★ wanayoo — archive 1999 http://www.wdvl.com/Authoring/Tools/Tutorial/HTTP.html

HTTP

August 16, 1999

Okay, so that is how data is sent from a software program on one computer, transferred through the network, and reassembled on the other end. TCP/IP pretty successfully handles network navigation. However, browsers and servers need to transfer more information than what is required in network navigation.

For example, browsers and servers must understand how to exchange web documents (multimedia files such as images, text, audio, etc). To do so, browsers and servers must speak their own standard protocol to help them exchange these files as well as meta information about those files.

Specifically, browsers and servers speak the HTTP protocol Where TCP/IP drives the internet, HTTP drives the web.

HTTP is a stateless, "request/response" protocol that specifies the means of transport as well as the process for maintaining the integrity of those multimedia files. It does so by using the Multipurpose Internet Mail Extensions (MIME) specification that describes the transfer and format of multimedia files.

The web consists of millions of hyperlinked files. These millions of files in turn, each may contain webs of links pointing to other files in the network. However, because the web is used to deliver files of all types (audio, video, text, etc), web browsers and servers need some way of defining what it is that is being linked to and transported.

To do so, MIME headers are used by HTTP to specify the contents of any transported file. The header will specify a file's type.

MIME Types and subtypes examples include the following:

MIME Type	MIME Subtype
Application	msword
Application	pdf
Audio	aif
Audio	mpeg
Image	gif
Image	jpeg
Image	bmp
Multipart	form-data
Multipart	mixed
Text	html
Text	plain

The list is hardly exhaustive of course, and in fact web servers and web browsers can easily extend the MIME specification to add new MIME types. All you need to do is make sure that the server is configured to output the new type and that browsers are configured to expect the new type.

Getting browsers to understand new types is usually a function of the options or preferences settings in that browser. Consider the Netscape preferences dialog.

[Setting recognized MIME headers in Netscape]

MIME types are specified as part of the "HTTP header" which defines meta-information about the document and the session. MIME type is specified in the Content-type field.

Thus, if a web server were to send a document of MIME type...

Content-type: application msword

the web browser would know that it should open Word and load the document.

Besides specifying the MIME type, the HTTP header also contains other important information including the following:

Header	Description
Accept	Specifies what types of media output the client is prepared to handle. Often, a client will specify several types of media that is can handle such as in the following case: Accept: image/gif Accept: image/jpeg Alternatively, a web browser might specify that it is prepared to handle "any" media using the following line: Accept: /
Authorization	Specifies whether or not the user has permission to access a secure area.
Content-encoding	Specifies that the message body is encrypted, compressed, or encoded.
Content-type	Indicates the media type of the information in the message body.
Content-length	Specifies the number of bytes in the message body for requests with a message body.
Date	Indicates the date and time of a request if the request has a message body.
From	Provides the email address of the user using the client if it is available
If-modified-since	Tells the server not to deal with the request if the document has not been modified since a given time.
Mime-version	Specifies the MIME version used to generate the message body.
Pragma	Contains any additional information that the client wishes to specify to the server.
Referer	Indicates the URL of the page from which the request was made. (This request header is misspelled in the protocol itself).
User-agent	Specifies the name and version number of the web browser making the request.

Transmission Control Protocol (TCP)
Introduction to the Web Application Development Environment (Tools)
Constructing a Web Application Development Environment Introduction to Transmission