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Versions: 00 01 02                                                      
Network Working Group                                       Jacob Palme
Internet Draft                                 Stockholm University/KTH
IETF status: To become an informational RFC
Expires: January 1998                                        March 1998

Advice on the implementation of In-Reply-To, References and Supersedes
e-mail and netnews headers

Status of this Document

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute working
documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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or to cite them other than as ``work in progress.''

To learn the current status of any Internet-Draft, please check the
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ftp.isi.edu (US West Coast).

Copyright (C) The Internet Society 1998. All Rights Reserved.


Separate Internets standards documents define the e-mail headers
In-Reply-To, References, Supersedes and Expires. This document, which
is an informational RFC, gives some advice on the implementation of
these features.

Table of Contents

1. User interface
2. Hard and soft Supersedes
3. Data base
4. Copyright
5. References
6. Author's Address

1.    User interface

The fields "In-Reply-To", "References" and "Supersedes" are all used to
convey information about references between different e-mail messages
or netnews articles.

A good way to implement these fields is to tell the recipient that two
messages reference each other, and to make it easy for readers to
traverse threads (series of linked messages) up and down.

It is also possible to have special features to see a whole thread (set
of related messages) graphically, or as an indented list, and to allow
users to traverse, print, save or do other actions on a thread.

Example of showing a thread as an indented list:

   This is entry no. 1, the start entry of the thread
     This is entry no. 2, a reply to entry no. 1
       This is entry no. 3, a reply to entry no. 2
     This is entry no. 4, a reply to entry no. 1

In the particular case of "Supersedes", a user who has not yet read
either the old or the new version, may be shown only the new version as
a new message, but with methods to easily find the old version.

A way to show this information to users is to show the "In-Reply-To",
"Supersedes" and "References" fields, possibly as buttons, and allow
the user to click on them to get to the referred-to messages.
Additionally, it is useful to add buttons to follow threads forward,
with texts like "Next in thread" or "Replies" or "This document is
referenced by" or "Superseding documents". This allows a user, when
reading a message, to see if someone else has already replied, and it
allows users to traverse threads downwards and not only upwards. Such
reverse buttons should not be sent in e-mail, they are just for local
handling in user mailbox databases. Note that their values may change
after a message has been submitted, when more new messages arrive which
reference it.

Example of showing a message with thread information:

   To: IETF-Announce: ;
   From: The IESG <iesg-secretary@ns.ietf.org>
   Subject: Last Call: The Auto-Submitted, Supersedes and Expires
      Headers in E-mail and Netnews to Proposed Standard
   In-Reply-To: <v04003a00b12335fb8686@ns.ietf.org>
   Replied-By: <v04003a00b12335fb8687@ns.ietf.org>
   Date: Thu, 05 Mar 1998 07:02:47 -0500
   Sender: scoya@cnri.reston.va.us

2.    Hard and soft Supersedes

By a hard supersedes is meant a Supersedes which causes deletion of the
superseded message. By a soft supersedes is meant a Supersedes which
still keeps both messages, and allows a user to see and use the
reference between them, somewhat similar to In-Reply-To and References.

Supersedes is best implemented as soft supersedes. Users of the
supersedes field should however be aware that some implementations,
especially in Usenet News, do implement it as hard supersedes.

Hard supersedes has the same security problem as the Cancel command of
Usenet News. They can be used to maliciously delete other people's
messages. Use of strong authentication of the author can reduce this

3.    Data base

In order to implement threads, a data base is needed which, given a
Message-ID, can find the message which this Message-ID refers to. This
data base has a very simple structure, just a single value mapped to
one or more messages. Note, however, that the same message can be
copied to more than one mailbox, so the data base should not be
restricted to only one location for each Message-ID.

Every time a message is added, moved, copied, deleted or purged, this
data base need to be updated.

When a new message arrives, the mailer can find the messages, to which
this message has references. Note that there is a risk that replies
arrive before the replied-to message, so a good implementation should
work even in this case.

A problem with these kinds of Message-ID data bases is that they tend
to become very large with time, and they easily collect garbage
(Message-ID-s of messages not any more available in the mailbox data

The two most common methods to implement such data bases are:

(a) Implement a large data base, but with some method of purging to
    avoid unlimited growth of the data base.

(b) Implement a smaller data base, where all objects are deleted
    after a certain time. A couple of months is enough if the
    techniques described in the next paragraph are used.

With implementation method (b), information about the references in the
form of "In-Reply-To", "References", "Supersedes", "Replied-By",
"Referenced-By" and "Superseded-By" should also be stored in the
message headers themselves. The reason method (b) works is that it is
very uncommon that a message has a reference to other than very recent
messages. Thus, the lack of "Replied-By", "Referenced-By" and
"Superseded-By" headers in these very uncommon cases is acceptable.

The advantage with method (b) is that a complex garbage collection
method, as for method (a), is not needed. A much simpler garbage
collection method can be used instead, just removing records after a
certain expiration time.

4.    Copyright

Copyright (C) The Internet Society (date). All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing the
copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of developing
Internet standards in which case the procedures for copyrights defined
in the Internet Standards process must be followed, or as required to
translate it into languages other than English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

5.    References

Ref.            Author, title
---------       --------------------------------------------------------

[AUTOLOOP]      J. Palme: "Loop control for the Auto-Submitted e-mail
                header", draft-palme-autosub-03.txt, July 1997.

[MIME1]         N. Freed, N. Borenstein, "Multipurpose Internet Mail
                Extensions (MIME) Part One: Format of Internet Message
                Bodies", RFC 2045, December 1996.
[MIME2]         N. Freed, N. Borenstein, "Multipurpose Internet Mail
                Extensions (MIME) Part Two:  Media Types", RFC 2046,
                December 1996.

[MIME3]         K. Moore, "MIME (Multipurpose Internet Mail Extensions)
                Part Three:  Message Header Extensions for Non-ASCII
                Text", RFC 2047, December 1996.

[MIME4]          N. Freed, J. Klensin, J. Postel, "Multipurpose Internet
                Mail Extensions (MIME) Part Four:  Registration
                Procedures", RFC 2048, January 1997.

[MIME5]         "Multipurpose Internet Mail Extensions (MIME) Part Five:
                Conformance Criteria and Examples", RFC 2049, December

[NEWFIELDS]     J. Palme: "The Auto-Submitted, Supersedes and Expires
                E-mail Headers", draft-ietf-mailext-new-fields-08.txt,
                July 1997.

[NEWS]          M.R. Horton, R. Adams: "Standard for interchange of
                USENET messages", RFC 1036, December 1987.

[RFC822]        D. Crocker: "Standard for the format of ARPA Internet
                text messages." STD 11, RFC 822, August 1982.

[SMTP]          J. Postel: "Simple Mail Transfer Protocol", STD 10, RFC
                821, August 1982.

6.    Author's Address

Jacob Palme                          Phone: +46-8-16 16 67
Stockholm University and KTH         Fax: +46-8-783 08 29
Electrum 230                         E-mail: jpalme@dsv.su.se
S-164 40 Kista, Sweden