cardano explorer, the wallet backend, and Daedalus itself. The new explorer has been redesigned to make it easier to use, as well as having an updated visual design and more information available. The improved wallet backend and associated services, collectively known as Adrestia, will allow exchanges and third-party developers to engage with the Cardano network using a collection of independent, self-contained libraries. The newly extended APIs have been explicitly designed to meet the needs of larger exchanges, enabling ada to be supported on even more platforms. And finally, a new and improved Daedalus release will see Yoroi wallet support, transaction filtering, parallel wallet restoration – not to mention some significant performance improvements.

How will it impact the Cardano network?

At the most fundamental level, the Byron reboot will improve the performance of the entire Cardano network. Transaction throughput capacity will increase, and the network will be able to handle much higher demand and transactions per second. The node improvements also make the node more efficient and reliable in terms of memory usage, increasing the viability of running a Cardano node on a low-spec machine or in poor network conditions, which, in turn, enables more users to participate in the Cardano network across the world.

What does it mean for Daedalus users?

First, there’ll be a new rolling release of Daedalus that is designed to allow users to test the new node functionality and wallet backend, and to provide us with feedback. We’ll be sharing more about how that will work in our monthly product update on Tuesday, March 31. Once the wallet has been user tested, feedback will be implemented in a new production version of Daedalus. Many of the issues users have experienced with Daedalus in the past were due to the underlying node, rather than Daedalus itself. The new node will go a long way to improving performance, and users should see Daedalus syncing and restoring wallets at significantly improved speeds.

What’s next?

The reboot commences with the deployment of a CLI and associated APIs to exchanges and wallet partners. The new node update will then be rolled out to more core and relay nodes on the Cardano mainnet within the next few weeks, followed by other reboot and continuing Daedalus improvements in the months ahead. The goal is to gradually and sustainably migrate the entire Cardano blockchain to working on the new node implementation, without any disruption or loss of service. After that comes the Haskell Shelley testnet, which will include onboarding stake pool operators from the Incentivized Testnet to help them set up and prepare for running their pools on the Shelley mainnet.

The Byron reboot is the culmination of over 18 months of work by several IOHK development teams and represents a significant investment in the network-critical infrastructure required to support the Shelley era of Cardano. For more details on the reboot and how it will all roll out, tune in to March’s product update on the IOHK Crowdcast stream on March 31. Cardano product director Aparna Jue and members of her team will join me to discuss the latest on the reboot and the next steps for the project.

Scalability is the greatest challenge to blockchain adoption. By applying a principled, evidence-based approach, we have arrived at a solution for Cardano and networks similar to it: Hydra. Hydra is the culmination of extensive research, and a decisive step in enabling decentralized networks to securely scale to global requirements.

What is scalability and how do we measure it?

Scaling a distributed ledger system refers to the capability of providing high transaction throughput, low latency, and minimal storage per node. These properties have been repeatedly touted as critical for the successful deployment of blockchain protocols as part of real-world systems. In terms of throughput, the VISA network reportedly handles an average of 1,736 payment transactions per second (TPS) with the capability of handling up to 24,000 TPS and is frequently used as a baseline comparison. Transaction latency is clearly desired to be as low as possible, with the ultimate goal of appearing instantaneous to the end-user. Other applications of distributed ledgers have a wide range of different requirements in terms of these metrics. When designing a general purpose distributed ledger, it is natural to strive to excel on all three counts.

Deploying a system that provides satisfactory scaling for a certain use case requires an appropriate combination of two independent aspects: adopting a proper algorithmic design and deploying it over a suitable underlying hardware and network infrastructure.

When evaluating a particular algorithmic design, considering absolute numbers in terms of specific metrics can be misleading. The reason is that such absolute quantities must refer to a particular underlying hardware and network configuration which can blur the advantages and disadvantages of particular algorithms. Indeed, a poorly designed protocol may still perform well enough when deployed over superior hardware and networking.

For this reason, it is more insightful to evaluate the ability of a protocol to reach the physical limits of the underlying network and hardware. This can be achieved by comparing the protocol with simple strawman protocols, in which all the design elements have been stripped away. For instance, if we want to evaluate the overhead of an encryption algorithm, we can compare the communication performance of two end-points using encryption against their performance when they simply exchange unencrypted messages. In such an experiment, the absolute message-per-second rate is unimportant. The important conclusion is the relative overhead that is added by the encryption algorithm. Moreover, in case the overhead approximates 0 for some configuration of the experimental setup, we can conclude that the algorithm approximates the physical limits of the underlying network’s message-passing ability for that particular configuration, and is hence optimal in this sense.

