[1] [[電子メイル]]や[[電子ニュース]]で、関連記事を書き記すのがこの [DFN[[CODE(822)@en[[[References:]]]]]] 欄です。 [2] 電子メイルでは、参照する記事について人間可読な情報を書く方法と、参照する[[記事の識別子]]を書く方法の2種類がありました。 (後者は電子ニュースの用法を輸入したとされています。) 電子メイルでは当初は (いまでも厳密には) [[Message-ID]] は必須でなく、しかも頭は機械も処理しつつも人間が読み書きする欄だったことが影響しているのでしょう。 現在の電子メイルの仕様である [[RFC2822]] は、 Message-ID を書く方法を採用し、かつ似た意味を持つ [[In-Reply-To:欄]]との使い分けも規定しています。 [3] 電子ニュースでは、参照する記事の Message-ID を書きます。 (電子ニュースでは Message-ID は必須です。) 電子ニュースでは In-Reply-To: 欄がありませんから、他の記事への単なる参照関係を表すにはこの欄を使うしかありません。 * レンダリング [11] この欄の情報は、[[スレッド]]表示のために使われます。 * 関連 [14] [CODE[In-Reply-To:]], [CODE[See-Also:]] [15] [CODE[Message-ID:]] * 歴史 [FIG(quote)[ [FIGCAPTION[ [6] RFC 724 ]FIGCAPTION] ::= ... | "References" ":" |... ::= | | "," ::= | ::= "<" ">" *c. References: >The contents of this field identify other correspondence which this message references. If message identifiers are used, they should be enclosed in angle brackets (<>). この欄の中身はこのメッセージが参照する他の手紙を同定するものです。 なおメッセージ識別子が使われる場合、角括弧 (<>) で囲むべきです。 ]FIG] [FIG(quote)[ [FIGCAPTION[ [7] RFC 733 ]FIGCAPTION] optional-field = ... / "References" ":" #(phrase / mach-id) /... *3. References >The contents of this field identify other correspondence which this message references. Note that if message identifiers are used, they must use the mach-id specification format. この欄の中身はこのメッセージが参照する他の手紙を同定するものです。 なおメッセージ識別子が使われる場合、 mach-id 詳述書式を 使わなければなりません。 ]FIG] [FIG(quote)[ [FIGCAPTION[ [8] RFC 822 ]FIGCAPTION] optional-field = ... / "References" ":" *(phrase / msg-id) /... *4.6.3. REFERENCES >The contents of this field identify other correspondence which this message references. Note that if message identifiers are used, they must use the msg-id specification format. この欄の中身はこのメッセージが参照する他の手紙を同定するものです。 なおメッセージ識別子が使われる場合、 msg-id 記述書式を使わなければ成りません。 ]FIG] **RFC 2822 See [[RFC2822の3.6.4]] [FIG(quote)[ [FIGCAPTION[ [9] RFC 850 2.2.6 References ]FIGCAPTION] >This field lists the message ID's of any articles prompting the submission of this article. It is required for all follow-up articles, and forbidden when a new subject is raised. Implementations should provide a follow-up command, which allows a user to post a follow-up article. This command should generate a Subject line which is the same as the original article, except that if the original subject does not begin with "Re: " or "re: ", the four characters "Re: " are inserted before the subject. If there is no References line on the original header, the References line should contain the message ID of the original article (including the angle brackets). If the original article does have a References line, the followup article should have a References line containing the text of the original References line, a blank, and the message ID of the original article. >The purpose of the References header is to allow articles to be grouped into conversations by the user interface program. This allows conversations within a newsgroup to be kept together, and potentially users might shut off entire conversations without unsubscribing to a newsgroup. User interfaces may not make use of this header, but all automatically generated followups should generate the References line for the benefit of systems that do use it, and manually generated followups (e.g. typed in well after the original article has been printed by the machine) should be encouraged to include them as well. **RFC 1036 2.2.5. References >This field lists the Message-ID's of any messages prompting the submission of this message. It is required for all follow-up messages, and forbidden when a new subject is raised. Implementations should provide a follow-up command, which allows a user to post a follow-up message. This command should generate a "Subject" line which is the same as the original message, except that if the original subject does not begin with "Re:" or "re:", the four characters "Re:" are inserted before the subject. If there is no "References" line on the original header, the "References" line should contain the Message-ID of the original message (including the angle brackets). If the original message does have a "References" line, the follow-up message should have a "References" line containing the text of the original "References" line, a blank, and the Message-ID of the original message. >The purpose of the "References" header is to allow messages to be grouped into conversations by the user interface program. This allows conversations within a newsgroup to be kept together, and potentially users might shut off entire conversations without unsubscribing to a newsgroup. User interfaces need not make use of this header, but all automatically generated follow-ups should generate the "References" line for the benefit of systems that do use it, and manually generated follow-ups (e.g., typed in well after the original message has been printed by the machine) should be encouraged to include them as well. >It is permissible to not include the entire previous "References" line if it is too long. An attempt should be made to include a reasonable number of backwards references. ]FIG] [FIG(quote)[ [FIGCAPTION[ [10] usefor-article (06) 6.10. References ]FIGCAPTION] >The References header lists optionally CFWS-separated message identifiers of precursors. The content syntax makes use of syntax defined in [RFC 2822]. References-content = msg-id *( CFWS msg-id ) NOTE: This differs from the syntax of [RFC 2822] by requiring at least one CFWS between the msg-ids (this was an [RFC 1036] requirement). >A followup MUST have a References header, and an article that is not a followup MUST NOT have a References header. In a followup, if the precursor did not have a References header, the followup's References-content MUST be formed by the message identifier of the precursor. A followup to an article which had a References header MUST have a References header containing the precursor's References- content (subject to trimming as described below) plus the precursor's message identifier appended to the end of the list (separated from it by CFWS). Followup agents SHOULD NOT trim message identifiers out of a References header unless the number of message identifiers exceeds 21, at which time trimming SHOULD be done by removing sufficient identifiers starting with the second so as to bring the total down to 21 (but the first message identifier MUST NOT be trimmed). However, it would be wrong to assume that References headers containing more than 21 message identifiers will not occur. *6.10.1. Examples References: References: References: <222@site1.example> <87tfbyv@site7.example> <67jimf@site666.example> References: ]FIG] [13] [CITE@en[RFC 5536 - Netnews Article Format]] ( ([TIME[2014-09-21 18:14:06 +09:00]] 版)) [5] [CITE@en[RFC 5537 - Netnews Architecture and Protocols]] ( ([TIME[2014-09-14 17:08:11 +09:00]] 版)) *メモ [4] [[返信]]の時に返信元記事の [[Message-ID]] を返信元記事の References: 欄の内容の前につける実装と後につける実装があります。最新の仕様や多くの実装にならうなら、後につける方が望ましいです。[[スレッド]]表示するときに末尾の Message-ID を親記事の Message-ID とみなす実装が多いようです。 [12] [[Gmail]] のように、表示時にこの欄を使わない [[MUA]] もあります。