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Modern Network Unicode
draft-bormann-dispatch-modern-network-unicode-03

Document Type Active Internet-Draft (individual)
Author Carsten Bormann
Last updated 2023-08-31
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draft-bormann-dispatch-modern-network-unicode-03
DISPATCH Working Group                                        C. Bormann
Internet-Draft                                    Universität Bremen TZI
Intended status: Standards Track                          31 August 2023
Expires: 3 March 2024

                         Modern Network Unicode
            draft-bormann-dispatch-modern-network-unicode-03

Abstract

   BCP18 (RFC 2277) has been the basis for the handling of character-
   shaped data in IETF specifications for more than a quarter of a
   century now.  It singles out UTF-8 (STD63, RFC 3629) as the "charset"
   that MUST be supported, and pulls in the Unicode standard with that.

   Based on this, RFC 5198 both defines common conventions for the use
   of Unicode in network protocols and caters for the specific
   requirements of the legacy protocol Telnet.  In applications that do
   not need Telnet compatibility, some of the decisions of RFC 5198 can
   be cumbersome.

   The present specification defines "Modern Network Unicode" (MNU),
   which is a form of RFC 5198 Network Unicode that can be used in
   specifications that require the exchange of plain text over networks
   and where just mandating UTF-8 may not be sufficient, but there is
   also no desire to import all of the baggage of RFC 5198.

   As characters are used in different environments, MNU is defined in a
   one-dimensional (1D) variant that is useful for identifiers and
   labels, but does not use a structure of lines.  A 2D variant is
   defined for text that is a sequence of text lines, such as plain text
   documents or markdown format.  Additional variances of these two base
   formats can be used to tailor MNU to specific areas of application.

Status

   The present version of this document represents the author's reaction
   to initial exposure on the art@ietf.org mailing list.  Some more
   editorial cleanup is probably desirable, but could not be achieved in
   time for the IETF105 Internet-Draft deadline.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 3 March 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  1D Modern Network Unicode . . . . . . . . . . . . . . . . . .   4
   3.  2D Modern Network Unicode . . . . . . . . . . . . . . . . . .   5
   4.  3D? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Variances . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  With CR-tolerant lines  . . . . . . . . . . . . . . . . .   6
     5.2.  With CR-LF  . . . . . . . . . . . . . . . . . . . . . . .   6
     5.3.  With Line or Paragraph Separators . . . . . . . . . . . .   6
     5.4.  With HT Characters  . . . . . . . . . . . . . . . . . . .   6
     5.5.  With CCC Characters . . . . . . . . . . . . . . . . . . .   6
     5.6.  With NFC  . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.7.  With NFKC . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.8.  With Unicode Version NNN  . . . . . . . . . . . . . . . .   7
     5.9.  With BOM  . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Using ABNF with Unicode . . . . . . . . . . . . . . . . . . .   7
   7.  IANA considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security considerations . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8

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     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Terminology  . . . . . . . . . . . . . . . . . . . .  10
   Appendix B.  History, Legacy  . . . . . . . . . . . . . . . . . .  12
   Appendix C.  Normalization  . . . . . . . . . . . . . . . . . . .  13
   Appendix D.  Relationship to RFC 5198 . . . . . . . . . . . . . .  13
     D.1.  Going beyond RFC 5198 . . . . . . . . . . . . . . . . . .  14
     D.2.  Byte order marks  . . . . . . . . . . . . . . . . . . . .  15
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   (Insert embellished copy of abstract here.)

   Complex specifications that use Unicode often come with detailed
   information on their Unicode usage; this level of detail generally is
   necessary to support some legacy applications.  New, simple protocol
   specifications generally do not have such a legacy or need such
   details, but can instead simply use common practice, informed by
   decades of using Unicode.  The present specification attempts to
   serve as a convenient reference for such protocol specifications,
   reducing their need for discussing Unicode to just pointing to the
   present specification and making a few simple choices.

   There is no intention that henceforth all new protocols "must" use
   the present specification.  It is offered as a standards-track
   specification simply so it can be normatively referenced from other
   standards-track specifications.

1.1.  Conventions

   Characters in this specification are named with their Unicode name
   notated in the usual form U+NNNN (where NNNN is a four-to-six-digit
   upper case hexadecimal number giving the Unicode scalar value), or
   with their ASCII names (such as CR, LF, HT, RS, NUL) [STD80].

      |  See [RFC5137] for a more detailed discussion of ways to refer
      |  in ASCII to Unicode characters (as well as to code points and
      |  code units in various transformation formats).