Hydra – 30,000-feet view

Hydra is an off-chain scalability architecture for distributed ledgers, which addresses all three of the scalability challenges mentioned above: high transaction throughput, low latency, and minimal storage per node. While Hydra is being designed in conjunction with the Ouroboros protocol and the Cardano ledger, it may be employed over other systems as well, provided they share the necessary salient characteristics with Cardano.

Despite being an integrated system aimed at solving one problem – scalability – Hydra consists of several subprotocols. This is necessary as the Cardano ecosystem itself is heterogenous and consists of multiple entities with differing technical capabilities: the system supports block producers with associated stake pools, high-throughput wallets as used by exchanges, but also end-users with a wide variety of computational performance and availability characteristics. It is unrealistic to expect that a one-shoe-fits-all, single-protocol approach is sufficient to provide overall scalability for such a diverse set of network participants.

The Hydra scalability architecture can be divided into four components: the head protocol, the tail protocol, the cross-head-and-tail communication protocol, as well as a set of supporting protocols for routing, reconfiguration, and virtualization. The centerpiece is the 'head' protocol, which enables a set of high-performance and high-availability participants (such as stake pools) to very quickly process large numbers of transactions with minimal storage requirements by way of a multiparty state channel – a concept that generalizes two-party payment channels as implemented in the context of the Lightning network. It is complemented by the 'tail' protocol, which enables those high-performance participants to provide scalability for large numbers of end-users who may use the system from low-power devices, such as mobile phones, and who may be offline for extended periods of time. While heads and tails can already communicate via the Cardano mainchain, the cross-head-and-tail communication protocol provides an efficient off–chain variant of this functionality. All this is tied together by routing and configuration management, while virtualisation facilitates faster communication generalizing head and tail communication.

The Hydra head protocol

The Hydra head protocol is the first component of the Hydra architecture to be publicly released. It allows a set of participants to create an off-chain state channel (called a head) wherein they can run smart contracts (or process simpler transactions) among each other without interaction with the underlying blockchain in the optimistic case where all head participants adhere to the protocol. The state channel offers very fast settlement and high transaction throughput; furthermore, it requires very little storage, as the off-chain transaction history can be deleted as soon as its resulting state has been secured via an off–chain 'snapshot' operation.

Even in the pessimistic case where any number of participants misbehave, full safety is rigorously guaranteed. At any time, any participant can initiate the head's 'closure' with the effect that the head's state is transferred back to the (less efficient) blockchain. We emphasize that the execution of any smart contracts can be seamlessly continued on-chain. No funds can be generated off-chain, nor can any single, responsive head participant lose any funds.

The state channels implemented by Hydra are isomorphic in the sense that they make use of the same transaction format and contract code as the underlying blockchain: contracts can be directly moved back and forth between channels and the blockchain. Thus, state channels effectively yield parallel, off-chain ledger siblings. In other words, the ledger becomes multi-headed.

Transaction confirmation in the head is achieved in full concurrency by an asynchronous off-chain certification process using multi-signatures. This high level of parallelism is enabled by use of the extended UTxO model (EUTxO). Transaction dependencies in the EUTxO model are explicit, which allows for state updates without unnecessary sequentialization of transactions that are independent of each other.

Experimental validation of the Hydra head protocol

As a first step towards experimentally validating the performance of the Hydra head protocol, we implemented a simulation. The simulation is parameterized by the time required by individual actions (validating transactions, verifying signatures, etc.), and carries out a realistic and timing-correct simulation of a cluster of distributed nodes forming a head. This results in realistic transaction confirmation time and throughput calculations.

We see that a single Hydra head achieves up to roughly 1,000 TPS, so by running 1,000 heads in parallel (for example, one for each stake pool of the Shelley release), we should achieve a million TPS. That’s impressive and puts us miles ahead of the competition, but why should we stop there? 2,000 heads will give us 2 million TPS – and if someone demands a billion TPS, then we can tell them to just run a million heads. Furthermore, various performance improvements in the implementation can improve the 1,000 TPS single head measurement, further adding to the protocol’s hypothetical performance.

