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Versions: 00 01 02 03                                                   
Network Working Group                                         S. Leonard
Internet-Draft                                             Penango, Inc.
Updates: 5234 (if approved)                                    C. Newman
Intended Status: Experimental                                     Oracle
Expires: May 4, 2017                                    October 31, 2016

                            Unicode in ABNF


   This experimental document adds support for Unicode strings in ABNF
   (Augmented Backus-Naur Form), and provides certain symbols related to
   Unicode code point ranges.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF). Note that other groups may also distribute working
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   at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft is a fork of

Copyright Notice

   Copyright (c) 2016 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
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   described in the Simplified BSD License.

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1.  Introduction

   Augmented Backus-Naur Form (ABNF) [RFC5234] is a formal syntax that
   is popular among many Internet specifications. Many Internet
   documents employ this syntax along with the Core Rules defined in
   Appendix B.1 of [RFC5234]. ABNF is defined in terms of ASCII
   [ASCII86, RFC0020]; however, Unicode [UNICODE] has become
   increasingly popular--even required--as the Internet has evolved over
   the last two decades. Unicode (as UTF-8) will be permitted in the RFC
   series [IABNA], while [RFC5198] established Net-Unicode as the
   standard form for the use of Unicode as "network text". Protocols
   that originally were ASCII-based have been, or are being, extended to
   support Unicode. However, protocols that use Unicode in some way
   (e.g., permit UTF-8 content in a production) use different ABNF
   expressions, some of which do not conform to the modern Unicode
   Standard 9.0.0, and therefore could introduce interoperability or
   security problems.

   Many parties have expressed interest in incorporating [UNICODE] into
   ABNF, yet the questions remain: "How?" and "To what extent?"

   This document proposes standardized techniques for expressing Unicode
   code points using ABNF. This document intends to be very conservative
   in its approach: a conforming implementation only needs to know how
   to map between the Unicode scalar values and any Unicode encoding
   form. The Unicode Character Database (UCD, Section 4.1 of [UNICODE])
   is intentionally not necessary. ABNF text that uses the syntax in
   this document needs to be in a Unicode encoding form (Conformance
   Clause D89 of [UNICODE]), but ABNF text that just uses the rules or
   terminal values can be expressed in ASCII [RFC0020].

2.  Unicode Code Points in ABNF

   (Consult Section 2.3 of [RFC5234] in relation to this paragraph.)
   Unicode has been expressed in several different ways in RFCs to-date.
   This document establishes that in contexts where Unicode is specified
   as the coded character set [RFC2130], the terminal values %x00-10FFFF
   are to be used to represent the Unicode code points. Only the Unicode
   scalar values are to be used in specifications that follow this
   document; surrogate code points (%xD800-DFFF) are not to be used
   [[NB: directly]]. This technique aligns ABNF with W3C EBNF [XMLEBNF]
   and Unicode EBNF [UNICODE].

   (Consult Section 2.4 and Appendix B.2 of [RFC5234] in relation to
   this paragraph.)
   In contexts where Unicode is specified as the character set, the
   ABNF-based grammar may have multiple external encodings. This
   document does not fix the encoding scheme. The obvious external

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   encoding is UTF-8 (see Net-Unicode [RFC5198]), but other encodings
   are possible. This document neither restricts productions to NFC, nor
   provides a syntax for normalization to NFC.

3.  Unicode Core Rule Update

   Appendix A furnishes Unicode Core Rules that include comprehensive
   support for certain Unicode ranges and characters. These Unicode Core
   Rules supplement the Core Rules of [RFC5234] and [ABNFMORE]; they are
   intended to be available whenever this document is invoked.

   The rules reflect broad categories of allowable and disallowable
   characters in protocols for interchange between systems, as the
   Internet community has evolved, and as of Unicode 9.0.0 in August
   2016 [UNICODE]. It is a design goal that a general-purpose ABNF
   grammar should not need to delve into the minutiae of Unicode
   character properties, which can be tailorable (i.e., language-
   specific), overridable, and unstable (between Unicode versions). It
   is a further design goal that a general-purpose ABNF grammar should
   not need to rely on sizeable external sources, namely the Unicode
   Character Database (Section 4.1 of [UNICODE]). To constrain this
   document's scope, character properties are not addressed further.