   The following definitions apply:

   Byte:  8-bit unit of information interchange, synonym for octet.

   Please see Appendix A for some more detailed term definitions that
   may be helpful to relate this specification with the Unicode
   Standard; please do read its first paragraph if the definitions in
   this section seem inadequate.

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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  1D Modern Network Unicode

   One-dimensional Modern Network Unicode (1D MNU) is the form of Modern
   Network Unicode that can be used for one-dimensional text, i.e., text
   without a structure of separate text lines.  It requires conformance
   to [STD63], as well as to the following four mandates:

   1.  Control codes (U+0000 to U+001F and U+007F to U+009F) MUST NOT be
       used.  (Note that this also excludes line endings, so a 1D MNU
       text string cannot extend beyond a single line.  See Section 3
       below if line structure is needed.)

   2.  The characters U+2028 and U+2029 MUST NOT be used.  (In case
       future Unicode versions add to the Unicode character categories
       Zl or Zp, any characters in these categories MUST NOT be used.)

   3.  Modern Network Unicode strongly RECOMMENDS that, except in
       unusual circumstances (see Appendix C), all text is transmitted
       in normalization form NFC.  This mandate should not be realized
       by routinely normalizing all text, but by encouraging text
       sources to use NFC if applicable.

   4.  As per the Unicode specification, the code points U+FFFE and
       U+FFFF MUST NOT be used.  Also, Byte Order Marks (leading U+FEFF
       characters) MUST NOT be used.

   Note that several control codes have been in use historically in
   ASCII documents even within a single text line.  For instance BS
   (U+0008) and CR (U+000D) have been used as a crude way to combine (by
   overtyping) characters; the present specification does not explicitly
   support this behavior anymore.  HT (U+0009, "TAB") has been used for
   a form of compressing stretches of blank space; by keeping its actual
   definition open it has also been used to enable users of text to
   specify variable blank space ("indentation").  The present
   specification RECOMMENDS not using a variance for 1D MNU to enable
   using BS, HT, or CR; legacy usage of HT may be more appropriate in
   certain forms of 2D text.

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3.  2D Modern Network Unicode

   Two-dimensional Modern Network Unicode (2D MNU) enhances 1D MNU by
   structuring the text into lines. 2D MNU does this is by using the
   control code LF (U+000A) for a line terminator.  The use of an
   unterminated last line is permitted, but not encouraged.  (This is
   not a variance.)

   Historically, the Internet NVT [RFC0854] and thus, much later,
   Network Unicode [RFC5198] have used the combination CR+LF as a line
   terminator, reflecting its heritage based on teletype functionality.
   The use of U+000D mostly introduces additional ways to create
   interoperability problems.  [XML] goes to great lengths to reduce the
   harm the presence of CR characters does to interchange of XML
   documents.  Pages 10 and 11 of [RFC0764] discuss how CR can actually
   only be used in the combinations CR+NUL and CR+LF. 2D MNU simply
   elides the CR.

   In other words, 2D MNU adds the use of line endings, represented by a
   single LF character (which is then the only control character
   allowed).  Variances are available for (1) allowing or even (2)
   requiring the use of CR before LF.

4.  3D?

   Three-dimensional forms of text have been used, e.g., by using U+000C
   FF ("form feed") to add a page structure to the RFC format [RFC2223],
   or by using U+001E RS ("record separator") to separate several (2D)
   JSON texts in a JSON sequence [RFC7464].

   As the need for this form of structure is now mostly being addressed
   by building data structures around text items, the present
   specification does not define a 3D MNU.  If needed, variances such as
   the use of FF or RS can be described in the documents specifying the
   3D format.

5.  Variances

   In addition to the basic 1D and 2D versions of MNU, this
   specification describes a number of variances that can be used in the
   forms such as "2D Modern Network Unicode with VVV", or "2D Modern
   Network Unicode with VVV, WWW, and ZZZ" for multiple variances used.
   Specifications that cannot directly use the basic MNU forms may be
   able to use MNU with one or more of these variances added.

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   An example for the usage of variances: Up to RFC 8649, RFCs were
   formatted in "2D Modern Network Unicode with CR-tolerant lines and FF
   characters", or exceptionally [RFC8265], "2D Modern Network Unicode
   with CR-tolerant lines and FF characters and BOM".

5.1.  With CR-tolerant lines

   The variance "with CR-tolerant lines" allows the sequence CR LF as
   well as a single LF character as a line ending.  This may enable
   existing texts to be used as MNU without processing at the sender
   side (substituting that by processing at the receiver side).  Note
   that, with this variance, a CR character cannot be used anywhere else
   but immediately preceding an LF character.

