23.2 MULTIPURPOSE INTERNET MAIL EXTENSION (MIME)
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23.2 MULTIPURPOSE INTERNET MAIL EXTENSION (MIME)
MIME is defined in RFC 1521 and 1522. The specifications include:
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Five new message header fields that provide information about the body of the message.
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A number of content formats to support multimedia mail.
The five header fields are:
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MIME Version 1.0 (conforms to RFC 1521 and 1522).
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Content type that describes data contained in the body.
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Content transfer encoding that indicates the type of transformation that has been used to represent the body of the message.
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Content ID to uniquely identify MIME content types.
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Content description: plain text description of object, such as audio, text, or video clip.
Some of the MIME content types are listed here.
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Type |
Subtype |
Description |
|---|---|---|
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Text |
Plain |
|
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Multipart |
Mixed |
To be presented to the
|
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Parallel |
No order is required. |
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|
Alternative |
Alternative version, best to be displayed. |
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|
Digest |
Similar to mixed, but RFC 822. |
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Message |
RFC822 |
|
|
Partial |
Fragment |
Large
|
|
External body |
Pointer to object |
|
|
Image |
JPEG |
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|
GIF |
||
|
Video |
MPEG |
|
|
Audio |
Basic |
8-bit at 8kHz |
|
Application |
PostScript |
|
|
Octet stream |
Binary data with 8-bit types. |
An example MIME header looks like this:
From: kvkk.prasad@ieee.org To: kvravi@ece.nus.sg Sub: demo MIME Version 1.0 Content type: multipart/mixed Content type: video/MPEG
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MIME is an extension to SMTP to provide multimedia message content to e-mails. In addition to plain text, images, executable files, audio, and video clips can be attached to the e-mail. MIME is defined in RFCs 1521 and 1522.
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Hence, MIME is an extension to the SMTP to provide multimedia support in the message of the mail using the various MIME types.
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23.3 HYPERTEXT TRANSFER PROTOCOL (HTTP)
To access the World Wide Web service, HTTP protocol is used, the latest version being HTTP 1.1. HTTP uses URL (Uniform Resource Locator), which represents the location and access method for a resource available on the Internet. For example:
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http://www.iseeyes.com
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ftp://rtfm.mit.edu/pub/Index.README
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The Uniform Resource Locator (URL) has six fields: protocol, domain name, port address, directory
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The format of a URL is
http://www.elearn.cdacindia.com:80:/hypertext/www/Addressing/Addressing.html#spot
1 2 3 4 5 6
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Protocol (such as HTTP, FTP)
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Domain name
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Port address
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Directory path
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Object name
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To reach a specific spot (or link)
Some of the most commonly used protocols along with port addresses are
|
FTP |
21 (port address) |
|
HTTP |
80 |
|
Telnet |
23 |
Figure 23.2 shows the procedure for accessing the resource corresponding to a URL. The client invokes the browser (such as Internet Explorer or Netscape Communicator) and specifies the URL. The URL is passed to a Domain Name System (DNS) server (Step 1), which gives the IP address of the server (Step 2) that has the resource corresponding to that URL. The server that has the resource is known as the origin server. Using that IP address, the HTTP request is sent to the Web server (Step 3) and the Web server gives the response in the form of an HTML document to the client via the ISP server (Step 4). Sometimes Web servers do not allow access to every
Figure 23.2:
Web access.
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When a user wants to access a resource by giving a URL, the IP address of the origin server is obtained using a Domain Name System. Then a TCP connection is established between the client and the origin server. The origin server sends the HTML document corresponding to that URL to the client.
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HTTP is a transaction-oriented client/server protocol. The client is the Web browser, and the server is the web server. The mechanism for HTTP protocol is shown in Figure 23.3. The origin server is the server on which the resource is located. There are three possibilities for interaction between the client and the origin server:
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Direct connection
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Proxy
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Relay
Figure 23.3:
Hypertext transfer protocol (HTTP).
In direct connection, a TCP connection is established between the client (user agent) and the origin server. The user agent sends an HTTP request, and the origin server sends the response.
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A proxy is used when the Internet server does not permit direct access to the users. The proxy acts as a client to the server and as a server to the client.
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A proxy acts as a server in interacting with
A tunnel
There are two types of messages:
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Request messages: |
GET, POST, DELETE |
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Response messages: |
OK, Accepted, Use Proxy |
Each message contains a header and a body.
HTTP defines status codes to
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OK |
Request successful. |
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No content |
No information to send back. |
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Moved permanently |
Resource URL has changed permanently. |
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Moved temporarily |
Resource URL has changed temporarily. |
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Use Proxy |
Resource must be accessed through proxy. |
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Unauthorized |
Access control is
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When a user wants to access a Web site, the URL given in the address field of the browser is sent to the ISP server, which gets the IP address corresponding to the URL. Then a TCP connection is established and a
GET
command is sent. The origin server sends back the resource (the HTML file). The HTML code containing the tags will be interpreted by the browser and displayed to the user. In case any error is
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HTTP is a simple transaction-oriented client/server protocol. The client sends a GET request along with a URL, and the server responds with a POST response containing the HTML document corresponding to that URL. In case there is a problem such as nonavailability of the content, nonavailability of the server, and so on, an error message is sent.
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The World Wide Web has become the most attractive service on the Internet because of HTTP. The information corresponding to a URL is obtained from the origin server in the form of Hypertext Markup Language (HTML), which is a simple text file with tags (called markup). These tags specify how the content has to be formatted and displayed (bold
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