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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP World Wide Web (WWW, "The Web") and the Hypertext Transfer Protocol (HTTP)                      9  TCP/IP Hypertext Transfer Protocol (HTTP)                           9  HTTP Messages, Message Formats, Methods and Status Codes

HTTP Generic Message Format 1 2 3 HTTP Response Message Format

The client initiates an HTTP session by opening a TCP connection to the HTTP server with which it wishes to communicate. It then sends request messages to the server, each of which specifies a particular type of action that the user of the HTTP client would like the server to take. Requests can be generated either by specific user action (such as clicking a hyperlink in a Web browser) or indirectly as a result of a prior action (such as a reference to an inline image in an HTML document leading to a request for that image.)

HTTP requests use a message format that is based on the generic message format described in the preceding topic, but specific to the needs of requests. The structure of this format is as follows (see Figure 317):

<request-line>
<general-headers>
<request-headers>
<entity-headers>
<empty-line>
[<message-body>]
[<message-trailers>]

Figure 317: HTTP Request Message Format

This diagram shows the structural elements of an HTTP request and an example of the sorts of headers a request might contain. Like most HTTP requests, this one carries no entity, so there are no entity headers and the message body is empty. See Figure 318 for the HTTP response format.

HTTP Generic Message Format 1 2 3 HTTP Response Message Format

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