usefor-article-09 February 2003

5.5.  Newsgroups

   The Newsgroups-header's content specifies the newsgroup(s) in which
   the article is intended to appear. It is an inheritable header
   (4.2.5.2) which then becomes the default Newsgroups-header of any
   followup, unless a Followup-To-header is present to prescribe
   otherwise.  Articles MUST NOT be passed between relaying agents or to
   serving agents unless the sending agent has been configured to supply
   and the receiving agent to receive at least one of the newsgroup-
   names in the Newsgroups-header.

   In order to allow newsgroup-names containing Non-ASCII characters,
   this section relies heavily on the provisions of the Unicode
   Standard.  All references to "Unicode" mean [UNICODE 3.2] or any
   standard that supersedes it. That document contains guarantees of
   strict future upwards compatibility (e.g. no character will be
   removed or change classification). Implementors should be aware that
   currently unassigned code points (Unicode category Cn) may become
   valid characters in future versions of Unicode. Since the poster of
   an article might have access to a newer version of that standard,
   relaying and serving agents MUST accept such characters, but posting
   agents (and indeed all agents) MUST NOT generate them (though they
   might well follow up to newsgroup-names containing them).

      header              =/ Newsgroups-header
      Newsgroups-header   = "Newsgroups"  ":" SP Newsgroups-content
                          *( ";" extension-parameter )
      Newsgroups-content  = [FWS] newsgroup-name
                     *( [FWS] ng-delim [FWS] newsgroup-name )
                     [FWS]
      newsgroup-name      = component *( "." component )
      component           = 1*component-grapheme
      ng-delim            = ","
      component-grapheme  = combiner-base *combiner-mark
      combiner-base       = combiner-ASCII / combiner-extended
      combiner-ASCII      = DIGIT / ALPHA / "+" / "-" / "_"
      combiner-extended   = <any character with a Unicode code value
                   of 0080 or greater but excluding any
                   character in Unicode categories
                   Cc, Cf, Cs, M* and Z*>
      combiner-mark       = <any character with a Unicode code value of
                   0080 or greater and in Unicode category M*>

        NOTE: the excluded characters in a combiner-extended are control
        characters (Cc), format control characters (Cf), surrogates
        (Cs), marks (M*) and separators (Z*). In particular, this
        excludes all whitespace characters.  To all intents and
        purposes, a component-grapheme is what a user might regard as a
        single "character" as displayed on his screen, though it might
        be transmitted as several actual characters (e.g. q-circumflex
        is two characters). Note also that, in some writing schemes,
        several component-graphemes will merge into one visible object
        of variable size.

   Each component MUST be invariant under Unicode normalization NFKC
   (cf. the weaker normalization requirement for other headers in
   section 4.4.1 which specified no more than normalization NFC, and see
   also the explanatory NOTE in that section).

        NOTE: As a result of of this restriction, a name has only one
        valid form. Implementations can assume that a straight (case
        sensitive) comparison of characters or octets is sufficient to
        compare two newsgroup-names.

        The requirement that names be invariant under NFKC, rather than
        NFC, means that all characters with a "compatibility
        decomposition" are forbidden (Unicode provides the property
        "NFKC_NO" to make this test easier).  The effect is to exclude
        variant forms of characters, such as superscripts and
        subscripts, wide and narrow forms, font variants, encircled
        forms, ligatures, and so on, as their use could cause confusion.

        There is insufficient experience in this area to determine
        whether this is the right long-term solution. Implementors
        should therefore be aware that a future version of this standard
        might reduce the requirement in the direction of NFC as opposed
        to NFKC.

        NOTE: An implementation is not required to apply NFKC, or any
        other normalization, to newsgroup-names. Only agencies that
        create new groups need to be careful to obey this restriction
        (7.2.1).  However, if a posting agent neglects to normalize a
        newsgroup-name entered manually, this may lead to the user
        posting to a non-existent group without understanding why.

   Newsgroup-names containing non-ASCII characters MUST be encoded in
   UTF-8. The use of [RFC 2047] encoding is inappropriate for reasons
   explained in section 4.4.1.

   Components beginning with underline ("_") are reserved for use by
   future versions of this standard and MUST NOT occur in newsgroup-
   names (whether in Newsgroups-headers or in newgroup control messages
   (7.2.1)).  However, such names MUST be accepted.

   Components beginning with "+" or "-" are reserved for use by
   implementations and MUST NOT occur in newsgroup-names (whether in
   Newsgroups-headers or in newgroup control messages). Implementors may
   assume that this rule will not change in any future version of this
   standard.

        NOTE: For example, implementors may safely use leading "+" and
        "-" to "escape" other entities within something that looks like
        a newsgroup-name.

