The application layer: clients interacting with servers to get work done

In network applications, a client and server work together to get useful work done.

Clients and servers communicate with each other across the Internet or any other network. (Client-server applications are also common on local area networks). The client requests some service and the server performs it (or returns an error message if there is a problem).

The Web is a familiar client-server application. To see a Web page, you run a Web client like Internet Explorer or Firefox. When you enter a URL and hit enter, the client encloses it in a Get message which it sends across the network to a Web server. If the document is on that Web server, it sends it back to the client. If not, it sends an error message (formatted as a Web page). When the client receives the page (pr error message), it formats and displays it.

  1. the user enters a URL and hits enter
  2. the client program sends a Get message containing the URL to the waiting server
  3. if the server has the requested document, it sends it back to the client
  4. if the server does not have the document, it sends an error message back to the client
  5. the client displays whatever was returned
  6. the server waits for the next request
This dialog between the client and the server follows a specific pattern or protocol. The protocol specifies the format of messages that can be sent and their meanings. The Web protocol is called the Hypertext Transfer Protocol (HTTP).

Note that when information is transferred from a server to a client as in this case, we speak of downloading and when we transfer information to a server we speak of uploading.

Surfing the Web with a Web client is a popular application, but there are many other application protocols, for example:

ProtocolApplication
HTTP, Hypertext Transferretrieving and viewing Web pages
FTP, File Transfer copy files from client to server or from server to client
SMTP, Simple Mail TransportSend email
POP, Post OfficeRead email

In some applications, a computer is programmed to be both a client and a server. These are called peer to peer applications.

Note that the terms "client" and "server" refer both to the computer hardware and the program it is running. When you are looking at things on the Internet, your computer is the client hardware and it is running a client program.

In the case of the Web, people also refer to a client program as a browser since it is used to browse thorough the contents of Web sites.

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