rfc2047.txt - rohrpost - A commandline mail client to change the world as we see it.
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rfc2047.txt (33262B)
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7 Network Working Group K. Moore
8 Request for Comments: 2047 University of Tennessee
9 Obsoletes: 1521, 1522, 1590 November 1996
10 Category: Standards Track
11
12
13 MIME (Multipurpose Internet Mail Extensions) Part Three:
14 Message Header Extensions for Non-ASCII Text
15
16 Status of this Memo
17
18 This document specifies an Internet standards track protocol for the
19 Internet community, and requests discussion and suggestions for
20 improvements. Please refer to the current edition of the "Internet
21 Official Protocol Standards" (STD 1) for the standardization state
22 and status of this protocol. Distribution of this memo is unlimited.
23
24 Abstract
25
26 STD 11, RFC 822, defines a message representation protocol specifying
27 considerable detail about US-ASCII message headers, and leaves the
28 message content, or message body, as flat US-ASCII text. This set of
29 documents, collectively called the Multipurpose Internet Mail
30 Extensions, or MIME, redefines the format of messages to allow for
31
32 (1) textual message bodies in character sets other than US-ASCII,
33
34 (2) an extensible set of different formats for non-textual message
35 bodies,
36
37 (3) multi-part message bodies, and
38
39 (4) textual header information in character sets other than US-ASCII.
40
41 These documents are based on earlier work documented in RFC 934, STD
42 11, and RFC 1049, but extends and revises them. Because RFC 822 said
43 so little about message bodies, these documents are largely
44 orthogonal to (rather than a revision of) RFC 822.
45
46 This particular document is the third document in the series. It
47 describes extensions to RFC 822 to allow non-US-ASCII text data in
48 Internet mail header fields.
49
50
51
52
53
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55
56
57
58 Moore Standards Track [Page 1]
59
60 RFC 2047 Message Header Extensions November 1996
61
62
63 Other documents in this series include:
64
65 + RFC 2045, which specifies the various headers used to describe
66 the structure of MIME messages.
67
68 + RFC 2046, which defines the general structure of the MIME media
69 typing system and defines an initial set of media types,
70
71 + RFC 2048, which specifies various IANA registration procedures
72 for MIME-related facilities, and
73
74 + RFC 2049, which describes MIME conformance criteria and
75 provides some illustrative examples of MIME message formats,
76 acknowledgements, and the bibliography.
77
78 These documents are revisions of RFCs 1521, 1522, and 1590, which
79 themselves were revisions of RFCs 1341 and 1342. An appendix in RFC
80 2049 describes differences and changes from previous versions.
81
82 1. Introduction
83
84 RFC 2045 describes a mechanism for denoting textual body parts which
85 are coded in various character sets, as well as methods for encoding
86 such body parts as sequences of printable US-ASCII characters. This
87 memo describes similar techniques to allow the encoding of non-ASCII
88 text in various portions of a RFC 822 [2] message header, in a manner
89 which is unlikely to confuse existing message handling software.
90
91 Like the encoding techniques described in RFC 2045, the techniques
92 outlined here were designed to allow the use of non-ASCII characters
93 in message headers in a way which is unlikely to be disturbed by the
94 quirks of existing Internet mail handling programs. In particular,
95 some mail relaying programs are known to (a) delete some message
96 header fields while retaining others, (b) rearrange the order of
97 addresses in To or Cc fields, (c) rearrange the (vertical) order of
98 header fields, and/or (d) "wrap" message headers at different places
99 than those in the original message. In addition, some mail reading
100 programs are known to have difficulty correctly parsing message
101 headers which, while legal according to RFC 822, make use of
102 backslash-quoting to "hide" special characters such as "<", ",", or
103 ":", or which exploit other infrequently-used features of that
104 specification.
105
106 While it is unfortunate that these programs do not correctly
107 interpret RFC 822 headers, to "break" these programs would cause
108 severe operational problems for the Internet mail system. The
109 extensions described in this memo therefore do not rely on little-
110 used features of RFC 822.