   According to a survey of all RFCs published through August 2016, many
   widely used Internet protocols rely on horizontal whitespace (HT and
   SP, or occasionally SP alone) and line breaks (usually CRLF,
   sometimes LF) as delimiters. Therefore, the rules specifically
   address horizontal whitespace and line breaks.

   Rules that both include and exclude the private-use characters
   (Section 23.5 of [UNICODE]) are provided. Private-use characters "are
   intended for open interchange, subject to interpretation by private
   agreement" (Section 23.7 of [UNICODE]). Therefore, there is no way
   within [UNICODE] itself to provide for a common interpretation of
   these code points. See also Section 4 of [RFC5198]. A protocol
   designer needs to establish that common interpretation in prose,
   provide for protocol elements that establish the common
   interpretation, or (explicitly) accept that a common interpretation
   is done outside of the designer's protocol.

4.  Case-Sensitive Unicode String Syntax

   This document extends ABNF with a new case-sensitive Unicode string
   literal. The type is denoted using a type prefix similar to the type
   prefixes used with numeric values and case-sensitive ASCII string
   literals. No syntax is provided for a case-insensitive Unicode string
   literal because doing so would require implementing Unicode caseless
   matching [UNICODE], which is language-dependent, Unicode version-

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   dependent, and very complicated overall. Caseless matching also
   requires the UCD.

   Add the contents of Section 4.1 to [RFC5234].

4.1.  Terminal Values - Literal Text Strings

   Literal case sensitive text strings in ABNF may be in the Unicode
   character set [UNICODE]. The following prefix is used:

         %su         =  case-sensitive, Unicode

   To be consistent with prior implementations of ABNF, having no prefix
   means that the string is case insensitive and in ASCII.

   [[ALT/DISCUSS: [RFC7405] %s"text" could be extended to support
   characters beyond ASCII. It is a strict superset of [RFC7405] and
   thus simpler. This document would leave [%i]"text" undefined for the
   time being, or, a collation from [RFC4790] could  be identified.]]

   The case-sensitive Unicode string can be comprised of any Graphic,
   Format, or Reserved code point. Control, Private-Use, Surrogate, and
   Noncharacter code points are excluded. Newline (line breaking)
   characters are also omitted. (See Table 2-3 of [UNICODE].)

   An example:

            rulename = %su"!100Q$"

   where the character ! is actually the Unicode code point U+00A5 YEN
   SIGN, and the character $ is actually the Unicode code point U+1F39F
   ADMISSION TICKETS, is equivalent to the rule:

            rulename = %xA5.

4.2.  ABNF Definition of ABNF - char-val

   char-val      =/  case-sensitive-Unicode-string

   ; ALT/DISCUSS: "%s", modify 7405
   case-sensitive-Unicode-string =
                     "%su" quoted-Unicode-string

   quoted-Unicode-string = DQUOTE *(%x20-21 / %x23-7E /
                           UVCHARBEYONDASCII) DQUOTE
                             ; quoted string of SP and VCHAR
                             ;  without DQUOTE, and UVCHAR
                             ;  beyond the ASCII range

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5.  Terminal Value Transformation Syntax for UTF-8 and UTF-16

   While Section 2 establishes terminal values %x00-10FFFF for Unicode,
   many Internet protocols incorporate Unicode using UTF-8 and define
   protocol elements using UTF-8 terminal values (i.e., values in the 8-
   bit range of %x00-FF, or more specifically, %x00-BF and %xC2-F4); see
   [RFC3629]. A smaller yet notable set of protocols use UTF-16.

   Writing out Unicode code points or ranges in UTF-8 or UTF-16 can be
   cumbersome and error-prone. This document therefore provides a
   "terminal value transformation syntax", so that the code points %x00-
   10FFFF can be written out natively, but the resulting ABNF represents
   8-bit or 16-bit units at the level of ABNF syntax. From there, a
   protocol can supply a specific mapping (encoding) of those values
   into a character set or other representation, consistent with Section
   2.3 of [RFC5234].

