rfc2646.txt - rohrpost - A commandline mail client to change the world as we see it.
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       rfc2646.txt (29175B)
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            7 Network Working Group                                 R. Gellens, Editor
            8 Request for Comments: 2646                                      Qualcomm
            9 Updates: 2046                                                August 1999
           10 Category: Standards Track
           11 
           12 
           13                     The Text/Plain Format Parameter
           14 
           15 Status of this Memo
           16 
           17    This document specifies an Internet standards track protocol for the
           18    Internet community, and requests discussion and suggestions for
           19    improvements.  Please refer to the current edition of the "Internet
           20    Official Protocol Standards" (STD 1) for the standardization state
           21    and status of this protocol.  Distribution of this memo is unlimited.
           22 
           23 Copyright Notice
           24 
           25    Copyright (C) The Internet Society (1999).  All Rights Reserved.
           26 
           27 Table of Contents
           28 
           29     1.  Abstract . . . . . . . . . . . . . . . . . . . . . . . . . .  2
           30     2.  Conventions Used in this Document . . . . . . . . . . . . .   2
           31     3.  The Problem  . . . . . . . . . . . . . . . . . . . . . . . .  2
           32       3.1.  Paragraph Text  . . . . . . . . . . . . . . . . . . . .   3
           33       3.2.  Embarrassing Line Wrap . . . . . . . . . . . . . . . . .  3
           34       3.3.  New Media Types . . . . . . . . . . . . . . . . . . . .   4
           35     4.  The Format Parameter to the Text/Plain Media Type  . . . . .  4
           36       4.1.  Generating Format=Flowed  . . . . . . . . . . . . . . .   5
           37       4.2.  Interpreting Format=Flowed . . . . . . . . . . . . . . .  6
           38       4.3.  Usenet Signature Convention . . . . . . . . . . . . . .   7
           39       4.4.  Space-Stuffing . . . . . . . . . . . . . . . . . . . . .  7
           40       4.5.  Quoting . . . . . . . . . . . . . . . . . . . . . . . .   8
           41       4.6.  Digital Signatures and Encryption  . . . . . . . . . . .  9
           42       4.7.  Line Analysis Table . . . . . . . . . . . . . . . . . .  10
           43       4.8.  Examples . . . . . . . . . . . . . . . . . . . . . . . . 10
           44     5.  ABNF  . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
           45     6.  Failure Modes  . . . . . . . . . . . . . . . . . . . . . . . 11
           46       6.1.  Trailing White Space Corruption . . . . . . . . . . . .  11
           47     7.  Security Considerations  . . . . . . . . . . . . . . . . . . 12
           48     8.  IANA Considerations . . . . . . . . . . . . . . . . . . . .  12
           49     9.  Internationalization Considerations  . . . . . . . . . . . . 12
           50    10.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . .  12
           51    11.  References . . . . . . . . . . . . . . . . . . . . . . . . . 13
           52    12.  Editor's Address  . . . . . . . . . . . . . . . . . . . . .  13
           53    13.  Full Copyright Statement . . . . . . . . . . . . . . . . . . 14
           54 
           55 
           56 
           57 
           58 Gellens                     Standards Track                     [Page 1]
           59 
           60 RFC 2646            The Text/Plain Format Parameter          August 1999
           61 
           62 
           63 1.  Abstract
           64 
           65    Interoperability problems have been observed with erroneous labelling
           66    of paragraph text as Text/Plain, and with various forms of
           67    "embarrassing line wrap." (See section 3.)
           68 
           69    Attempts to deploy new media types, such as Text/Enriched [RICH] and
           70    Text/HTML [HTML] have suffered from a lack of backwards compatibility
           71    and an often hostile user reaction at the receiving end.
           72 
           73    What is required is a format which is in all significant ways
           74    Text/Plain, and therefore is quite suitable for display as
           75    Text/Plain, and yet allows the sender to express to the receiver
           76    which lines can be considered a logical paragraph, and thus flowed
           77    (wrapped and joined) as appropriate.
           78 
           79    This memo proposes a new parameter to be used with Text/Plain, and,
           80    in the presence of this parameter, the use of trailing whitespace to
           81    indicate flowed lines.  This results in an encoding which appears as
           82    normal Text/Plain in older implementations, since it is in fact
           83    normal Text/Plain.
