In the client-server model, the device requesting the information is called a client and the device responding to the request is called a server. Client and server processes are considered to be in the application layer. The client begins the exchange by requesting data from the server, which responds by sending one or more streams of data to the client. Application layer protocols describe the format of the requests and responses between clients and servers. In addition to the actual data transfer, this exchange may also require user authentication and the identification of a data file to be transferred.
One example of a client-server network is using an ISP’s email service to send, receive and store email. The email client on a home computer issues a request to the ISP’s email server for any unread mail. The server responds by sending the requested email to the client.
Although data is typically described as flowing from the server to the client, some data always flows from the client to the server. Data flow may be equal in both directions, or may even be greater in the direction going from the client to the server. For example, a client may transfer a file to the server for storage purposes. As shown in the figure, data transfer from a client to a server is referred to as an upload and data from a server to a client as a download.