111
112
113
114 Moore Standards Track [Page 2]
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116 RFC 2047 Message Header Extensions November 1996
117
118
119 Instead, certain sequences of "ordinary" printable ASCII characters
120 (known as "encoded-words") are reserved for use as encoded data. The
121 syntax of encoded-words is such that they are unlikely to
122 "accidentally" appear as normal text in message headers.
123 Furthermore, the characters used in encoded-words are restricted to
124 those which do not have special meanings in the context in which the
125 encoded-word appears.
126
127 Generally, an "encoded-word" is a sequence of printable ASCII
128 characters that begins with "=?", ends with "?=", and has two "?"s in
129 between. It specifies a character set and an encoding method, and
130 also includes the original text encoded as graphic ASCII characters,
131 according to the rules for that encoding method.
132
133 A mail composer that implements this specification will provide a
134 means of inputting non-ASCII text in header fields, but will
135 translate these fields (or appropriate portions of these fields) into
136 encoded-words before inserting them into the message header.
137
138 A mail reader that implements this specification will recognize
139 encoded-words when they appear in certain portions of the message
140 header. Instead of displaying the encoded-word "as is", it will
141 reverse the encoding and display the original text in the designated
142 character set.
143
144 NOTES
145
146 This memo relies heavily on notation and terms defined RFC 822 and
147 RFC 2045. In particular, the syntax for the ABNF used in this memo
148 is defined in RFC 822, as well as many of the terminal or nonterminal
149 symbols from RFC 822 are used in the grammar for the header
150 extensions defined here. Among the symbols defined in RFC 822 and
151 referenced in this memo are: 'addr-spec', 'atom', 'CHAR', 'comment',
152 'CTLs', 'ctext', 'linear-white-space', 'phrase', 'quoted-pair'.
153 'quoted-string', 'SPACE', and 'word'. Successful implementation of
154 this protocol extension requires careful attention to the RFC 822
155 definitions of these terms.
156
157 When the term "ASCII" appears in this memo, it refers to the "7-Bit
158 American Standard Code for Information Interchange", ANSI X3.4-1986.
159 The MIME charset name for this character set is "US-ASCII". When not
160 specifically referring to the MIME charset name, this document uses
161 the term "ASCII", both for brevity and for consistency with RFC 822.
162 However, implementors are warned that the character set name must be
163 spelled "US-ASCII" in MIME message and body part headers.
164
165
166
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168
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170 Moore Standards Track [Page 3]
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172 RFC 2047 Message Header Extensions November 1996
173
174
175 This memo specifies a protocol for the representation of non-ASCII
176 text in message headers. It specifically DOES NOT define any
177 translation between "8-bit headers" and pure ASCII headers, nor is
178 any such translation assumed to be possible.
179
180 2. Syntax of encoded-words
181
182 An 'encoded-word' is defined by the following ABNF grammar. The
183 notation of RFC 822 is used, with the exception that white space
184 characters MUST NOT appear between components of an 'encoded-word'.
185
186 encoded-word = "=?" charset "?" encoding "?" encoded-text "?="
187
188 charset = token ; see section 3
189
190 encoding = token ; see section 4
191
192 token = 1*<Any CHAR except SPACE, CTLs, and especials>
193
194 especials = "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "
195 <"> / "/" / "[" / "]" / "?" / "." / "="
196
197 encoded-text = 1*<Any printable ASCII character other than "?"
198 or SPACE>
199 ; (but see "Use of encoded-words in message
200 ; headers", section 5)
201
202 Both 'encoding' and 'charset' names are case-independent. Thus the
203 charset name "ISO-8859-1" is equivalent to "iso-8859-1", and the
204 encoding named "Q" may be spelled either "Q" or "q".
205
206 An 'encoded-word' may not be more than 75 characters long, including
207 'charset', 'encoding', 'encoded-text', and delimiters. If it is
208 desirable to encode more text than will fit in an 'encoded-word' of
209 75 characters, multiple 'encoded-word's (separated by CRLF SPACE) may
210 be used.
