In a client-server architecture, there is an always-on host, called the server, which services requests from many other hosts, called clients. The client hosts can be either sometimes-on or always-on. A classic example is the Web application for which an always-on Web server services requests from browsers running on client hosts. When a Web server receives a request for an object from a client host, it responds by sending the requested object to the client host. Note that with the client- server architecture, clients do not directly communicate with each other; for example, in the Web application, two browsers do not directly communicate. Another characteristic of the client-server architecture is that the server has a fixed, well- known address, called an IP address (which we'll discuss soon). Because the server has a fixed, well-known address, and because the server is always on, a client can always contact the server by sending a packet to the server's address. Some of the better-known applications with a client-server architecture include the Web, F1'R Telnet, and e-mail.

Often in a client-server application, a single server host is incapable of keeping up with all the requests from its clients. For example, a popular social-networking site can quickly become overwhelmed if it has only one server handling all of its requests. For this reason, a cluster of hosts--sometimes referred to as a server farm--is often used to create a powerful virtual server in client-server architectures. Application services that are based on the client-server architecture are often infrastructure intensive, since they require the service providers to purchase, install, and maintain server farms. Additionally, the service providers must pay recurring interconnection and bandwidth costs for sending an4 receiving data to and from the Internet. Popular services such as search engines (e.g., Google), Internet commerce (e.g., Amazon and e-Bay), Web- based v-mail (e.g, Yahoo Mail), social networking (e.g., MySpace and Facebook), and video sharing (e.g., YouTube) are infrastructure intensive and costly to provide.

In a P2P architecture, there is minimal (or no) reliance on always-on infrastructure servers. Instead the application exploits direct communication between pairs of intermittently connected hosts, called peers. The peers are not owned by the service provider, but are instead desktops and laptops controlled by users, with most of the peers residing in homes, universities, and offices. Because the peers communicate without passing through a dedicated server, the architecture is called peer-to-peer. Many of today's most popular and traffic-intensiye applications are based on P2P architectures. These applications include file distribution (e.g., BitTorrent), file searching/sharing (e.g., eMule and LimeWire), Intemet telephony (e.g., Skype), and IPTV (e.g., PPLive).

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