           84 
           85 2.  Conventions Used in this Document
           86 
           87    The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT",
           88    and "MAY" in this document are to be interpreted as described in "Key
           89    words for use in RFCs to Indicate Requirement Levels" [KEYWORDS].
           90 
           91 3.  The Problem
           92 
           93    The Text/Plain media type is the lowest common denominator of
           94    Internet email, with lines of no more than 997 characters (by
           95    convention usually no more than 80), and where the CRLF sequence
           96    represents a line break [MIME-IMT].
           97 
           98    Text/Plain is usually displayed as preformatted text, often in a
           99    fixed font.  That is, the characters start at the left margin of the
          100    display window, and advance to the right until a CRLF sequence is
          101    seen, at which point a new line is started, again at the left margin.
          102    When a line length exceeds the display window, some clients will wrap
          103    the line, while others invoke a horizontal scroll bar.
          104 
          105    Text which meets this description is defined by this memo as "fixed".
          106 
          107    Some interoperability problems have been observed with this media
          108    type:
          109 
          110 
          111 
          112 
          113 
          114 Gellens                     Standards Track                     [Page 2]
          115 
          116 RFC 2646            The Text/Plain Format Parameter          August 1999
          117 
          118 
          119 3.1.  Paragraph Text
          120 
          121    Many modern programs use a proportional-spaced font and CRLF to
          122    represent paragraph breaks.  Line breaks are "soft", occurring as
          123    needed on display.  That is, characters are grouped into a paragraph
          124    until a CRLF sequence is seen, at which point a new paragraph is
          125    started.  Each paragraph is displayed, starting at the left margin
          126    (or paragraph indent), and continuing to the right until a word is
          127    encountered which does not fit in the remaining display width.  This
          128    word is displayed at the left margin of the next line.  This
          129    continues until the paragraph ends (a CRLF is seen).  Extra vertical
          130    space is left between paragraphs.
          131 
          132    Text which meets this description is defined by this memo as
          133    "flowed".
          134 
          135    Numerous software products erroneously label this media type as
          136    Text/Plain, resulting in much user discomfort.
          137 
          138 3.2.  Embarrassing Line Wrap
          139 
          140    As Text/Plain messages get quoted in replies or forwarded messages,
          141    the length of each line gradually increases, resulting in
          142    "embarrassing line wrap." This results in text which is at best hard
          143    to read, and often confuses attributions.
          144 
          145       Example:
          146 
          147             >>>>>>This is a comment from the first message to show a
          148             >quoting example.
          149             >>>>>This is a comment from the second message to show a
          150             >quoting example.
          151             >>>>This is a comment from the third message.
          152             >>>This is a comment from the fourth message.
          153 
          154    It can be confusing to assign attribution to lines 2 and 4 above.
          155 
          156    In addition, as devices with display widths smaller than 80
          157    characters become more popular, embarrassing line wrap has become
          158    even more prevalent, even with unquoted text.
          159 
          160 
          161 
          162 
          163 
          164 
          165 
          166 
          167 
          168 
          169 
          170 Gellens                     Standards Track                     [Page 3]
          171 
          172 RFC 2646            The Text/Plain Format Parameter          August 1999
          173 
          174 
          175    Example:
          176 
          177             This is paragraph text that is
          178             meant to be flowed across
          179             several lines.
          180             However, the sending mailer is
          181             converting it to fixed text at
          182             a width of 72
          183             characters, which causes it to
          184             look like this when shown on a
          185             PDA with only
          186             30 character lines.
          187 
          188 3.3.  New Media Types
          189 
          190    Attempts to deploy new media types, such as Text/Enriched [RICH] and
          191    Text/HTML [HTML] have suffered from a lack of backwards compatibility
          192    and an often hostile user reaction at the receiving end.
          193 
          194    In particular, Text/Enriched requires that open angle brackets ("<")
          195    and hard line breaks be doubled, with resulting user unhappiness when
          196    viewed as Text/Plain.  Text/HTML requires even more alteration of
          197    text, with a corresponding increase in user complaints.
          198 
          199    A proposal to define a new media type to explicitly represent the
          200    paragraph form suffered from a lack of interoperability with
          201    currently deployed software.  Some programs treat unknown subtypes of
          202    Text as an attachment.
          203 
          204    What is desired is a format which is in all significant ways
          205    Text/Plain, and therefore is quite suitable for display as
          206    Text/Plain, and yet allows the sender to express to the receiver
          207    which lines can be considered a logical paragraph, and thus flowed
          208    (wrapped and joined) as appropriate.
