[Docs] [txt|pdf] [draft-klensin-net...] [Diff1] [Diff2] [Errata]

PROPOSED STANDARD
Errata Exist
Network Working Group                                         J. Klensin
Request for Comments: 5198                                  M. Padlipsky
Obsoletes: 698                                                March 2008
Updates: 854
Category: Standards Track


                 Unicode Format for Network Interchange

Status of This Memo

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

Abstract

   The Internet today is in need of a standardized form for the
   transmission of internationalized "text" information, paralleling the
   specifications for the use of ASCII that date from the early days of
   the ARPANET.  This document specifies that format, using UTF-8 with
   normalization and specific line-ending sequences.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Requirement for a Standardized Text Stream Format  . . . .  2
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Net-Unicode Definition . . . . . . . . . . . . . . . . . . . .  3
   3.  Normalization  . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Versions of Unicode  . . . . . . . . . . . . . . . . . . . . .  5
   5.  Applicability and Stability of this Specification  . . . . . .  7
     5.1.  Use in IETF Applications Specifications  . . . . . . . . .  7
     5.2.  Unicode Versions and Applicability . . . . . . . . . . . .  7
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   Appendix A.  History and Context . . . . . . . . . . . . . . . . . 11
   Appendix B.  The ASCII NVT Definition  . . . . . . . . . . . . . . 12
   Appendix C.  The Line-Ending Problem . . . . . . . . . . . . . . . 14
   Appendix D.  A Note about Related Future Work  . . . . . . . . . . 14
   References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     Normative References . . . . . . . . . . . . . . . . . . . . . . 15
     Informative References . . . . . . . . . . . . . . . . . . . . . 16






Klensin & Padlipsky         Standards Track                     [Page 1]

RFC 5198                    Network Unicode                   March 2008


1.  Introduction

1.1.  Requirement for a Standardized Text Stream Format

   Historically, Internet protocols have been largely ASCII-based and
   references to "text" in protocols have assumed ASCII text and
   specifically text in Network Virtual Terminal ("NVT") or "Network
   ASCII" form (see Appendix A and Appendix B).  Protocols and formats
   that have moved beyond ASCII have included arrangements to
   specifically identify the character set and often the language being
   used.

   In our more internationalized world, "text" clearly no longer equates
   unambiguously to "network ASCII".  Fortunately, however, we are
   converging on Unicode [Unicode] [ISO10646] as a single international
   interchange character coding and no longer need to deal with per-
   script standards for character sets (e.g., one standard for each of
   Arabic, Cyrillic, Devanagari, etc., or even standards keyed to
   languages that are usually considered to share a script, such as
   French, German, or Swedish).  Unfortunately, though, while it is
   certainly time to define a Unicode-based text type for use as a
   common text interchange format, "use Unicode" involves even more
   ambiguity than "use ASCII" did decades ago.

   Unicode identifies each character by an integer, called its "code
   point", in the range 0-0x10ffff.  These integers can be encoded into
   byte sequences for transmission in at least three standard and
   generally-recognized encoding forms, all of which are completely
   defined in The Unicode Standard and the documents cited below:

   o  UTF-8 [RFC3629] defines a variable-length encoding that may be
      applied uniformly to all code points.

   o  UTF-16 [RFC2781] encodes the range of Unicode characters whose
      code points are less than 65536 straightforwardly as 16-bit
      integers, and provides a "surrogate" mechanism for encoding larger
      code points in 32 bits.

   o  UTF-32 (also known as UCS-4) simply encodes each code point as a
      32-bit integer.

   Older forms and nomenclature, such as the 16-bit UCS-2, are now
   strongly discouraged.

   As with ASCII, any of these forms may be used with different line-
   ending conventions.  That flexibility can be an additional source of
   confusion with, e.g., index (offset) references into documents based
   on character counts.



Klensin & Padlipsky         Standards Track                     [Page 2]

RFC 5198                    Network Unicode                   March 2008


   This document proposes to establish "Net-Unicode" as a new
   standardized text transmission form for the Internet, to serve as an
   internationalized alternative for NVT ASCII when specified in new --
   and, where appropriate, updated -- protocols.  UTF-8 [RFC3629] is
   chosen for the coding because it has good compatibility properties
   with ASCII and for other reasons discussed in the existing IETF
   character set policy [RFC2277].  "Net-Unicode" is specified in
   Section 2; the subsequent sections of the document provide background
   and explanation.

