Multipurpose Internet Mail Extensions(MIME)

MIME composite type

A MIME type can be composite, i.e for a collection of different types of data a It has MIME type: multipart/mixed In this case the parts are delimited by named boundary in the email message Each part has its own MIME type The characters "--" are used to start each opart and to end the list of parts

Encoding declaration

A header field is used to declare the encoding Content-transfer-encoding <encoding>

Media subtypes

Each types can have many subtypes, giving a finer classification The full media type is written: type/subtype Common subtypes of text type MIME include: text/plain text/enriched text/html text/css

MIME

HTTP and email were originally designed to carry ASCII text Now they carry data of different kinds Must make the type of data explicit, so that an application can be chosen to process it MIME specifies how to declare the type of data being transferred MIME allows data to be encoded to allow non ASCII data transfer over ASCII protocols

Content type declaration

In HTTP, email and other headers, the MIME type of the content is declared as: Content-type: type/subtype: options This lets the receiving host knowe how to process the message data For example Content=Type: text/plain: charset=ISO-8859-1

Base64

In base64, ebery 24 bits of data is split into 4 6 bits chunks Each chunk is turned into a letter 0 → A 1 → B etc Where data does not divide into 24 bit chunks, = is used to encode the missing chunks

Media types

MIME provides a high level classification of data into 7 media types text Image Audio Vide application multipart Messages New types can be created but must start with X- to mark them as experimental

MIME in HTTP

MIME types are used in HTTP messages e.g The type of entity in an HTTP response: Content-Type: test/plain The types accepted in response by an HTTP GET request: Accepted: text/plain, text/html

MIME encoding types

Mime data can be encoded in one of 5 ways 7 bit: ASCII compatible, fits in lines of 1000 characters 8 bit: 8-bit text character encoding, fits in lines of 1000 characters binary: Encoded directly in binary form (cannot be used for direct transmission in SMTP) quoted-printable: Non-standard ASCII text base64:Binary data encoded as ASCII

Parameterised types

Optional parameters can be added to media type to give even more detail This takes the form type/subtype:options text/plain: charset=ISO-8859-1

MIME encoding

Protocols have limits on The format of the data The structure of the data Have to encode data to make it transferable Have to declare what encoding was used, so that receiver can decode it (if necessary MIME allows for such declarations '