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Network Working Group                                 R. Gellens, EditorRequest for Comments: 2646                                      QualcommUpdates:2046                                                August 1999Category: Standards TrackThe Text/Plain Format ParameterStatus 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 (1999).  All Rights Reserved.Table of Contents1.  Abstract . . . . . . . . . . . . . . . . . . . . . . . . . .22.  Conventions Used in this Document . . . . . . . . . . . . .23.  The Problem  . . . . . . . . . . . . . . . . . . . . . . . .23.1.  Paragraph Text  . . . . . . . . . . . . . . . . . . . .33.2.  Embarrassing Line Wrap . . . . . . . . . . . . . . . . .33.3.  New Media Types . . . . . . . . . . . . . . . . . . . .44.  The Format Parameter to the Text/Plain Media Type  . . . . .44.1.  Generating Format=Flowed  . . . . . . . . . . . . . . .54.2.  Interpreting Format=Flowed . . . . . . . . . . . . . . .64.3.  Usenet Signature Convention . . . . . . . . . . . . . .74.4.  Space-Stuffing . . . . . . . . . . . . . . . . . . . . .74.5.  Quoting . . . . . . . . . . . . . . . . . . . . . . . .84.6.  Digital Signatures and Encryption  . . . . . . . . . . .94.7.  Line Analysis Table . . . . . . . . . . . . . . . . . .104.8.  Examples . . . . . . . . . . . . . . . . . . . . . . . .105.  ABNF  . . . . . . . . . . . . . . . . . . . . . . . . . . .116.  Failure Modes  . . . . . . . . . . . . . . . . . . . . . . .116.1.  Trailing White Space Corruption . . . . . . . . . . . .117.  Security Considerations  . . . . . . . . . . . . . . . . . .128.  IANA Considerations . . . . . . . . . . . . . . . . . . . .129.  Internationalization Considerations  . . . . . . . . . . . .1210.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . .1211.  References . . . . . . . . . . . . . . . . . . . . . . . . .1312.  Editor's Address  . . . . . . . . . . . . . . . . . . . . .1313.  Full Copyright Statement . . . . . . . . . . . . . . . . . .14Gellens                     Standards Track                     [Page 1]

RFC 2646            The Text/Plain Format Parameter          August 19991.  Abstract   Interoperability problems have been observed with erroneous labelling   of paragraph text as Text/Plain, and with various forms of   "embarrassing line wrap." (Seesection 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 can be considered a logical paragraph, and thus flowed   (wrapped and joined) as appropriate.   This memo proposes a new parameter to be used with Text/Plain, and,   in the presence of this parameter, the use of trailing whitespace to   indicate flowed lines.  This results in an encoding which appears as   normal Text/Plain in older implementations, since it is in fact   normal Text/Plain.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].3.  The Problem   The Text/Plain media type is the lowest common denominator of   Internet email, with lines of no more than 997 characters (by   convention usually no more than 80), and where the CRLF sequence   represents a line break [MIME-IMT].   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 media   type:Gellens                     Standards Track                     [Page 2]

RFC 2646            The Text/Plain Format Parameter          August 19993.1.  Paragraph Text   Many modern programs use a proportional-spaced font and 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 media type as   Text/Plain, resulting in much user discomfort.3.2.  Embarrassing Line Wrap   As Text/Plain messages get quoted in replies or forwarded messages,   the length of each line gradually increases, resulting in   "embarrassing line wrap." This results in text which is at best hard   to read, and often confuses attributions.      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 80   characters become more popular, embarrassing line wrap has become   even more prevalent, even with unquoted text.Gellens                     Standards Track                     [Page 3]

RFC 2646            The Text/Plain Format Parameter          August 1999   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.   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 Parameter to the Text/Plain Media Type   This document defines a new MIME parameter for use with Text/Plain:      Name:  Format      Value:  Fixed, Flowed   (Neither the parameter name nor its value are case sensitive.)   If not specified, a value of Fixed is assumed.  The semantics of the   Fixed value are the usual associated with Text/Plain [MIME-IMT].Gellens                     Standards Track                     [Page 4]

