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rfc:rfc3676

Network Working Group R. Gellens Request for Comments: 3676 Qualcomm Obsoletes: 2646 February 2004 Category: Standards Track

             The Text/Plain Format and DelSp Parameters

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

 This specification establishes two parameters (Format and DelSP) to
 be used with the Text/Plain media type.  In the presence of these
 parameters, trailing whitespace is used to indicate flowed lines and
 a canonical quote indicator is used to indicate quoted lines.  This
 results in an encoding which appears as normal Text/Plain in older
 implementations, since it is in fact normal Text/Plain, yet provides
 for superior wrapping/flowing, and quoting.
 This document supersedes the one specified in RFC 2646, "The
 Text/Plain Format Parameter", and adds the DelSp parameter to
 accommodate languages/coded character sets in which ASCII spaces are
 not used or appear rarely.

Table of Contents

 1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.  Conventions Used in this Document . . . . . . . . . . . . . .   2
 3.  The Problem . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Paragraph Text. . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Embarrassing Line Wrap  . . . . . . . . . . . . . . . .   3
     3.3.  New Media Types . . . . . . . . . . . . . . . . . . . .   4
 4.  The Format and DelSp Parameters . . . . . . . . . . . . . . .   5
     4.1.  Interpreting Format=Flowed. . . . . . . . . . . . . . .   6
     4.2.  Generating Format=Flowed  . . . . . . . . . . . . . . .   7
     4.3.  Usenet Signature Convention . . . . . . . . . . . . . .   9
     4.4.  Space-Stuffing  . . . . . . . . . . . . . . . . . . . .   9

Gellens Standards Track [Page 1] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

     4.5.  Quoting . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.6.  Digital Signatures and Encryption . . . . . . . . . . .  11
     4.7.  Examples. . . . . . . . . . . . . . . . . . . . . . . .  12
 5.  Interoperability. . . . . . . . . . . . . . . . . . . . . . .  12
 6.  ABNF. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
 7.  Failure Modes . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Trailing White Space Corruption . . . . . . . . . . . .  14
 8.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
 9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
 10. Internationalization Considerations . . . . . . . . . . . . .  15
 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  15
 12. Normative References. . . . . . . . . . . . . . . . . . . . .  16
 13. Informative References. . . . . . . . . . . . . . . . . . . .  16
 Appendix A: Changes from RFC 2646 . . . . . . . . . . . . . . . .  18
 Author's Address. . . . . . . . . . . . . . . . . . . . . . . . .  19
 Full Copyright Statement. . . . . . . . . . . . . . . . . . . . .  20

1. Introduction

 Interoperability problems have been observed with erroneous labelling
 of paragraph text as Text/Plain, and with various forms of
 "embarrassing line wrap".  (See Section 3.)
 Attempts to deploy new media types, such as Text/Enriched [Rich] and
 Text/HTML [HTML] have suffered from a lack of backwards compatibility
 and an often hostile user reaction at the receiving end.
 What is required is a format which is in all significant ways
 Text/Plain, and therefore is quite suitable for display as
 Text/Plain, and yet allows the sender to express to the receiver
 which lines are quoted and which lines are considered a logical
 paragraph, and thus eligible to be flowed (wrapped and joined) as
 appropriate.

2. Conventions Used in this Document

 The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT",
 and "MAY" in this document are to be interpreted as described in "Key
 words for use in RFCs to Indicate Requirement Levels" [KEYWORDS].
 The term "paragraph" is used here to mean a series of lines which are
 logically to be treated as a unit for display purposes and eligible
 to be flowed (wrapped and joined) as appropriate to fit in the
 display window and when creating text for replies, forwarding, etc.