   Whenever there is a choice, Unicode SHOULD be used with the text
   encoding specified here.  This combination is preferred to the
   double-byte encoding of "extended ASCII" [RFC0698] or the assorted
   per-language or per-country character coding systems.

1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Net-Unicode Definition

   The Network Unicode format (Net-Unicode) is defined as follows.
   Parts of this definition are deliberately informal, providing
   guidance for specific profiles or rules in the protocols that
   reference this one rather than firm rules that apply globally.

   1.  Characters MUST be encoded in UTF-8 as defined in [RFC3629].

   2.  If the protocol has the concept of "lines", line-endings MUST be
       indicated by the sequence Carriage-Return (CR, U+000D) followed
       by Line-Feed (LF, U+000A), often known just as CRLF.  CR SHOULD
       NOT appear except when followed by LF.  The only other allowed
       context in which CR is permitted is in the combination CR NUL,
       which is not recommended (see the note at the end of this
       section).

   3.  The control characters in the ASCII range (U+0000 to U+001F and
       U+007F to U+009F) SHOULD generally be avoided.  Space (SP,
       U+0020), CR, LF, and Form Feed (FF, U+000C) are exceptions to
       this principle, but use of all but the first requires care as
       discussed elsewhere in this document.  The so-called "C1
       Controls" (U+0080 through U+009F), which did not appear in ASCII,
       MUST NOT appear.

       FF should be used only with caution: it does not have a standard
       and universal interpretation and, in particular, if its use



Klensin & Padlipsky         Standards Track                     [Page 3]

RFC 5198                    Network Unicode                   March 2008


       assumes a page length, such assumptions may not be appropriate in
       international contexts (e.g., considering 8.5x11 inch paper
       versus A4).  Other control characters are used to affect display
       format, control devices, or to structure files.  None of those
       uses is appropriate for streams of plain text.

   4.  Before transmission, all character sequences SHOULD be normalized
       according to Unicode normalization form "NFC" (see Section 3).

   5.  As suggested in Section 6 of RFC 3629, the Byte Order Mark
       ("BOM") signature MUST NOT appear at the beginning of these text
       strings.

   6.  Systems conforming to this specification MUST NOT transmit any
       string containing any code point that is unassigned in the
       version of Unicode on which they are dependent.  The version of
       NFC and the version of Unicode used by that system MUST be
       consistent.

   The use of LF without CR is questionable; see Appendix B for more
   discussion.  The newer control characters IND (U+0084) and NEL ("Next
   Line", U+0085) might have been used to disambiguate the various line-
   ending situations, but, because their use has not been established on
   the Internet, because many protocols require CRLF, and because IND
   and NEL fall within the "C1 Controls" group (see below), they MUST
   NOT be used.  Similar observations apply to the yet newer line and
   paragraph separators at U+2028 and U+2029 and any future characters
   that might be defined to serve these functions.  For this
   specification and protocols that depend on it, lines end in CRLF and
   only in CRLF.  Anything that does not end in CRLF is either not a
   line or is severely malformed.

   The NVT specification contained a number of additional provisions,
   e.g., for the optional use of backspacing and "bare CR" (sent as CR
   NUL) to generate overstruck character sequences.  The much greater
   number of precomposed characters in Unicode, the availability of
   combining characters, and the growing use of markup conventions of
   various types to show, e.g., emphasis (rather than attempting to do
   that via the use of special characters), should make such sequences
   largely unnecessary.  These sequences SHOULD be avoided if at all
   possible.  However, because they were optional in NVT applications
   and this specification is an NVT superset, they cannot be prohibited
   entirely.  The most important of these rules is that CR MUST NOT
   appear unless it is immediately followed by LF (indicating end of
   line) or NUL.  Because NUL (an octet whose value is all zeros, i.e.,
   %x00 in the notation of [RFC5234]) is hostile to programming
   languages that use that character as a string delimiter, the CR NUL
   sequence SHOULD be avoided for that reason as well.



Klensin & Padlipsky         Standards Track                     [Page 4]

RFC 5198                    Network Unicode                   March 2008


3.  Normalization

   There are cases where strings of Unicode are fundamentally
   equivalent, essentially representing the same text.  These are called
   "canonical equivalents" in the Unicode Standard.  For example, the
   following pairs of strings are canonically equivalent:

   U+2126 OHM SIGN
   U+03A9 GREEK CAPITAL LETTER OMEGA

   U+0061 LATIN SMALL LETTER A, U+0300 COMBINING GRAVE ACCENT
   U+00E0 LATIN SMALL LETTER A WITH GRAVE

   Comparison of strings becomes much easier if any such cases are
   always represented by a single unique form.  The Unicode Consortium
   specifies a normalization form, known as NFC [NFC], which provides
   the necessary mappings and mechanisms to convert all canonically
   equivalent sequences to a single unique form.  Typically, this form
   produces precomposed characters for any sequences that can be
   represented in that fashion.  It also reorders other combining marks
   so that they have a unique and unambiguous order.

