rfc1642.ps

中、英文RFC文档大全打包下载完全版 .

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-.062(Goldsmith & Davis)A
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-.04(Rationale)A
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-.008(. The pad character \322=\323 is excluded because UTF-7 is designed for use within)A
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-.007(header fields as set forth in RFC 1522. Since the only readable encoding in RFC 1522 is)A
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-.003(\322Q\323 \(based on RFC 1521\325s Quoted-Printable\), the \322=\323 character is not available for use)A
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-.011(\(without a lot of escape sequences\). This was very unfortunate but unavoidable.The \322=\323)A
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-.005(character could otherwise have been used as the UTF-7 escape character as well \(rather)A
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-.005(than using \322+\323\).)A
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-.001(Note that all characters in US-ASCII have the same value in Unicode when zero-extended to)A
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-.048(16 bits.)A
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(UTF-7 Definition)S
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-.01(A UTF-7 stream represents 16-bit Unicode characters in 7-bit US-ASCII as follows:)A
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-.002(Rule 1: \(direct encoding\) Unicode characters in set D above may be encoded directly as)A
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-.01(their ASCII equivalents. Unicode characters in Set O may optionally be encoded directly)A
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-.009(as their ASCII equivalents, bearing in mind that many of these characters are illegal in)A
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-.008(header fields, or may not pass correctly through some mail gateways.)A
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-.006(Rule 2: \(Unicode shifted encoding\) Any Unicode character sequence may be encoded)A
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-.002(using a sequence of characters in set B, when preceded by the shift character \322)A
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(+)S
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-.129(\323 \(US-)A
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-.006(ASCII character value decimal 43\). The \322)A
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(+)S
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-.006(\323\312signals that subsequent octets are to be)A
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-.01(interpreted as elements of the Modified Base64 alphabet until a character not in that)A
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-.005(alphabet is encountered. Such characters include control characters such as carriage)A
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-.002(returns and line feeds; thus, a Unicode shifted sequence always terminates at the end of a)A
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-.011(line. As a special case, if the sequence terminates with the character \322)A
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(-)S
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-.097(\323 \(US-ASCII)A
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-.001(decimal 45\) then that character is absorbed; other terminating characters are not absorbed)A
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-.023(and are processed normally.)A
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-.04(Rationale)A
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-.005(. A terminating character is necessary for cases where the next character after)A
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-.005(the Modified Base64 sequence is part of character set B. It can also enhance readability)A
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-.031(by delimiting encoded sequences.)A
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-.024(Also as a special case, the sequence \322)A
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-.397(+-)A
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-.015(\323 may be used to encode the character \322)A
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(+)S
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-.33(\323. A)A
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(\322)S
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(+)S
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-.001(\323 character followed immediately by any character other than members of set B or \322)A
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(-)S
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(\323)S
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-.038(is an ill-formed sequence.)A
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-.002(Unicode is encoded using Modified Base64 by first converting Unicode 16-bit quantities)A
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-.01(to an octet stream \(with the most significant octet first\). Text with an odd number of)A
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-.031(octets is ill-formed.)A
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-.04(Rationale)A
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-.012(. ISO/IEC 10646-1:1993\(E\) specifies that when characters in the UCS-2 form)A
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-.009(are serialized as octets, that the most significant octet appear first. This is also in keeping)A
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-.011(with common network practice of choosing a canonical format for transmission.)A
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-.097(RFC 1642)A
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-.164(UTF-7)A
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-.062(Goldsmith & Davis)A
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-.063([Page )A
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-.002(Next, the octet stream is encoded by applying the Base64 content transfer encoding)A
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-.003(algorithm as defined in RFC 1521, modified to omit the \322=\323 pad character. Instead, when)A
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-.01(encoding, zero bits are added to pad to a Base64 character boundary. When decoding,)A
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-.002(any bits at the end of the Modified Base64 sequence that do not constitute a complete 16-)A
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-.001(bit Unicode character are discarded. If such discarded bits are non-zero the sequence is)A
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-.064(ill-formed.)A
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-.04(Rationale)A
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-.008(. The pad character \322=\323 is not used when encoding Modified Base64 because)A
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-.009(of the conflict with its use as an escape character for the Q content transfer encoding in)A
