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Multipurpose Internet Mail Extensions ( MIME ) HEADER. MANOJ TALAGADADEEVI 04/ 03 / 2007. INTRODUCTION. An Internet Standard that extends the format of e-mail to support: text in character sets other than US-ASCII ; non-text attachments;
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Multipurpose Internet Mail Extensions (MIME) HEADER MANOJ TALAGADADEEVI 04/ 03 / 2007
INTRODUCTION An Internet Standard that extends the format of e-mail to support: • text in character sets other than US-ASCII; • non-text attachments; • multi-part message bodies; and • header information in non-ASCII character sets.
MIME HEADER FIELD • A header field is required to define the type of data contained in the message body. • There are five header fields. MIME – Version Content – Type Content-Transfer-Encoding Content-ID Content-Description The first three header fields below are required and the last two are optional.
MIME - Version • Describes the version of the MIME message format. • Declare the version of the Internet message body format standard in use. • The presence of this header field is an assertion that the message has been composed in compliance with this document. • This field can be included in the document as below: MIME-Version: 1.0
Content – Type • Describes the MIME content type and subtype. There are seven defined types which include: • Application - Executable or binary code is being transmitted which a specific program may be required to run. • Audio - Audio data. • Image - Graphic non-moving images. • Message - Encapsulates a message as defined by RFC 822. • Multipart - Combines several parts of the document which have different types. • Text - (Default) Specifies text content. • Video - Moving images.
Content-Transfer-Encoding Header Field • Indicate the type of transformation that has been used in order to represent the body in an acceptable manner for transport. • Designed to specify an invertible mapping between the "native" representation of a type of data and a representation that can be readily exchanged using 7 bit mail transport protocols, such as those defined by RFC 821 (SMTP).
The field's value is a single token specifying the type of encoding. • Formally: encoding := "Content-Transfer-Encoding" ":" mechanism mechanism := "7bit" ; case-insensitive / "quoted-printable" / "base64" / "8bit" / "binary" / x-token
Options: • 7-Bit - Data is sent as US-ASC data. • 8-Bit - 8 bit characters are included in short lines. • BASE64 - Used for binary files. Three bytes are transformed into 4 ASC characters in lines limited to a length of 76 characters. • Binary - Long lines are sent using 8 bit characters. These lines may not be transportable using SMTP. • Quoted-Printable - Used tor ASC text. Line length is limited to 76 characters. • X-Token - Defines private encoding values prefixed with an "X-".
Content-ID Header Field • Allows a body of information to refer to another. • It is syntactically identical to the "Message-ID" header field: id := "Content-ID" ":" msg-id • The Content-ID value may be used for uniquely identifying MIME entities in several contexts, particularly for cacheing data referenced by the message / external – body mechanism.
Content-Description • Possible description for a body of information. • In the form : description := "Content-Description" ":" *text • The description is presumed to be given in the US-ASCII character set
