Taxonomy of trust: Categorizing P2P reputation systems

Abstract

The field of peer-to-peer reputation systems has exploded in the last few years. Our goal is to organize existing ideas and work to facilitate system design. We present a taxonomy of reputation system components, their properties, and discuss how user behavior and technical constraints can conflict. In our discussion, we describe research that exemplifies compromises made to deliver a useable, implementable system.

Introduction

The development of any complex computer architecture can be a challenge. This is especially true of a complex distributed algorithm that is run by autonomous untrusted agents, yet is expected to be relatively reliable, efficient, and secure. Such is the task of designing a complete reputation system for use in peer-to-peer networks. To accomplish the task, it is necessary to break down the problem into separate simpler problems of constructing a mechanism that provides a specific set of functions or properties, allowing developers to “divide and conquer” the problem of reputation system design.

Our primary goal is to provide a useful taxonomy of the field of peer-to-peer reputation design. To accomplish this goal, we identify the three basic components of a reputation system, break them down into the necessary separate mechanisms, and categorize properties we feel the mechanisms need to provide in order for the reputation system to fulfill its function. For each mechanism we list possible design choices proposed by the research community.

In the process, we give examples of research in the area of trust and reputation. A variety of research papers and implementations are referenced to illustrate ideas and provide the reader avenues for further investigation. We often draw on work done by the Peers research group [1] at Stanford University and do not pretend to produce a complete survey of the research area. We feel this overview will be of particular interest to those who are unfamiliar with the breadth of issues relating to reputation system design for peer-to-peer networks.

Taxonomies related to trust and reputation systems (either in part or as a whole) have been proposed by others (e.g., [2]) and will be discussed in the text when appropriate.

The following section defines terms we use throughout the paper. We begin our taxonomy by classifying the assumptions and constraints of the system in Section 3. These assumptions include expected user behavior, as well as the goals of adversaries in the system and their capabilities. How effectively a reputation system can deal with adversaries may be constrained by the technical limitations imposed on the implementation by the target system environment. These issues determine the necessary properties and powers of the reputation system.

Next, we break down the functionality of a reputation system into the three components shown in Table 1. In general, a reputation system assists agents in choosing a reliable peer (if possible) to transact with when one or more have offered the agent a service or resource. To provide this function, a reputation system collects information on the transactional behavior of each peer (information gathering), scores and ranks the peers based on expected reliability (scoring and ranking), and allows the system to take action against malicious peers while rewarding contributors (response). Each component requires separate system mechanisms (listed in Table 1). For each mechanism we study the possible desired properties and then discuss the implementation limitations and trade-offs that may prevent some of the properties from being met. In the discussion we will reference existing solutions or research to illustrate how different mechanism designs achieve certain properties within the given system constraints.

The three functionalities, gathering, scoring and response are covered in turn in Sections 4 Gathering information, 5 Reputation scoring and ranking, 6 Taking action.