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Network Working Group                                          M. Duerst
Internet-Draft                                  Aoyama Gakuin University
Expires: April 27, 2006                                 October 24, 2005

                   The Archived-At Message Header Field
Status of this Memo
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    This Internet-Draft will expire on April 27, 2006.
Copyright Notice
    Copyright (C) The Internet Society (2005).
    This memo defines a new email header field, Archived-At:, to provide
    a direct link to the archived form of an individual email message.

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Table of Contents
    1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
    2.  Syntax Definition  . . . . . . . . . . . . . . . . . . . . . .  3
    3.  Multiple Archived-At Header Fields . . . . . . . . . . . . . .  4
    4.  Formats of Archived Message  . . . . . . . . . . . . . . . . .  4
    5.  Implementation Considerations  . . . . . . . . . . . . . . . .  4
    6.  Usage Considerations . . . . . . . . . . . . . . . . . . . . .  5
    7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
    8.  IANA considerations  . . . . . . . . . . . . . . . . . . . . .  7
    9.  Change Log . . . . . . . . . . . . . . . . . . . . . . . . . .  7
      9.1.  Changes from -03.txt to -04.txt  . . . . . . . . . . . . .  7
      9.2.  Changes from -02.txt to -03.txt  . . . . . . . . . . . . .  7
    10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
    11. Normative References . . . . . . . . . . . . . . . . . . . . .  8
    Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .  9
    Intellectual Property and Copyright Statements . . . . . . . . . . 10

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1.  Introduction
    [RFC2369] defines a number of header fields that can be added to
    email messages sent by email distribution lists.  One of them is the
    'List-Archive' header field that describes how to access archives for
    the list.  This allows to access the archives as a whole, but not the
    individual message.
    There is often a need or desire to refer to the archived form of a
    single message.  For more detailled usage scenarios, please see
    Section 6.  This memo defines a new header, 'Archived-At', to refer
    to a single message at an archived location.  This can help to find a
    message to a mailing list in the archive of that mailing list much
    more quickly and directly.  It can also be used independently of
    mailing lists, for example in connection with legal requirements to
    archive certain messages.
    In this document, the key words "MUST", "MUST NOT", "REQUIRED",
    and "OPTIONAL" are to be interpreted as described in [RFC2119].

2.  Syntax Definition
    The name of the header field is 'Archived-At'.  The content of the
    header field mostly consist of an URI [STD66] enclosed in angle
    brackets ('<', '>'), with internal whitespace being ignored.  MTAs
    MUST NOT insert whitespace within the angle brackets, but client
    applications should treat any whitespace, which might have been
    inserted by poorly behaved MTAs, as characters to ignore.  The URI
    points to an archived version of the message.  See Section 4 for some
    more details.
    This header field is subject to the encoding and character
    restrictions for mail headers as described in [RFC2822].
    Additionally, the URI content is further restricted to the set of URI
    safe characters [STD66].
    More formally, the header field is defined as follows in ABNF
    according to [RFC4234]:
       archived-at = "Archived-At:" *WSP '<' URI '>' CRLF ; URI not empty
    where URI is defined in [STD66] and CRLF and WSP are defined in

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    For backward-compatibility, the following MAY also be parsed (but
    MUST not be generated):
       obs-archived-at = "X-Archived-At:"  *WSP URI CRLF ; URI not empty
    This syntax is kept simple in that only one URI per header field is
    allowed.  In this point, the syntax is different from [RFC2369].

3.  Multiple Archived-At Header Fields
    Each Archived-At header field only contains a single URI.  If it is
    desired to list multiple URIs where an archived copy of the message
    can be found, a separate Archived-At field is required for each URI.
    Multiple Archived-At header fields with the same URI SHOULD be
    avoided.  An Archived-At header field SHOULD only be created if the
    message is actually being made available at the URI given in the
    header field.
    If a message is forwarded from a list to a sublist, and both lists
    support adding the Archived-At header field, then the sublist SHOULD
    add a new Archived-At header field without removing the already
    existing one(s), unless the header field is exactly the same as an
    already existing one, in which case the new header field SHOULD NOT
    be added.

4.  Formats of Archived Message
    There is no restriction on the format used to serve the archived
    message from the URI in the 'Archived-At' header field.  It is
    expected that in many cases, the archived message will be served as
    (X)HTML, as plain text, or in their original form as message/rfc822
    [RFC2046].  Some forms of URIs may imply the format in which the
    archived message is served, although this should not be relied upon.
    If the protocol used to retrieve the message allows for content
    negotiation, then it is also possible to serve the archived message
    in several different formats.  As an example, a HTTP URI in an
    'Archived-At' header may allow to serve the archived message both as
    text/html for human consumption in a browser and as message/rfc822
    for use by a mail user agent without loss of information.

5.  Implementation Considerations
    This section shortly discusses some implementation considerations.
    Mailing list expanders and email archives are often separate pieces

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    of software.  It may therefore be difficult to create an
    'Archived-At' header field in the mailing list expander software.
    One way to address this difficulty is to have the mailing list
    expander software generate an unambiguous URI, e.g. an URI based on
    the message id of the incoming email, and to set up the archiving
    system so that it redirects requests for such URIs to the actual
    Such a system has been implemented and is in productive use at W3C.
    As an example, the URI
    containing significant part of the message id
    "<0I5U00G08DFGCR@mailsj-v1.corp.adobe.com>", is redirected to the URI
    of this message in the W3C mailing-list archive at
    Source code for this implementation is available at
    http://dev.w3.org/cvsweb/search/, in particular
    http://dev.w3.org/cvsweb/search/cgi/mid.pl and
    When using the message id to create an address for the archived mail,
    care has to be taken to escape characters in the message id that are
    not allowed in the URI, or to remove them, as done above for the "<"
    and ">" delimiters.