RFC 2646            The Text/Plain Format Parameter          August 1999   A 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.   This section discusses flowed text;section 5 provides a formal   definition.   Because flowed lines are all-but-indistinguishable from fixed lines,   currently deployed software treats flowed lines as normal Text/Plain   (which is what they are).  Thus, no interoperability problems are   expected.   Note that this memo describes an on-the-wire format.  It does not   address formats for local file storage.4.1.  Generating Format=Flowed   When generating Format=Flowed text, lines SHOULD be shorter than 80   characters.  As suggested values, any paragraph longer than 79   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 reason for the restriction to 79 or fewer characters between   CRLFs on the wire is to ensure that all lines, even when displayed by   a non-flowed-aware program, will fit in a standard 80-column screen   without having to be wrapped.  The limit is 79, not 80, because while   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   between words.  Because a soft line break is a SP CRLF sequence, the   generating agent creates one by inserting a CRLF after the occurance   of a space.   A generating agent SHOULD NOT insert white space into a word (a   sequence of printable characters not containing spaces).  If faced   with a word which exceeds 79 characters (but less than 998   characters, the [SMTP] limit on line length), the agent SHOULD send   the word as is and exceed the 79-character limit on line length.Gellens                     Standards Track                     [Page 5]

RFC 2646            The Text/Plain Format Parameter          August 1999   A generating agent SHOULD:      1.  Ensure all lines (fixed and flowed) are 79 characters or          fewer in length, counting the trailing space but not          counting the CRLF, unless a word by itself exceeds 79          characters.      2.  Trim spaces before user-inserted hard line breaks.      3.  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 sections4.4 and4.5 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.   [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 (seeSection 4.6).   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.4.2.  Interpreting Format=Flowed   If the first character of a line is a quote mark (">"), the line is   considered to be quoted (seesection 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).Gellens                     Standards Track                     [Page 6]

RFC 2646            The Text/Plain Format Parameter          August 1999   If the first character of a line is a space, the line has been   space-stuffed (seesection 4.4).  Logically, this leading space is   deleted before examining the line further (that is, before checking   for flowed).   If the line ends in one or more spaces, the line is flowed.   Otherwise it is fixed.  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 (seesection 4.5).   A line consisting of one or more spaces (after deleting a stuffed   space) is considered a flowed line.4.3.  Usenet Signature Convention   There is a convention in Usenet news 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) line consisting of   DASH DASH SP is not considered flowed.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, 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 " SHOULD be space-stuffed.  Other   lines MAY be space-stuffed as desired.   (Note that space-stuffing is similar to dot-stuffing as specified in   [SMTP].)Gellens                     Standards Track                     [Page 7]

RFC 2646            The Text/Plain Format Parameter          August 1999   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.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 ">"   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.  A sequence of quoted lines of the same   quote depth SHOULD be encoded as a paragraph, with the last line   generated as fixed and prior lines generated as flowed.   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.  Consecutive lines with the   same quoting depth are considered one paragraph and are reformatted   together.  To re-quote after reformatting, a quote indicator   containing the same number of close angle brackets originally present   is prefixed to each line.Gellens                     Standards Track                     [Page 8]

RFC 2646            The Text/Plain Format Parameter          August 1999   On reception, if a change in quoting 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 ignore   the flowed indicator and treat the line as fixed.  That is, the   change in quote depth ends the 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):      > 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 should 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 SHOULD NOT create this situation; a receiving   agent SHOULD handle it using quote-depth wins.4.6.  Digital Signatures and Encryption   If a message is digitally signed or encrypted it is important that   cryptographic processing use the on-the-wire Format=Flowed format.   That is, during generation the message SHOULD be prepared for   transmission, including addition of soft line breaks, space-stuffing,   and [Quoted-Printable] encoding (to protect soft line breaks) before   being digitally signed or encrypted; similarly, on receipt the   message SHOULD have the signature verified or be decrypted before   [Quoted-Printable] decoding and removal of stuffed spaces, soft line   breaks and quote marks, and reflowing.Gellens                     Standards Track                     [Page 9]

