Saturday, 14 February 2015

Post 27: Resources, Media Types, and Content Labeling

On the Internet you'll encounter lots of different resources, e. g. text, image, audio, video, etc. In order for the server to host and the client to display the resource correctly both have to be specific about the type of the resource. In order for the browser to correctly display these content for the user the resource has to be labeled accordingly.
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When a host responds to a HTTP request it returns the resource and specifies it's content type (also called media type). For content type specification HTTP uses the MIME (Multipurpose Internet Mail Extension) standard. The specification is done by labeling the resource so the client knows the resource's content. (As a site note: You may wonder why MIME stands for “Multipurpose Internet Mail Extension”. The reason is MIME was originally used for email communication but as it turned out to be very useful for labeling content types as well.)

Here are a few examples for HTTP labels:
- webpages are labeled with “text/html”; “text” is primary media type and “html” is media subtype.
- jpeg images are labeled with “image/jpeg”;
- png images are labeled with “image/png”;
- etc.

These content types are standard MIME types and will appear in the HTTP responses.

When the browser is requesting a resource it must know what type of resource to display. To get this information the browser will look at these location one after the other. If he doesn't find it in one of the location he will search in the next one:
1st: content type specified by the host in the HTTP response message
2nd: scan the first 200 bytes of the HTTP response message and trying to “guess” the content type
3rd: read the file extension

Here I explained to you the specification of the content media type for the client to correctly represent or display the resource. In the next post I'll explain that the client also has a say what kind of resources the server is sending to him.

Source(s):
HTTP Succinctly by Scott Allen Syncfusion
Wikipedia
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