6.  Usage Considerations
    It may at first seem strange to have a pointer to an archived form of
    a message in a header field of that same message.  After all, if one
    has the message, why would one need a pointer to it?  It turns out
    that such pointers can be extremely useful.  This section describes
    some of the scenarios for their use.
    One may want to refer to messages in a non-message context, such as
    on a Web page or in an instant messaging context.  In such a case,
    being able to take the URI out of the Archived-At header avoids
    having to search the correct message in the archive.
    One may want to refer to other messages in a message context.  While
    refering to a single message is often done by replying to that
    message, when referring to more than one message, providing a pointer
    to these messages is a widespread practice.  The Archived-At header
    makes it easier to provide these pointers.
    One may want to find related messages, for which one may need to go

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    to the archive if one has not received potentially related messages.
    Also, it may often be easier to find related messages in an archive
    than in an email client.  The Archived-At header makes it easier to
    find the starting point in the archive to find related messages.
    Please note that in the above usage scenarios, it is mostly the human
    reader, rather than the email client software, that makes use of the
    URI in the Archived-At header.  However, this does not rule out the
    use of the URI in the Archived-At header by the email client or other
    software if such use is found helpful.

7.  Security Considerations
    There are many potential security issues when activating and
    dereferencing an URI.  For more details, please see [STD66].  In the
    context of this proposal, the following are particularly relevant: An
    intruder may get access to the message transmission and be able to
    insert an URI pointing to some malicious content.  Also, somebody may
    be able to construct a message that when received directly is
    harmless, but that when accessed via the URI produces some problems
    because in the format used in the archive, some content has not been
    adequately escaped.
    Because the Archived-At header field points to some archived form of
    the message itself, which in turn may contain the Archived-At field,
    there is a potential for a denial-of-service attack on the server
    pointed to by the URI in the Archived-At header field if the archived
    form of the message is downloaded automatically, and further URIs in
    that message are followed and downloaded recursively without checking
    for already downloaded resources.  However, this kind of scenario can
    easily be avoided by implementations.  First, the URI in the
    Archived-At header field should not be dereferenced automatically,
    and second, appropriate measures for loop detection should be used.

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8.  IANA considerations
    IANA is herewith requested to register the Archived-At header field
    in the Message Header Fields Registry ([RFC3864]) as follows:
       Header field name:
       Applicable protocol:
          mail (RFC 2822), optionally also netnews (RFC 1036)
       Author/Change controller:
       Specification document(s):
          Internet-Draft draft-duerst-archived-at-02.txt
          (Note to RFC Editor: Replace this with
           RFC YYYY (RFC number of this spec))
       Related information:

9.  Change Log
9.1.  Changes from -03.txt to -04.txt
       Updated from RFC 2234 to RFC 4234.
       Updated author's address/affiliation.
9.2.  Changes from -02.txt to -03.txt
       Fixed syntax to allow spaces after colon.
       Avoided misunderstanding with message ids ('<' and '>' are part of
       the message id)
       Added reference to RFC 2119 (keywords)
       Changed "MUST" to "SHOULD" for existence of message

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10.  Acknowledgements
    The members of the W3C system team, in particular Gerald Oskoboiny,
    Olivier Thereaux, and Jose Kahan, created the mid-based email archive
    lookup system and the experimental form of the Archived-At header.
    Pete Resnik provided the motivation for writing up this memo.
    Discussion on the ietf-822@imc.org mailing list, in particular
    contributions by Arnt Gulbrandsen, Graham Klyne, Bruce Lilly, Charles
    Lindsey, and Keith Moore, has led to further improvements of the
11.  Normative References
    [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
               Extensions (MIME) Part Two: Media Types", RFC 2046,
               November 1996.
    [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
    [RFC2369]  Baer, J. and G. Neufeld, "The Use of URLs as Meta-Syntax
               for Core Mail List Commands and their Transport through
               Message Header Fields", RFC 2369, July 1998.
    [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
               April 2001.
    [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
               Procedures for Message Header Fields", RFC 3864, BCP 90,
               September 2004.
    [RFC4234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
               Specifications: ABNF", RFC 4234, October 2005.
    [STD66]    Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
               Resource Identifier (URI): Generic Syntax", STD 66,
               RFC 3986, January 2005.

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Author's Address
    Martin Duerst (Note: Please write "Duerst" with u-umlaut wherever
                  possible, for example as "D&#252;rst" in XML and HTML.)
    Aoyama Gakuin University
    5-10-1 Fuchinobe
    Sagamihara, Kanagawa  229-8558
    Phone: +81 466 49 1170
    Fax:   +81 466 49 1171
    Email: mailto:duerst@it.aoyama.ac.jp
    URI:   http://www.sw.it.aoyama.ac.jp/D%C3%BCrst/

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    This document and the information contained herein are provided on an

Copyright Statement
    Copyright (C) The Internet Society (2005).  This document is subject
    to the rights, licenses and restrictions contained in BCP 78, and
    except as set forth therein, the authors retain all their rights.

    Funding for the RFC Editor function is currently provided by the
    Internet Society.

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