Gellens Standards Track [Page 2] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

3. The Problem

 The Text/Plain media type is the lowest common denominator of
 Internet email, with lines of no more than 998 characters (by
 convention usually no more than 78), and where the carriage-return
 and line-feed (CRLF) sequence represents a line break (see [MIME-IMT]
 and [MSG-FMT]).
 Text/Plain is usually displayed as preformatted text, often in a
 fixed font.  That is, the characters start at the left margin of the
 display window, and advance to the right until a CRLF sequence is
 seen, at which point a new line is started, again at the left margin.
 When a line length exceeds the display window, some clients will wrap
 the line, while others invoke a horizontal scroll bar.
 Text which meets this description is defined by this memo as "fixed".
 Some interoperability problems have been observed with this format:

3.1. Paragraph Text

 Many modern programs use a proportional-spaced font, and use CRLF to
 represent paragraph breaks.  Line breaks are "soft", occurring as
 needed on display.  That is, characters are grouped into a paragraph
 until a CRLF sequence is seen, at which point a new paragraph is
 started.  Each paragraph is displayed, starting at the left margin
 (or paragraph indent), and continuing to the right until a word is
 encountered which does not fit in the remaining display width.  This
 word is displayed at the left margin of the next line.  This
 continues until the paragraph ends (a CRLF is seen).  Extra vertical
 space is left between paragraphs.
 Text which meets this description is defined by this memo as
 "flowed".
 Numerous software products erroneously label this format as
 Text/Plain, resulting in much user discomfort.

3.2. Embarrassing Line Wrap

 As Text/Plain messages are quoted in replies or forwarded messages,
 each line gradually increases in length, eventually being arbitrarily
 hard wrapped, resulting in "embarrassing line wrap".  This produces
 text which is, at best, hard to read, and often confuses
 attributions.

Gellens Standards Track [Page 3] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

 Example:
    >>>>>>This is a comment from the first message to show a
    >quoting example.
    >>>>>This is a comment from the second message to show a
    >quoting example.
    >>>>This is a comment from the third message.
    >>>This is a comment from the fourth message.
 It can be confusing to assign attribution to lines 2 and 4 above.
 In addition, as devices with display widths smaller than 79 or 80
 characters become more popular, embarrassing line wrap has become
 even more prevalent, even with unquoted text.
 Example:
    This is paragraph text that is
    meant to be flowed across
    several lines.
    However, the sending mailer is
    converting it to fixed text at
    a width of 72
    characters, which causes it to
    look like this when shown on a
    PDA with only
    30 character lines.

3.3. New Media Types

 Attempts to deploy new media types, such as Text/Enriched [Rich] and
 Text/HTML [HTML] have suffered from a lack of backwards compatibility
 and an often hostile user reaction at the receiving end.
 In particular, Text/Enriched requires that open angle brackets ("<")
 and hard line breaks be doubled, with resulting user unhappiness when
 viewed as Text/Plain.  Text/HTML requires even more alteration of
 text, with a corresponding increase in user complaints.
 A proposal to define a new media type to explicitly represent the
 paragraph form suffered from a lack of interoperability with
 currently deployed software.  Some programs treat unknown subtypes of
 TEXT as an attachment.

Gellens Standards Track [Page 4] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

 What is desired is a format which is in all significant ways
 Text/Plain, and therefore is quite suitable for display as
 Text/Plain, and yet allows the sender to express to the receiver
 which lines can be considered a logical paragraph, and thus flowed
 (wrapped and joined) as appropriate.

4. The Format and DelSp Parameters

 This specification defines two MIME parameters for use with
 Text/Plain:
    Name:  Format
    Value:  Fixed, Flowed
    Name:  DelSp
    Value:  Yes, No
 (Neither the parameter names nor values are case sensitive.)
 If Format is not specified, or if the value is not recognized, a
 value of Fixed is assumed.  The semantics of the Fixed value are the
 usual associated with Text/Plain [MIME-IMT].
 A Format value of Flowed indicates that the definition of flowed text
 (as specified in this memo) was used on generation, and MAY be used
 on reception.
 Note that because Format is a parameter of the Text/Plain content-
 type, any content-transfer-encoding used is irrelevant to the
 processing of flowed text.
 If DelSp is not specified, or if its value is not recognized, a value
 of No is assumed.  The use of DelSp without a Format value of Flowed
 is undefined.  When creating messages, DelSp SHOULD NOT be specified
 in Text content types other than Text/Plain with Format = Flowed.
 When receiving messages, DelSp SHOULD be ignored if used in a Text
 content type other than Text/Plain with Format = Flowed.
 This section discusses flowed text; section 6 provides a formal
 definition.
 Section 5 discusses interoperability.
 Note that this memo describes an on-the-wire format.  It does not
 address formats for local file storage.

