usefor-article-03 February 2000

6.13.2.1.  Message version numbers

   According to [MESSFOR], and omitting the obsolete forms, the syntax
   of the left hand side of a msg-id (the part before the "@") is given
   by:

      id-left-side        = dot-atom-text / no-fold-quote
   Consider this to be replaced by:

      id-left-side        = ( atom-text / no-fold-quote )
                     *( dollars-sequence )
      dollars-sequence    = version-number / random-dollars-sequence
      version-number      = "$" %d118 "=" 1*DIGIT ; $v=digits
      random-dollars-sequence
                = "$" 1*atom-text

   Whilst this is admittedly ambiguous ("$" is already a possible value
   of atom-text) and does not in fact change what is allowable as an
   id-left-side, it does serve to allow dollars-sequences such as
   version-number (and any others that may be added by extensions to
   this standard) to be distinguished within a message identifier and
   utilized by agents which can understand them.  Observe that no-fold-
   quotes cannot occur within a dollars-sequence.

   Posters and/or posting agents when replacing (or superseding)
   articles SHOULD arrange that the message identifier of the
   replacement follows the following convention, generating what are
   known as "version-number" message identifiers. This is to enable the
   new version of the article to be retrieved by its original message
   identifier, notably when it occurs in a URL of the form
   <news:message-identifier> [RFC 1738].

   1. If the id-left-side of the most recent predecessor's message
      identifier contains a leftmost version-number "$v=<n>", where <n>
      is an integer version number, possibly followed by one or more
      random-dollars-sequences, the replacement message identifier
      should be obtained by replacing the <n> with the integer <n+1> and
      providing a different random-dollars-sequence(s). For example
      <foo$v=3$XYZ@faq-site.example> becomes <foo$v=4$PQR@faq-
      site.example>.

   2. If the id-left-side of the predecessor's message identifier does
      not contain a version-number, the replacement message identifier
      should be obtained by appending the string "$v=1", preferably
      followed by a random-dollars-sequence(s), to that id-left-side.
      For example <foo@faq-site.example> becomes <foo$v=1$ABC@faq-
      site.example>.

   Any random-dollars-sequence so added MUST NOT start with "$<l>=" for
   any letter <l>.

        NOTE: The presence of a random-dollars-sequence following the
        version-number is intended to prevent a malicious poster from
        preempting the posting of a replacement article by guessing its
        likely message identifier.

   Attempts to fetch a replaced (or superseded) article by its message
   identifier SHOULD retrieve instead its most recent successor which
   has used the version-number convention. Some indication that a newer
   version than was asked for has been delivered SHOULD be provided.
   This is intended to ensure that "news:" URLs [RFC-1738] will continue
   to work even when an article has been replaced, but agents SHOULD
   then draw attention to the fact that the message identifier retrieved
   differed from that requested.