   Agencies responsible for the administration of particular hierarchies
   Ought to place additional restrictions on the characters they allow
   in newsgroup-names within those hierarchies (such as to accord with
   the languages commonly used within those hierarchies, or to avoid
   perceived ambiguities pertinent to those languages). Where there is
   no such specific policy, the following restrictions SHOULD be applied
   to newsgroup-names.

        NOTE: These restrictions are intended to reflect existing
        practice, with some additions to accommodate foreseeable
        enhancements, and are intended both to avoid certain technical
        difficulties and to avoid unnecessary confusion. It may well be
        that experience will allow future extensions to this standard to
        relax some or all of these restrictions.

   The specific restrictions (to be applied in the absence of
   established policies to the contrary) are:

   1. The following characters are forbidden, subject to the comments
      and notes at the end of the list:

      characters in category Cn (Other, Not assigned)         [1]
      characters in category Co (Other, Private Use)          [2]
      characters in category Lt (Letter, Titlecase)           [3]
      characters in category Lu (Letter, Uppercase)           [3]
      characters in category Me (Mark, Enclosing)             [4]
      characters in category Pd (Punctuation, Dash)           [4][5]
      characters in category Pe (Punctuation, Close)          [4]
      characters in category Pf (Punctuation, Final quote)    [4]
      characters in category Pi (Punctuation, Initial quote)  [4]
      characters in category Po (Punctuation, Other)          [4]
      characters in category Ps (Punctuation, Open)           [4]
      characters in category Sc (Symbol, Currency)            [4]
      characters in category Sk (Symbol, Modifier)            [4]
      characters in category Sm (Symbol, Math)                [4][5]
      characters in category So (Symbol, Other)               [4]

      [1] As new characters are added to Unicode, the code point moves
from category Cn to some other category. As stated above,
implementors should be prepared for this.

      [2] Specific private use characters can be used within a hierarchy
or co-operating subnet that has agreed meanings for them.

      [3] Traditionally, newsgroup-names have been written in lowercase.
Posting agents Ought Not to convert uppercase or titlecase
characters to the corresponding lowercase forms except under
the explicit instructions of the poster.

      [4] Traditionally newsgroup-names have only used letters, digits,
and the three special characters "+", "-" and "_". These
categories correspond to characters outside that set.

      [5] Although the characters "+" and "-" are within categories Pd
and Sm, they are not forbidden.

   2. A component name is forbidden to consist entirely of digits.

        NOTE: This requirement was in [RFC 1036] but nevertheless
        several such groups have appeared in practice and implementors
        should be prepared for them. A common implementation technique
        uses each component as the name of a directory and uses numeric
        filenames for each article within a group. Such an
        implementation needs to be careful when this could cause a clash
        (e.g. between article 123 of group xxx.yyy and the directory for
        group xxx.yyy.123).
   3. A component is limited to 30 component-graphemes and a newsgroup-
      name to 71 component-graphemes (counting also the '.'s separating
      the components). Whilst there is no longer any technical reason to
      limit the length of a component (formerly, it was limited to 14
      octets) nor of a newsgroup-name, it should be noted that these
      names are also used in the newsgroups-line (7.2.1.2) where an
      overall policy limit applies and, moreover, excessively long names
      can be exceedingly inconvenient in practical use.

   Serving and relaying agents MUST accept any newsgroup-name that meets
   the above requirements, even if they violate one or more of the
   policy restrictions. Posting and injecting agents MAY reject articles
   containing newsgroup-names that do not meet these restrictions, and
   posting agents MAY attempt to correct them (but only with the
   explicit agreement of the poster for anything more than NFC or NFKC
   normalization). However, because of the large and changing tables
   required to do these checks and corrections throughout the whole of
   Unicode, this standard does not require them to do so. Rather, the
   onus is placed on those who create new newsgroups (7.2.1) to check
   the mandatory requirements, to consider the effects of relaxing the
   other restrictions, and to consider how all this may affect
   propagation of the group.

   Since future extensions to this standard and the Unicode standard,
   including a possible relaxation of the NFKC normalization, plus any
   relaxations of the default restrictions introduced by specific
   hierarchies might invalidate some such checks, warnings, and
   adjustments, implementations MUST incorporate means to disable them.

      NOTE: The newsgroup-name as encoded in UTF-8 should be regarded as
      the canonical form. Reading agents may convert it to whatever
      character set they are able to display and serving agents may
      possibly need to convert it to some form more suitable as a
      filename. Simple algorithms for both kinds of conversion are
      readily available.  Observe that the syntax does not allow
      comments within the Newsgroups-header; this is to simplify
      processing by relaying and serving agents which have a requirement
      to process this header extremely rapidly.