Gellens Standards Track [Page 5] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

4.1. Interpreting Format=Flowed

 If the first character of a line is a quote mark (">"), the line is
 considered to be quoted (see Section 4.5).  Logically, all quote
 marks are counted and deleted, resulting in a line with a non-zero
 quote depth, and content.  (The agent is of course free to display
 the content with quote marks or excerpt bars or anything else.)
 Logically, this test for quoted lines is done before any other tests
 (that is, before checking for space-stuffed and flowed).
 If the first character of a line is a space, the line has been
 space-stuffed (see Section 4.4).  Logically, this leading space is
 deleted before examining the line further (that is, before checking
 for flowed).
 If the line ends in a space, the line is flowed.  Otherwise it is
 fixed.  The exception to this rule is a signature separator line,
 described in Section 4.3.  Such lines end in a space but are neither
 flowed nor fixed.
 If the line is flowed and DelSp is "yes", the trailing space
 immediately prior to the line's CRLF is logically deleted.  If the
 DelSp parameter is "no" (or not specified, or set to an unrecognized
 value), the trailing space is not deleted.
 Any remaining trailing spaces are part of the line's content, but the
 CRLF of a soft line break is not.
 A series of one or more flowed lines followed by one fixed line is
 considered a paragraph, and MAY be flowed (wrapped and unwrapped) as
 appropriate on display and in the construction of new messages (see
 Section 4.5).
 An interpreting agent SHOULD allow for three exceptions to the rule
 that paragraphs end with a fixed line.  These exceptions are
 improperly constructed messages: a flowed line SHOULD be considered
 to end the paragraph if it is followed by a line of a different quote
 depth (see 4.5) or by a signature separator (see 4.3); the end of the
 body also ends the paragraph.
 A line consisting of one or more spaces (after deleting a space
 acting as stuffing) is considered a flowed line.
 An empty line (just a CRLF) is a fixed line.
 Note that, for Unicode text, [Annex-14] provides guidance for
 choosing at which characters to wrap a line.

Gellens Standards Track [Page 6] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

4.2. Generating Format=Flowed

 When generating Format=Flowed text, lines SHOULD be 78 characters or
 shorter, including any trailing white space and also including any
 space added as part of stuffing (see Section 4.4).  As suggested
 values, any paragraph longer than 78 characters in total length could
 be wrapped using lines of 72 or fewer characters.  While the specific
 line length used is a matter of aesthetics and preference, longer
 lines are more likely to require rewrapping and to encounter
 difficulties with older mailers.  (It has been suggested that 66
 character lines are the most readable.)
 The restriction to 78 or fewer characters between CRLFs on the wire
 is to conform to [MSG-FMT].
 (In addition to conformance to [MSG-FMT], there is a historical need
 that all lines, even when displayed by a non-flowed-aware program,
 will fit in a standard 79- or 80-column screen without having to be
 wrapped.  The limit is 78, not 79 or 80, because while 79 or 80 fit
 on a line, the last column is often reserved for a line-wrap
 indicator.)
 When creating flowed text, the generating agent wraps, that is,
 inserts 'soft' line breaks as needed.  Soft line breaks are added at
 natural wrapping points, such as between words.  A soft line break is
 a SP CRLF sequence.
 There are two techniques for inserting soft line breaks.  The older
 technique, established by RFC 2646, creates a soft line break by
 inserting a CRLF after the occurrence of a space.  With this
 technique, soft line breaks are only possible where spaces already
 occur.  When this technique is used, the DelSp parameter SHOULD be
 used; if used it MUST be set to "no".
 The newer technique, suitable for use even with languages/coded
 character sets in which the ASCII space character is rare or not
 used, creates a soft line break by inserting a SP CRLF sequence.
 When this technique is used, the DelSp parameter MUST be used and
 MUST be set to "yes".  Note that because of space-stuffing (see
 Section 4.4), when this technique is used and a soft line break is
 inserted at a point where a SP already exists (such as between
 words), if the SP CRLF sequence is added immediately before the SP,
 the pre-existing SP becomes leading and thus requires stuffing.  It
 is RECOMMENDED that agents avoid this by inserting the SP CRLF
 sequence following the existing SP.
 Generating agents MAY use either method within each Text/Plain body
 part.