5.2.  With CR-LF

   The variance "with CR-LF lines" requires the sequence CR LF as a line
   ending; a single LF character is not allowed.  This is appropriate
   for certain existing environments that have already made that
   determination.

5.3.  With Line or Paragraph Separators

   For 2D applications, and even for 1D applications that need to
   address text in 2D forms, it may be useful to enable the use of
   U+2028 and U+2029; this should be explicitly called out.

5.4.  With HT Characters

   In some cases, the use of HT characters ("TABs") cannot be completely
   excluded.  The variance "with HT characters" allows their use,
   without attempting to define their meaning (e.g., equivalence with
   spaces, column definitions, etc.).

5.5.  With CCC Characters

   Some applications of MNU may need to add specific control characters,
   such as RS [RFC7464] or FF characters [RFC2223].  This variance is
   spelled with the ASCII name of the control character for CCC, e.g.,
   "with RS characters".

5.6.  With NFC

   This variance turns the encouragement of using NFC normalization form
   into a requirement.  Please see Appendix C for why this should be an
   exceptional case.

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5.7.  With NFKC

   Some applications require a stronger form of normalization than NFC.
   The variance "with NFKC" swaps out NFC and uses NFKC instead.  This
   is probably best used in conjunction with "with Unicode version NNN".

5.8.  With Unicode Version NNN

   Some applications need to be sure that a certain Unicode version is
   used.  The variance "with Unicode version NNN" (where NNN is a
   Unicode version number) defines the Unicode version in use as NNN.
   Also, it requires that only characters assigned in that Unicode
   version are being used.

   See Section 4 of [RFC5198] for more discussion of Unicode versions.

5.9.  With BOM

   In some environments, UTF-8 files need to be distinguished from files
   in other formats, and one convention that has been used in certain
   environments was to prepend a "Byte Order Mark" (U+FEFF) as a
   distinguishing mark (as UTF-8 is not influenced by different byte
   orders, this mark does not serve a purpose in UTF-8).  Sometimes
   these BOMs are included for interchange to ensure local copies of the
   file include the distinguishing mark.  This variance enables this
   usage.

6.  Using ABNF with Unicode

   Internet STD 68, [STD68], defines Augmented BNF for Syntax
   Specifications: ABNF.  Since the late 1970s, ABNF has often been used
   to formally describe the pieces of text that are meant to be used in
   an Internet protocol.  ABNF was developed at a time when character
   coding grew more and more complicated, and even in its current form,
   discusses encoding of characters only briefly (Section 2.4 of
   [STD68]).  This discussion offers no information about how this
   should be used today (it actually still refers to 16-bit Unicode!).

   The best current practice of using ABNF for Unicode-based protocols
   is as follows: ABNF is used as a grammar for describing sequences of
   Unicode code points, valued from 0x0 to 0x10FFFF.  The actual
   encoding (as UTF-8) is never seen on the ABNF level; see Section 9.4
   of [RFC6020] for a recent example of this.  Approaches such as
   representing the rules of UTF-8 encoding in ABNF (see Section 3.5 of
   [RFC5255] for an example) add complexity without benefit and are NOT
   RECOMMENDED.

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   ABNF features such as case-insensitivity in literal text strings
   essentially do not work for general Unicode; text string literals
   therefore (and by the definition in Section 2.3 of [STD68]) are
   limited to ASCII characters.  That is often not actually a problem in
   text-based protocol definitions.  Still, characters beyond ASCII need
   to be allowed in many productions.  ABNF does not have access to
   Unicode character categories and thus will be limited in its
   expressiveness here.  The core rules defines in Appendix B of [STD68]
   are limited to ASCII as well; new rules will therefore need to be
   defined in any protocol employing modern Unicode.

   The present specification recommends defining ABNF rules as in these
   examples:

   ; 1D modern unicode character:
   uchar = %x20-7E ; exclude C0
         / %xA0-2027 ; exclude DEL, C1
         / %x202A-D7FF ; exclude U+2028/2029
         / %xE000-FFFD ; exclude U+FFFE/FFFF
         / %x10000-10FFFD ; could exclude more non-characters

   ; 2D modern unicode with lines -- newline:
   unl = %x0A
   u2dchar = unl / uchar
   ; alternatively, CR-tolerant newline:
   ucrnl = [%x0D] %x0A
   u2dcrchar = ucrnl / uchar

   ; if really needed, HT-tolerant 1D unicode character:
   utchar = %x09 / uchar

7.  IANA considerations

   This specification places no requirements on IANA.

