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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 Entities, Transfers, Coding Methods and Content Management

HTTP Data Length Issues, "Chunked" Transfers and Message Trailers 1 2 3 HTTP Features, Capabilities and Issues

Many Internet resources have only one representation, meaning a single way in which they are stored or made available. In this situation, a client request to a server is an “all or nothing” proposition. The client may specify conditions under which it would like the server to send the resource, using the “If-” series of request headers. If the condition is met, the resource will be sent in the server’s response in the one form in which it exists; if the condition is not met, no entity will be returned.

Other resources, however, may have multiple representations. The most common example would be a document that is available in multiple languages, or that is stored using more than one character set. Similarly, a graphical image might exist in two different formats: one a Tagged Image File Format (TIFF) file for those wanting maximum image quality despite the large size of TIFF images; and a more compact JPEG file for those who need to see the image quickly and don’t care as much about its quality level.

To provide flexibility in allowing clients to obtain the best version of resources that exist in multiple forms, HTTP/1.1 defines a set of features that are collectively called content negotiation. The standard defines two basic methods by which this negotiation may be performed:

• Server-Driven Negotiation: In this technique, the client includes headers in its request that provide guidance to the server about its desired representation for the resource. The server uses an algorithm that processes this information and provides the version of the resource that it feels best matches the client’s preferences.
  
  
• Agent-Driven Negotiation: This method puts the client in charge of the negotiation process. It first sends a preliminary request for the resource to the server. If the resource is available in multiple forms, the server typically sends back a 300 (“Multiple Choices”) response, which contains a list of the various representations in which the resource is available. The client then sends a second request for the one that it prefers.

HTTP Data Length Issues, "Chunked" Transfers and Message Trailers 1 2 3 HTTP Features, Capabilities and Issues