HTTP headers

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Suppose Jake clicks on this link on a page:

Superdogs

Figure 1. Superdogs

Here’s the conversation between the browser and the server.

Client: 
GET superdogs.html HTTP/1.1
Host: www.evildoom.com

Server: 
HTTP/1.1 200 OK
Last-Modified: Mon, 25 Dec 2006 22:22:22 GMT
Content-Type: text/html
(Web page content)

Figure 2. HTTP conversation

The HTTP response consists of three parts:

  • Response status (line 6 in Figure 2)
  • Response headers (lines 7 and 8)
  • Data (line 9 and beyond)

The status is the first line of the response, like HTTP/1.1 200 OK. There are other codes. For example, what if the server can’t find the data the browser asked for? The server will send 404, a code meaning “page not found.”

The third piece – the data – is the content the client wants. We’ll talk more about that on the next page.

The response headers describe the server, the data, or the request. For instance, the header Last-Modified tells the browser when the resource (superdogs.html) was last changed. The header Server tells the browser what the server is (e.g., Server: Apache/1.3.3.7 (Unix)). The header Content-Length tells the browser how much data to expect.

One of the most important headers is Content-Type. This tells the browser what type of data it is being sent. Text? An image? A sound? An OpenOffice spreadsheet? The browser will look at the type to decide how to show the data to the user.

HTTP uses the Internet media type standard to indicate content type. A typical type indicator has two parts. For instance, text/plain means that the content is just plain text. text/html means the content is in the HTML language, which we’ll see on the next page. image/jpeg means the content is an image in the JPEG format. image/png means that the content is an image as well, but in the PNG format. audio/wav means that the content is sound, in the WAV format.

The Internet media type standard is maintained by the IANA (Internet Assigned Numbers Authority). It’s a standards group, like the W3C.

When a user sees a Web page in a browser, s/he doesn’t see the headers. But the headers can affect the way the content is shown. For example, suppose a browser gets this content:

<h1>Hi there!</h1>

If the content type header is:

Content-Type: text/html

The browser will interpret the data as HTML, and the user will see:

Hi there!

But if the header is:

Content-Type: text/plain

The browser will interpret the data as text, and the user will see:

Hi there!


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