So, can we just reach any TPS number that we want? In theory the answer is a solid yes, and that points to a problem with the dominant usage of TPS as a metric to compare systems. While it is tempting to reduce the complexity of assessing protocol performance to a single number, in practice this leads to an oversimplification. Without further context, a TPS number is close to meaningless. In order to properly interpret it, and make comparisons, you should at least ask for the size of the cluster (which influences the communication overhead); its geographic distribution (which determines how much time it takes for information to transit through the system); how the quality of service (transaction confirmation times, providing data to end users) is impacted by a high rate of transactions; how large and complicated the transactions are (which has an impact on transaction validation times, message propagation time, requirements on the local storage system, and composition of the head participants); and what kind of hardware and network connections were used in the experiments. Changing the complexity of transactions alone can change the TPS by a factor of three, as can be seen in the figures in the paper (refer to Section 7 – Simulations).

Clearly, we need a better standard. Is the Hydra head protocol a good protocol design? What we need to ask is whether it reaches the physical limits of the network, not a mere TPS number. Thus, for this first iteration of the evaluation of the Hydra head protocol, we used the following approach to ensure that the data we provide is properly meaningful:

1. We clearly list all the parameters that influence the simulation: transaction size, time to validate a single transaction, time needed for cryptographic operations, allocated bandwidth per node, cluster size and geographical distribution, and limits on the parallelism in which transactions can be issued. Without this controlled environment, it would be impossible to reproduce our numbers.
2. We compare the protocol’s performance to baselines that provide precise and absolute limits of the underlying network and hardware infrastructure. How well we approach those limits tells us how much room there would be for further improvements. This follows the methodology explained above using the example of an encryption algorithm.

We use two baselines for Hydra. The first, Full Trust, is universal: it applies to any protocol that distributes transactions amongst nodes and insists that each node validate transactions one after the other – without even ensuring consensus. This yields a limit on TPS by simply adding the message delivery and validation times. How well we approach this limit tells us what price we are paying for consensus, without relying on comparison with other protocols. The second baseline, Hydra Unlimited, yields a TPS limit specifically for the head protocol and also provides the ideal latency and storage for any protocol. We achieve that by assuming that we can send enough transactions in parallel to completely amortize network round-trip times and that all actions can be carried out when needed, without resource contention. The baseline helps us answer the question of what can be achieved under ideal circumstances with the general design of Hydra (for a given set of values of the input parameters) as well as evaluate confirmation latency and storage overhead against any possible protocol. More details and graphs for those interested can be found in our paper (again, Section 7 – Simulations).

What comes next?

Solving the scalability question is the holy grail for the whole blockchain space. The time has come to apply a principled, evidence-based approach in designing and engineering blockchain scalability solutions. Comparing scalability proposals against well-defined baselines can be a significant aide in the design of such protocols. It provides solid evidence for the appropriateness of the design choices and ultimately leads to the engineering of effective and performant distributed ledger protocols that will provide the best possible absolute metrics for use cases of interest. While the Hydra head protocol is implemented and tested, we will, in time, release the rest of the Hydra components following the same principled approach.

As a last note, Hydra is the joint effort of a number of researchers, whom I'd like to thank. These include Manuel Chakravarty, Sandro Coretti, Matthias Fitzi, Peter Gaži, Philipp Kant, and Alexander Russel. The research was also supported, in part, by EU Project No.780477, PRIVILEDGE, which we gratefully acknowledge.

With the recent BFT update to the Byron mainnet, and the freshly-published Hydra paper, you’ll probably have heard a lot about Ouroboros: the ground-breaking proof-of-stake consensus protocol used by Cardano. Developed as a more energy efficient and sustainable alternative to proof of work, upon which earlier cryptocurrencies – Bitcoin and, currently, Ethereum – are built, Ouroboros was the first blockchain consensus protocol to be developed through peer-reviewed research.

Led by Aggelos Kiayias at the University of Edinburgh, Ouroboros and its subsequent implementations – BFT, Praos, Genesis, Hydra – provide a new baseline to solve some of the world’s greatest challenges, securely and at scale.