   Of the various normalization forms defined as part of Unicode, NFC is
   closest to actual use in practice, minimizes side-effects due to
   considering characters equivalent that may not be equivalent in all
   situations, and typically requires the least work when converting
   from non-Unicode encodings.

   The section above requires that, except in very unusual
   circumstances, all Net-Unicode strings be transmitted in normalized
   form.  Recognition of the fact that some implementations of
   applications may rely on operating system libraries over which they
   have little control and adherence to the robustness principle
   suggests that receivers of such strings should be prepared to receive
   unnormalized ones and to not react to that in excessive ways.

4.  Versions of Unicode

   Unicode changes and expands over time.  Large blocks of space are
   reserved for future expansion.  New versions, which appear at regular
   intervals, add new scripts and characters.  Occasionally they also
   change some property definitions.  In retrospect, one of the
   advantages of ASCII [ASCII] when it was chosen was that the code
   space was full when the Standard was first published.  There was no
   practical way to add characters or change code point assignments
   without being obviously incompatible.





Klensin & Padlipsky         Standards Track                     [Page 5]

RFC 5198                    Network Unicode                   March 2008


   While there are some security issues if people deliberately try to
   trick the system (see Section 6), Unicode version changes should not
   have a significant impact on the text stream specification of this
   document for the following reasons:

   o  The transformation between Unicode code table positions and the
      corresponding UTF-8 code is algorithmic; it does not depend on
      whether a code point has been assigned or not.

   o  The normalization recommended here, NFC (see Section 3), performs
      a very limited set of mappings, much more limited than those of
      the more extensive NFKC used in, e.g., Nameprep [RFC3491].

   The NFC tables may be updated over time as new characters are added,
   but the Unicode Consortium has guaranteed the stability of all NFC
   strings.  That is, if a string does not contain any unassigned
   characters, and it is normalized according to NFC, it will always be
   normalized according to all future versions of the Unicode Standard.
   The stability of the Net-Unicode format is thus guaranteed when any
   implementation that converts text into Net-Unicode format does not
   permit unassigned characters.

   Because Unicode code points that are reserved for private use do not
   have standard definitions or normalization interpretations, they
   SHOULD be avoided in strings intended for Internet interchange.

   Were Unicode to be changed in a way that violated these assumptions,
   i.e., that either invalidated the byte string order specified in RFC
   3629 or that changed the stability of NFC as stated above, this
   specification would not apply.  Put differently, this specification
   applies only to versions of Unicode starting with version 5.0 and
   extending to, but not including, any version for which changes are
   made in either the UTF-8 definition or to NFC stability.  Such
   changes would violate established Unicode policies and are hence
   unlikely, but, should they occur, it would be necessary to evaluate
   them for compatibility with this specification and other Internet
   uses of NFC.

   If the specification of a protocol references this one, strings that
   are received by that protocol and that appear to be UTF-8 and are not
   otherwise identified (e.g., by charset labeling) SHOULD be treated as
   using UTF-8 in conformance with this specification.









Klensin & Padlipsky         Standards Track                     [Page 6]

RFC 5198                    Network Unicode                   March 2008


5.  Applicability and Stability of this Specification

5.1.  Use in IETF Applications Specifications

   During the development of this specification, there was some
   confusion about where it would be useful given that, e.g., the
   individual MIME media types used in email and with HTTP have their
   own rules about UTF-8 character types and normalization, and the
   application transport protocols impose their own conventions about
   line endings.  There are three answers.  The first is that, in
   retrospect, it would have been better to have those protocols and
   content types standardized in the way specified here, even though it
   is certainly too late to change them at this time.  The second is
   that we have several protocols that are dependent on either the
   original Telnet design or other arrangements requiring a standard,
   interoperable, string definition without specific content-labels of
   one sort or another.  Whois [RFC3912] is an example member of this
   group.  As consideration is given to upgrading them for non-ASCII
   use, this specification provides a normative reference that provides
   the same stability that NVT has provided the ASCII forms.  This
   specification is intended for use by other specifications that have
   not yet defined how to use Unicode.  Having a preferred standard
   Internet definition for Unicode text streams -- rather than just one
   for transmission codings -- may help improve the specification and
   interoperability of protocols to be developed in the future.  This
   specification is not intended for use with specifications that
   already allow the use of UTF-8 and precisely define that use.

