8-bit clean describes a computer system that correctly handles 8-bit character encodings, such as the ISO 8859 series and the UTF-8 encoding of Unicode.
Up to the early 1990s, many programs and data transmission channels assumed that all characters would be represented as numbers between 0 and 127 (7 bits); for example, the ASCII
standard used only 7 bits per character, avoiding an 8-bit representation in order to save on data transmission costs. On computers and data links using 8-bit bytes this left the top bit
of each byte
free for use as a parity bit
, flag bit
, or meta data control bit. 7-bit systems and data links are unable to handle more complex character codes which are commonplace in non-English language
-speaking countries with larger
cannot be transmitted through 7-bit data channels directly. To work around this, binary-to-text encodings have been devised which use only 7-bit ASCII characters. Some of these encodings are uuencoding, Ascii85, SREC, BinHex, kermit and MIME's Base64. EBCDIC-based systems cannot handle all characters used in UUencoded data. However, the base64 encoding does not have this problem.
SMTP and NNTP 8-bit cleanness
Historically, various media were used to transfer messages, some of them only supporting 7-bit data, so an 8-bit message had high chances to be mojibake
during transmission in the 20th century. But some implementations really did not care about formal discouraging of 8-bit data and allowed high bit set bytes to pass through.
Many early communications protocol standards, such as , , for SMTP, for NNTP, , , , were designed to work over such "7-bit" communication links. They specifically mention the use of ASCII character set "transmitted as a 8-bit byte with the high-order bit cleared to zero" and some of these
explicitly restrict all data to 7-bit characters.
- Appendix A, : 4.5.2., : Appendix B, : 4.
For the first few decades of email networks (1971 to the early 1990s),
most email messages were plain text in the 7-bit US-ASCII character set.
According to , the original definition of SMTP limits Internet Mail to
lines (1000 characters or less) of 7-bit US-ASCII characters.
"SMTP as defined in limits the sending of Internet Mail to US-ASCII characters."
"E-mail with Attachments".
"The Perl Journal".
"When mail was standardized way back in 1982 with RFC822, ...
The only limits placed on the body were the character set (7-bit ASCII) and the maximum line length (1000 characters)."
Later the format of email messages was re-defined
in order to support
messages that are not entirely US-ASCII text
(text messages in character sets
other than US-ASCII,
and non-text messages,
such as audio and images).
"Multipurpose Internet Mail Extensions, or MIME, redefines the format of messages"
The Internet community generally adds features by "extension", allowing communication in both directions between upgraded machines and not-yet-upgraded machines, rather than declaring formerly standards-compliant legacy software to be "broken" and insisting that all software worldwide be upgraded to the latest standard.
In the mid-1990s, people objected to "just send 8 bits (to SMTP servers)",
perhaps because of a perception that "just send 8 bits"
is an implicit declaration that ISO 8859-1 become the new "standard encoding", forcing everyone in the world to use the same character set.
Instead, the recommended way to take advantage of 8-bit-clean links between machines is to use the ESMTP () 8BITMIME extension.
] Despite this, some Mail Transfer Agents, notably Exim and qmail, relay mail to servers that do not advertise 8BITMIME without performing the conversion to 7-bit MIME (typically quoted-printable, "Q-P conversion") required by . This "just-send-8" attitude does not in fact cause problems in practice, since virtually all modern email servers are 8-bit clean.