The Nervos Network Positioning Paper
1.Purpose of This Paper
The Nervos Network is made up of a number of protocols and innovations. It’s important to have clear documentation and technical specifications on key protocol design and implementations - for which we utilize an RFC (request for comment) process. However, we feel it’s equally important that we help our communities to understand what we try to accomplish, the trade-offs we have made, and how we have arrived at our current design decisions.
We start this document with a detailed examination of the problems that public permissionless blockchains face today and the existing solutions attempting to solve them. We hope this provides the necessary context for our readers to understand our own rationale on how best to approach these challenges, and our underlying design decisions. We then provide a high-level walkthrough of all parts of the Nervos Network, with a focus on how they work together to support the overall vision of the network.
Scalability, sustainability and interoperability are among the largest challenges public permissionless blockchains face today. While many projects claim to have solutions to these problems, it’s important to understand where these problems come from and put solutions in the context of possible trade-offs.
Bitcoin was the first public permissionless blockchain, designed to be used as peer-to-peer electronic cash. Ethereum made more use cases possible and created a general purpose decentralized computing platform. However, both of these platforms impose limitations on their transaction capabilities - Bitcoin caps its block size and Ethereum caps its block gas limit. These are necessary steps to ensure long-term decentralization, however they also limit the capabilities of both platforms.
The blockchain community has proposed many scalability solutions in recent years. In general, we can divide these solutions into two categories: on-chain scaling and off-chain scaling.
On-chain scaling solutions aim to expand the throughput of the consensus process and create blockchains with native throughput that rivals centralized systems. Off-chain scaling solutions only use the blockchain as a secure asset and settlement platform, while moving nearly all transactions to upper layers.
2.1.1 On-chain Scaling with a Single Blockchain
The most straightforward way to increase the throughput of a blockchain is to increase its supply of block space. With additional block space, more transactions can flow through the network and be processed. Increasing the supply of block space in response to increased transaction demand also allows for transaction fees to remain low.
Bitcoin Cash (BCH) adopts this approach to scale its peer-to-peer payment network. The Bitcoin Cash protocol began with a maximum block size of 8 MB, which was later increased to 32 MB, and which will continue to be increased indefinitely as transaction demand increases. For reference, following Bitcoin’s (BTC) implementation of Segregated Witness in August 2017, the Bitcoin protocol now allows for an average block size of around 2 MB.
In the scope of a datacenter, the math works out. If 7.5 billion people each create 2 on-chain transactions per day, the network will require production of 26 GB blocks every 10 minutes, leading to a blockchain growth rate of 3.75 TB per day or 1.37 PB per year. These storage and bandwidth requirements are reasonable for any cloud service today.
However, constraining node operation to a datacenter environment leads to a single viable network topology and forces compromises in security (the fork rate of the blockchain will increase as data transmission requirements across the network increase), as well as decentralization (the full node count will be reduced as the cost of consensus participation increases).
From an economic standpoint, an ever-increasing block size does alleviate fee pressure felt by users. Analysis of the Bitcoin network has shown that fees remain flat until a block is about 80% full, and then rise exponentially.
Though placing the burden of a growing network’s costs on its operators may seem to be a reasonable decision, it could be short-sighted for two reasons:
- Suppression of transaction fees forces miners to rely predominantly on compensation from new coin issuance (block rewards). Unless inflation is a permanent part of the protocol, new coin issuance will eventually stop (when the total coin hard-cap is reached), and miners will receive neither block rewards nor significant transaction fees. The economic impact of this will severely compromise the security model of the network.
- The cost of running a full node becomes prohibitively expensive. This removes the ability of regular users to independently verify a blockchain’s history and transactions, forcing reliance on service providers such as exchanges and payment processors to ensure the integrity of the blockchain. This trust requirement negates the core value proposition of public permissionless blockchains as peer-to-peer, trustless distributed systems.
Transaction cost optimized platforms such as Bitcoin Cash face significant competition from other blockchains (permissioned and permissionless), as well as traditional payment systems. Design decisions that improve security or censorship resistance will incur associated costs and in turn increase the cost of using the platform. Taking into account a competitive landscape, as well as the network’s stated objectives, it is likely that lower costs will be the overarching goal of the network, at the expense of any other considerations.
This goal is consistent with our observations of transactional network usage. Users of these systems are indifferent to significant long-run trade-offs because they will only utilize the network for a short time. Once their goods or services have been received and their payment has been settled, these users no longer have any concern for the network’s effective operation. The acceptance of these trade-offs is apparent in the widespread use of centralized crypto-asset exchanges, as well as more centralized blockchains. These systems are popular primarily for their convenience and transactional efficiency.
