Purposeful Omissions

Nested Encoding

In some early draft version encoding on the entire message was allowed. At the end, however, encodings are forbidden when the Content-type is "multipart" or "message".

There were several reasons for this decision:

• The overall structure of a message might not be visible without a decoding operation.
• There might be nested encodings, where the same data has to be decoded several times.

One consequence of this decision may be increased complexity for future gateways.

Uuencode

At first "uuencode" was considered instead of the BASE64 scheme.

It was rejected for several reasons:

• It is not well-specified anywhere.
• Its output is not robust across many mail gateways.
• Sometimes uuencoded files arrive at remote sites in a form which cannot be decoded.

The BASE64 format was designed to avoid the problems associated with uuencode.

Compression

Although it was felt that a compressed encoding mechanism should be supported (based on the UNIX compress facility), it was not specified due to the following reasons:

• The uncertain legal situation regarding the compression algorithms.
• A lack of compression expertise among the authors of the MIME standard.

Number of Content-types

There was a significant initial controversy about the overall number of content-types to be permitted. Some favored an unlimited number, while others wanted assurances that mail readers would have a good prospect of recognizing the content-type. The subtype mechanism was a successful compromise that met the needs of both camps.