   The syntax is:
    %t8(...)    for 8-bit UTF-8 (transform to %x00-BF and %xC2-F4)
    %t16(...)   for 16-bit UTF-16 (transform to %x00-D7FF,
                %xD800-DBFF %xDC00-DFFF, and %xE000-FFFF)
    %t16le(...) for 8-bit UTF-16LE (transform to %x00.00-%xFF.FF,
    %t16be(...) for 8-bit UTF-16BE (transform to %x00.00-%xFF.FF,

   [[NB: Other possibilities: !t8 ~t8 $t8 #t8 -t8]]

   A transform is applied by recursively driving it into the elements,
   transforming terminal values from the original code point to the
   corresponding Unicode Transformation Format over an 8-bit (or 16-bit)
   field. The transforms in this document distribute over ABNF
   operators. "%t16" outputs 16-bit terminal values from %x00-FFFF,
   meaning that the endianness is not specified: a protocol needs to
   specify this or furnish a protocol slot for 16-bit code units. In
   contrast, "%t16be" and "%t16le" output 8-bit terminal values: each
   terminal value in the input will correspond to two or four terminal
   values in the output.

   If a transform is used on a terminal value outside the Unicode scalar
   value range (see the proposed Core Rule <UNICODE>), the resulting
   terminal value can be neither satisfied nor produced.

   A "reverse transformation syntax" to go from 8-bit or 16-bit terminal
   values to reassembled Unicode code points is not proposed at this

5.1.  Examples

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   Example 1: The following rules are equivalent; see [RFC3629]:

    UTF8-MB     = UTF8-2 / UTF8-3 / UTF8-4  ; from RFC 3629

                       ; %x80-D7FF / %xE000-10FFFF
    UTF8-MB     = %t8( BEYONDASCII )

   Example 2: The code point U+1F430 RABBIT FACE can be represented as
   %x1F430. It can also be represented as %xD83D.DC30 or %t16( %x1F430 )
   when UTF-16 is intended.

5.2.  Advantages and Features

   Using transformation syntax offers several advantages:

   The generic ABNF syntax of a textual protocol can take full advantage
   of the Unicode character set; the syntax is not dependent on a
   particular encoding form.

   Specifying ranges of characters becomes unwieldy when explicitly
   defined in terms of code units in a Unicode encoding form, e.g., as
   UTF-8 code units (octets) for characters beyond ASCII, or as UTF-16
   code units (16-bit words) for supplementary characters. Trying to
   specify Punycode in ABNF would be, for all intents and purposes,
   impossible! (Note: it's not actually impossible, but very difficult
   and not particularly useful.)

   Protocols that have arbitrary binary slots (e.g., BINARYMIME) are
   inherently incompatible with Section 2 syntax, but compatibility can
   be achieved by using transformation syntax.

   Protocol designers can effectively exploit the "holes" in UTF-8,
   because octets C0, C1, and F5-FF are never seen in UTF-8. These
   octets provide natural delimiters for arbitrary runs of UTF-8. An
   advantage of using such octets as delimiters is that checking for
   these octets has to be done anyway for security reasons, so a
   designer can save cycles by incorporating this part of a check for
   well-formed Unicode into a protocol. Such delimiters can only be
   expressed outside of "%t8", since a "%t8" transform will never
   produce those terminal values.

   (UTF-16 also has such "holes", namely, in unpaired surrogates. But
   using unpaired surrogates as delimiters may suffer from other
   security pitfalls; in any event, UTF-16 is far less common in IETF

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6.  Comment Syntax

   This document extends ABNF to have Unicode comments. Comments are
   treated as specification prose, so they may be normative depending on
   the context. Comment text allows for the same repertoire of
   characters as RFC text. The RFC Editors can regulate comments to the
   same extent as specification prose, including disallowing certain
   characters or code points.

6.1.  Comment:  ; Comment

   (No changes to the text of Section 3.9 of [RFC5234] are needed.)

