What Is A Mime Encoding ?
| Multipurpose Internet Mail Extensions |
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What Is MIME? Multipurpose Internet Mail Extensions, or MIME, is an extension of the RFC-822 format. MIME format messages are also sent across the Internet by way of the SMTP protocol, but the MIME format provides for much greater flexibility than RFC-822. MIME provides a means to interchange text in languages with different character sets. Multimedia e-mail can be sent between different computer systems that use the SMTP protocol. MIME allows you to send and receive e-mail messages containing:
The MIME extensions are described in RFC-1521. MIME is fully backward-compatible with RFC-822. If a MIME message is sent to someone whose system lacks MIME capabilities, the recipient can see the message, even if the message is partially or completely unreadable. In most instances the user would then need a MIME-compliant e-mail program (such as the GroupWise Internet Agent) to read MIME messages. The MIME format includes a mail header identical to the RFC-822 header. Part of that header is a field that tells a MIME-aware mail program that the message is in MIME format. There can be one or more bodies after the header. Each body can have its own subheader that defines the content of the sub body. These bodies may be defined as almost any desired type. There are also seven predefined body types. Multimedia data types such as audio clips, video clips and pictures, along with binary files and any other file type may be encoded and included in the body of the mail message. MIME specifies several encoding schemes to ensure that the recipient's mail program will correctly decode the message. These encoding schemes include Base64 and printed quotable. MIME is largely hidden to users. Once you have installed the GroupWise Internet Agent, inbound Internet messages that are in either RFC-822 or MIME format are automatically processed by the Internet Agent into their original form and delivered to the recipient. Sending mail is the only situation in which the user might need to know the difference between RFC-822 and MIME format. If you set up your Internet Agent program with two foreign domains, one defined to send in MIME format, the other defined to send in RFC-822 format, you can easily choose the best way to send the message. If you know the recipient's e-mail system has MIME capability, you can send the message through the MIME foreign domain. MIME Encoding When a message is sent or received in MIME format, attachments are encoded or decoded because the SMTP protocol sends only US-ASCII characters. Binary files cannot be sent using SMTP without being encoded in some manner. By default, the Internet Agent can encode numerous file types with various encoding methods. A file type is a particular kind of file. For example, a WordPerfect* 6.1 file, a JPEG graphics file, and an executable are all different types of files. Virtually any program that generates data will save that data to a particular file type. It is important to be able to specify the kind of encoding for a given type of file. When using RFC-822 format, the Internet Agent encodes non-US ASCII attachments with UUencode. This can cause two problems that MIME overcomes. First, if the recipient of the mail message does not have a UUdecode program that uses the same version of UUdecode as does GroupWise, the recipient may be unable to decode the attachments. Second, if an attachment is sent that is not US-ASCII, but is close enough that one could read the message in a text editor, it will also be UUencoded. This kind of message could have been read by the recipient if it had not been UUencoded. MIME overcomes these problems by providing the ability to map various file content types to MIME content types and encode them with MIME-compatible encoding schemes. One MIME-compatible encoding scheme is called quoted printable. A word processing document might be sent with this encoding. If the recipient does not have a MIME-compliant mail program, the recipient can still read most of the document. It may have various control characters and other formatting information scattered through it, but the message will be readable. For example, if a WordPerfect 5.1 document is attached to a mail message and sent through the Internet Agent in MIME format, the recipient will be able to read the message even if the recipient does not have a MIME-compliant mail reader. Remember, however, that not all documents will be readable. Also, if the recipient does have a MIME-compliant mail program, the document will be decoded from quoted printable back into its original form. MIME overcomes the problem of various versions of UUencode by using more standardized encoding methods. The primary encoding method used by MIME is Base64. Attachments that do not have a file type specified will be encoded in Base64. Because MIME-compliant mail programs use Base64 encoding, the attachment will correctly return to its original form. |
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