8.  Security considerations

   The security considerations of [RFC5198] apply.

   A variance "with NUL characters" would create specific security
   considerations as discussed in the security considerations of
   [RFC5198] and should therefore only be used in circumstances that
   absolutely do require it.

9.  References

9.1.  Normative References

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   [BCP18]    Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, DOI 10.17487/RFC2277,
              January 1998, <https://www.rfc-editor.org/rfc/rfc2277>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [STD63]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/rfc/rfc3629>.

   [STD68]    Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/rfc/rfc5234>.

   [STD80]    Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <https://www.rfc-editor.org/rfc/rfc20>.

9.2.  Informative References

   [RFC0764]  Postel, J., "Telnet Protocol specification", RFC 764,
              DOI 10.17487/RFC0764, June 1980,
              <https://www.rfc-editor.org/rfc/rfc764>.

   [RFC0854]  Postel, J. and J. Reynolds, "Telnet Protocol
              Specification", STD 8, RFC 854, DOI 10.17487/RFC0854, May
              1983, <https://www.rfc-editor.org/rfc/rfc854>.

   [RFC2223]  Postel, J. and J. Reynolds, "Instructions to RFC Authors",
              RFC 2223, DOI 10.17487/RFC2223, October 1997,
              <https://www.rfc-editor.org/rfc/rfc2223>.

   [RFC5137]  Klensin, J., "ASCII Escaping of Unicode Characters",
              BCP 137, RFC 5137, DOI 10.17487/RFC5137, February 2008,
              <https://www.rfc-editor.org/rfc/rfc5137>.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
              <https://www.rfc-editor.org/rfc/rfc5198>.

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   [RFC5255]  Newman, C., Gulbrandsen, A., and A. Melnikov, "Internet
              Message Access Protocol Internationalization", RFC 5255,
              DOI 10.17487/RFC5255, June 2008,
              <https://www.rfc-editor.org/rfc/rfc5255>.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/rfc/rfc6020>.

   [RFC7464]  Williams, N., "JavaScript Object Notation (JSON) Text
              Sequences", RFC 7464, DOI 10.17487/RFC7464, February 2015,
              <https://www.rfc-editor.org/rfc/rfc7464>.

   [RFC8265]  Saint-Andre, P. and A. Melnikov, "Preparation,
              Enforcement, and Comparison of Internationalized Strings
              Representing Usernames and Passwords", RFC 8265,
              DOI 10.17487/RFC8265, October 2017,
              <https://www.rfc-editor.org/rfc/rfc8265>.

   [UNICODE]  The Unicode Consortium, "The Unicode® Standard: Version
              15.0 — Core Specification", September 2022,
              <https://www.unicode.org/versions/Unicode15.0.0/
              UnicodeStandard-15.0.pdf>.  For convenience, this
              bibliography entry points to what was the most recent
              version of Unicode at the time of writing.  It is,
              however, intended to be a generic reference to the most
              recent version of Unicode, which always can be found at
              http://www.unicode.org/versions/ latest/
              (http://www.unicode.org/versions/latest/).

   [XML]      Bray, T., Paoli, J., Sperberg-McQueen, C.M., Maler, E.,
              and F. Yergeau, "Extensible Markup Language (XML) 1.0
              (Fifth Edition)", November 2008,
              <http://www.w3.org/TR/2008/REC-xml-20081126/>.

Appendix A.  Terminology

   In order for the main body of this specification to stay readable for
   its target audience, we banish what could be considered excessive
   detail into appendices.  Expert readers who know a lot more about
   Coded Character Sets and Unicode than the target audience is likely
   interested in are requested to hold back the urge to request re-
   introduction of excessive detail into the main body.  For instance,
   this specification has a local definition of "Unicode character" that
   is useful for protocol designers, but is not actually defined by the
   Unicode consortium; the details are here.

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   Some definitions below reference definitions given in the Unicode
   standard and are simplified from those, which see if the full detail
   is needed.

   Grapheme:  The smallest functional unit of a writing system.  For
      computer representation, graphemes are represented by
      _characters_, sometimes a single character, sometimes decomposed
      into multiple characters.  In visible presentation, graphemes are
      made visible as _glyphs_; for computer use, glyphs are collected
      into _typefaces_ (fonts).  The term grapheme is used to abstract
      from the specific glyph used — a specific glyph from a typeface
      may be considered one of several _allographs_ for a grapheme.