5.2.  Unicode Versions and Applicability

   The IETF faces a practical dilemma with regard to versions of
   Unicode.  Each new version brings with it new characters and
   sometimes new combining characters.  Version 5.0 introduces the new
   concept of sequences of characters named as if they were individual
   characters (see [NamedSequences]).  The normalization represented by
   NFC is stable if all strings are transmitted and stored in normalized
   form if corrections are never made to character definitions or
   normalization tables and if unassigned code points are never used.
   The latter is important because an unassigned code point always
   normalizes to itself.  However, if the same code point is assigned to
   a character in a future version, it may participate in some other
   normalization mapping (some specific difficulties in this regard are
   discussed in [RFC4690]).  It is worth noting that transmission in
   normalized form is not required by either the IETF's UTF-8 Standard
   [RFC3629] or by standards dependent on the current version of
   Stringprep [RFC3454].





Klensin & Padlipsky         Standards Track                     [Page 7]

RFC 5198                    Network Unicode                   March 2008


   All would be well with this as described in Section 4 except for one
   problem: Applications typically do not perform their own conversions
   to Unicode and may not perform their own normalizations but instead
   rely on operating system or language library functions -- functions
   that may be upgraded or otherwise changed without changes to the
   application code itself.  Consequently, there may be no plausible way
   for an application to know which version of Unicode, or which version
   of the normalization procedures, it is utilizing, nor is there any
   way by which it can guarantee that the two will be consistent.

   Because of per-version changes in definitions and tables, Stringprep
   and documents depending on it are now tied to Unicode Version 3.2
   [Unicode32] and full interoperability of Internet Standard UTF-8
   [RFC3629], when used with normalization as specified here, is
   dependent on normalization definitions and the definition of UTF-8
   itself not changing after Unicode Version 5.0.  These assumptions
   seem fairly safe, but they are still assumptions.  Rather than being
   linked to the latest available version of Unicode, version 5.0
   [Unicode] or broader concepts of version independence based on
   specific assumptions and conditions, this specification could
   reasonably have been tied, like Stringprep and Nameprep to Unicode
   3.2 [Unicode32] or some more recent intermediate version, but, in
   addition to the obvious disadvantages of having different IETF
   standards tied to different versions of Unicode, the library-based
   application implementation behavior described above makes these
   version linkages nearly meaningless in practice.

   In theory, one can get around this problem in four ways:

   1.  Freeze on a particular version of Unicode and try to insist that
       applications enforce that version by, e.g., containing lists of
       unassigned characters and prohibiting their use.  Of course, this
       would prohibit evolution to include newly-added scripts and the
       tables of unassigned code points would be cumbersome.

   2.  Require that every Unicode "text" string or file start with a
       version indication, somewhat akin to the "byte order mark"
       indicator.  It is unlikely that this provision would be
       practical.  More important, it would require that each
       application implementation be prepared to either support multiple
       normalization tables and versions or that it reject text from
       Unicode versions with which it was not prepared to deal.

   3.  Devise a different set of normalization rules that would, e.g.,
       guarantee that no character assigned to a previously-unassigned
       code point in Unicode was ever normalized to anything but itself
       and use those rules instead of NFC.  It is not clear whether or
       not such a set of rules is possible or whether some other



Klensin & Padlipsky         Standards Track                     [Page 8]

RFC 5198                    Network Unicode                   March 2008


       completely stable set of rules could be devised, perhaps in
       combination with restrictions on the ways in which characters
       were added in future versions of Unicode.

   4.  Devise a normalization process that is otherwise equivalent to
       NFC but that rejects code points that are unassigned in the
       current version of Unicode, rather than mapping those code points
       to themselves.  This would still leave some risk of incompatible
       corrections in Unicode and possibly a few edge cases, but it is
       probably stable enough for Internet use in the overwhelming
       number of cases.  This process has been discussed in the Unicode
       Consortium under the name "Stable NFC".

   None of these approaches seems ideal: the ideal procedure would be as
   stable and predictable as ASCII has been.  But that level is simply
   not feasible as long as Unicode continues to evolve by the addition
   of new code points and scripts.  The fourth option listed above
   appears to be a reasonable compromise.