211
212 While there is no limit to the length of a multiple-line header
213 field, each line of a header field that contains one or more
214 'encoded-word's is limited to 76 characters.
215
216 The length restrictions are included both to ease interoperability
217 through internetwork mail gateways, and to impose a limit on the
218 amount of lookahead a header parser must employ (while looking for a
219 final ?= delimiter) before it can decide whether a token is an
220 "encoded-word" or something else.
221
222
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224
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226 Moore Standards Track [Page 4]
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228 RFC 2047 Message Header Extensions November 1996
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230
231 IMPORTANT: 'encoded-word's are designed to be recognized as 'atom's
232 by an RFC 822 parser. As a consequence, unencoded white space
233 characters (such as SPACE and HTAB) are FORBIDDEN within an
234 'encoded-word'. For example, the character sequence
235
236 =?iso-8859-1?q?this is some text?=
237
238 would be parsed as four 'atom's, rather than as a single 'atom' (by
239 an RFC 822 parser) or 'encoded-word' (by a parser which understands
240 'encoded-words'). The correct way to encode the string "this is some
241 text" is to encode the SPACE characters as well, e.g.
242
243 =?iso-8859-1?q?this=20is=20some=20text?=
244
245 The characters which may appear in 'encoded-text' are further
246 restricted by the rules in section 5.
247
248 3. Character sets
249
250 The 'charset' portion of an 'encoded-word' specifies the character
251 set associated with the unencoded text. A 'charset' can be any of
252 the character set names allowed in an MIME "charset" parameter of a
253 "text/plain" body part, or any character set name registered with
254 IANA for use with the MIME text/plain content-type.
255
256 Some character sets use code-switching techniques to switch between
257 "ASCII mode" and other modes. If unencoded text in an 'encoded-word'
258 contains a sequence which causes the charset interpreter to switch
259 out of ASCII mode, it MUST contain additional control codes such that
260 ASCII mode is again selected at the end of the 'encoded-word'. (This
261 rule applies separately to each 'encoded-word', including adjacent
262 'encoded-word's within a single header field.)
263
264 When there is a possibility of using more than one character set to
265 represent the text in an 'encoded-word', and in the absence of
266 private agreements between sender and recipients of a message, it is
267 recommended that members of the ISO-8859-* series be used in
268 preference to other character sets.
269
270 4. Encodings
271
272 Initially, the legal values for "encoding" are "Q" and "B". These
273 encodings are described below. The "Q" encoding is recommended for
274 use when most of the characters to be encoded are in the ASCII
275 character set; otherwise, the "B" encoding should be used.
276 Nevertheless, a mail reader which claims to recognize 'encoded-word's
277 MUST be able to accept either encoding for any character set which it
278 supports.
279
280
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282 Moore Standards Track [Page 5]
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284 RFC 2047 Message Header Extensions November 1996
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286
287 Only a subset of the printable ASCII characters may be used in
288 'encoded-text'. Space and tab characters are not allowed, so that
289 the beginning and end of an 'encoded-word' are obvious. The "?"
290 character is used within an 'encoded-word' to separate the various
291 portions of the 'encoded-word' from one another, and thus cannot
292 appear in the 'encoded-text' portion. Other characters are also
293 illegal in certain contexts. For example, an 'encoded-word' in a
294 'phrase' preceding an address in a From header field may not contain
295 any of the "specials" defined in RFC 822. Finally, certain other
296 characters are disallowed in some contexts, to ensure reliability for
297 messages that pass through internetwork mail gateways.
298
299 The "B" encoding automatically meets these requirements. The "Q"
300 encoding allows a wide range of printable characters to be used in
301 non-critical locations in the message header (e.g., Subject), with
302 fewer characters available for use in other locations.
303
304 4.1. The "B" encoding
305
306 The "B" encoding is identical to the "BASE64" encoding defined by RFC
307 2045.
