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[1] [[電子ニュース]]の[[メッセージ]]の [DFN[[CODE(822)@en[Xref:]] [[頭欄]]]]は、
[[ニュース鯖]]が[[メッセージ]]の識別番号を保持するために使われています。

* 仕様書

[REFS[
- [2] [CITE@en[RFC 5536 - Netnews Article Format]]
( ([TIME[2014-09-21 18:14:06 +09:00]] 版))
<https://tools.ietf.org/html/rfc5536#section-3.2.14>
]REFS]

* URL

[3] [[emacs-w3m]] の [[shimbun]] では、元記事の [[URI]] を入れています。

;; [4] [[shimbun]] の出力は[[ネットニュース]]の[[メッセージ]]ではありませんが、
その構文が流用されていました。

* 歴史

** RFC 1036

[FIG(quote)[ [5] RFC 1036 2.2.13.  Xref

This line contains the name of the host (with domains omitted) and a
white space separated list of colon-separated pairs of newsgroup
names and message numbers.  These are the newsgroups listed in the
"Newsgroups" line and the corresponding message numbers from the
spool directory.

この行はホスト名 (ドメインは省略)と
コロンで区切った新聞組名とメッセージ番号の組み合わせの
空白間隔で区切った一覧を入れます。これは「Newsgroups」(新聞組)
行に列せられた新聞組とスプール・ディレクトリの対応する
メッセージ番号です。

This is only of value to the local system, so it should not be
transmitted.  For example, in:

これは単に局地系統の値であるだけで、転送されるべきものではありません。
例えば、

[PRE[
               Path: seismo!lll-crg!lll-lcc!pyramid!decwrl!reid
               From: reid@decwrl.DEC.COM (Brian Reid)
               Newsgroups: news.lists,news.groups
               Subject: USENET READERSHIP SUMMARY REPORT FOR SEP 86
               Message-ID: <5658@decwrl.DEC.COM>
               Date: 1 Oct 86 11:26:15 GMT
               Organization: DEC Western Research Laboratory
               Lines: 441
               Approved: reid@decwrl.UUCP
               Xref: seismo news.lists:461 news.groups:6378
]PRE]

the "Xref" line shows that the message is message number 461 in the
newsgroup news.lists, and message number 6378 in the newsgroup
news.groups, on host seismo.  This information may be used by
certain user interfaces.

で「Xref」行はこのメッセージがホスト seismo において新聞組 news.lists 
のメッセージ番号461, news.groups のメッセージ番号6378 であることを
表します。この情報は利用者界面で使っても構いません。
]FIG]

** [[son-of-RFC1036]]

[FIG(quote)[ [6] 6.12. Xref

The Xref header content indicates where an article was filed
          by the last relayer to process it:

Xref-content     = relayer 1*( space location )
               relayer          = relayer-name
               location         = newsgroup-name ":" article-locator
               article-locator  = 1*<ASCII printable character>

The relayer's name is included so that software  can  deter-
          mine  which  relayer generated the header (and specifically,
          whether it really was the one  that  filed  the  copy  being
          examined).   The locations specify what newsgroups the arti-
          cle was filed under (which may  differ  from  those  in  the
          Newsgroups  header)  and where it was filed under them.  The
          exact form of an article locator is implementation-specific.

NOTE:  Reading agents can exploit this information
               to avoid presenting the same article to  a  reader
               several   times.   The  information  is  sometimes
               available in system databases, but  having  it  in
               the article is convenient.  Relayers traditionally
               generate an Xref header only  if  the  article  is
               cross-posted, but this is not mandatory, and there
               is at  least  one  new  application  ("mirroring":
               keeping  news  databases  on  two hosts identical)
               where the header is useful in all articles.

NOTE: The traditional form of an  article  locator
               is  a  decimal number, with articles in each news-
               group  numbered  consecutively  starting  from  1.
               NNTP  [rrr] demands that such a model be provided,
               and there may be other software which expects  it,
               but  it  seems desirable to permit flexibility for
               unorthodox implementations.

A relayer inserting an Xref  header  into  an  article  MUST
          delete  any  previous  Xref  header.  A relayer which is not
          inserting its own Xref header  SHOULD  delete  any  previous
          Xref  header.   A  relayer  MAY  delete the Xref header when
          passing an article on to another relayer.

Xref 頭を記事に挿入する中継者は前の Xref 頭を削除し''なければなりません''。
自分の Xref 頭を挿入しない中継者は前の Xref 頭を削除する''べき''です。
中継者は Xref 頭を他の中継者に渡す時に Xref 頭を削除しても
''構いません''。

NOTE: RFC 1036 specified that the Xref header  was
               not  transmitted  when  an  article  was passed to
               another relayer, but the  major  news  implementa-
               tions  have  never  obeyed this rule, and applica-
               tions like mirroring depend on this  disobedience.

参考: RFC 1036 は Xref 頭は記事が他の中継者に渡される時に
転送されないとしていましたが、有名ニュース実装はこの規則に従わず、
鏡化(ミラーリング)のような応用はこの規格違反に依存してきました。

A  relayer MUST use the same name in Xref headers as it uses
          in Path headers.  Reading agents MUST ignore an Xref  header
          containing  a  relayer  name  that differs from the one that
          begins the path list.

中継者は Xref 頭に Path 頭と同じ名前を使わ''なければなりません''。
読み代理者は経路表の最初の中継者名と異なる Xref 頭を無視し
''なければなりません''。

]FIG]

** [[draft-usefor-article]]

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