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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 Request Message Format 1 2 3 HTTP Methods

Headers

The response message will always include a number of headers that provide extra information about it. Response message headers fall into these categories:

• General Headers: General headers that refer to the message itself and are not specific to response messages or the entity in the message body. These are the same as the generic headers that can appear in request messages (though certain headers appear more often in responses and others are more common in requests).
  
  
• Response Headers: These headers provide additional data that expands upon the summary result information in the status line. The server may also return extra result information in the body of the message, especially when an error occurs, as discussed below.
  
  
• Entity Headers: These are headers that describe the entity contained in the body of the response, if any. These are the same entity headers that can appear in a request message, but they are seen more often in response messages. The reason for this is simply that responses more often carry an entity than requests, because most requests are to retrieve a resource.

Response headers are of course used only in response messages, while the others are general with respect to message type. See the section describing HTTP headers for more details.

Most response messages contain an entity in the message body. In the case of a successful request to retrieve a resource, this is the resource itself. Responses indicating unsuccessful requests usually contain detailed error information, often in the form of an HTML-formatted error message.

HTTP Request Message Format 1 2 3 HTTP Methods