   The inclusion of folding white space within a Newsgroups-content is a
   newly introduced feature in this standard. It MUST be accepted by all
   conforming implementations (relaying agents, serving agents and
   reading agents).  Posting agents should be aware that such postings
   may be rejected by overly-critical old-style relaying agents. When a
   sufficient number of relaying agents are in conformance, posting
   agents SHOULD generate such whitespace in the form of <CRLF WSP> so
   as to keep the length of lines in the relevant headers (notably
   Newsgroups and Followup-To) to no more than than 79 characters (or
   other agreed policy limit - see 4.5).  Before such critical mass
   occurs, injecting agents MAY reformat such headers by removing
   whitespace inserted by the posting agent, but relaying agents MUST
   NOT do so.
   Posters SHOULD use only the names of existing newsgroups in the
   Newsgroups-header. However, it is legitimate to cross-post to
   newsgroups which do not exist on the posting agent's host, provided
   that at least one of the newsgroups DOES exist there, and followup
   agents SHOULD accept this (posting agents MAY accept it, but Ought at
   least to alert the poster to the situation and request confirmation).
   Relaying agents MUST NOT rewrite Newsgroups-headers in any way, even
   if some or all of the newsgroups do not exist on the relaying agent's
   host. Serving agents MUST NOT create new newsgroups simply because an
   unrecognized newsgroup-name occurs in a Newsgroups-header (see 7.2.1
   for the correct method of newsgroup creation).

   The Newsgroups-header is intended for use in Netnews articles rather
   than in email messages. It MAY be used in an email message to
   indicate that it is a copy also posted to the listed newsgroups, in
   which case the inclusion of a Posted-And-Mailed header (6.9) would
   also be appropriate. However, it SHOULD NOT be used in an email-only
   reply to a Netnews article (thus the "inheritable" property of this
   header applies only to followups to a newsgroup, and not to followups
   to the poster). Moreover, if a newsgroup-name contains any non-ASCII
   character, it may need to be encoded using the mechanism defined in
   section 5.5.2.  See also the further discussion in section 8.8.1.1.

--- ../usefor-article-08/Newsgroups.out          August 2002
+++ ../usefor-article-09/Newsgroups.out          February 2003
@@ -24,7 +24,7 @@
 
       header              =/ Newsgroups-header
       Newsgroups-header   = "Newsgroups"  ":" SP Newsgroups-content
-                          *( ";" other-parameter )
+                          *( ";" extension-parameter )
       Newsgroups-content  = [FWS] newsgroup-name
                      *( [FWS] ng-delim [FWS] newsgroup-name )
                      [FWS]
@@ -84,7 +84,8 @@
         posting to a non-existent group without understanding why.
 
    Newsgroup-names containing non-ASCII characters MUST be encoded in
-   UTF-8 and not according to [RFC 2047].
+   UTF-8. The use of [RFC 2047] encoding is inappropriate for reasons
+   explained in section 4.4.1.
 
    Components beginning with underline ("_") are reserved for use by
    future versions of this standard and MUST NOT occur in newsgroup-
@@ -100,6 +101,7 @@
         NOTE: For example, implementors may safely use leading "+" and
         "-" to "escape" other entities within something that looks like
         a newsgroup-name.
+
    Agencies responsible for the administration of particular hierarchies
    Ought to place additional restrictions on the characters they allow
    in newsgroup-names within those hierarchies (such as to accord with
@@ -152,6 +154,7 @@
       [4] Traditionally newsgroup-names have only used letters, digits,
 and the three special characters "+", "-" and "_". These
 categories correspond to characters outside that set.
+
       [5] Although the characters "+" and "-" are within categories Pd
 and Sm, they are not forbidden.
 
@@ -165,7 +168,6 @@
         implementation needs to be careful when this could cause a clash
         (e.g. between article 123 of group xxx.yyy and the directory for
         group xxx.yyy.123).
-
    3. A component is limited to 30 component-graphemes and a newsgroup-
       name to 71 component-graphemes (counting also the '.'s separating
       the components). Whilst there is no longer any technical reason to
@@ -204,6 +206,7 @@
       comments within the Newsgroups-header; this is to simplify
       processing by relaying and serving agents which have a requirement
       to process this header extremely rapidly.
+
    The inclusion of folding white space within a Newsgroups-content is a
    newly introduced feature in this standard. It MUST be accepted by all
    conforming implementations (relaying agents, serving agents and
@@ -217,7 +220,6 @@
    occurs, injecting agents MAY reformat such headers by removing
    whitespace inserted by the posting agent, but relaying agents MUST
    NOT do so.
-
    Posters SHOULD use only the names of existing newsgroups in the
    Newsgroups-header. However, it is legitimate to cross-post to
    newsgroups which do not exist on the posting agent's host, provided
@@ -239,5 +241,5 @@
    header applies only to followups to a newsgroup, and not to followups
    to the poster). Moreover, if a newsgroup-name contains any non-ASCII
    character, it may need to be encoded using the mechanism defined in
-   section 5.5.2.  See also the further discussion in section 8.8.1.
+   section 5.5.2.  See also the further discussion in section 8.8.1.1.