Understanding the Byzantine Generals Problem: The Importance of Knowledge and Communication in Distributed Systems

The Byzantine Generals Problem, also known as the FBA, is a thought experiment that illustrates the importance of knowledge and communication in distributed systems. The problem aims to determine if consensus can be reached among generals in a decentralized army, and it was first introduced in the 1970s by computer scientist Robert Shostak, who is known for his contributions to distributed computing. The problem is often explained using the analogy of a group of generals who must come to a consensus on how to invade a town.

A Classical Analogy

Decentralized Network Challenges: Addressing Correctness, Agreement, and Utility in Distributed Systems

Before diving into the Byzantine Generals analogy, it’s essential to understand the fundamental concepts and challenges of distributed computing and how they apply to distributed payment systems. In a distributed system, multiple nodes process conflicting information in order to make a decision. However, these conflicting requests can lead to inconsistencies in the system, which can compromise the validity of transactions on the network. To overcome these challenges, a decentralized network must address three key areas: (1) correctness, (2) agreement, and (3) utility.

1. Correctness refers to the ability of the system to identify and validate legitimate transactions while preventing fraudulent ones.
2. Agreement deals with the underlying truth of the network and addresses the issue of double-spending by ensuring that there is only one set of globally valid transactions.
3. Utility relates to the balance between the computational costs of participating in the network and the speed of transactions on the distributed payment system.

For a decentralized network to function effectively, the consensus algorithm must achieve positive results in all three of these areas.

The Byzantine Generals Problem: The Foundation of the XRPL’s Consensus Protocol

The issue of receiving conflicting requests from various dispersed nodes can be complex, and it is well-illustrated by the Byzantine Generals Problem, a thought experiment proposed by Robert Shostak. This problem served as the foundation for the creation of the XRPL’s consensus protocol.

The classic version of the Byzantine Generals Problem goes as follows: several units of the Byzantine army are stationed outside an enemy city, each commanded by a separate general. The generals must agree on whether to attack or retreat, but can only communicate using messengers. In this scenario, there are two uncertainties: (1) the communication between the generals can be distorted or halted by messengers travelling through hostile territories, and (2) some generals may be traitors acting in bad faith.

In the context of a distributed system, a failing messenger represents a failing node, and a traitor general represents a malicious actor within the network. Although the problem is presented with only two options (attack or retreat), a successful outcome is determined by which plan receives the most votes. Even with only two options, failure may originate from multiple sources within the system, known as a ‘Byzantine’ failure.

The Byzantine Generals Problem highlights the need for distributed payment systems to be robust enough to withstand both standard and Byzantine failures. Such a system must effectively address the three challenges of correctness, agreement, and utility, allowing for only minimal errors.

The Role of Quorum Slices in the XRPL Consensus Protocol: Building on the Byzantine Generals Problem

The Byzantine Generals Problem has been a crucial building block in the development of the XRPL Consensus Protocol. It allows the network to develop a consensus mechanism that can sustain an undetermined number of unaffiliated participants or nodes.

While reaching an agreement in a system typically requires a majority of participants or nodes (known as a quorum) to conform, the FBA operates differently. To achieve consensus in an FBA, each participant or node must extend its trust to a smaller group of other participants or nodes (known as a quorum slice). Quorum slices facilitate trust between a node and its appointed validator nodes, and if this is done throughout the ledger, it leads to a state of agreement in the distributed ledger. They can be viewed as the (d)UNL validators on the XRPL.

The concept of “trust”, depicted by the grey square in the image, is configured in the configuration file of a node. Since each node has a unique configuration file, they may have different sets of validators and UNLs that they trust. This can lead to the formation of dynamic and independent quorum slices and quorums within the network. However, for a valid quorum to form, it must have overlapping connections between nodes, resulting in agreements. This overlap is known as a “quorum intersection” and is crucial for a sustainable peer-to-peer consensus mechanism, as it enables transactions to be proposed, accepted, and recognized throughout the network.

When quorums do not overlap, some nodes may be excluded, leading to a situation called “disjoint” quorums. This is an undesirable outcome in a distributed system as it can result in multiple independent agreements on contradictory transactions, undermining the overall consensus of the entire network. However, the XRPL Consensus Protocol has a built-in mechanism to handle this situation by having a maximum fault tolerance of approximately 20%. As long as this percentage is not exceeded within the quorums, the validators within the (d)UNLs will continue to approve new transaction proposals from network participants and nodes.

The XRPL Consensus Protocol: Ensuring Correctness and Tolerance of Faulty Processes

The effectiveness of a consensus algorithm is often measured by the percentage of faulty processes that can be tolerated on the network before it begins to fail. To achieve “correctness”, the XRPL Consensus Protocol requires that more than 80% of the (d)UNL validators agree on a transaction, ensuring that no fraudulent transactions will be approved as long as 80% of the (d)UNL is honest (Schwartz, 2018).

Over the years, various algorithms with increased complexity have been proposed for the Byzantine Generals Problem, with some even allowing for up to 33% fault tolerance before a fork is formed. The XRPL Consensus Protocol includes several key characteristics such as a mandatory 2-second window for all nodes to propose their initial candidate sets, a network split detection algorithm, and a default UNL provided to all users (dUNL) to reduce the likelihood of malicious collusion on the network. This default list of nodes ensures that even naive users will participate in a consensus process that achieves agreement, correctness, and utility with a high probability (Schwartz, 2018).

Different Use Cases, Different Solutions: The Unique Characteristics of the XRPL Consensus Protocol

Comparing blockchain systems and consensus algorithms can be misleading because each one is designed to solve different problems and address different use cases. The XRPL, for example, is optimized for scalability as a global payments gateway, which is distinct from the use cases of other systems like Bitcoin, which was designed as a decentralized store of value, or Avalanche, which was created for low-cost smart contract capability. Each system chooses the consensus/security model that best suits its specific network and purpose.