6.  Security Considerations

   This specification provides a standard form for the use of Unicode as
   "network text".  Most of the same security issues that apply to
   UTF-8, as discussed in [RFC3629], apply to it, although it should be
   slightly less subject to some risks by virtue of requiring NFC
   normalization and generally being somewhat more restrictive.
   However, shifts in Unicode versions, as discussed in Section 5.2, may
   introduce other security issues.

   Programs that receive these streams should use extreme caution about
   assuming that incoming data are normalized, since it might be
   possible to use unnormalized forms, as well as invalid UTF-8, as part
   of an attack.  In particular, firewalls and other systems that
   interpret UTF-8 streams should be developed with the clear knowledge
   that an attacker may deliberately send unnormalized text, for
   instance, to avoid detection by naive text-matching systems.

   NVT contains a requirement, of necessity repeated here (see
   Section 2), that the CR character be immediately followed by either
   LF or ASCII NUL (an octet with all bits zero).  NUL may be
   problematic for some programming languages that use it as a string
   terminator, and hence a trap for the unwary, unless caution is used.
   This may be an additional reason to avoid the use of CR entirely,
   except in sequence with LF, as suggested above.

   The discussion about Unicode versions above (see Section 4 and
   Section 5.2) makes several assumptions about future versions of
   Unicode, about NFC normalization being applied properly, and about



Klensin & Padlipsky         Standards Track                     [Page 9]

RFC 5198                    Network Unicode                   March 2008


   UTF-8 being processed and transmitted exactly as specified in RFC
   3629.  If any of those assumptions are not correct, then there are
   cases in which strings that would be considered equivalent do not
   compare equal.  Robust code should be prepared for those
   possibilities.

7.  Acknowledgments

   Many thanks to Mark Davis, Martin Duerst, and Michel Suignard for
   suggestions about Unicode normalization that led to the format
   described here, and especially to Mark for providing the paragraphs
   that describe the role of NFC.  Thanks also to Mark, Doug Ewell,
   Asmus Freytag for corrected text describing Unicode transmission
   forms, and to Tim Bray, Carsten Bormann, Stephane Bortzmeyer, Martin
   Duerst, Frank Ellermann, Clive D.W. Feather, Ted Hardie, Bjoern
   Hoehrmann, Alfred Hoenes, Kent Karlsson, Bill McQuillan, George
   Michaelson, Chris Newman, and Marcos Sanz for a number of helpful
   comments and clarification requests.

































Klensin & Padlipsky         Standards Track                    [Page 10]

RFC 5198                    Network Unicode                   March 2008


Appendix A.  History and Context

   This subsection contains a review of prior work in the ARPANET and
   Internet to establish a standard text type, work that establishes the
   context and motivation for the approach taken in this document.  The
   text is explanatory rather than normative: nothing in this section is
   intended to change or update any current specification.  Those who
   are uninterested in this review and analysis can safely skip this
   section.

   One of the earlier application design decisions made in the
   development of ARPANET, a decision that was carried forward into the
   Internet, was the decision to standardize on a single and very
   specific coding for "text" to be passed across the network [RFC0020].
   Hosts on the network were then responsible for translating or mapping
   from whatever character coding conventions were used locally to that
   common intermediate representation, with sending hosts mapping to it
   and receiving ones mapping from it to their local forms as needed.
   It is interesting to note that at the time the ARPANET was being
   developed, participating host operating systems used at least three
   different character coding standards: the antiquated BCD (Binary
   Coded Decimal), the then-dominant major manufacturer-backed EBCDIC
   (Extended BCD Interchange Code), and the then-still emerging ASCII
   (American Standard Code for Information Interchange).  Since the
   ARPANET was an "open" project and EBCDIC was intimately linked to a
   particular hardware vendor, the original Network Working Group agreed
   that its standard should be ASCII.  That ASCII form was precisely
   "7-bit ASCII in an 8-bit field", which was in effect a compromise
   between hosts that were natively 7-bit oriented (e.g., with five
   seven-bit characters in a 36-bit word), those that were 8-bit
   oriented (using eight-bit characters) and those that placed the
   seven-bit ASCII characters in 9-bit fields with two leading zero bits
   (four characters in a 36-bit word).

