The Hypertext Transfer Protocol is the application-layer protocol that implements the World Wide Web. While the Web itself has many different facets, HTTP is only concerned with one basic function: the transfer of hypertext documents and other files from Web servers to Web clients. In terms of actual communication, clients are chiefly concerned with making requests to servers, which respond to those requests.

Thus, even though HTTP includes a lot of functionality to meet the needs of clients and servers, when you boil it down, what see is a very simple, client/server, request/response protocol. In this respect, HTTP more closely resembles a rudimentary protocol like BOOTP or ARP than it does other application-layer protocols like FTP and SMTP, which all involve multiple communication steps and command/reply sequences.

In its simplest form, the operation of HTTP involves only an HTTP client, usually a Web browser on a client machine, and an HTTP server, more commonly known as a Web server. After a TCP connection is created, the two steps in communication are as follows:

1. Client Request: The HTTP client sends a request message formatted according to the rules of the HTTP standard—an HTTP Request. This message specifies the resource that the client wishes to retrieve, or includes information to be provided to the server.
   
   
2. Server Response: The server reads and interprets the request. It takes action relevant to the request and creates an HTTP Response message, which it sends back to the client. The response message indicates whether the request was successful, and may also contain the content of the resource that the client requested, if appropriate.
   

   Figure 315: HTTP Client/Server Communication

   In its simplest form, HTTP communication consists of an HTTP Request message sent by a client to a server, which replies with an HTTP Response.

    

   
   

In HTTP/1.0, each TCP connection involves only one such exchange, as shown in Figure 315; in HTTP/1.1, multiple exchanges are possible, as we'll see in the next topic. Note also that the server may in some cases respond with one or preliminary responses prior to sending the full response. This may occur if the server sends a preliminary response using the “100 Continue” status code prior to the “real” reply. See the topic on HTTP status codes for more information.