Gellens Standards Track [Page 7] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

 Regardless of which technique is used, a generating agent SHOULD NOT
 insert a space in an unnatural location, such as into a word (a
 sequence of printable characters, not containing spaces, in a
 language/coded character set in which spaces are common).  If faced
 with such a word which exceeds 78 characters (but less than 998
 characters, the [SMTP] limit on line length), the agent SHOULD send
 the word as is and exceed the 78-character limit on line length.
 A generating agent SHOULD:
 o  Ensure all lines (fixed and flowed) are 78 characters or fewer in
    length, counting any trailing space as well as a space added as
    stuffing, but not counting the CRLF, unless a word by itself
    exceeds 78 characters.
 o  Trim spaces before user-inserted hard line breaks.
 A generating agent MUST:
 o  Space-stuff lines which start with a space, "From ", or ">".
 In order to create messages which do not require space-stuffing, and
 are thus more aesthetically pleasing when viewed as Format=Fixed, a
 generating agent MAY avoid wrapping immediately before ">", "From ",
 or space.
 (See Sections 4.4 and 4.5 for more information on space-stuffing and
 quoting, respectively.)
 A Format=Flowed message consists of zero or more paragraphs, each
 containing one or more flowed lines followed by one fixed line.  The
 usual case is a series of flowed text lines with blank (empty) fixed
 lines between them.
 Any number of fixed lines can appear between paragraphs.
 When placing soft line breaks in a paragraph, generating agents MUST
 NOT place them in a way that causes any line of the paragraph to be a
 signature separator line, because paragraphs cannot contain signature
 separator lines (see Sections 4.3 and 6).
 [Quoted-Printable] encoding SHOULD NOT be used with Format=Flowed
 unless absolutely necessary (for example, non-US-ASCII (8-bit)
 characters over a strictly 7-bit transport such as unextended
 [SMTP]).  In particular, a message SHOULD NOT be encoded in Quoted-
 Printable for the sole purpose of protecting the trailing space on
 flowed lines unless the body part is cryptographically signed or
 encrypted (see Section 4.6).

Gellens Standards Track [Page 8] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

 The intent of Format=Flowed is to allow user agents to generate
 flowed text which is non-obnoxious when viewed as pure, raw
 Text/Plain (without any decoding); use of Quoted-Printable hinders
 this and may cause Format=Flowed to be rejected by end users.

4.3. Usenet Signature Convention

 There is a long-standing convention in Usenet news which also
 commonly appears in Internet mail of using "-- " as the separator
 line between the body and the signature of a message.  When
 generating a Format=Flowed message containing a Usenet-style
 separator before the signature, the separator line is sent as-is.
 This is a special case; an (optionally quoted or quoted and stuffed)
 line consisting of DASH DASH SP is neither fixed nor flowed.
 Generating agents MUST NOT end a paragraph with such a signature
 line.
 A receiving agent needs to test for a signature line both before the
 test for a quoted line (see Section 4.5) and also after logically
 counting and deleting quote marks and stuffing (see Section 4.4) from
 a quoted line.

4.4. Space-Stuffing

 In order to allow for unquoted lines which start with ">", and to
 protect against systems which "From-munge" in-transit messages
 (modifying any line which starts with "From " to ">From "),
 Format=Flowed provides for space-stuffing.
 Space-stuffing adds a single space to the start of any line which
 needs protection when the message is generated.  On reception, if the
 first character of a line is a space, it is logically deleted.  This
 occurs after the test for a quoted line (which logically counts and
 deletes any quote marks), and before the test for a flowed line.
 On generation, any unquoted lines which start with ">", and any lines
 which start with a space or "From " MUST be space-stuffed.  Other
 lines MAY be space-stuffed as desired.
 (Note that space-stuffing is conceptually similar to dot-stuffing as
 specified in [SMTP].)

