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The TCP/IP Guide

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Table Of Contents  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

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HTTP Request Message Format
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HTTP Response Message Format
(Page 3 of 3)


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.

Note: Entity headers may appear in a response to describe the resource that is the subject of the request, even if the entity itself is not sent in the message. This occurs when the HEAD method is used to request only the headers associated with an entity.

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.

Key Concept: Each HTTP request sent by a client leads to a server returning one or more HTTP responses. Each response message starts with a status line that contains the server’s HTTP version number, and a numeric status code and text reason phrase that indicate the result of processing the client’s request. The message then contains headers related to the response, followed by a blank line and then the optional message body. Since most HTTP requests ask for a server to return a file or other resource, many HTTP responses carry an entity in the message body.

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HTTP Request Message Format
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HTTP Methods
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Version 3.0 - Version Date: September 20, 2005

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