HTTP message headers are very important, because they are the mechanism that HTTP uses to allow devices to specify the details of client requests and server responses. These headers, however, are only the means to an end, which is the transfer of resources such as files, form input and program output from one device to another. When a resource is carried in the body of an HTTP message, it is called an entity. HTTP defines special rules for how these entities are identified, encoded and transferred.

In this section, I take a detailed look at how HTTP handles entities. I begin with a discussion of entities in general terms, and a look at how their contents are identified; this includes an examination of the relationship between HTTP and MIME. I discuss the issues behind the transfer of entities between clients and servers, and the difference between content encodings and transfer encodings. I describe the special issues associated with identifying the length of entities in HTTP messages, and detail the special “chunked” transfer coding and message trailers. Finally, I describe the methods by which devices can perform content negotiation, and how quality values allow clients to intelligently select different variations of a resource.