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

Headers

After the request line come any of the headers that the client wants to include in the message; it is in these headers that details are provided to the server about the request. The headers all use the same structure, but are organized into categories based on the functions they serve, and whether they are specific to one kind of message or not:

• General Headers: General headers refer mainly to the message itself, as opposed to its contents, and are used to control its processing or provide the recipient with extra information. They are not particular to either request or response messages, so they can appear in either. They are likewise not specifically relevant to any entity the message may be carrying.
  
  
• Request Headers: These headers convey to the server more details about the nature of the client's request, and give the client more control over how the request is handled. For example, special request headers can be used by the client to specify a conditional request—one that is only filled if certain criteria are met. Others can tell the server which formats or encodings the client is able to process in a response message.
  
  
• Entity Headers: These are headers that describe the entity contained in the body of the request, if any.

Request headers are obviously used only in request messages, but both general headers and entity headers can appear in either a request or a response message. Since there are so many headers and most are not particular to one message type, they are described in detail in their own complete section.

HTTP Generic Message Format 1 2 3 HTTP Response Message Format