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 (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.