6.2.  ABNF Definition of ABNF - comment

         ; given:
         comment      =    ";" *(WSP / VCHAR) CRLF

         ; increment (unambiguous grammar):
         comment      =/   ";" *(UWSP / UVCHAR / PUACHAR)
                               *(UWSP / UVCHAR / PUACHAR) CRLF

         ; or redefine:
         comment      =    ";" *(UWSP / UVCHAR / PUACHAR) CRLF

7.  Notational Conventions

   For readability it is advisable to express a Unicode code point as
   the character itself, the numeric terminal value, and the name or a
   name alias. Only one expression is used for the formal ABNF notation:
   either the character itself (Section 4) or the numeric terminal value
   (Section 2). The other expressions can be incorporated into an
   adjacent comment.

   The suggested notational convention for the adjacent comment follows
   Appendix A of [UNICODE]. The comment text is comprised of one or more
   WSP characters, optionally either the character itself or "U+" syntax
   followed by exactly one SP, and the name or a name alias in ALL-CAPS
   ASCII. Multiple characters can be notated in sequence on multiple
   comment lines or on a single comment line. It is neither advisable
   nor necessary to notate characters in the ASCII range. Examples of
   the notation include:

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         ; U+2206 INCREMENT
         ; U+2030 PER MILLE SIGN
         change-in-temp = %su"$" 3DIGIT %su"%"

         ;       # EURO SIGN    ZWJ    / VULGAR FRACTION ONE HALF
         euros = %x20AC 3DIGIT [%x200D.BD]

   where the characters $, %, #, and / are actually the respective
   Unicode characters mentioned in the comments.

8.  Effects on RFC 5234

   Formally, this document updates [RFC5234] but does not modify it in
   situ. Authors need to reference this document if they want to include
   these enhancements; bare references to [RFC5234] do not include this
   specification (or, for that matter, [RFC7405]). This directive
   follows a model whereby document authors can choose whether to invoke
   particular enhancements to ABNF. As time goes on, the IETF can
   determine how often these enhancements are invoked, and can decide
   whether to include them as part of a revision to the base [RFC5234].

   A bare reference to this document invokes the case-sensitive Unicode
   literal string syntax enhancement, the Unicode comment syntax
   enhancement, and the Unicode Core Rules of Appendix A (i.e., the Core
   Rules do not have to be further referenced). Nevertheless, document
   authors are free to qualify a reference to this document to invoke
   each feature selectively.

   Appendix A of this document is meant to supplement Appendix B.1 of
   [RFC5234] and Appendix A of [ABNFMORE]; therefore, concurrently
   referencing those documents is a good idea. Document authors who
   reference this document should use the rules of Appendix A, and
   should not attempt to redefine or provide incremental alternatives to
   them (except for backwards compatibility with prior documents).

9.  IANA Considerations

   This document implies no IANA considerations.

10.  Security Considerations

   While the Unicode Core Rules themselves may not be security-relevant,
   the use of C1 control characters could very well be security-
   relevant, because they may trigger special functions on various
   devices, while being invisible in other contexts. Similarly, case-
   sensitive Unicode string syntax allows for a broad range of code
   points, many of which represent characters that are confusable with
   other characters, or can only be inferred by visible yet subtle

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   changes in the surrounding graphemes (or worse, semantic changes that
   do not have visual representations).

   Protocols using Unicode should evaluate the applicability of Unicode
   security considerations [UTR#36].

11.  References

11.1. Normative References

   [ASCII86]  American National Standards Institute, "Coded Character
              Set -- 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.

   [RFC0020]  Cerf, V., "ASCII format for network interchange", RFC 20,
              October 1969.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, March 2008.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [UNICODE]  The Unicode Consortium, "The Unicode Standard, Version
              9.0.0", The Unicode Consortium, August 2016.

11.2. Informative References

   [IABNA]    Flanagan, H., "The Use of Non-ASCII Characters in RFCs",
              draft-iab-rfc-nonascii-02 (work in progress), April 2016.