Some smart contract platforms have taken similar approaches to scaling blockchain throughput, allowing only a limited set of “super computer” validators to participate in the consensus process and independently validate the blockchain.
Though compromises in regard to decentralization and network security allow for cheaper transactions and may be convenient for a set of users, the compromised long-term security model, cost barrier to independently verify transactions, and the likely concentration and entrenchment of node operators lead us to believe that this is not a proper approach for scaling public blockchains.
2.1.2 On-chain Scaling through Multiple Chains
On-chain scaling through multiple chains can be accomplished through sharding, as seen in Ethereum 2.0, or application chains, as seen in Polkadot. These designs effectively partition the global state and transactions of the network into multiple chains, allowing each chain to quickly reach local consensus, and later the entirety of the network to reach global consensus through the consensus of the “Beacon Chain” or the “Relay Chain”.
These designs allow the multiple chains to utilize a shared security model, while allowing high throughput and fast transactions inside shards (Ethereum) or para-chains (Polkadot). Though each of these systems is a network of interconnected blockchains, they differ in regard to the protocols running on each chain. In Ethereum 2.0, every shard runs the same protocol, while in Polkadot, each para-chain can run a customized protocol, created through the Substrate framework.
In these multi-chain architectures, each dApp (or instance of a dApp) only resides on a single chain. Though developers today are accustomed to the ability to build dApps that seamlessly interact with any other dApp on the blockchain, design patterns will need to adapt to new multi-chain architectures. If a dApp is split across different shards, mechanisms will be required to keep state synced across different instances of the dApp (residing on different shards). Additionally, though layer 2 mechanisms can be deployed for fast cross-shard communication, cross-shard transactions will require global consensus and introduce confirmation latency.
With these asynchronous transactions, the infamous “train-and-hotel” problem arises. When two transactions must be atomic (for example booking a train ticket and a hotel room on two different shards), new solutions are required. Ethereum introduces contract “yanking”, in which a dependent contract is deleted on one shard, created on a second shard (that contains the other dependent contract), and both transactions are then executed on the second shard. However, the yanked contract would then be unavailable on the original shard, introducing usability issues, and again requiring new design patterns.
Sharding has its own advantages and challenges. If shards can be truly independent and cross-shard needs are minimal, a blockchain can linearly scale its throughput by increasing the number of shards. This is best suited for self-contained applications that don’t require outside state or collaboration with other applications.
A sharded architecture can be problematic for applications that are developed by composing together “building block” applications (this is known as the “composability problem”). Composability is especially relevant in the decentralized finance (DeFi) space, where more advanced products tend to be built on top of other building block products.
On a more technical note, sharding typically requires a “1 + N” topology, in which N chains connect to one meta-chain, introducing an upper bound on the number of shards a meta-chain can support without itself running into scalability issues.
We observe significant value in a unified global state, allowing an ecosystem of interdependent applications to emerge and developers to innovate at the edges, similar to web developers’ use of libraries for lower-level concerns and open APIs for service integration. A much simpler development experience is enabled when developers don’t have to consider synchronicity (in cross-shard asset transfer or messaging passing), as well as a superior user experience, resulting from consistency in the architectural concerns of blockchain interactions.
We recognize that sharding is a promising scalability solution (in particular for less interdependent applications), however we believe it is beneficial to have a design that concentrates the most valuable state on a single blockchain, allowing composability. With this design, off-chain scaling approaches are utilized to allow for higher throughput.
2.1.3 Off-chain Scaling through Layer 2
In layer 2 protocols, the base layer blockchain acts as a settlement (or commitment) layer, while a second layer network routes cryptographic proofs that allow participants to “take delivery of” the cryptocurrency. All activities of the second layer are cryptographically secured by the underlying blockchain and the base layer is only used to settle amounts entering/exiting the second layer network, and for dispute resolution. These designs operate without delegation of custody (or risk of loss) of funds and enable instant, nearly free transactions.
These technologies demonstrate how a store of value network such as Bitcoin could be used for everyday payments. The most typical example of a layer 2 solution in practice is a payment channel between a customer and a coffee shop. Let’s assume Alice visits the Bitcoin Coffee Shop every morning. At the beginning of the month, she deposits funds into a Lightning payment channel she has opened with the coffee shop. As she visits each day, she cryptographically signs the coffee shop’s right to take some of the funds, in exchange for her coffee. These transactions happen instantly and are completely peer-to-peer, “off-chain”, allowing for a smooth customer experience. The Lightning channel is trustless, Alice or the coffee shop can close the channel at any time, taking the funds they are owed at that time.
