Network Topologies

Several network topologies can be used for connecting P2P systems. In this section, we discuss the major P2P network topologies in order to explain how information can be transmitted between peers effectively.

The Hierarchical Topology:

One of the most common topologies is the hierarchy. Every time you type a website URL into your browser , you are using a system called DNS, or Domain
Name Server. This system is set up in a hierarchy, with root servers at the very top levels. The hierarchy topology looks like Figure 1.1. For several years now, critics have called for an overhaul of the DNS architecture because the root servers represent a single point of failure. However, because the entire system is based on replication and the chance of the DNS system going down is very small, no real work has occurred in this area.

The Ring Topology:

Token Ring is a network topology that uses the concept of passing a single token around to the computers connected in a ring pattern. When a machine receives the token, it can send information out onto the network. The ring topology isn’t used much anymore for common networks, but does provide an interesting pattern for load-balancing a single-server system or hierarchy. The top rung of a hierarchy topology could actually be a ring of servers that balance the network requests. Figure 1.2 shows what a ring topology looks like.

The Client-Server (Centralized) Topology:

By far the most common topology is the client-server, or centralized, topology. The terminology of client-server has been with us for many years; more recently, the term centralized has been used to describe a system in which a single computer, the server, makes services available over the network. Client machines contact the server when the services are needed. Obviously, the more clients in the system, the larger the server must be. At some point, the server will need to be replicated in order to handle the traffic volume from all clients. Figure 1.3 shows an example of the centralized topology.

The Decentralized Topology:


The decentralized topology is a network topology that comes closest to being truly peer-to-peer. There is no central authority, only individual computers that are able to connect and communicate with any of the other computers on the network. When a packet of information starts its travels on the Internet, it is basically traveling through a decentralized topology. Information within the packet itself tells each computer where to send the packet next. Figure 1.4 shows an example of a decentralized network topology. Basically, all of the peers in the system act as both clients and servers, handling query requests and downloads while also making media searches and download request themselves.

The Hybrid Topology:

In the hybrid topology , we have an example of a situation where the individual computers are considered clients when they need information. The client that needs information will contact a central server (the centralized servers are distributed in the example shown in Figure1.5) to obtain the “name” of another client where the requested information is stored. The requesting client will then contact the client with the information directly. With the hybrid, a computer can be either a client or a server. This is the topology used for the Napster [8] [16] system—individual peers contact a localized server for searching and proceed to contact peers directly for media downloading.