          209 
          210 4.  The Format Parameter to the Text/Plain Media Type
          211 
          212    This document defines a new MIME parameter for use with Text/Plain:
          213 
          214       Name:  Format
          215       Value:  Fixed, Flowed
          216 
          217    (Neither the parameter name nor its value are case sensitive.)
          218 
          219    If not specified, a value of Fixed is assumed.  The semantics of the
          220    Fixed value are the usual associated with Text/Plain [MIME-IMT].
          221 
          222 
          223 
          224 
          225 
          226 Gellens                     Standards Track                     [Page 4]
          227 
          228 RFC 2646            The Text/Plain Format Parameter          August 1999
          229 
          230 
          231    A value of Flowed indicates that the definition of flowed text (as
          232    specified in this memo) was used on generation, and MAY be used on
          233    reception.
          234 
          235    This section discusses flowed text; section 5 provides a formal
          236    definition.
          237 
          238    Because flowed lines are all-but-indistinguishable from fixed lines,
          239    currently deployed software treats flowed lines as normal Text/Plain
          240    (which is what they are).  Thus, no interoperability problems are
          241    expected.
          242 
          243    Note that this memo describes an on-the-wire format.  It does not
          244    address formats for local file storage.
          245 
          246 4.1.  Generating Format=Flowed
          247 
          248    When generating Format=Flowed text, lines SHOULD be shorter than 80
          249    characters.  As suggested values, any paragraph longer than 79
          250    characters in total length could be wrapped using lines of 72 or
          251    fewer characters.  While the specific line length used is a matter of
          252    aesthetics and preference, longer lines are more likely to require
          253    rewrapping and to encounter difficulties with older mailers.  It has
          254    been suggested that 66 character lines are the most readable.
          255 
          256    (The reason for the restriction to 79 or fewer characters between
          257    CRLFs on the wire is to ensure that all lines, even when displayed by
          258    a non-flowed-aware program, will fit in a standard 80-column screen
          259    without having to be wrapped.  The limit is 79, not 80, because while
          260    80 fit on a line, the last column is often reserved for a line-wrap
          261    indicator.)
          262 
          263    When creating flowed text, the generating agent wraps, that is,
          264    inserts 'soft' line breaks as needed.  Soft line breaks are added
          265    between words.  Because a soft line break is a SP CRLF sequence, the
          266    generating agent creates one by inserting a CRLF after the occurance
          267    of a space.
          268 
          269    A generating agent SHOULD NOT insert white space into a word (a
          270    sequence of printable characters not containing spaces).  If faced
          271    with a word which exceeds 79 characters (but less than 998
          272    characters, the [SMTP] limit on line length), the agent SHOULD send
          273    the word as is and exceed the 79-character limit on line length.
          274 
          275 
          276 
          277 
          278 
          279 
          280 
          281 
          282 Gellens                     Standards Track                     [Page 5]
          283 
          284 RFC 2646            The Text/Plain Format Parameter          August 1999
          285 
          286 
          287    A generating agent SHOULD:
          288 
          289       1.  Ensure all lines (fixed and flowed) are 79 characters or
          290           fewer in length, counting the trailing space but not
          291           counting the CRLF, unless a word by itself exceeds 79
          292           characters.
          293       2.  Trim spaces before user-inserted hard line breaks.
          294       3.  Space-stuff lines which start with a space, "From ", or
          295           ">".
          296 
          297    In order to create messages which do not require space-stuffing, and
          298    are thus more aesthetically pleasing when viewed as Format=Fixed, a
          299    generating agent MAY avoid wrapping immediately before ">", "From ",
          300    or space.
          301 
          302    (See sections 4.4 and 4.5 for more information on space-stuffing and
          303    quoting, respectively.)
          304 
          305    A Format=Flowed message consists of zero or more paragraphs, each
          306    containing one or more flowed lines followed by one fixed line.  The
          307    usual case is a series of flowed text lines with blank (empty) fixed
          308    lines between them.
          309 
          310    Any number of fixed lines can appear between paragraphs.
          311 
          312    [Quoted-Printable] encoding SHOULD NOT be used with Format=Flowed
          313    unless absolutely necessary (for example, non-US-ASCII (8-bit)
          314    characters over a strictly 7-bit transport such as unextended SMTP).
