When we discussed the HTTP request-response model in the Understanding the HTTP request-response model section, we talked about the HTTP header parameters (the name-value pairs) that define the operating parameters of an HTTP transaction. In this section, we will cover the header parameter used for describing the content types present in the request and the response message body.

The Content-Type header in an HTTP request or response describes the content type for the message body. The Accept header in the request tells the server the content types that the client is expecting in the response body. The content types are represented using the internet media type. The internet media type (also known as the MIME type) indicates the type of data that a file contains. Here is an example:

Content-Type: text/html 

This header indicates that the body content is presented in the html format. The format of the content type values is a primary type/subtype followed by optional semicolon-delimited attribute-value pairs (known as parameters).

The internet media types are broadly classified into the following categories on the basis of the primary (or initial) Content-Type header:

• text: This type indicates that the content is a plain text and no special software is required to read the contents. The subtype represents more specific details about the content, which can be used by the client for special processing, if any. For instance, Content-Type: text/html indicates that the body content is html, and the client can use this hint to kick off an appropriate rendering engine while displaying the response.
• multipart: As the name indicates, this type consists of multiple parts of independent data types. For instance, Content-Type: multipart/form-data is used for submitting forms that contain the files, non-ASCII data, and binary data.
• message: This type encapsulates more messages. It allows messages to contain other messages or pointers to other messages. For instance, the Content-Type: message/partial content type allows for large messages to be broken up into smaller messages. The full message can then be read by the client (user agent) by putting all the broken messages together.
• image: This type represents the image data. For instance, Content-Type: image/png indicates that the body content is a .png image.
• audio: This type indicates the audio data. For instance, Content-Type: audio/mpeg indicates that the body content is MP3 or other MPEG audio.
• video: This type indicates the video data. For instance, Content-Type: video/mp4 indicates that the body content is an MP4 video.
• application: This type represents the application data or binary data. For instance, Content-Type: application/json; charset=utf-8 designates the content to be in the JavaScript Object Notation (JSON) format, encoded with UTF-8 character encoding.

We may need to use some of these content types in the next chapters while developing the RESTful web services. This hint will be used by the client to correctly process the response body.

The next topic, a simple but important one, is on HTTP status codes.