   More standardization was suggested in the first preliminary
   description of the Telnet protocol [RFC0097].  With the iterations of
   that protocol [RFC0137] [RFC0139] and the drawing together of an
   essentially formal definition somewhat later [RFC0318], a standard
   abstraction, the Network Virtual Terminal (NVT) was established.  NVT
   character-coding conventions (initially called "Telnet ASCII" and
   later called "NVT ASCII", or, more casually, "network ASCII")
   included the requirement that Carriage Return followed by Line Feed
   (CRLF) be the common representation for ending lines of text (given
   that some participating "Host" operating systems used the one
   natively, some the other, at least one used both, and a few used
   neither (preferring variable-length lines with counts or special
   delimiters or markers instead) and specified conventions for some
   other characters.  Also, since NVT ASCII was restricted to seven-bit



Klensin & Padlipsky         Standards Track                    [Page 11]

RFC 5198                    Network Unicode                   March 2008


   characters, use of the high-order bit in octets was reserved for the
   transmission of control signaling information.

   At a very high level, the concept was that a system could use
   whatever character coding and line representations were appropriate
   locally, but text transmitted over the network as text must conform
   to the single "network virtual terminal" convention.  Virtually all
   early Internet protocols that presume transfer of "text" assume this
   virtual terminal model, although different ones assume or limit it in
   different ways.  Telnet, the command stream and ASCII Type in FTP
   [RFC0542], the message stream in SMTP transfer [RFC2821], and the
   strings passed to finger [RFC0742] and whois [RFC0954] are the
   classic examples.  More recently, HTTP [RFC1945] [RFC2616] follows
   the same general model but permits 8-bit data and leaves the line end
   sequence unspecified (the latter has been the source of a significant
   number of problems).

Appendix B.  The ASCII NVT Definition

   The main body of this specification is intended as an update to, and
   internationalized version of, the Net-ASCII definition.  The
   specification is self-contained in that parts of the Net-ASCII
   definition that are no longer recommended are not included above.
   Because Net-ASCII evolved somewhat over time and there has been
   debate about which specification is the "official" Net-ASCII, it is
   appropriate to review the key elements of that definition here.  This
   review is informal with regard to the contents of Net-ASCII and
   should not be considered as a normative update or summary of the
   earlier specifications (Section 2 does specify some normative updates
   to those specifications and some comments below are consistent with
   it).

   The first part of the section titled "THE NVT PRINTER AND KEYBOARD"
   in RFC 854 [RFC0854] is generally, although not universally,
   considered to be the normative definition of the (ASCII) Network
   Virtual Terminal and hence of Net-ASCII.  It includes not only the
   graphic ASCII characters but a number of control characters.  The
   latter are given Internet-specific meanings that are often more
   specific than the definitions in the ASCII specification.  In today's
   usage, and for the present specification, the following
   clarifications and updates to that list should be noted.  Each one is
   accompanied by a brief explanation of the reason why the original
   specification is no longer appropriate.

   1.  The "defined but not required" codes -- BEL (U+0007), BS
       (U+0008), HT (U+0009), VT (U+000B), and FF (U+000C) -- and the
       undefined control codes ("C0") SHOULD NOT be used unless required
       by exceptional circumstances.  Either their original "network



Klensin & Padlipsky         Standards Track                    [Page 12]

RFC 5198                    Network Unicode                   March 2008


       printer" definitions are no longer in general use, common
       practice has evolved away from the formats specified there, or
       their use to simulate characters that are better handled by
       Unicode is no longer appropriate.  While the appearance of some
       of these characters on the list may seem surprising, BS now has
       an ambiguous interpretation in practice (erasing in some systems
       but not in others), the width associated with HT varies with the
       environment, and VT and FF do not have a uniform effect with
       regard to either vertical positioning or the associated
       horizontal position result.  Of course, telnet escapes are not
       considered part of the data stream and hence are unaffected by
       this provision.

   2.  In Net-ASCII, CR MUST NOT appear except when immediately followed
       by either NUL or LF, with the latter (CR LF) designating the "new
       line" function.  Today and as specified above, CR should
       generally appear only when followed by LF.  Because page layout
       is better done in other ways, because NUL has a special
       interpretation in some programming languages, and to avoid other
       types of confusion, CR NUL should preferably be avoided as
       specified above.