4.5. Quoting

 In Format=Flowed, the canonical quote indicator (or quote mark) is
 one or more close angle bracket (">") characters.  Lines which start
 with the quote indicator are considered quoted.  The number of ">"

Gellens Standards Track [Page 9] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

 characters at the start of the line specifies the quote depth.
 Flowed lines which are also quoted may require special handling on
 display and when copied to new messages.
 When creating quoted flowed lines, each such line starts with the
 quote indicator.
 Note that because of space-stuffing, the lines
     >> Exit, Stage Left
 and
     >>Exit, Stage Left
 are semantically identical; both have a quote-depth of two, and a
 content of "Exit, Stage Left".
 However, the line
     > > Exit, Stage Left
 is different.  It has a quote-depth of one, and a content of
 "> Exit, Stage Left".
 When generating quoted flowed lines, an agent needs to pay attention
 to changes in quote depth.  All lines of a paragraph MUST be
 unquoted, or else they MUST all be quoted and have the same quote
 depth.  Therefore, whenever there is a change in quote depth, or a
 change from quoted to unquoted, or change from unquoted to quoted,
 the line immediately preceding the change MUST NOT be a flowed line.
 If a receiving agent wishes to reformat flowed quoted lines (joining
 and/or wrapping them) on display or when generating new messages, the
 lines SHOULD be de-quoted, reformatted, and then re-quoted.  To de-
 quote, the number of close angle brackets in the quote indicator at
 the start of each line is counted.  To re-quote after reformatting, a
 quote indicator containing the same number of close angle brackets
 originally present are prefixed to each line.
 On reception, if a change in quote depth occurs on a flowed line,
 this is an improperly formatted message.  The receiver SHOULD handle
 this error by using the 'quote-depth-wins' rule, which is to consider
 the paragraph to end with the flowed line immediately preceding the
 change in quote depth.
 In other words, whenever two adjacent lines have different quote
 depths, senders MUST ensure that the earlier line is not flowed (does
 not end in a space), and receivers finding a flowed line there SHOULD
 treat it as the last line of a paragraph.
 For example, consider the following sequence of lines (using '*' to
 indicate a soft line break, i.e., SP CRLF, and '#' to indicate a hard
 line break, i.e., CRLF):

Gellens Standards Track [Page 10] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

    > Thou villainous ill-breeding spongy dizzy-eyed*
    > reeky elf-skinned pigeon-egg!*     <--- problem ---<
    >> Thou artless swag-bellied milk-livered*
    >> dismal-dreaming idle-headed scut!#
    >>> Thou errant folly-fallen spleeny reeling-ripe*
    >>> unmuzzled ratsbane!#
    >>>> Henceforth, the coding style is to be strictly*
    >>>> enforced, including the use of only upper case.#
    >>>>> I've noticed a lack of adherence to the coding*
    >>>>> styles, of late.#
    >>>>>> Any complaints?#
 The second line ends in a soft line break, even though it is the last
 line of the one-deep quote block.  The question then arises as to how
 this line is to be interpreted, considering that the next line is the
 first line of the two-deep quote block.
 The example text above, when processed according to quote-depth wins,
 results in the first two lines being considered as one quoted, flowed
 section, with a quote depth of 1; the third and fourth lines become a
 quoted, flowed section, with a quote depth of 2.
 A generating agent MUST NOT create this situation; a receiving agent
 SHOULD handle it by giving preference to the quote depth.

4.6. Digital Signatures and Encryption

 If a message is digitally signed or encrypted it is important that
 cryptographic processing use the same text for signature verification
 and/or decryption as was used for signature generation and/or
 encryption.  Since the use of format=flowed allows text to be altered
 (by adding or removing line breaks and trailing spaces) between
 composition and transmission, and between reception and display,
 interoperability problems or security vulnerabilities may arise if
 originator and recipient do not both use the on-the-wire format for
 cryptographic processing.
 The implications of the interaction between format=flowed and any
 specific cryptographic process depend on the details of the
 cryptographic processing and should be understood before using
 format=flowed in conjunction with signed and/or encrypted messages.
 Note that [OpenPGP] specifies (in Section 7.1) that "any trailing
 whitespace (spaces, and tabs, 0x09) at the end of any line is ignored
 when the cleartext signature is calculated."

Gellens Standards Track [Page 11] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

 Thus it would be possible to add, in transit, a format=flowed header
 to a regular, format=fixed vanilla PGP (not [OpenPGP-MIME]) signed
 message and add arbitrary trailing space characters without this
 addition being detected.  This would change the rendering of the
 article by a client which supported format=flowed.
 Therefore, the use of [OpenPGP] with format=flowed messages is
 strongly discouraged. [OpenPGP-MIME] is recommended instead.

