8-bit clean
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8-bit clean is an attribute of
History
Until the early 1990s, many programs and data transmission channels were character-oriented and treated some characters, e.g.,
Binary files of octets 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 garbled 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. Such implementations are said to be 8-bit clean. In general, a communications protocol is said to be 8-bit clean if it correctly passes through the high bit of each byte in the communication process.
Many early
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.[2]
The
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).[6] The header field Content-Transfer-Encoding=binary[a] requires an 8-bit clean transport.
MIME encoding of non-ASCII data.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[
See also
- 32-bit clean
- MIME § Content-Transfer-Encoding
- Telnet § 8-bit data
Notes
- CRLFhas special significance.
References
- RFC 1056: 4.
- ^ John Beck. "Email Explained". 2011.
- .
The maximum total length of a text line including the <CRLF> is 1000 characters (but not counting the leading dot duplicated for transparency).
- .
SMTP as defined in RFC 821 limits the sending of Internet Mail to US-ASCII characters.
- ^ Dan Sugalski. "E-mail with Attachments". "The Perl Journal". Summer 1999. "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)."
- ^ , redefines the format of messages
- .
- .
- .
- .
- IETF-SMTP mail list. Archived from the originalon 20 March 2012. Retrieved 3 April 2010.
- ^ "comp.mail.mime FAQ, part 3 'What's ESMTP, and how does it affect MIME?'". Usenet FAQs. 8 August 1997. Archived from the original on 18 January 2012. Retrieved 3 April 2010.
- .
- ^ "The 8BITMIME extension".