   Character:  Only defined as "abstract character" in [UNICODE]: A unit
      of information used for the organization, control, or
      representation of textual data (definition D7).  Most characters
      are used to stand for graphemes or to build graphemes out of
      several characters.  Characters are usually used in ordered
      sequences, which are referred to as "character strings".

   Character set, Coded Character Set:  A mapping from code points to
      characters.  Note that the shorter form term "character set" is
      easily misunderstood for a set of characters, which are called
      "character repertoire"

   Character repertoire:  A set of characters, in the sense of what is
      available as characters to choose from.

   Code point:  The (usually numeric) value used in a coded character
      set to refer to a character.  Note that code points may undergo
      some encoding before interchange, see Unicode transformation
      format below.  Code points can also be allocated to refer to
      internal constructs of a Unicode transformation format instead of
      referring to characters.

      Unicode uses unsigned numbers between 0 and 1114111 (0x10FFFF) as
      code points.

   Unicode transformation format (UTF):  Character (and character
      strings) usually are interchanged as a sequence of bytes.  A
      Unicode Transformation Format or Unicode Encoding Scheme specifies
      a byte serialization for a Unicode encoding form.  (See definition
      D94 in Section 3.10, Unicode Encoding Schemes.)

   Code unit:  A bit combination used for encoding text for processing

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      or interchange.  The Unicode Standard uses 8-bit code units in the
      UTF-8 encoding form, 16-bit code units in the UTF-16 encoding
      form, and 32-bit code units in the UTF-32 encoding form.  (See
      Definition D77 in Section 3.9, Unicode Encoding Forms.)

   Unicode scalar value:  Unicode code points except high-surrogate and
      low-surrogate code points (see Appendix B).  In other words, the
      ranges of integers 0 to 0xD7FF and 0xE000 to 0x10FFFF16 inclusive.
      (See Definition D76 in Section 3.9, Unicode Encoding Forms.)

   Unicode Character:  For the purposes of the present specification, we
      use "Unicode Character" as a synonym for "Unicode scalar value".
      Note that this term includes Unicode scalar values that are not
      Assigned Characters; for the purposes of a specification using the
      present document, it is rarely useful to make this distinction, as
      Unicode evolves and could assign characters not assigned.

Appendix B.  History, Legacy

   Some of the complexity of using Unicode is, unsurprisingly, rooted in
   its long history of development.

   Unicode originally was introduced around 1988 as a 16-bit character
   standard.  By the early 1990s, a specification had been published,
   and a number of environments eagerly picked up Unicode.  Unicode
   characters were represented as 16-bit values (UCS-2), enabling a
   simple character model using these uniformly 16-bit values as the
   characters.  Programming language environments such as those of Java,
   JavaScript and C# are now intimately entangled with this character
   model.

   In the mid-1990s, it became clear that 16 bits would not suffice.
   Unicode underwent an extension to 31 bits, and then back to ~ 21 bits
   (0 to 0x10FFFF).  For the 16-bit environments, this was realized by
   switching to UTF-16, a "Unicode transformation format" (UTF-16).
   This reassigned some of the 16-bit code points not for characters,
   but for 2 sets of 1024 "surrogates" that would be used in pairs to
   represent characters that do not fit into 16 bits, each supplying 10
   bits to together add 2^20 code points to the 2^16 already available
   (this leads to the surprising upper limit of 0x10FFFF).

   In parallel, UTF-8 was created as an ASCII-compatible form of Unicode
   representation that would become the dominant form of text in the Web
   and other interchange environments.

   The UCS-2 based character models of the legacy 16-bit platforms in
   many cases couldn't be updated for fully embracing UTF-16 right away.
   For instance, only much later did ECMAScript introduce the "u"

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   (Unicode) flag for regular expressions to have them actually match
   "Unicode" characters.  So, on these platforms, UTF-16 is handled in a
   UCS-2 character model, and sometimes orphaned surrogates leak out
   instead of Unicode characters as "code points" in interfaces that are
   not meant to impose these implementation limitations on the outside
   world.

   UTF-8 is unaffected by this UTF-16 quirk and of course doesn't
   support encoding surrogates.  (UTF-8 is careful to allow a single
   representation only for each Unicode character, and enabling the
   alternative use of surrogate pairs would violate that invariant,
   while isolated surrogates don't mean anything in Unicode.)

   Newly designed IETF protocols typically do not have to consider these
   problems, but occasionally there are attempts to include isolated
   surrogate code points into what we call Unicode characters here.