4.7. Examples

 The following example contains three paragraphs:
    `Take some more tea,' the March Hare said to Alice, very
    earnestly.
    `I've had nothing yet,' Alice replied in an offended tone, `so I
    can't take more.'
    `You mean you can't take LESS,' said the Hatter: `it's very easy
    to take MORE than nothing.'
 This could be encoded as follows (using '*' to indicate a soft line
 break, that is, SP CRLF sequence, and '#' to indicate a hard line
 break, that is, CRLF):
    `Take some more tea,' the March Hare said to Alice, very*
    earnestly.#
    #
    `I've had nothing yet,' Alice replied in an offended tone, `so*
    I can't take more.'#
    #
    `You mean you can't take LESS,' said the Hatter: `it's very*
    easy to take MORE than nothing.'#
 To show an example of quoting, here we have the same exchange,
 presented as a series of direct quotes:
    >>>Take some more tea.#
    >>I've had nothing yet, so I can't take more.#
    >You mean you can't take LESS, it's very easy to take*
    >MORE than nothing.#

5. Interoperability

 Because flowed lines are all-but-indistinguishable from fixed lines,
 software which does not recognize Format=Flowed treats flowed lines
 as normal Text/Plain (which is what they are).  Thus, Format=Flowed

Gellens Standards Track [Page 12] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

 interoperates with older clients, although flowed lines will have
 trailing white space inserted.
 If a space-stuffed message is received by an agent which handles
 Format=Flowed, the space-stuffing is reversed and thus the message
 appears unchanged.  An agent which is not aware of Format=Flowed will
 of course not undo any space-stuffing; thus Format=Flowed messages
 may appear with a leading space on some lines (those which start with
 a space, ">" which is not a quote indicator, or "From ").  Since
 lines which require space-stuffing rarely occur, and the aesthetic
 consequences of unreversed space-stuffing are minimal, this is not
 expected to be a significant problem.
 If some lines begin with one or more spaces, the generating agent MAY
 space-stuff all lines, to maintain the relative indentation of the
 lines when viewed by clients which are not aware of Format=Flowed.
 Messages generated with DelSp=yes and received by clients which are
 aware of Format=Flowed but are not aware of the DelSp parameter will
 have an extra space remaining after removal of soft line breaks.
 Thus, when generating text in languages/coded character sets in which
 spaces are common, the generating agent MAY always use the DelSp=no
 method.
 Hand-aligned text, such as ASCII tables or art, source code, etc.,
 SHOULD be sent as fixed, not flowed lines.

6. ABNF

 The constructs used in Text/Plain; Format=Flowed body parts are
 described using Augmented Backus-Naur Form [ABNF], including the core
 rules defined in Appendix A.
 Note that the SP (space) and ">" characters are encoded according to
 the charset parameter.

flowed-body = *( paragraph / fixed-line / sig-sep ) paragraph = 1*flowed-line fixed-line

                 ; all lines in paragraph MUST be unquoted or
                 ; have same quote depth

flowed-line = ( flowed-line-qt / flowed-line-unqt ) flow CRLF flowed-line-qt = quote ( ( stuffing stuffed-flowed ) /

                         unstuffed-flowed )

flowed-line-unqt = ( stuffing stuffed-flowed ) / unstuffed-flowed stuffed-flowed = *text-char unstuffed-flowed = non-sp-quote *text-char fixed-line = fixed-line-qt / fixed-line-unqt fixed-line-qt = quote ( ( stuffing stuffed-fixed ) /

Gellens Standards Track [Page 13] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

                         unstuffed-fixed ) CRLF

fixed-line-unqt = ( stuffed-fixed / unstuffed-fixed ) CRLF stuffed-fixed = *text-char non-sp unstuffed-fixed = non-sp-quote [ *text-char non-sp ] sig-sep = [ quote [stuffing] ] "–" SP CRLF quote-mark = ">" quote = 1*quote-mark stuffing = SP ; space-stuffed, added on generation if

