what is MIME? multipurpose internet mail extensions. how is works?

 MULTIPURPOSE INTERNET MAIL EXTENSIONS (MIME)

·        Multipurpose Internet Mail Extensions (MIME) is a supplementary protocol that allows the non-ASCII data such as picture, images and video/audio content to be sent through e-mail.

·        MIME was invented to overcome the following limitations of SMTP:

1.      SMTP cannot transfer executable files and binary objects.

2.      SMTP cannot transmit text data of other   language, e.g. French, Japanese, Chinese etc. as these are represented in 8-bit codes.

3.      SMTP services may reject mails having size greater than a certain size.

4.      SMTP cannot handle non-textual data such as pictures, images, and IP services may reject mails having size greater than a certain size video/audio content.

·        MIME transforms non-ASCII data at sender site to NVT ASCII data and delivers them to client MTA so that it can be sent through internet. This message is then transformed into its original form at the receiver site. (see fig.).

·        Therefore MIME is a set of software functions that transforms non-ASCI data to ASCII data.

·        The MIME specification includes the following elements:

1.      Message header fields. Five message header fields are defined. These fields provide information about the body of the message.

2.      Content formats. A number of content formats are defined, thus standardizing representations that support multimedia electronic mail.

3.      Transfer encoding. Transfer encoding are defined that enable the conversion of any content format into a form that is protected from alteration by the mail system.

MIME Header

The five header fields defined in MIME are as follows:

1.      MIME-version. It indicates the MIME version being used. The current version is 1.1. It is represented as : MIME-version: 1.1.

2.      Content-type. It describes the type and subtype of the data in the body of the message. The content type and content subtype are separated by slash. This field describes how the object in the body is to be interpreted. The default value is plaintext in US ASCII. Content type field is represented as:

Context-type: <type/subtype; parameters>

There are seven different types and fourteen sub-types of content.

Type	Sub-type	Description
Text	Plain	Uniformatted text in US ASCII or ISO 8859
Image	Jpeg Gif	Image in JPEG format Image in GIF format
Video	Mpeg	MPEG format
Audio	Basic	Single channel encoding of voice at 8 KHz.
Message	Rfc822	The body is an encapsulated messafe that confirms to RFC 822.
	Partial External body	Large mail is fragmented. Contains pointer to an object that exists elsewhere and is accessible via FTP, TFTP etc.
Multipart	Mixed Parallel Alternate Digest	The different parts are independent but are to be transmitted together. They should be presented to receiver in the order they appear in mail message. Same as mixed but order not defined. The different parts are alternate versions of the same information Similar to mixed, but the default type/subtype of each part is message/rfc822.
Application	Post script Octet-stream	Adobe postscript General binary data consisting of 8 bit bytes(octets).

         























 3.      Content-transfer encoding. it describes how the object within the body has been encoded to US ASCII to make it acceptable for mail transfer. Thus it specifies the method used to encode the message into Os and 1s for transport. The content transfer encoding field is represented as:

Content-transfer-encoding : <type>

The various encoding methods used are given in the table below 

Type	Description
7 bit	The body contains the 7 bit ASCII characters with maximum length 1000 characters.
8 bit	There can be non ASCII 8 bit characters but the maximum length of the body is limited to 1000 characters/
Binary	Binary 8 bit characters without limitation of  1000 characters in the body.
Quoted printable	This is usefil when data consists of largely printable characters. Characters  in the range decimal equivalent 33 to 61 in ASCII are represented in ASCII. Others are represented as two-digit hex representation preceeded by ‘=’ sign. Non-text characters are replaced with six digit hex sequence.
Base 64	6 bit block of input data is encoded into 8 bit block of output.

4.      Content-Id. It is used to uniquely identify the MIME entities in multiple contexts i.e. it uniquely identifies the whole message in a multiple- message environment. This field is represented as

Content-id: id <content-id>

5.      Content-description. It is a plaintext description of the object within the body. It specifies whether the body is image, audio or video. This field is represented as:

Content-description: <description>

The various fields in the MIME header are shown in the figure.