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Incomputer networking, a system is8-bit clean if it processes8-bitcharacter encodings without altering thehigh bit or treating anybyte as anin-band control code. This property can describe both acommunications protocol and the software and devices that implement such protocols. Although many early email systems only supported 7-bit data, the vast majority of modern email systems are 8-bit clean.
Until the early 1990s, many programs and data transmission channels werecharacter-oriented and treated some characters likeend-of-text (ETX) ascontrol characters. Others assumed a stream of seven-bit characters, with values between 0 and 127; for example, theASCII standard used only seven bits per character,avoiding an eight-bit representation in order to save on data transmission costs. On computers and data links using8-bit bytes, this left the topbit of each byte free for use as aparity bit,flag bit, or metadata control bit. Seven-bit systems and data links are unable to directly handle more complex character codes which are commonplace in non-English-speaking countries with largeralphabets.
Binary files consisting of 8-bitoctets cannot be transmitted through 7-bit data channels directly. To work around this,binary-to-text encodings have been devised which use only 7-bitASCII characters. Some of these encodings areuuencoding,Ascii85,SREC,BinHex,kermit andMIME'sBase64.EBCDIC-based systems cannot handle all characters used in UUencoded data.[clarification needed (seetalk)] However, the base64 encoding does not have this problem.
Historically, various media were used to transfer messages, some of which only supported 7-bit data, so an 8-bit message had high chances to begarbled during transmission in the 20th century. Some implementations ignored the formal discouraging of 8-bit data and allowed bytes with thehigh bit set to pass through. Such implementations are said to be 8-bit clean. In general, acommunications 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 communications protocol standards, such asRFC 780,788,821,2821,5321 (forSMTP),RFC 977 (forNNTP) andRFC 1056, were designed to work over such "7-bit" communication links. They specifically require the use of ASCII "transmitted as an 8-bit byte with the high-order bit cleared to zero", and some of these[1] explicitly restrictall data to 7-bit characters.
For the first few decades of email networks (1971 to the early 1990s), most email messages wereplain text in the 7-bit US-ASCII character set.[2]
The RFC 788 definition of SMTP, like its predecessor RFC 780, limits Internet Mail to lines (1000 characters or less) of 7-bit US-ASCII characters.[3][4][5][6]
Later, the format of email messages was redefined 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.
RFC 3977[7] specifies that "NNTP operates over any reliable bi-directional 8-bit-wide data stream channel" and changes the character set for commands toUTF-8. However, RFC 5536[8] still limits the character set to ASCII, including RFC 2047[9] and RFC 2231[10] MIME encoding of non-ASCII data.
The Internet community generally adds features byextension, 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 requiring that all software worldwide be upgraded to the latest standard. The recommended way to take advantage of 8-bit clean links between machines is to use the ESMTP (RFC 1869)8BITMIME extension[11][12] for message bodies and the SMTPSMTPUTF8[13] extension for message headers. Despite this, somemail transfer agents, notablyExim andqmail, relay mail to servers that do not advertise 8BITMIME without performing the conversion to 7-bit MIME (typicallyquoted-printable, "Q-P conversion") required byRFC 6152. This "just-send-8" attitude does not, in fact, cause problems in practice because virtually all modern email servers are 8-bit clean.[14]
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.
Multipurpose Internet Mail Extensions, orMIME, redefines the format of messages