   [RFC1345]  Simonsen, K., "Character Mnemonics and Character Sets",
              RFC 1345, June 1992.

   [RFC2130]  Weider, C., Preston, C., Simonsen, K., Alvestrand, H.,
              Atkinson, R., Crispin, M., and P. Svanberg, "The Report of
              the IAB Character Set Workshop held 29 February - 1 March,
              1996", RFC 2130, April 1997.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November

   [RFC4790]  Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
              Application Protocol Collation Registry", RFC 4790, March

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC

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              7405, December 2014.

   [UTR#36]   Davis, M. and M. Suignard, "Unicode Security
              Considerations", Unicode Technical Report #36, September
              2014, <http://unicode.org/reports/tr36/>.

   [XMLEBNF]  Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", Section 6, W3C Recommendation REC-xml-20081126,
              November 2008, <http://www.w3.org/TR/2008/REC-xml-

Appendix A. Comprehensive Unicode Core Rules

   Certain basic rules are in uppercase, such as SP, HTAB, CRLF, DIGIT,
   ALPHA, etc.

      ; D76 Unicode scalar value

      UNICODE      = <U+0000-U+D7FF / U+E000-U+10FFFF>
      BEYONDASCII  = <U+0080-U+D7FF / U+E000-U+10FFFF>
      BEYONDG0     = <U+0080-U+D7FF / U+E000-U+10FFFF>

      C1           = <U+0080-U+009F>
      BEYONDC1     = <U+00A0-U+D7FF / U+E000-U+10FFFF>
      G1           = <U+00A0-U+00FF>  ; 96-set
      BEYONDG1     = <U+0100-U+D7FF / U+E000-U+10FFFF>
      LATIN1       = <U+0000-U+00FF>
      BEYONDLATIN1 = <U+0100-U+D7FF / U+E000-U+10FFFF>

      ; C2 D14 noncharacter (sentinel)
      ; Section 23.7 Noncharacters, see also NUL

                      U+1FFFE-U+1FFFF / U+2FFFE-U+2FFFF /
                      U+3FFFE-U+1FFFF / U+4FFFE-U+4FFFF /
                      U+5FFFE-U+1FFFF / U+6FFFE-U+6FFFF /
                      U+7FFFE-U+1FFFF / U+8FFFE-U+8FFFF /
                      U+9FFFE-U+1FFFF / U+AFFFE-U+AFFFF /
                      U+BFFFE-U+1FFFF / U+CFFFE-U+CFFFF /
                      U+DFFFE-U+1FFFF / U+EFFFE-U+EFFFF /
                      U+FFFFE-U+FFFFF / U+10FFFE-U+10FFFF>

      ; UCHAR rules are analogous to CHAR

      UCHARBEYONDBMP = <U+10000-U+1FFFD / U+20000-U+2FFFD /
                      U+30000-U+3FFFD / U+40000-U+4FFFD /

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                      U+50000-U+5FFFD / U+60000-U+6FFFD /
                      U+70000-U+7FFFD / U+80000-U+8FFFD /
                      U+90000-U+9FFFD / U+A0000-U+AFFFD /
                      U+B0000-U+BFFFD / U+C0000-U+CFFFD /
                      U+D0000-U+DFFFD / U+E0000-U+EFFFD /
                      U+F0000-U+FFFFD / U+100000-U+10FFFD>

      UCHARBEYONDLATIN1 = <U+0100-U+D7FF / U+E000-U+FDCF /
                           U+FDF0-U+FFFD> / UCHARBEYONDBMP

      UCHARBEYONDC1  = <U+00A0-U+D7FF / U+E000-U+FDCF / U+FDF0-U+FFFD>
                       / UCHARBEYONDBMP


      UCHAR        = <U+0001-U+D7FF / U+E000-U+FDCF / U+FDF0-U+FFFD> /

      ; D49 private-use
      ; Section 23.5 Private-Use Characters

      ; Primary Private Use Area (in BMP)
      PPUACHAR     = <U+E000-U+F8FF>
      ; Supplementary Private Use Area-A
      SPUAACHAR    = <U+F0000-U+FFFFF>
      ; Supplementary Private Use Area-B
      SPUABCHAR    = <U+100000-U+10FFFF>