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-.014(RFC 1522 header fields, as mentioned above.)A
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-.005(Rule 3: The space \(decimal 32\), tab \(decimal 9\), carriage return \(decimal 13\), and line)A
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-.001(feed \(decimal 10\) characters may be directly represented by their ASCII\312equivalents.)A
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-.01(However, note that MIME\312content transfer encodings have rules concerning the use of)A
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-.002(such characters. Usage that does not conform to the restrictions of RFC 822, for example,)A
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-.002(would have to be encoded using MIME content transfer encodings other than 7bit or 8bit,)A
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-.006(such as quoted-printable, binary, or base64.)A
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-.002(Given this set of rules, Unicode characters which may be encoded via rules 1 or 3 take one)A
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-.005(octet per character, and other Unicode characters are encoded on average with 2 2/3 octets)A
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-.002(per character plus one octet to switch into Modified Base64 and an optional octet to switch)A
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-.111(out.)A
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-.055(Example)A
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-.005(. The Unicode sequence \322A<NOT\312IDENTICAL\312TO><ALPHA>.\323)A
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-.004(\(hexadecimal 0041,2262,0391,002E\) may be encoded as follows:)A
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-.223(A+ImIDkQ.)A
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-.055(Example)A
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-.006(. The Unicode sequence \322Hi Mom <WHITE SMILING FACE>!\323 \(hexadecimal)A
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-.013(0048,0069,0020,004D,006F,004D,0020,263A,0021\) may be encoded as follows:)A
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-.215(Hi Mom +Jjo-!)A
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-.055(Example)A
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-.01(. The Unicode sequence representing the Han characters for the Japanese word)A
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-.003(\322nihongo\323 \(hexadecimal 65E5,672C,8A9E\) may be encoded as follows:)A
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-.22(+ZeVnLIqe-)A
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-.01(Use of Character Set UTF-7 Within MIME)A
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-.002(Character set UTF-7 is safe for mail transmission and therefore may be used with any)A
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-.005(content transfer encoding in MIME \(except where line length and line break restrictions are)A
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-.009(violated\). Specifically, the 7 bit encoding for bodies and the Q encoding for headers are both)A
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-.012(acceptable. The MIME character set identifier is UNICODE-1-1-UTF-7.)A
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-.097(RFC 1642)A
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-.164(UTF-7)A
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-.062(Goldsmith & Davis)A
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-.055(Example)A
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-.012(. Here is a text portion of a MIME message containing the Unicode sequence)A
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-.006(\322Hi Mom <WHITE SMILING FACE>!\323 \(hexadecimal 0048, 0069, 0020, 004D, 006F,)A
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-.014(004D, 0020, 263A, 0021\).)A
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f3_12 sf
-.202(Content-Type: text/plain; charset=UNICODE-1-1-UTF-7)A
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-.215(Hi Mom +Jjo-!)A
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f1_12 sf
-.055(Example)A
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f0_12 sf
-.012(. Here is a text portion of a MIME message containing the Unicode sequence)A
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-.002(representing the Han characters for the Japanese word \322nihongo\323 \(hexadecimal)A
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-.021(65E5,672C,8A9E\).)A
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f3_12 sf
-.202(Content-Type: text/plain; charset=UNICODE-1-1-UTF-7)A
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-.22(+ZeVnLIqe-)A
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f1_12 sf
-.055(Example)A
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f0_12 sf
-.012(. Here is a text portion of a MIME message containing the Unicode sequence)A
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-.015(\322A<NOT\312IDENTICAL\312TO><ALPHA>.\323 \(hexadecimal 0041,2262,0391,002E\).)A
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f3_12 sf
-.202(Content-Type: text/plain; charset=UNICODE-1-1-UTF-7)A
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-.223(A+ImIDkQ.)A
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f1_12 sf
-.055(Example)A
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f0_12 sf
-.012(. Here is a text portion of a MIME message containing the Unicode sequence)A
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-.01(\322Item 3 is <POUND\312SIGN>1.\323  \(hexadecimal 0049, 0074, 0065, 006D, 0020, 0033,)A
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-.024(0020, 0069, 0073, 0020, 00A3, 0031, 002E\).)A
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-.202(Content-Type: text/plain; charset=UNICODE-1-1-UTF-7)A
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-.211(Item 3 is +AKM-1.)A
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-.002(Note that to achieve the best interoperability with systems that may not support Unicode or)A
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-.006(MIME, when preparing text for mail transmission line breaks should follow Internet)A
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-.003(conventions. This means that lines should be short and terminated with the proper SMTP)A
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-.008(CRLF sequence. Unicode LINE SEPARATOR \(hexadecimal 2028\) and PARAGRAPH)A
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-.011(SEPARATOR \(hexadecimal 2029\) should be converted to SMTP line breaks. Ideally, this)A
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-.009(would be handled transparently by a Unicode-aware user agent.)A
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-.01(This preparation is not absolutely necessary, since UTF-7 and the appropriate MIME)A
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-.01(content transfer encoding can handle text that does not follow Internet conventions, but)A
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-.006(readability by systems without Unicode or MIME will be impaired. See RFC 1521 for an in-)A