                    ; needed, deleted on reception

flow = SP ; space before CRLF indicates flowed line,

                    ; if DelSp=yes, space was added on generation
                    ; and is deleted on reception

non-sp-quote = < any character except NUL, CR, LF, SP, quote-mark > non-sp = non-sp-quote / quote-mark text-char = non-sp / SP

 That is, a Format=Flowed message body consists of any number of
 paragraphs and/or fixed lines and/or signature separator lines;
 paragraphs need at least one flowed line and are terminated by a
 fixed line; the fixed line terminating the paragraph is part of the
 paragraph.  (There are some exceptions to this described in the
 text.)
 Without at least one flowed line, there is a series of fixed lines,
 each independent.  There is no paragraph.
 With at least one flowed line, there is a paragraph, and the received
 lines can be reformed and flowed to fit the display window size.
 This can only be done if the lines are part of a logical grouping,
 the paragraph.
 Note that the definitions of flowed-line and sig-sep are potentially
 ambiguous: a signature separator line matches both, but is treated as
 a signature separator line and not a flowed line.

7. Failure Modes

7.1. Trailing White Space Corruption

 There are systems in existence which alter trailing whitespace on
 messages which pass through them.  Such systems may strip, or in
 rarer cases, add trailing whitespace, in violation of RFC 2821 [SMTP]
 Section 4.5.2.
 Stripping trailing whitespace has the effect of converting flowed
 lines to fixed lines, which results in a message no worse than if
 Format=Flowed had not been used.

Gellens Standards Track [Page 14] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

 Adding trailing whitespace to a Format=Flowed message may result in a
 malformed display or reply.
 Since most systems which add trailing white space do so to create a
 line which fills an internal record format, the result is almost
 always a line which contains an even number of characters (counting
 the added trailing white space).
 One possible avoidance, therefore, would be to define Format=Flowed
 lines to use either one or two trailing space characters to indicate
 a flowed line, such that the total line length is odd.  However,
 considering the scarcity of such systems today, it is not worth the
 added complexity.

8. Security Considerations

 Any security considerations which apply to Text/Plain also apply to
 Text/Plain with Format=Flowed.
 Section 4.6 discusses the interaction between Format=Flowed and
 digital signatures or encryption.

9. IANA Considerations

 IANA has added a reference to this specification in the Text/Plain
 Media Type registration.

10. Internationalization Considerations

 The line wrap and quoting specifications of Format=Flowed may not be
 suitable for certain charsets, such as for Arabic and Hebrew
 characters that read from right to left.  Care needs to be taken in
 applying format=flowed in these cases, as format=fixed combined with
 [quoted-printable] encoding may be more suitable.
 The DelSp parameter was added specifically to permit Format=Flowed to
 be used with languages/coded character sets in which the ASCII space
 character is rarely used, or not used at all.

11. Acknowledgments

 The DelSp parameter was developed during a series of discussions
 among a number of people, including Harald Alvestrand, Grant Baillie,
 Ian Bell, Steve Dorner, Patrik Faltstrom, Eric Fischer, Ned Freed,
 Alexey Melnikov, John Myers, and Pete Resnick.

Gellens Standards Track [Page 15] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

 Corrections and clarifications to RFC 2646 and early versions of this
 document were pointed out by several people, including Adam Costello,
 Jutta Degener, Tony Hansen, Simon Josefsson, Dan Kohn, Ragho
 Mahalingam, Keith Moore, Greg Troxel, and Dan Wing.
 I'm told that NeXT's mail application used a very similar mechanism
 (without support for non-Western languages) in 1992.

12. Normative References

 [ABNF]             Crocker, D., Ed. and P. Overell, "Augmented BNF
                    for Syntax Specifications: ABNF", RFC 2234,
                    November 1997.
 [KEYWORDS]         Bradner, S., "Key words for use in RFCs to
                    Indicate Requirement Levels", BCP 14, RFC 2119,
                    March 1997.
 [MIME-IMT]         Freed, N. and N. Borenstein, "Multipurpose
                    Internet Mail Extensions (MIME) Part Two:  Media
                    Types", RFC 2046, November 1996.
 [Quoted-Printable] Freed, N. and N. Borenstein, "Multipurpose
                    Internet Mail Extensions (MIME) Part One:  Format
                    of Internet Message Bodies", RFC 2045, November
                    1996.