          315    In particular, a message SHOULD NOT be encoded in Quoted-Printable
          316    for the sole purpose of protecting the trailing space on flowed lines
          317    unless the body part is cryptographically signed or encrypted (see
          318    Section 4.6).
          319 
          320    The intent of Format=Flowed is to allow user agents to generate
          321    flowed text which is non-obnoxious when viewed as pure, raw
          322    Text/Plain (without any decoding); use of Quoted-Printable hinders
          323    this and may cause Format=Flowed to be rejected by end users.
          324 
          325 4.2.  Interpreting Format=Flowed
          326 
          327    If the first character of a line is a quote mark (">"), the line is
          328    considered to be quoted (see section 4.5).  Logically, all quote
          329    marks are counted and deleted, resulting in a line with a non-zero
          330    quote depth, and content. (The agent is of course free to display the
          331    content with quote marks or excerpt bars or anything else.)
          332    Logically, this test for quoted lines is done before any other tests
          333    (that is, before checking for space-stuffed and flowed).
          334 
          335 
          336 
          337 
          338 Gellens                     Standards Track                     [Page 6]
          339 
          340 RFC 2646            The Text/Plain Format Parameter          August 1999
          341 
          342 
          343    If the first character of a line is a space, the line has been
          344    space-stuffed (see section 4.4).  Logically, this leading space is
          345    deleted before examining the line further (that is, before checking
          346    for flowed).
          347 
          348    If the line ends in one or more spaces, the line is flowed.
          349    Otherwise it is fixed.  Trailing spaces are part of the line's
          350    content, but the CRLF of a soft line break is not.
          351 
          352    A series of one or more flowed lines followed by one fixed line is
          353    considered a paragraph, and MAY be flowed (wrapped and unwrapped) as
          354    appropriate on display and in the construction of new messages (see
          355    section 4.5).
          356 
          357    A line consisting of one or more spaces (after deleting a stuffed
          358    space) is considered a flowed line.
          359 
          360 4.3.  Usenet Signature Convention
          361 
          362    There is a convention in Usenet news of using "-- " as the separator
          363    line between the body and the signature of a message.  When
          364    generating a Format=Flowed message containing a Usenet-style
          365    separator before the signature, the separator line is sent as-is.
          366    This is a special case; an (optionally quoted) line consisting of
          367    DASH DASH SP is not considered flowed.
          368 
          369 4.4.  Space-Stuffing
          370 
          371    In order to allow for unquoted lines which start with ">", and to
          372    protect against systems which "From-munge" in-transit messages
          373    (modifying any line which starts with "From " to ">From "),
          374    Format=Flowed provides for space-stuffing.
          375 
          376    Space-stuffing adds a single space to the start of any line which
          377    needs protection when the message is generated.  On reception, if the
          378    first character of a line is a space, it is logically deleted.  This
          379    occurs after the test for a quoted line, and before the test for a
          380    flowed line.
          381 
          382    On generation, any unquoted lines which start with ">", and any lines
          383    which start with a space or "From " SHOULD be space-stuffed.  Other
          384    lines MAY be space-stuffed as desired.
          385 
          386    (Note that space-stuffing is similar to dot-stuffing as specified in
          387    [SMTP].)
          388 
          389 
          390 
          391 
          392 
          393 
          394 Gellens                     Standards Track                     [Page 7]
          395 
          396 RFC 2646            The Text/Plain Format Parameter          August 1999
          397 
          398 
          399    If a space-stuffed message is received by an agent which handles
          400    Format=Flowed, the space-stuffing is reversed and thus the message
          401    appears unchanged.  An agent which is not aware of Format=Flowed will
          402    of course not undo any space-stuffing, thus Format=Flowed messages
          403    may appear with a leading space on some lines (those which start with
          404    a space, ">" which is not a quote indicator, or "From ").  Since
          405    lines which require space-stuffing rarely occur, and the aesthetic
          406    consequences of unreversed space-stuffing are minimal, this is not
          407    expected to be a significant problem.
          408 
          409 4.5.  Quoting
          410 
          411    In Format=Flowed, the canonical quote indicator (or quote mark) is
          412    one or more close angle bracket (">") characters.  Lines which start
          413    with the quote indicator are considered quoted.  The number of ">"
          414    characters at the start of the line specifies the quote depth.
          415    Flowed lines which are also quoted may require special handling on
          416    display and when copied to new messages.