      ; TODO: possible alternates: PUCHAR, PUA

      ; Unicode-y VCHAR: like VCHAR, attempts to capture
      ; "all standardized graphic and formatting
      ; characters/code points for open interchange,
      ; excluding white space and controls"
      ; EXCLUDES: Noncharacters (some Cn), Cs, Co, Cc, Z (Zs, Zl, Zp)

      UVCHARBEYONDBMP = <U+10000-U+1FFFD / U+20000-U+2FFFD /
                       U+30000-U+3FFFD / U+40000-U+4FFFD /
                       U+50000-U+5FFFD / U+60000-U+6FFFD /
                       U+70000-U+7FFFD / U+80000-U+8FFFD /
                       U+90000-U+9FFFD / U+A0000-U+AFFFD /
                       U+B0000-U+BFFFD / U+C0000-U+CFFFD /
                       U+D0000-U+DFFFD / U+E0000-U+EFFFD>

      UVCHARBEYONDLATIN1 = <U+0100-U+167F / U+1681-U+1FFF /
                            U+200B-U+2027 / U+202A-U+202E /
                            U+2030-U+205E / U+2060-U+2FFF /

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                            U+3001-U+D7FF /
                            U+F900-U+FDCF / U+FDF0-U+FFFD> /

      UVCHARBEYONDASCII = <U+00A1-U+167F / U+1681-U+1FFF /
                           U+200B-U+2027 / U+202A-U+202E /
                           U+2030-U+205E / U+2060-U+2FFF /
                           U+3001-U+D7FF /
                           U+F900-U+FDCF / U+FDF0-U+FFFD> /



      ; horizontal white space only (Zs beyond ASCII),
      ; NO line breaks (Cc, Zl, Zp)
      ; cf Section 5.8 Newline Guidelines with RFC 5198
      ; see also SP
      UWSPBEYONDASCII = <U+00A0 / U+1680 / U+2000-U+200A /
                         U+202F / U+205F / U+3000>

      ; includes HT

      ; C1 Controls
      PAD          = <U+0080> ; gov't health warning: figment
      HOP          = <U+0081> ; gov't health warning: figment
      BPH          = <U+0082>
      NBH          = <U+0083>
      IND          = <U+0084>
      NEL          = <U+0085>
      ; NLF          CRLF, CR, LF, NEL (not LS or PS)
      ;  --probably unnecessary for Internet usage:
      ;  CRLF is already the standard
      SSA          = <U+0086>
      ESA          = <U+0087>
      HTS          = <U+0088>
      HTJ          = <U+0089>
      VTS          = <U+008A>
      PLD          = <U+008B>
      PLU          = <U+008C>
      RI           = <U+008D>
      SS2          = <U+008E>
      SS3          = <U+008F>
      DCS          = <U+0090>
      PU1          = <U+0091>
      PU2          = <U+0092>

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      STS          = <U+0093>
      CCH          = <U+0094>
      MW           = <U+0095>
      SPA          = <U+0096>
      EPA          = <U+0097>
      SOS          = <U+0098>
      SGCI         = <U+0099> ; or SGC, gov't health warning: figment
      SCI          = <U+009A>
      CSI          = <U+009B>
      ST           = <U+009C>
      OSC          = <U+009D>
      PM           = <U+009E>
      APC          = <U+009F>

      ; Latin1
      NBSP         = <U+00A0>
      SHY          = <U+00AD>

      ; Zl, Zp
      ; NB: These are excluded from both UVCHAR and UWSP
      LS           = <U+2028>
      PS           = <U+2029>

Authors' Addresses

   Sean Leonard
   Penango, Inc.
   5900 Wilshire Boulevard
   21st Floor
   Los Angeles, CA  90036

   EMail: dev+ietf@seantek.com
   URI:   http://www.penango.com/

   Chris Newman
   440 E. Huntington Dr., Suite 400
   Arcadia, CA  91006

   EMail: chris.newman@oracle.com

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