Yet recognition begins with education, and we cannot rely on what a technology achieves to convey the how. In this article, we present an overview of the how of Ouroboros. We’ll examine the tangibles and cover what each implementation introduces, to further the community’s understanding of the protocol, and illustrate why it’s such a game changer. A detailed analysis of each implementation can be found in the corresponding whitepapers below. For a broad-stroke explanation of Ouroboros and its implementations, however, read on.

• Ouroboros Classic
• Ouroboros BFT
• Ouroboros Genesis
• Ouroboros Praos
• Ouroboros Hydra

A word on consensus protocols, and why Ouroboros is different

It’s reasonable to assume that anybody new to the space might be confused by the term 'consensus protocol'. Put simply, a consensus protocol is the system of laws and parameters that govern the behavior of distributed ledgers: a ruleset by which each network participant plays.

Public blockchains aren’t controlled by any single, central authority. Instead, a consensus protocol is used to allow distributed network participants to agree on the history of the network captured on the blockchain – to reach consensus on what has happened, and continue from a single source of truth.

That single source of truth provides a single record. This is why blockchains are sometimes referred to as trustless: instead of requiring participants to trust one another, trust is built into the protocol. Unknown actors may interact and transact with each other without relying on an intermediary to mediate, or for there to be a prerequisite exchange of personal data.

Ouroboros is a proof-of-stake protocol, which is distinct from proof of work. Rather than relying on 'miners' to solve computationally complex equations to create new blocks – and rewarding the first to do so – proof of stake selects participants (in the case of Cardano, stake pools) to create new blocks based on the stake they control in the network.

Networks using Ouroboros are many times more energy efficient than those using proof of work – and, through Ouroboros, Cardano is able to achieve unparalleled energy efficiency. At the same level of decentralization – for example, 100 pools, which exceeds bitcoin’s current network – Cardano could consume as little as 0.01567GWh (gigawatt-hours) per year. Bitcoin, meanwhile, would require 67,000 GWh per year (according to current statistics). This is based on Ouroboros' ability to run on a Raspberry Pi, which has a power consumption of 15 to 18W (watts). In theory, this equates to more than four-million times the energy efficiency. The resulting difference in energy use can be analogized to that between a household and a small country: one can be scaled to the mass market; the other cannot.

Now, let’s take a closer look at how the Ouroboros protocol works, and what each new implementation adds.

Ouroboros Classic

We start with Ouroboros: the first implementation of the Ouroboros protocol, published in 2017. This first implementation (referred to as Ouroboros Classic) laid the foundations for the protocol as an energy-efficient rival to proof of work, introduced the mathematical framework to analyze proof of stake, and introduced a novel incentive mechanism to reward participants in a proof-of-stake setting.

More than this, however, what separated Ouroboros from other blockchain, and, specifically, proof-of-stake protocols was its ability to generate unbiased randomness in the protocol’s leader selection algorithm, and the subsequent security assurances that provided. Randomness prevents the formation of patterns, and is a critical part of maintaining the protocol’s security. Whenever a behavior can be predicted, it can be exploited – and though Ouroboros ensures transparency, it prevents coercion. Significantly, Ouroboros was the first blockchain protocol to be developed with this type of rigorous security analysis.

How Ouroboros works

A comprehensive explanation of how Ouroboros works can be found in its research paper. To summarize, Ouroboros divides the blockchain into slots and epochs. In Cardano, each slot lasts for 20 seconds and each epoch – which is an aggregation of slots – represents approximately five days’ worth of slots.

Central to Ouroboros’ design is the recognition that attacks are inevitable. As such, the protocol has tolerance built-in to prevent attackers from propagating alternative versions of the blockchain, and assumes that an adversary may send arbitrary messages to any participant at any time. In fact, the protocol is guaranteed to be secure so long as more than 51% of the stake is controlled by honest participants (that is, those following the protocol).

A slot leader is elected for each slot, who is responsible for adding a block to the chain and passing it to the next slot leader. To protect against adversarial attempts to subvert the protocol, each new slot leader is required to consider the last few blocks of the received chain as transient: only the chain that precedes the prespecified number of transient blocks is considered settled. This is also referred to as the settlement delay. Among other things, this means that a stakeholder can go offline and still be synced to the blockchain, so long as it’s not for more than the settlement delay.