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-.013(depth discussion of mail interoperability issues.)A
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-.006(Lines should never be broken in the middle of a UTF-7 shifted sequence, since such)A
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-.009(sequences may not cross line breaks. Therefore, UTF-7 encoding should take place after line)A
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-.009(breaking. If a line containing a shifted sequence is too long after encoding, a MIME content)A
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-.006(transfer encoding such as Quoted Printable can be used to encode the text. Another)A
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-.097(RFC 1642)A
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-.164(UTF-7)A
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-.062(Goldsmith & Davis)A
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-.063([Page )A
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-.006(possibility is to perform line breaking and UTF-7 encoding at the same time, so that lines)A
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-.008(containing shifted sequences already conform to length restrictions.)A
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(Discussion)S
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-.006(In this section we will motivate the introduction of UTF-7 as opposed to the alternative of)A
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-.006(using the existing transformation formats of Unicode \(e.g., UTF-8\) with MIME\325s content)A
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-.006(transfer encodings. Before discussing this, it will be useful to list some assumptions about)A
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-.002(character frequency within typical natural language text strings that we use to estimate)A
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-.01(typical storage requirements:)A
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-.01(1. Most Western European languages use roughly 7/8 of their letters from US-ASCII and)A
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-.01(1/8 from Latin 1 \(ISO-8859-1\).)A
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-.002(2. Most non-European alphabet-based languages \(e.g., Greek\) use about 1/6 of their)A
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-.002(letters from ASCII \(since white space is in the 7-bit area\) and the rest from their)A
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-.035(alphabets.)A
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-.002(3. East Asian ideographic-based languages \(including Japanese\) use essentially all of)A
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-.017(their characters from the Han or CJK syllabary area.)A
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-.009(4. Non-directly encoded punctuation characters do not occur frequently enough to affect)A
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-.089(the results.)A
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-.005(Notice that current 8 bit standards, such as ISO-8859-x, require use of a content transfer)A
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-.01(encoding. For comparison with the subsequent discussion, the costs break down as follows)A
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-.002(\(note that many of these figures are approximate since they depend on the exact composition)A
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-.081(of the text\):)A
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-.043(8859-x in Base64)A
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-.081(Text type)A
148 488.5 -.5 .5 194.5 488 .5 148 488 @a
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-.039(Average octets/character)A
256 488.5 -.5 .5 375.5 488 .5 256 488 @a
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-.165(All)A
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(1.33)S
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-.039(8859-x in Quoted Printable)A
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-.081(Text type)A
148 552.5 -.5 .5 194.5 552 .5 148 552 @a
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-.039(Average octets/character)A
256 552.5 -.5 .5 375.5 552 .5 256 552 @a
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-.093(US-ASCII)A
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(1)S
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-.063(Western European)A
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(1.25)S
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-.079(Other)A
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(2.67)S
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-.009(Note also that Unicode encoded in Base64 takes a constant 2.67 octets per character. For)A
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-.006(purposes of comparison, we will look at UTF-8\312in Base64 and Quoted Printable, and UTF-7.)A
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-.006(UTF-1 gives results substantially similar to UTF-8. Also note that fixed overhead for long)A
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-.003(strings is relative to 1/n, where n is the encoded string length in octets.)A
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-.097(RFC 1642)A
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-.164(UTF-7)A
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-.062(Goldsmith & Davis)A
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-.021(UTF-8 in Base64 )A
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-.081(Text type)A
148 105.5 -.5 .5 194.5 105 .5 148 105 @a
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-.039(Average octets/character)A
256 105.5 -.5 .5 375.5 105 .5 256 105 @a
148 116 :M
-.093(US-ASCII)A
256 116 :M
(1.33)S
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-.063(Western European)A
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(1.5)S
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-.043(Some Alphabetics)A
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(2.44)S
148 152 :M
-.072(All others)A
256 152 :M
(4)S
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-.026(UTF-8 in Quoted Printable)A
148 204 :M
-.081(Text type)A
148 205.5 -.5 .5 194.5 205 .5 148 205 @a
256 204 :M
-.039(Average octets/character)A
256 205.5 -.5 .5 375.5 205 .5 256 205 @a
148 216 :M
-.093(US-ASCII)A
256 216 :M
(1)S
148 228 :M
-.063(Western European)A
256 228 :M
(1.63)S
148 240 :M
-.043(Some Alphabetics)A
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(5.17)S
148 252 :M
-.072(All others)A
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-.497(7-9)A
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-.164(UTF-7)A
148 304 :M
-.081(Text type)A
148 305.5 -.5 .5 194.5 305 .5 148 305 @a
256 304 :M
-.039(Average octets/character)A
256 305.5 -.5 .5 375.5 305 .5 256 305 @a
148 316 :M
-.027(Most US-ASCII)A
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(1)S
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-.063(Western European)A
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(1.5)S
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