Payment channel technologies such as Lightning are only one example of an off-chain scaling technique; there are many maturing technologies that can safely scale blockchain throughput in this way. While payment channels include off-chain agreements to channel balances between two parties, state channels include off-chain agreements to arbitrary state between channel participants. This generalization can be the basis of scalable, trustless, decentralized applications. A single state channel can even be utilized by multiple applications, allowing for even greater efficiency. When one party is ready to exit the channel, they can submit the agreed upon cryptographic proof to the blockchain, which will then execute the agreed state transitions.
A side-chain is another construction that allows for increased throughput, though via trusted third party blockchain operators. With a two-way peg to a blockchain with reliable, trustless consensus, funds can be moved back and forth between the main-chain and side-chain. This allows for a high volume of trusted transactions on the side-chain, with later net settlement on the main-chain. Side-chain transactions have minimal fees, fast confirmation and high throughput. Though side-chains offer a superior experience in some regard, they do compromise on security. There is however, a great deal of research into trustless side-chains, which can provide the same performance improvements without compromising security.
An example of a trustless side-chain technology is Plasma (covered in 5.3), a side-chain architecture that leverages a trust root on a blockchain with broad global consensus. Plasma chains offer the same performance improvements as centralized side-chains, however do so while offering security guarantees. In the event a Plasma chain operator is malicious or malfunctioning, users are provided a mechanism that allows them to safely withdraw their side-chain assets to the main-chain. This is done without the cooperation of the Plasma chain operator, offering users the convenience of side-chain transactions, as well as the security of a layer 1 blockchain.
Off-chain scaling allows for decentralization, security and scalability. By moving everything except settlement transactions and disputes off-chain, a public blockchain’s limited global consensus is efficiently utilized. Diverse layer 2 protocols can be implemented based on application requirements, affording flexibility to developers and users. As more participants are added to the network, performance is not impacted and all parties can share the security guarantees offered by layer 1 consensus.
Sustaining the long-term operation of an autonomous, ownerless public blockchain presents quite the challenge. Incentives must be balanced among diverse stakeholders and the system must be designed in a way that allows for widespread full node operation and public verifiability. Hardware requirements must remain reasonable, while supporting an open, global network.
The incentives and controls of a blockchain’s native asset must be able to balance the appreciation demands of long-term holders with the compensation demands of miners or validators securing the network.
Additionally, once a public blockchain is in operation, it is very difficult to change the underlying rules governing the protocol. From the start, the system must be designed to be sustainable. In this interest, we have conducted a thorough inventory of the challenges in building sustainable, permissionless blockchains.
One of the largest long-term threats public blockchains face is an ever-increasing barrier of independent participation and transaction verification, reflected in the cost of full node operation. Full nodes allow blockchain participants to independently verify the on-chain state/history, and hold miners or validators of the network accountable by refusing to route invalid blocks. As the cost of full nodes increases and their numbers decline, participants in the network are increasingly forced to rely on professional service operators to provide both history and current state, eroding the fundamental trust model of open and permissionless blockchains.
For a full node to keep up with the progression of the blockchain, it must have adequate computational throughput to validate transactions, bandwidth throughput to receive transactions, and storage capacity to store the entire global state. To control a full node’s operating cost, the protocol has to take measures to bound the throughput or capacity growth of all three of these resources. Most blockchain protocols bound their computational or bandwidth throughput, but very few bound the growth of the global state. As these chains grow in size and length of operation, full node operation costs will irreversibly increase.
2.2.2 Economic Models
While there has been a lot of research into consensus protocols in recent years, we believe crypto-economics is an understudied field. Broadly speaking, current crypto-economic models for layer 1 protocols are primarily focused on incentives and punishments to ensure network consensus, and native tokens are mostly used to pay transaction fees or to satisfy staking requirements that provide Sybil resistance.
We believe that a well-designed economic model should go beyond the consensus process and ensure the long-term sustainability of the protocol as well. In particular, the economic model should be designed with the following goals:
- the network should have a sustainable way to raise revenue to compensate service providers (typically miners or validators), ensuring that the network remains sustainably secure
- the network should have a sustainable way to maintain a low barrier to participation, ensuring that the network remains decentralized over time
- the resources of the public network should be efficiently and fairly allocated
- the blockchain’s native token must have a convincing intrinsic value
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