Within the Ouroboros protocol, each network node stores a copy of the transaction mempool – where transactions are added if they are consistent with existing transactions – and the blockchain. The locally stored blockchain is replaced when the node becomes aware of an alternative, longer valid chain.

The drawback of Ouroboros Classic was that it was susceptible to adaptive attackers – a significant threat in a real-world setting that was resolved with Ouroboros Praos – and had no secure way for a new participant to bootstrap from the blockchain, which was resolved with Ouroboros Genesis.

Ouroboros BFT

Ouroboros BFT came next. Ouroboros BFT (Byzantine Fault Tolerance) is a simple protocol that was used by Cardano during the Byron reboot, which was the transition of the old Cardano codebase to the new. Ouroboros BFT will help prepare Cardano’s network for Shelley’s release and, with that, its decentralization.

Rather than requiring nodes to be online all of the time, Ouroboros BFT assumes a federated network of servers – the blockchain – and synchronous communication between the servers, providing ledger consensus in a simpler and more deterministic manner.

Additional benefits include instant proof of settlement, transaction settlement at network speed – which means the determiner for transactions is the speed of your network connection to a OBFT node – and instant confirmation in a single round trip of communication. Each of these results in significant performance improvements.

Ouroboros Praos

Ouroboros Praos builds upon – and provides substantial security and scalability improvements to – Ouroboros Classic.

As with Ouroboros Classic, Ouroboros Praos processes transaction blocks by dividing chains into slots, which are aggregated into epochs. Unlike Ouroboros Classic, however, Praos is analyzed in a semi-synchronous setting and is secure against adaptive attackers.

It assumes two possibilities: that adversaries can delay honest participant messages for longer than one slot, and that an adversary may send arbitrary messages to any participant at any time.

Through private-leader selection and forward-secure, key-evolving signatures, Praos ensures that a strong adversary cannot predict the next slot leader and launch a focused attack (such as a DDoS attack) to subvert the protocol. Praos is also able to tolerate adversarially-controlled message delivery delays and a gradual corruption of individual participants in an evolving stakeholder population, which is critical for maintaining network security in a global setting, provided that an honest majority of stake is maintained.

Ouroboros Genesis

Then, we have Ouroboros Genesis. Genesis further improves upon Ouroboros Praos by adding a novel chain selection rule, which enables parties to bootstrap from a genesis block – without, significantly, the need for trusted checkpoints or assumptions about past availability. Genesis also provides proof of the protocol’s Universal Composability, which demonstrates that the protocol can be composed with other protocols in arbitrary configurations in a real-world setting, without losing its security properties. This significantly contributes to its security and sustainability, and that of the networks using it.

Ouroboros Hydra

Last is Ouroboros Hydra. Hydra is an off-chain scalability architecture that addresses three key scalability challenges: high-transaction output, low latency, and minimal storage per node.

The recently released Hydra whitepaper proposes and outlines the introduction of multi-party state channels, which offers parallel transaction processing to dramatically improve Cardano’s transaction-per-second (TPS) output, and instant confirmation of transactions. Reflecting the implementation’s namesake, the paper refers to off-chain ledger siblings – state channels – as heads, which makes the ledger multi-headed.

Ouroboros Hydra enables Cardano to scale horizontally, increasing performance by incorporating additional nodes, rather than vertically, through the addition of more powerful hardware. Early simulations show that each head is able to perform up to 1,000 TPS. With 1,000 heads, this could be as high as 1,000,000 TPS. Once implemented, Ouroboros Hydra will allow Cardano to scale to unrivalled levels – to the level of, for example, global payment systems.

While Hydra is being designed in conjunction with the Ouroboros protocol and the Cardano ledger, it may also be employed over other systems, provided they share the necessary characteristics with Cardano.

The future of Ouroboros

Ouroboros, named after the symbol of infinity, is the backbone of the Cardano ecosystem. The protocol serves as a foundation and staging point for self-propagating systems that cyclically transform and grow, supplanting existing systems – financial and otherwise – and disintermediating the power structures upon which they rely. It is the beginning of a new standard, defined not from the center but, instead, from the margins.

Its future is as its past: a tireless effort to explore, iterate, and optimize, and drive positive change through rigorous research. Each step in its journey – after Hydra comes Ouroboros Crypsinous and Ouroboros Chronos – is a new evolution, and takes us closer to our vision of a fairer, securer, and more sustainable world.