   3.  LF CR SHOULD NOT appear except as a side-effect of multiple CR LF
       sequences (e.g., CR LF CR LF).

   4.  The historical NVT documents do not call out either "bare LF" (LF
       without CR) or HT for special treatment.  Both have generally
       been understood to be problematic.  In the case of LF, there is a
       difference in interpretation as to whether its semantics imply
       "go to same position on the next line" or "go to the first
       position on the next line" and interoperability considerations
       suggest not depending on which interpretation the receiver
       applies.  At the same time, misinterpretation of LF is less
       harmful than misinterpretation of "bare" CR: in the CR case, text
       may be erased or made completely unreadable; in the LF one, the
       worst consequence is a very funny-looking display.  Obviously, HT
       is problematic because there is no standard way to transmit
       intended tab position or width information in running text.
       Again, the harm is unlikely to be great if HT is simply
       interpreted as one or more spaces, but, in general, it cannot be
       relied upon to format information.

   It is worth noting that the telnet IAC character (an octet consisting
   of all ones, i.e., %xFF) itself is not a problem for UTF-8 since that
   particular octet cannot appear in a valid UTF-8 string.  However,
   while few of them have been used, telnet permits other command-
   introducer characters whose bit sequences in an octet may be part of
   valid UTF-8 characters.  While it causes no ambiguity in UTF-8,



Klensin & Padlipsky         Standards Track                    [Page 13]

RFC 5198                    Network Unicode                   March 2008


   Unicode assigns a graphic character ("Latin Small Letter Y with
   Diaeresis") to U+00FF (octets C3 B0 in UTF-8).  Some caution is
   clearly in order in this area.

Appendix C.  The Line-Ending Problem

   The definition of how a line ending should be denoted in plain text
   strings on the wire for the Internet has been controversial from even
   before the introduction of NVT.  Some have argued that recipients
   should be required to interpret almost anything that a sender might
   intend as a line ending as actually a line ending.  Others have
   pointed out that this would lead to some ambiguities of
   interpretation and presentation and would violate the principle that
   we should minimize the number of forms that are permitted on the wire
   in order to promote interoperability and eliminate the "every
   recipient needs to understand every sender format" problem.  The
   design of this specification, like that of NVT, takes the latter
   approach.  Its designers believe that there is little point in a
   standard if it is to specify "anyone can do whatever they like and
   the receiver just needs to cope".

   A further discussion of the nature and evolution of the line-ending
   problem appears in Section 5.8 of the Unicode Standard [Unicode] and
   is suggested for additional reading.  If we were starting with the
   Internet today, it would probably be sensible to follow the
   recommendation there and use LS (U+2028) exclusively, in preference
   to CRLF.  However, the installed base of use of CRLF and the
   importance of forward compatibility with NVT and protocols that
   assume it makes that impossible, so it is necessary to continue using
   CRLF as the "New Line Function" ("NLF", see the terminology section
   in that reference).

Appendix D.  A Note about Related Future Work

   Consideration should be given to a Telnet (or SSH [RFC4251]) option
   to specify this type of stream and an FTP extension [RFC0959] to
   permit a new "Unicode text" data TYPE.














Klensin & Padlipsky         Standards Track                    [Page 14]

RFC 5198                    Network Unicode                   March 2008


References

Normative References

   [ISO10646]        International Organization for Standardization,
                     "Information Technology - Universal Multiple-Octet
                     Coded Character Set (UCS) - Part 1: Architecture
                     and Basic Multilingual Plane", ISO/
                     IEC 10646-1:2000, October 2000.

   [NFC]             Davis, M. and M. Duerst, "Unicode Standard Annex
                     #15: Unicode Normalization Forms", October 2006,
                     <http://www.unicode.org/reports/tr15/>.

   [RFC2119]         Bradner, S., "Key words for use in RFCs to Indicate
                     Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3629]         Yergeau, F., "UTF-8, a transformation format of ISO
                     10646", STD 63, RFC 3629, November 2003.

   [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 5.0", 2007.

                     Boston, MA, USA: Addison-Wesley.  ISBN
                     0-321-48091-0

   [Unicode32]       The Unicode Consortium, "The Unicode Standard,
                     Version 3.0", 2000.

                     (Reading, MA, Addison-Wesley, 2000.  ISBN 0-201-
                     61633-5).  Version 3.2 consists of the definition
                     in that book as amended by the Unicode Standard
                     Annex #27: Unicode 3.1
                     (http://www.unicode.org/reports/tr27/) and by the
                     Unicode Standard Annex #28: Unicode 3.2
                     (http://www.unicode.org/reports/tr28/).











Klensin & Padlipsky         Standards Track                    [Page 15]

RFC 5198                    Network Unicode                   March 2008


Informative References

   [ASCII]           American National Standards Institute (formerly
                     United States of America Standards Institute), "USA
                     Code for Information Interchange", ANSI X3.4-1968,
                     1968.