Appendix C.  Normalization

   Please see Section 3 of [RFC5198] for a brief introduction to Unicode
   normalization and Normalization Form C (NFC).  However, since that
   section was written, additional experience with normalization has led
   to the realization that the Unicode normalization rules do not always
   preserve certain details in certain writing systems.

   Therefore, the implementation approach of routinely normalizing all
   text before interchange has fallen out of favor.  Instead,
   implementations are encouraged to use text sources that already
   generally use NFC except where normalization would have been harmful.
   Where two text strings needs to be compared, it may be appropriate to
   employ normalization to both text strings for the purposes of the
   comparison only.

   Additional complications can come from the fact that some
   implementations of applications may rely on operating system
   libraries over which they have little control.  The need to maintain
   interoperability in such environments suggests that receivers of MNU
   should be prepared to receive unnormalized text and should not react
   to that in excessive ways; however, there also is no expectation for
   receivers to go out of their way doing so.

Appendix D.  Relationship to RFC 5198

   Of the mandates listed in Section 2, the third and fourth requirement
   are also posed by [RFC5198], while the first two remove further
   legacy compatibility considerations.

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   [RFC5198] contains some discussion and background material that the
   present document does not attempt to repeat; the interested reader
   may therefore want to consult it as an informative reference.

   Mandates of [RFC5198] that are specific to a version of Unicode are
   not picked up in this specification, e.g., there is no check for
   unassigned code points.  Some implementations may want to add such a
   check; however, in general, this can hinder further evolution as it
   may become hard to use new characters as long as not every component
   on the way has been upgraded. (see also Section 5.8.)

D.1.  Going beyond RFC 5198

   The handling of line endings (with 2D MNU prescribing LF as the line
   terminator, and adding or specifying CRLF line endings as variances)
   may be controversial.  In particular, calling out CR-tolerance as an
   extra (and often undesirable) feature may seem novel to some readers.
   The handling as specified here is much closer to the way line endings
   are handled on the software side than the cumbersome rules of
   [RFC5198].  More generally speaking, one could say that the present
   specification is intended to be used by state of the art protocols
   going forward, maybe less so by existing protocols.

   Even in the "with CR-tolerant lines" variance, the CR character is
   only allowed as an embellishment of an immediately following LF
   character.  This reflects the fact that overprinting has only seen
   niche usage for quite a number of decades now.

   Unicode Line and Paragraph separators probably seemed like a good
   idea at the time, but have not taken hold.  Today, their occurrence
   is more likely to trigger a bug or even serve as an attack.

   HT characters ("TABs") were needed on ASR33 terminals to speed up
   processing of blank spaces at 110 bit/s line speed.  Unless some
   legacy applications require compatibility with this ancient and
   frequently varied convention, HT characters are no longer appropriate
   in Modern Network Unicode.  In support of legacy compatibility cases
   that do require tolerating their use, the "with HT characters"
   variance is defined.

   The version-nonspecific nature of MNU creates some fuzziness that may
   be undesirable but is more realistic in environments where
   applications choose the Unicode version with the Unicode library that
   happens to be available to them.

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D.2.  Byte order marks

   For UTF-8, there is no encoding ambiguity and thus no need for a byte
   order mark.  However, some systems have made regular of a leading
   U+FEFF character in UTF-8 files, anyway, often in order to mark the
   file as UTF-8 in case other character codings are also in use and
   metadata is not available.  This destroys the ASCII compatibility of
   UTF-8; it also creates problems when systems then start to expect a
   BOM in UTF-8 input and none is provided.  Section 6 of [STD63] also
   RECOMMENDS not using Byte Order Marks with UTF-8 when it is clear
   that this charset is being used, but does not phrase this as an
   unambiguous mandate, so we add that here (as did [RFC5198]), unless
   permitted by a variance.

   Some background on the prohibition of byte order marks: The 16-bit
   and 32-bit encodings for Unicode are available in multiple byte
   orders.  The byte order in use in a specific piece of text can be
   provided by metadata (such as a media type) or by prefixing the text
   with a "Byte Order Mark", U+FEFF.  Since code point U+FFFE is never
   used in Unicode, this unambiguously identifies the byte order.

Acknowledgements

   Klaus Hartke and Henk Birkholz drove the author out of his mind
   enough to make him finally write this up.  James Manger, Tim Bray and
   Martin Thomson provided comments on an early version of this draft.

Author's Address

   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany
   Phone: +49-421-218-63921
   Email: cabo@tzi.org

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