TABLE OF CONTENTS P2P has led to a recent renewal of interest in decentralized systems. Although the underlying Internet itself is the largest decentralized computer system in the world, most systems have employed a completely centralized topology in the 1990s through the massive growth of the Web.
With the emergence of P2P in early 2000, there has been a shift into employing the use of radically decentralized architectures, such as Gnutella [6]. In practice however, extreme architectural choices in either direction are seldom the way to build a usable system. Most current P2P file-sharing software, for example, use a hybrid of the two approaches.
In this chapter, we look at the ways in which peers are organized within ad-hoc, pervasive, multi-hop networks. Social networks are introduced and compared against their P2P counterparts. A summary of the typical topologies for P2P networks is given and then an analysis is undertaken using Gnutella as a test-case example, describing the evolution of the Gnutella system into using a hybrid topology.
Before we analyse P2P networks, let's first take a look at a unique social experiment conducted in 1967 by Harvard professor Stanley Milgram [154], with some notes from [62]. He performed a social-networking experiment within the United States to find out how many social hops it would take for messages to traverse through the population, which was at the time, around 200 million people.
He posted 160 letters to randomly chosen people living in Omaha, Ne-braska and asked them to try...
