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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 Generic Message Format
(Page 2 of 2)

Header Format

There are many dozens of message headers defined in HTTP, which are organized into groups by function as we will soon see. Almost all of these are optional; the one exception is the Host header, which must be present in each request in HTTP/1.1. Headers may be sent in any order, and all follow the same header format used in e-mail messages:

<header-name>: <header-value>
Message Body Contents

The message body is optional, because it is only needed for certain types of messages. The body may carry a set of information to be communicated between the client and server, such as a detailed error message in a response. More commonly, when present, it carries a file or other resource, which is formally called an entity in the HTTP standard. Entities are most often found in the body of a response message, since most client requests ask for a server to send a file or other resource. However, they can also be found in certain requests. HTTP supports many kinds of entities and provides encoding methods to support a wide range of media. Special headers describe the characteristics of the entity, if present. HTTP entities and related issues are described in detail in their own section of the Guide.

Key Concept: All HTTP messages conform to a structure called the generic message format. This format is based on the RFC 822 and MIME electronic mail message standards, though HTTP does not follow those formats precisely. Each HTTP message begins with a start line, then contains a number of message headers, followed by an empty line and optionally a message body. The body of the message may contain a resource such as a file to be communicated between client and server, called an entity.

Chunking and Message Trailers

HTTP/1.1 uses persistent connections by default, so messages are sent in a steady stream from client to server and server to client. This requires that some means be used to mark where one message ends and the next begins, which is usually accomplished in one of two ways. The first is using a special header that indicates the length of the message so the receiving device knows when the entire message has been received. The second is a method called “chunking”, where a message is broken into pieces for transmission and the length of each piece indicated in the message body.

When chunking is done, a set of message trailers may follow the body of the message. Trailers are in fact the same as headers except for their position in the file, but may only be used for entity headers. See the topic on data length issues, “chunked” transfers and trailers for more details on all of these matters.

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Version 3.0 - Version Date: September 20, 2005

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