                     ANSI X3.4-1968 has been replaced by newer versions
                     with slight modifications, but the 1968 version
                     remains definitive for the Internet.  ISO 646
                     International Reverence Version (IRV)
                     [ISO.646.1991] is usually considered equivalent to
                     ASCII.

   [ISO.646.1991]    International Organization for Standardization,
                     "Information technology - ISO 7-bit coded character
                     set for information interchange", ISO Standard 646,
                     1991.

   [NamedSequences]  The Unicode Consortium, "NamedSequences-4.1.0.txt",
                     2005, <http://www.unicode.org/Public/UNIDATA/
                     NamedSequences.txt>.

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

   [RFC0097]         Melvin, J. and R. Watson, "First Cut at a Proposed
                     Telnet Protocol", RFC 97, February 1971.

   [RFC0137]         O'Sullivan, T., "Telnet Protocol - a proposed
                     document", RFC 137, April 1971.

   [RFC0139]         O'Sullivan, T., "Discussion of Telnet Protocol",
                     RFC 139, May 1971.

   [RFC0318]         Postel, J., "Telnet Protocols", RFC 318,
                     April 1972.

   [RFC0542]         Neigus, N., "File Transfer Protocol", RFC 542,
                     August 1973.

   [RFC0698]         Mock, T., "Telnet extended ASCII option", RFC 698,
                     July 1975.

   [RFC0742]         Harrenstien, K., "NAME/FINGER Protocol", RFC 742,
                     December 1977.





Klensin & Padlipsky         Standards Track                    [Page 16]

RFC 5198                    Network Unicode                   March 2008


   [RFC0854]         Postel, J. and J. Reynolds, "Telnet Protocol
                     Specification", STD 8, RFC 854, May 1983.

   [RFC0954]         Harrenstien, K., Stahl, M., and E. Feinler,
                     "NICNAME/WHOIS", RFC 954, October 1985.

   [RFC0959]         Postel, J. and J. Reynolds, "File Transfer
                     Protocol", STD 9, RFC 959, October 1985.

   [RFC1945]         Berners-Lee, T., Fielding, R., and H. Nielsen,
                     "Hypertext Transfer Protocol -- HTTP/1.0",
                     RFC 1945, May 1996.

   [RFC2277]         Alvestrand, H., "IETF Policy on Character Sets and
                     Languages", BCP 18, RFC 2277, January 1998.

   [RFC2616]         Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                     Masinter, L., Leach, P., and T. Berners-Lee,
                     "Hypertext Transfer Protocol -- HTTP/1.1",
                     RFC 2616, June 1999.

   [RFC2781]         Hoffman, P. and F. Yergeau, "UTF-16, an encoding of
                     ISO 10646", RFC 2781, February 2000.

   [RFC2821]         Klensin, J., "Simple Mail Transfer Protocol",
                     RFC 2821, April 2001.

   [RFC3454]         Hoffman, P. and M. Blanchet, "Preparation of
                     Internationalized Strings ("stringprep")",
                     RFC 3454, December 2002.

   [RFC3491]         Hoffman, P. and M. Blanchet, "Nameprep: A
                     Stringprep Profile for Internationalized Domain
                     Names (IDN)", RFC 3491, March 2003.

   [RFC3912]         Daigle, L., "WHOIS Protocol Specification",
                     RFC 3912, September 2004.

   [RFC4251]         Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
                     Protocol Architecture", RFC 4251, January 2006.

   [RFC4690]         Klensin, J., Faltstrom, P., Karp, C., and IAB,
                     "Review and Recommendations for Internationalized
                     Domain Names (IDNs)", RFC 4690, September 2006.







Klensin & Padlipsky         Standards Track                    [Page 17]

RFC 5198                    Network Unicode                   March 2008


Authors' Addresses

   John C Klensin
   1770 Massachusetts Ave, #322
   Cambridge, MA  02140
   USA

   Phone: +1 617 491 5735
   EMail: john-ietf@jck.com


   Michael A. Padlipsky
   8011 Stewart Ave.
   Los Angeles, CA  90045
   USA

   Phone: +1 310-670-4288
   EMail: the.map@alum.mit.edu

































Klensin & Padlipsky         Standards Track                    [Page 18]

RFC 5198                    Network Unicode                   March 2008


Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.












Klensin & Padlipsky         Standards Track                    [Page 19]


Html markup produced by rfcmarkup 1.108, available from http://tools.ietf.org/tools/rfcmarkup/