          417 
          418    When creating quoted flowed lines, each such line starts with the
          419    quote indicator.
          420 
          421    Note that because of space-stuffing, the lines
          422        >> Exit, Stage Left
          423    and
          424        >>Exit, Stage Left
          425    are semantically identical; both have a quote-depth of two, and a
          426    content of "Exit, Stage Left".
          427 
          428    However, the line
          429        > > Exit, Stage Left
          430    is different.  It has a quote-depth of one, and a content of
          431    "> Exit, Stage Left".
          432 
          433    When generating quoted flowed lines, an agent needs to pay attention
          434    to changes in quote depth.  A sequence of quoted lines of the same
          435    quote depth SHOULD be encoded as a paragraph, with the last line
          436    generated as fixed and prior lines generated as flowed.
          437 
          438    If a receiving agent wishes to reformat flowed quoted lines (joining
          439    and/or wrapping them) on display or when generating new messages, the
          440    lines SHOULD be de-quoted, reformatted, and then re-quoted.  To
          441    de-quote, the number of close angle brackets in the quote indicator
          442    at the start of each line is counted.  Consecutive lines with the
          443    same quoting depth are considered one paragraph and are reformatted
          444    together.  To re-quote after reformatting, a quote indicator
          445    containing the same number of close angle brackets originally present
          446    is prefixed to each line.
          447 
          448 
          449 
          450 Gellens                     Standards Track                     [Page 8]
          451 
          452 RFC 2646            The Text/Plain Format Parameter          August 1999
          453 
          454 
          455    On reception, if a change in quoting depth occurs on a flowed line,
          456    this is an improperly formatted message.  The receiver SHOULD handle
          457    this error by using the 'quote-depth-wins' rule, which is to ignore
          458    the flowed indicator and treat the line as fixed.  That is, the
          459    change in quote depth ends the paragraph.
          460 
          461    For example, consider the following sequence of lines (using '*' to
          462    indicate a soft line break, i.e., SP CRLF, and '#' to indicate a hard
          463    line break, i.e., CRLF):
          464 
          465       > Thou villainous ill-breeding spongy dizzy-eyed*
          466       > reeky elf-skinned pigeon-egg!*     <--- problem ---<
          467       >> Thou artless swag-bellied milk-livered*
          468       >> dismal-dreaming idle-headed scut!#
          469       >>> Thou errant folly-fallen spleeny reeling-ripe*
          470       >>> unmuzzled ratsbane!#
          471       >>>> Henceforth, the coding style is to be strictly*
          472       >>>> enforced, including the use of only upper case.#
          473       >>>>> I've noticed a lack of adherence to the coding*
          474       >>>>> styles, of late.#
          475       >>>>>> Any complaints?#
          476 
          477    The second line ends in a soft line break, even though it is the last
          478    line of the one-deep quote block.  The question then arises as to how
          479    this line should be interpreted, considering that the next line is
          480    the first line of the two-deep quote block.
          481 
          482    The example text above, when processed according to quote-depth wins,
          483    results in the first two lines being considered as one quoted, flowed
          484    section, with a quote depth of 1; the third and fourth lines become a
          485    quoted, flowed section, with a quote depth of 2.
          486 
          487    A generating agent SHOULD NOT create this situation; a receiving
          488    agent SHOULD handle it using quote-depth wins.
          489 
          490 4.6.  Digital Signatures and Encryption
          491 
          492    If a message is digitally signed or encrypted it is important that
          493    cryptographic processing use the on-the-wire Format=Flowed format.
          494    That is, during generation the message SHOULD be prepared for
          495    transmission, including addition of soft line breaks, space-stuffing,
          496    and [Quoted-Printable] encoding (to protect soft line breaks) before
          497    being digitally signed or encrypted; similarly, on receipt the
          498    message SHOULD have the signature verified or be decrypted before
          499    [Quoted-Printable] decoding and removal of stuffed spaces, soft line
          500    breaks and quote marks, and reflowing.