308
309 4.2. The "Q" encoding
310
311 The "Q" encoding is similar to the "Quoted-Printable" content-
312 transfer-encoding defined in RFC 2045. It is designed to allow text
313 containing mostly ASCII characters to be decipherable on an ASCII
314 terminal without decoding.
315
316 (1) Any 8-bit value may be represented by a "=" followed by two
317 hexadecimal digits. For example, if the character set in use
318 were ISO-8859-1, the "=" character would thus be encoded as
319 "=3D", and a SPACE by "=20". (Upper case should be used for
320 hexadecimal digits "A" through "F".)
321
322 (2) The 8-bit hexadecimal value 20 (e.g., ISO-8859-1 SPACE) may be
323 represented as "_" (underscore, ASCII 95.). (This character may
324 not pass through some internetwork mail gateways, but its use
325 will greatly enhance readability of "Q" encoded data with mail
326 readers that do not support this encoding.) Note that the "_"
327 always represents hexadecimal 20, even if the SPACE character
328 occupies a different code position in the character set in use.
329
330 (3) 8-bit values which correspond to printable ASCII characters other
331 than "=", "?", and "_" (underscore), MAY be represented as those
332 characters. (But see section 5 for restrictions.) In
333 particular, SPACE and TAB MUST NOT be represented as themselves
334 within encoded words.
335
336
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338 Moore Standards Track [Page 6]
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340 RFC 2047 Message Header Extensions November 1996
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342
343 5. Use of encoded-words in message headers
344
345 An 'encoded-word' may appear in a message header or body part header
346 according to the following rules:
347
348 (1) An 'encoded-word' may replace a 'text' token (as defined by RFC 822)
349 in any Subject or Comments header field, any extension message
350 header field, or any MIME body part field for which the field body
351 is defined as '*text'. An 'encoded-word' may also appear in any
352 user-defined ("X-") message or body part header field.
353
354 Ordinary ASCII text and 'encoded-word's may appear together in the
355 same header field. However, an 'encoded-word' that appears in a
356 header field defined as '*text' MUST be separated from any adjacent
357 'encoded-word' or 'text' by 'linear-white-space'.
358
359 (2) An 'encoded-word' may appear within a 'comment' delimited by "(" and
360 ")", i.e., wherever a 'ctext' is allowed. More precisely, the RFC
361 822 ABNF definition for 'comment' is amended as follows:
362
363 comment = "(" *(ctext / quoted-pair / comment / encoded-word) ")"
364
365 A "Q"-encoded 'encoded-word' which appears in a 'comment' MUST NOT
366 contain the characters "(", ")" or "
367 'encoded-word' that appears in a 'comment' MUST be separated from
368 any adjacent 'encoded-word' or 'ctext' by 'linear-white-space'.
369
370 It is important to note that 'comment's are only recognized inside
371 "structured" field bodies. In fields whose bodies are defined as
372 '*text', "(" and ")" are treated as ordinary characters rather than
373 comment delimiters, and rule (1) of this section applies. (See RFC
374 822, sections 3.1.2 and 3.1.3)
375
376 (3) As a replacement for a 'word' entity within a 'phrase', for example,
377 one that precedes an address in a From, To, or Cc header. The ABNF
378 definition for 'phrase' from RFC 822 thus becomes:
379
380 phrase = 1*( encoded-word / word )
381
382 In this case the set of characters that may be used in a "Q"-encoded
383 'encoded-word' is restricted to: <upper and lower case ASCII
384 letters, decimal digits, "!", "*", "+", "-", "/", "=", and "_"
385 (underscore, ASCII 95.)>. An 'encoded-word' that appears within a
386 'phrase' MUST be separated from any adjacent 'word', 'text' or
387 'special' by 'linear-white-space'.
388
389
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394 Moore Standards Track [Page 7]
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396 RFC 2047 Message Header Extensions November 1996
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398
399 These are the ONLY locations where an 'encoded-word' may appear. In
400 particular:
401
402 + An 'encoded-word' MUST NOT appear in any portion of an 'addr-spec'.
403
404 + An 'encoded-word' MUST NOT appear within a 'quoted-string'.