13. Informative References

 [Annex-14]         Unicode Standard Annex #14, "Line Breaking
                    Properties"
                    <URL:http://www.unicode.org/unicode/reports/tr14/>
 [MSG-FMT]          Resnick, P., Ed., "Internet Message Format", RFC
                    2822, April 2001.
 [OpenPGP]          Callas, J., Donnerhacke, L., Finney, H. and R.
                    Thayer, "OpenPGP Message Format", RFC 2440,
                    November 1998.
 [OpenPGP-MIME]     Elkins, M., "MIME Security with Pretty Good
                    Privacy (PGP)", RFC 2015, October 1996.
                    Elkins, M., Del Torto, D., Levien, R. and J.
                    Roessler, "MIME Security with OpenPGP", RFC 3156,
                    August 2001.

Gellens Standards Track [Page 16] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

 [Rich]             Resnick, P. and A. Walker, "The text/enriched MIME
                    Content-type", RFC 1896, February 1996.
 [SMTP]             Klensin, J., Ed., "Simple Mail Transfer Protocol",
                    RFC 2821, April 2001.

Gellens Standards Track [Page 17] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

Appendix A: Changes from RFC 2646

 Substantive:
 o  Added DelSp parameter to handle languages and coded character sets
    in which space is less common or not used.
 o  Updated text on generating and interpreting to accommodate the
    DelSp parameter.
 o  Changed the limits of 79 or 80 to be 78 in conformance with RFC
    2822.
 o  Added text on generating to clarify that the 78-character limit
    includes trailing white space and stuffing.
 o  Changed sig-sep in ABNF to allow stuffing.
 o  Changed fixed-line to allow empty lines in ABNF.
 o  Added explanatory text following ABNF.
 o  Moved text from Abstract to new Introduction; rewrote Abstract.
 o  Moved interoperability text to new section, and updated.
 o  Clarified Security Considerations.
 o  Text on digital signatures now discusses that OpenPGP ignores
    trailing white space.
 o  Mention Unicode Annex 14.
 o  Added mention of quoting to Abstract and Introduction.
 o  Deleted line analysis table.
 o  Added recommendations for OpenPGP and OpenPGP-MIME.
 o  Rewrote ABNF rules to remove most ambiguity and note remaining
    case.
 o  Added note that c-t-e is irrelevant to flowed text processing.
 o  Added text indicating that end of data terminates a paragraph.
 o  Moved sig-sep out of fixed-line ABNF.
 o  Changed some SHOULDs to MUSTs (space-stuffing, quoted paragraphs).
 o  Added note to ABNF that space and ">" are encoded according to
    charset.
 o  Mentioned exceptions in section on interpreting.
 o  Clarified and made consistent treatment of signature separator
    lines.
 Editorial:
 o  Added mention of NeXT's mail application to Acknowledgments.
 o  Updated Acknowledgments.
 o  Updated [SMTP] reference to 2821.
 o  Added Notices.
 o  Split References into Normative and Informative.
 o  Improved text wording in some areas.
 o  Standardize on "quote depth", not "quoting depth".
 o  Moved section on interpreting before section on generating.
 o  Reworded non-normative "should"s.
 o  Noted meaning of "paragraph".

Gellens Standards Track [Page 18] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

 The DelSp parameter was added specifically to permit Format=Flowed to
 be used with languages/coded character sets in which the ASCII space
 character is rarely used, or not used at all.  The DelSp mechanism
 was selected despite having been initially rejected as too much of a
 kludge, because among the many different techniques proposed, it
 allows for maximum interoperability among clients which support
 neither this specification nor RFC 2646, those which do support RFC
 2646 but not this specification, and those that do support this
 specification; this set is multiplied by those that handle
 languages/coded character sets in which spaces are common, and in
 which they are uncommon or not used.

Author's Address

 Randall Gellens
 QUALCOMM Incorporated
 5775 Morehouse Drive
 San Diego, CA  92121
 USA
 Phone: +1 858 651 5115
 EMail: randy@qualcomm.com

Gellens Standards Track [Page 19] RFC 3676 Text/Plain Format and DelSp Parameters February 2004

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Gellens Standards Track [Page 20]

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