          501 
          502 
          503 
          504 
          505 
          506 Gellens                     Standards Track                     [Page 9]
          507 
          508 RFC 2646            The Text/Plain Format Parameter          August 1999
          509 
          510 
          511 4.7.  Line Analysis Table
          512 
          513    Lines contained in a Text/Plain body part with Format=Flowed can be
          514    analyzed by examining the start and end of the line.  If the line
          515    starts with the quote indicator, it is quoted.  If the line ends with
          516    one or more space characters, it is flowed.  This is summarized by
          517    the following table:
          518 
          519       Starts          Ends in
          520       with            One or             Line
          521       Quote           More Spaces        Type
          522       ------          -----------        ---------------
          523       no              no                 unquoted, fixed
          524       yes             no                 quoted,   fixed
          525       no              yes                unquoted, flowed
          526       yes             yes                quoted,   flowed
          527 
          528 4.8.  Examples
          529 
          530    The following example contains three paragraphs:
          531 
          532       `Take some more tea,' the March Hare said to Alice, very
          533       earnestly.
          534 
          535       `I've had nothing yet,' Alice replied in an offended tone, `so I
          536       can't take more.'
          537 
          538       `You mean you can't take LESS,' said the Hatter: `it's very easy
          539       to take MORE than nothing.'
          540 
          541    This could be encoded as follows (using '*' to indicate a soft line
          542    break, that is, SP CRLF sequence, and '#' to indicate a hard line
          543    break, that is, CRLF):
          544 
          545       `Take some more tea,' the March Hare said to Alice, very*
          546       earnestly.*
          547       #
          548       `I've had nothing yet,' Alice replied in an offended tone, `so*
          549       I can't take more.'*
          550       #
          551       `You mean you can't take LESS,' said the Hatter: `it's very*
          552       easy to take MORE than nothing.'#
          553 
          554 
          555 
          556 
          557 
          558 
          559 
          560 
          561 
          562 Gellens                     Standards Track                    [Page 10]
          563 
          564 RFC 2646            The Text/Plain Format Parameter          August 1999
          565 
          566 
          567    To show an example of quoting, here we have the same exchange,
          568    presented as a series of direct quotes:
          569 
          570                 >>>Take some more tea.#
          571                 >>I've had nothing yet, so I can't take more.#
          572                 >You mean you can't take LESS, it's very easy to take*
          573                 >MORE than nothing.#
          574 
          575 5.  ABNF
          576 
          577    The constructs used in Text/Plain; Format=Flowed body parts are
          578    described using [ABNF], including the Core Rules:
          579 
          580       paragraph     = 1*flowed-line fixed-line
          581       fixed-line    = fixed / sig-sep
          582       fixed         = [quote] [stuffing] *text-char non-sp CRLF
          583       flowed-line   = flow-qt / flow-unqt
          584       flow-qt       = quote [stuffing] *text-char 1*SP CRLF
          585       flow-unqt     = [stuffing] *text-char 1*SP CRLF
          586       non-sp        = %x01-09 / %x0B / %x0C / %x0E-1F / %x21-7F
          587                          ; any 7-bit US-ASCII character, excluding
          588                          ; NUL, CR, LF, and SP
          589       quote         = 1*">"
          590       sig-sep       = [quote] "--" SP CRLF
          591       stuffing      = [SP] ; space-stuffed, added on generation if
          592                            ; needed, deleted on reception
          593       text-char     = non-sp / SP
          594 
          595 6.  Failure Modes
          596 
          597 6.1.  Trailing White Space Corruption
          598 
          599    There are systems in existence which alter trailing whitespace on
          600    messages which pass through them.  Such systems may strip, or in
          601    rarer cases, add trailing whitespace, in violation of RFC 821 [SMTP]
          602    section 4.5.2.
          603 
          604    Stripping trailing whitespace has the effect of converting flowed
          605    lines to fixed lines, which results in a message no worse than if
          606    Format=Flowed had not been used.
          607 
          608    Adding trailing whitespace to a Format=Flowed message may result in a
          609    malformed display or reply.
          610 
          611    Since most systems which add trailing white space do so to create a
          612    line which fills an internal record format, the result is almost
          613    always a line which contains an even number of characters (counting
          614    the added trailing white space).
          615 
          616 
          617 
          618 Gellens                     Standards Track                    [Page 11]
          619 
          620 RFC 2646            The Text/Plain Format Parameter          August 1999
          621 
          622 
          623    One possible avoidance, therefore, would be to define Format=Flowed
          624    lines to use either one or two trailing space characters to indicate
          625    a flowed line, such that the total line length is odd.  However,
          626    considering the scarcity of such systems today, it is not worth the
          627    added complexity.