405
406 + An 'encoded-word' MUST NOT be used in a Received header field.
407
408 + An 'encoded-word' MUST NOT be used in parameter of a MIME
409 Content-Type or Content-Disposition field, or in any structured
410 field body except within a 'comment' or 'phrase'.
411
412 The 'encoded-text' in an 'encoded-word' must be self-contained;
413 'encoded-text' MUST NOT be continued from one 'encoded-word' to
414 another. This implies that the 'encoded-text' portion of a "B"
415 'encoded-word' will be a multiple of 4 characters long; for a "Q"
416 'encoded-word', any "=" character that appears in the 'encoded-text'
417 portion will be followed by two hexadecimal characters.
418
419 Each 'encoded-word' MUST encode an integral number of octets. The
420 'encoded-text' in each 'encoded-word' must be well-formed according
421 to the encoding specified; the 'encoded-text' may not be continued in
422 the next 'encoded-word'. (For example, "=?charset?Q?=?=
423 =?charset?Q?AB?=" would be illegal, because the two hex digits "AB"
424 must follow the "=" in the same 'encoded-word'.)
425
426 Each 'encoded-word' MUST represent an integral number of characters.
427 A multi-octet character may not be split across adjacent 'encoded-
428 word's.
429
430 Only printable and white space character data should be encoded using
431 this scheme. However, since these encoding schemes allow the
432 encoding of arbitrary octet values, mail readers that implement this
433 decoding should also ensure that display of the decoded data on the
434 recipient's terminal will not cause unwanted side-effects.
435
436 Use of these methods to encode non-textual data (e.g., pictures or
437 sounds) is not defined by this memo. Use of 'encoded-word's to
438 represent strings of purely ASCII characters is allowed, but
439 discouraged. In rare cases it may be necessary to encode ordinary
440 text that looks like an 'encoded-word'.
441
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450 Moore Standards Track [Page 8]
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452 RFC 2047 Message Header Extensions November 1996
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454
455 6. Support of 'encoded-word's by mail readers
456
457 6.1. Recognition of 'encoded-word's in message headers
458
459 A mail reader must parse the message and body part headers according
460 to the rules in RFC 822 to correctly recognize 'encoded-word's.
461
462 'encoded-word's are to be recognized as follows:
463
464 (1) Any message or body part header field defined as '*text', or any
465 user-defined header field, should be parsed as follows: Beginning
466 at the start of the field-body and immediately following each
467 occurrence of 'linear-white-space', each sequence of up to 75
468 printable characters (not containing any 'linear-white-space')
469 should be examined to see if it is an 'encoded-word' according to
470 the syntax rules in section 2. Any other sequence of printable
471 characters should be treated as ordinary ASCII text.
472
473 (2) Any header field not defined as '*text' should be parsed
474 according to the syntax rules for that header field. However,
475 any 'word' that appears within a 'phrase' should be treated as an
476 'encoded-word' if it meets the syntax rules in section 2.
477 Otherwise it should be treated as an ordinary 'word'.
478
479 (3) Within a 'comment', any sequence of up to 75 printable characters
480 (not containing 'linear-white-space'), that meets the syntax
481 rules in section 2, should be treated as an 'encoded-word'.
482 Otherwise it should be treated as normal comment text.
483
484 (4) A MIME-Version header field is NOT required to be present for
485 'encoded-word's to be interpreted according to this
486 specification. One reason for this is that the mail reader is
487 not expected to parse the entire message header before displaying
488 lines that may contain 'encoded-word's.
489
490 6.2. Display of 'encoded-word's
491
492 Any 'encoded-word's so recognized are decoded, and if possible, the
493 resulting unencoded text is displayed in the original character set.
494
495 NOTE: Decoding and display of encoded-words occurs *after* a
496 structured field body is parsed into tokens. It is therefore
497 possible to hide 'special' characters in encoded-words which, when
498 displayed, will be indistinguishable from 'special' characters in the
499 surrounding text. For this and other reasons, it is NOT generally
500 possible to translate a message header containing 'encoded-word's to
501 an unencoded form which can be parsed by an RFC 822 mail reader.