RFC 2646            The Text/Plain Format Parameter          August 19994.7.  Line Analysis Table   Lines contained in a Text/Plain body part with Format=Flowed can be   analyzed by examining the start and end of the line.  If the line   starts with the quote indicator, it is quoted.  If the line ends with   one or more space characters, it is flowed.  This is summarized by   the following table:      Starts          Ends in      with            One or             Line      Quote           More Spaces        Type      ------          -----------        ---------------      no              no                 unquoted, fixed      yes             no                 quoted,   fixed      no              yes                unquoted, flowed      yes             yes                quoted,   flowed4.8.  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.'#Gellens                     Standards Track                    [Page 10]

RFC 2646            The Text/Plain Format Parameter          August 1999   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.  ABNF   The constructs used in Text/Plain; Format=Flowed body parts are   described using [ABNF], including the Core Rules:      paragraph     = 1*flowed-line fixed-line      fixed-line    = fixed / sig-sep      fixed         = [quote] [stuffing] *text-char non-sp CRLF      flowed-line   = flow-qt / flow-unqt      flow-qt       = quote [stuffing] *text-char 1*SP CRLF      flow-unqt     = [stuffing] *text-char 1*SP CRLF      non-sp        = %x01-09 / %x0B / %x0C / %x0E-1F / %x21-7F                         ; any 7-bit US-ASCII character, excluding                         ; NUL, CR, LF, and SP      quote         = 1*">"      sig-sep       = [quote] "--" SP CRLF      stuffing      = [SP] ; space-stuffed, added on generation if                           ; needed, deleted on reception      text-char     = non-sp / SP6.  Failure Modes6.1.  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 ofRFC 821 [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.   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).Gellens                     Standards Track                    [Page 11]

RFC 2646            The Text/Plain Format Parameter          August 1999   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.7.  Security Considerations   This parameter introduces no security considerations beyond those   which apply to Text/Plain.Section 4.6 discusses the interaction between Format=Flowed and   digital signatures or encryption.8.  IANA Considerations   IANA is requested to add a reference to this specification in the   Text/Plain Media Type registration.9.  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 should be taken in   applying format=flowed in these cases, as format=fixed combined with   quoted-printable encoding may be more suitable.10.  Acknowledgments   This proposal evolved from a discussion of Chris Newman's   Text/Paragraph draft which took place on the IETF 822 mailing list.   Special thanks to Ian Bell, Steve Dorner, Brian Kelley, Dan Kohn,   Laurence Lundblade, and Dan Wing for their reviews, comments,   suggestions, and discussions.11.  References   [ABNF]             Crocker, D. and  P. Overell, "Augmented BNF for                      Syntax Specifications: ABNF",RFC 2234, November                      1997.   [KEYWORDS]         S. Bradner, "Key words for use in RFCs to Indicate                      Requirement Levels",BCP 14,RFC 2119, March 1997.   [RICH]             Resnick, P. and A. Walker, "The text/enriched MIME                      Content-type",RFC 1896, February 1996.Gellens                     Standards Track                    [Page 12]

RFC 2646            The Text/Plain Format Parameter          August 1999   [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.   [SMTP]             Postel, J., "Simple Mail Transfer Protocol", STD                      10,RFC 821,  August 1982.   [HTML]             Berners-Lee, T. and D. Connolly, "Hypertext Markup                      Language -- 2.0",RFC 1866, November 1995.12.  Editor's Address   Randall Gellens   QUALCOMM Incorporated   5775 Morehouse Dr.   San Diego, CA  92121-2779   USA   Phone: +1 619 651 5115   EMail: randy@qualcomm.comGellens                     Standards Track                    [Page 13]

RFC 2646            The Text/Plain Format Parameter          August 199913.  Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Gellens                     Standards Track                    [Page 14]