          628 
          629 7.  Security Considerations
          630 
          631    This parameter introduces no security considerations beyond those
          632    which apply to Text/Plain.
          633 
          634    Section 4.6 discusses the interaction between Format=Flowed and
          635    digital signatures or encryption.
          636 
          637 8.  IANA Considerations
          638 
          639    IANA is requested to add a reference to this specification in the
          640    Text/Plain Media Type registration.
          641 
          642 9.  Internationalization Considerations
          643 
          644    The line wrap and quoting specifications of Format=Flowed may not be
          645    suitable for certain charsets, such as for Arabic and Hebrew
          646    characters that read from right to left.  Care should be taken in
          647    applying format=flowed in these cases, as format=fixed combined with
          648    quoted-printable encoding may be more suitable.
          649 
          650 10.  Acknowledgments
          651 
          652    This proposal evolved from a discussion of Chris Newman's
          653    Text/Paragraph draft which took place on the IETF 822 mailing list.
          654    Special thanks to Ian Bell, Steve Dorner, Brian Kelley, Dan Kohn,
          655    Laurence Lundblade, and Dan Wing for their reviews, comments,
          656    suggestions, and discussions.
          657 
          658 11.  References
          659 
          660    [ABNF]             Crocker, D. and  P. Overell, "Augmented BNF for
          661                       Syntax Specifications: ABNF", RFC 2234, November
          662                       1997.
          663 
          664    [KEYWORDS]         S. Bradner, "Key words for use in RFCs to Indicate
          665                       Requirement Levels", BCP 14, RFC 2119, March 1997.
          666 
          667    [RICH]             Resnick, P. and A. Walker, "The text/enriched MIME
          668                       Content-type", RFC 1896, February 1996.
          669 
          670 
          671 
          672 
          673 
          674 Gellens                     Standards Track                    [Page 12]
          675 
          676 RFC 2646            The Text/Plain Format Parameter          August 1999
          677 
          678 
          679    [MIME-IMT]         Freed, N. and N. Borenstein, "Multipurpose
          680                       Internet Mail Extensions (MIME) Part Two: Media
          681                       Types", RFC 2046, November 1996.
          682 
          683    [Quoted-Printable] Freed, N. and N. Borenstein, "Multipurpose
          684                       Internet Mail Extensions (MIME) Part One: Format
          685                       of Internet Message Bodies", RFC 2045, November
          686                       1996.
          687 
          688    [SMTP]             Postel, J., "Simple Mail Transfer Protocol", STD
          689                       10, RFC 821,  August 1982.
          690 
          691    [HTML]             Berners-Lee, T. and D. Connolly, "Hypertext Markup
          692                       Language -- 2.0", RFC 1866, November 1995.
          693 
          694 
          695 12.  Editor's Address
          696 
          697    Randall Gellens
          698    QUALCOMM Incorporated
          699    5775 Morehouse Dr.
          700    San Diego, CA  92121-2779
          701    USA
          702 
          703    Phone: +1 619 651 5115
          704    EMail: randy@qualcomm.com
          705 
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          729 
          730 Gellens                     Standards Track                    [Page 13]
          731 
          732 RFC 2646            The Text/Plain Format Parameter          August 1999
          733 
          734 
          735 13.  Full Copyright Statement
          736 
          737    Copyright (C) The Internet Society (1999).  All Rights Reserved.
          738 
          739    This document and translations of it may be copied and furnished to
          740    others, and derivative works that comment on or otherwise explain it
          741    or assist in its implementation may be prepared, copied, published
          742    and distributed, in whole or in part, without restriction of any
          743    kind, provided that the above copyright notice and this paragraph are
          744    included on all such copies and derivative works.  However, this
          745    document itself may not be modified in any way, such as by removing
          746    the copyright notice or references to the Internet Society or other
          747    Internet organizations, except as needed for the purpose of
          748    developing Internet standards in which case the procedures for
          749    copyrights defined in the Internet Standards process must be
          750    followed, or as required to translate it into languages other than
          751    English.
          752 
          753    The limited permissions granted above are perpetual and will not be
          754    revoked by the Internet Society or its successors or assigns.
          755 
          756    This document and the information contained herein is provided on an
          757    "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
          758    TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
          759    BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
          760    HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
          761    MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
          762 
          763 Acknowledgement
          764 
          765    Funding for the RFC Editor function is currently provided by the
          766    Internet Society.
          767 
          768 
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          787