502
503
504
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506 Moore Standards Track [Page 9]
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508 RFC 2047 Message Header Extensions November 1996
509
510
511 When displaying a particular header field that contains multiple
512 'encoded-word's, any 'linear-white-space' that separates a pair of
513 adjacent 'encoded-word's is ignored. (This is to allow the use of
514 multiple 'encoded-word's to represent long strings of unencoded text,
515 without having to separate 'encoded-word's where spaces occur in the
516 unencoded text.)
517
518 In the event other encodings are defined in the future, and the mail
519 reader does not support the encoding used, it may either (a) display
520 the 'encoded-word' as ordinary text, or (b) substitute an appropriate
521 message indicating that the text could not be decoded.
522
523 If the mail reader does not support the character set used, it may
524 (a) display the 'encoded-word' as ordinary text (i.e., as it appears
525 in the header), (b) make a "best effort" to display using such
526 characters as are available, or (c) substitute an appropriate message
527 indicating that the decoded text could not be displayed.
528
529 If the character set being used employs code-switching techniques,
530 display of the encoded text implicitly begins in "ASCII mode". In
531 addition, the mail reader must ensure that the output device is once
532 again in "ASCII mode" after the 'encoded-word' is displayed.
533
534 6.3. Mail reader handling of incorrectly formed 'encoded-word's
535
536 It is possible that an 'encoded-word' that is legal according to the
537 syntax defined in section 2, is incorrectly formed according to the
538 rules for the encoding being used. For example:
539
540 (1) An 'encoded-word' which contains characters which are not legal
541 for a particular encoding (for example, a "-" in the "B"
542 encoding, or a SPACE or HTAB in either the "B" or "Q" encoding),
543 is incorrectly formed.
544
545 (2) Any 'encoded-word' which encodes a non-integral number of
546 characters or octets is incorrectly formed.
547
548 A mail reader need not attempt to display the text associated with an
549 'encoded-word' that is incorrectly formed. However, a mail reader
550 MUST NOT prevent the display or handling of a message because an
551 'encoded-word' is incorrectly formed.
552
553 7. Conformance
554
555 A mail composing program claiming compliance with this specification
556 MUST ensure that any string of non-white-space printable ASCII
557 characters within a '*text' or '*ctext' that begins with "=?" and
558 ends with "?=" be a valid 'encoded-word'. ("begins" means: at the
559
560
561
562 Moore Standards Track [Page 10]
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564 RFC 2047 Message Header Extensions November 1996
565
566
567 start of the field-body, immediately following 'linear-white-space',
568 or immediately following a "(" for an 'encoded-word' within '*ctext';
569 "ends" means: at the end of the field-body, immediately preceding
570 'linear-white-space', or immediately preceding a ")" for an
571 'encoded-word' within '*ctext'.) In addition, any 'word' within a
572 'phrase' that begins with "=?" and ends with "?=" must be a valid
573 'encoded-word'.
574
575 A mail reading program claiming compliance with this specification
576 must be able to distinguish 'encoded-word's from 'text', 'ctext', or
577 'word's, according to the rules in section 6, anytime they appear in
578 appropriate places in message headers. It must support both the "B"
579 and "Q" encodings for any character set which it supports. The
580 program must be able to display the unencoded text if the character
581 set is "US-ASCII". For the ISO-8859-* character sets, the mail
582 reading program must at least be able to display the characters which
583 are also in the ASCII set.
584
585 8. Examples
586
587 The following are examples of message headers containing 'encoded-
588 word's:
589
590 From: =?US-ASCII?Q?Keith_Moore?= <moore@cs.utk.edu>
591 To: =?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?= <keld@dkuug.dk>
592 CC: =?ISO-8859-1?Q?Andr=E9?= Pirard <PIRARD@vm1.ulg.ac.be>
593 Subject: =?ISO-8859-1?B?SWYgeW91IGNhbiByZWFkIHRoaXMgeW8=?=
594 =?ISO-8859-2?B?dSB1bmRlcnN0YW5kIHRoZSBleGFtcGxlLg==?=
595
596 Note: In the first 'encoded-word' of the Subject field above, the
597 last "=" at the end of the 'encoded-text' is necessary because each
598 'encoded-word' must be self-contained (the "=" character completes a
599 group of 4 base64 characters representing 2 octets). An additional
600 octet could have been encoded in the first 'encoded-word' (so that
601 the encoded-word would contain an exact multiple of 3 encoded
602 octets), except that the second 'encoded-word' uses a different
603 'charset' than the first one.
604
605 From: =?ISO-8859-1?Q?Olle_J=E4rnefors?= <ojarnef@admin.kth.se>
606 To: ietf-822@dimacs.rutgers.edu, ojarnef@admin.kth.se
607 Subject: Time for ISO 10646?
608
609 To: Dave Crocker <dcrocker@mordor.stanford.edu>
610 Cc: ietf-822@dimacs.rutgers.edu, paf@comsol.se
611 From: =?ISO-8859-1?Q?Patrik_F=E4ltstr=F6m?= <paf@nada.kth.se>
612 Subject: Re: RFC-HDR care and feeding
613
614
615
616
617
618 Moore Standards Track [Page 11]
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620 RFC 2047 Message Header Extensions November 1996
621
622
623 From: Nathaniel Borenstein <nsb@thumper.bellcore.com>
624 (=?iso-8859-8?b?7eXs+SDv4SDp7Oj08A==?=)
625 To: Greg Vaudreuil <gvaudre@NRI.Reston.VA.US>, Ned Freed
626 <ned@innosoft.com>, Keith Moore <moore@cs.utk.edu>
627 Subject: Test of new header generator
628 MIME-Version: 1.0
629 Content-type: text/plain; charset=ISO-8859-1
630
631 The following examples illustrate how text containing 'encoded-word's
632 which appear in a structured field body. The rules are slightly
633 different for fields defined as '*text' because "(" and ")" are not
634 recognized as 'comment' delimiters. [Section 5, paragraph (1)].
635
636 In each of the following examples, if the same sequence were to occur
637 in a '*text' field, the "displayed as" form would NOT be treated as
638 encoded words, but be identical to the "encoded form". This is
639 because each of the encoded-words in the following examples is
640 adjacent to a "(" or ")" character.
641
642 encoded form displayed as
643 ---------------------------------------------------------------------
644 (=?ISO-8859-1?Q?a?=) (a)
645
646 (=?ISO-8859-1?Q?a?= b) (a b)
647
648 Within a 'comment', white space MUST appear between an
649 'encoded-word' and surrounding text. [Section 5,
650 paragraph (2)]. However, white space is not needed between
651 the initial "(" that begins the 'comment', and the
652 'encoded-word'.
653
654
655 (=?ISO-8859-1?Q?a?= =?ISO-8859-1?Q?b?=) (ab)
656
657 White space between adjacent 'encoded-word's is not
658 displayed.
659
660 (=?ISO-8859-1?Q?a?= =?ISO-8859-1?Q?b?=) (ab)
661
662 Even multiple SPACEs between 'encoded-word's are ignored
663 for the purpose of display.
664
665 (=?ISO-8859-1?Q?a?= (ab)
666 =?ISO-8859-1?Q?b?=)
667
668 Any amount of linear-space-white between 'encoded-word's,
669 even if it includes a CRLF followed by one or more SPACEs,
670 is ignored for the purposes of display.
671
672
673
674 Moore Standards Track [Page 12]
675
676 RFC 2047 Message Header Extensions November 1996
677
678
679 (=?ISO-8859-1?Q?a_b?=) (a b)
680
681 In order to cause a SPACE to be displayed within a portion
682 of encoded text, the SPACE MUST be encoded as part of the
683 'encoded-word'.
684
685 (=?ISO-8859-1?Q?a?= =?ISO-8859-2?Q?_b?=) (a b)
686
687 In order to cause a SPACE to be displayed between two strings
688 of encoded text, the SPACE MAY be encoded as part of one of
689 the 'encoded-word's.
690
691 9. References
692
693 [RFC 822] Crocker, D., "Standard for the Format of ARPA Internet Text
694 Messages", STD 11, RFC 822, UDEL, August 1982.
695
696 [RFC 2049] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
697 Extensions (MIME) Part Five: Conformance Criteria and Examples",
698 RFC 2049, November 1996.
699
700 [RFC 2045] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
701 Extensions (MIME) Part One: Format of Internet Message Bodies",
702 RFC 2045, November 1996.
703
704 [RFC 2046] Borenstein N., and N. Freed, "Multipurpose Internet Mail
705 Extensions (MIME) Part Two: Media Types", RFC 2046,
706 November 1996.
707
708 [RFC 2048] Freed, N., Klensin, J., and J. Postel, "Multipurpose
709 Internet Mail Extensions (MIME) Part Four: Registration
710 Procedures", RFC 2048, November 1996.
711
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730 Moore Standards Track [Page 13]
731
732 RFC 2047 Message Header Extensions November 1996
733
734
735 10. Security Considerations
736
737 Security issues are not discussed in this memo.
738
739 11. Acknowledgements
740
741 The author wishes to thank Nathaniel Borenstein, Issac Chan, Lutz
742 Donnerhacke, Paul Eggert, Ned Freed, Andreas M. Kirchwitz, Olle
743 Jarnefors, Mike Rosin, Yutaka Sato, Bart Schaefer, and Kazuhiko
744 Yamamoto, for their helpful advice, insightful comments, and
745 illuminating questions in response to earlier versions of this
746 specification.
747
748 12. Author's Address
749
750 Keith Moore
751 University of Tennessee
752 107 Ayres Hall
753 Knoxville TN 37996-1301
754
755 EMail: moore@cs.utk.edu
756
757
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785
786 Moore Standards Track [Page 14]
787
788 RFC 2047 Message Header Extensions November 1996
789
790
791 Appendix - changes since RFC 1522 (in no particular order)
792
793 + explicitly state that the MIME-Version is not requried to use
794 'encoded-word's.
795
796 + add explicit note that SPACEs and TABs are not allowed within
797 'encoded-word's, explaining that an 'encoded-word' must look like an
798 'atom' to an RFC822 parser.values, to be precise).
799
800 + add examples from Olle Jarnefors (thanks!) which illustrate how
801 encoded-words with adjacent linear-white-space are displayed.
802
803 + explicitly list terms defined in RFC822 and referenced in this memo
804
805 + fix transcription typos that caused one or two lines and a couple of
806 characters to disappear in the resulting text, due to nroff quirks.
807
808 + clarify that encoded-words are allowed in '*text' fields in both
809 RFC822 headers and MIME body part headers, but NOT as parameter
810 values.
811
812 + clarify the requirement to switch back to ASCII within the encoded
813 portion of an 'encoded-word', for any charset that uses code switching
814 sequences.
815
816 + add a note about 'encoded-word's being delimited by "(" and ")"
817 within a comment, but not in a *text (how bizarre!).
818
819 + fix the Andre Pirard example to get rid of the trailing "_" after
820 the =E9. (no longer needed post-1342).
821
822 + clarification: an 'encoded-word' may appear immediately following
823 the initial "(" or immediately before the final ")" that delimits a
824 comment, not just adjacent to "(" and ")" *within* *ctext.
825
826 + add a note to explain that a "B" 'encoded-word' will always have a
827 multiple of 4 characters in the 'encoded-text' portion.
828
829 + add note about the "=" in the examples
830
831 + note that processing of 'encoded-word's occurs *after* parsing, and
832 some of the implications thereof.
833
834 + explicitly state that you can't expect to translate between
835 1522 and either vanilla 822 or so-called "8-bit headers".
836
837 + explicitly state that 'encoded-word's are not valid within a
838 'quoted-string'.
839
840
841
842 Moore Standards Track [Page 15]
843