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Network Working Group M. Duerst
Request for Comments: 5064 Aoyama Gakuin University
Category: Standards Track December 2007
The Archived-At Message Header Field
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This memo defines a new email header field, Archived-At:, to provide
a direct link to the archived form of an individual email message.
Table of Contents
1. Introduction ....................................................2
2. Header Field Definition .........................................2
2.1. Syntax .....................................................2
2.2. Multiple Archived-At Header Fields .........................3
2.3. Interaction with Message Fragmentation and Reassembly ......3
2.4. Syntax Extension for Internationalized Message Headers .....3
2.5. The X-Archived-At Header Field .............................4
3. Implementation and Usage Considerations .........................4
3.1. Formats of Archived Message ................................4
3.2. Implementation Considerations ..............................4
3.3. Usage Considerations .......................................5
4. Security Considerations .........................................6
5. IANA Considerations .............................................7
5.1. Registration of the Archive-At Header Field ................7
5.2. Registration of the X-Archived-At Header Field .............7
6. Acknowledgments .................................................8
7. References ......................................................8
7.1. Normative References .......................................8
7.2. Informative References .....................................8
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RFC 5064 The Archived-At Message Header Field December 2007
1. Introduction
[RFC2369] defines a number of header fields that can be added to
Internet messages such as those sent by email distribution lists or
in netnews [RFC1036]. One of them is the List-Archive header field
that describes how to access archives for the list. This allows
access to the archives as a whole, but not an individual message.
There is often a need or desire to refer to the archived form of a
single message. For more detailed usage scenarios, please see
Section 3.3. This memo defines a new header, Archived-At, to refer
to a single message at an archived location. This provides quick
access to the location of a mailing list message in the list archive.
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",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in [RFC2119].
2. Header Field Definition
2.1. Syntax
For the Archived-At header field, the field name is "Archived-At".
The field body consist of a URI [STD66] enclosed in angle brackets
('<', '>'). The URI MAY contain folding whitespace (FWS, [RFC2822]),
which is ignored. Mail Transfer Agents (MTAs) MUST NOT insert
whitespace within the angle brackets, but client applications SHOULD
ignore any whitespace, which might have been inserted by poorly
behaved MTAs. The URI points to an archived version of the message.
See Section 3.1 for more details.
This header field is subject to the encoding and character
restrictions for mail headers as described in [RFC2822].
More formally, the header field is defined as follows in Augmented
BNF (ABNF) according to [RFC4234]:
archived-at = "Archived-At:" [FWS] "<" folded-URI ">" CRLF
folded-URI = <URI, but free insertion of FWS permitted>
where URI is defined in [STD66], and CRLF and FWS are defined in
[RFC2822].
To convert a folded-URI to a URI, first apply standard [RFC2822]
unfolding rules (replacing FWS with a single SP), and then delete any
remaining un-encoded SP characters.
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RFC 5064 The Archived-At Message Header Field December 2007
This syntax is kept simple in that only one URI per header field is
allowed. In this respect, the syntax is different from [RFC2369].
Also, comments are not allowed.
2.2. 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 per URI is required.
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.
2.3. Interaction with Message Fragmentation and Reassembly
[RFC2046] allows for the fragmentation and reassembly of messages.
Archived-At header fields are to be treated in the same way as
Comments header fields, i.e., copied to the first fragment message
header on fragmentation and back from there to the header of the
reassembled message.
This treatment has been chosen for compatibility with existing
infrastructure. It means that Archived-At header fields in the first
fragment message MAY refer to an archived version of the whole,
unfragmented message. To avoid confusion, Archived-At headers SHOULD
NOT be added to fragment messages.
2.4. Syntax Extension for Internationalized Message Headers
There are some efforts to allow non-ASCII text directly in message
header field bodies. In such contexts, the URI non-terminal in the
syntax defined in Section 2.1 is to be replaced by an
Internationalized Resource Identifier (IRI) as defined in [RFC3987].
The specifics of the actual octet encoding of the IRI will follow the
rules for general direct encoding of non-ASCII text. For conversion
between IRIs and URIs, the procedures defined in [RFC3987] are to be
applied.
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RFC 5064 The Archived-At Message Header Field December 2007
2.5. The X-Archived-At Header Field
For backwards compatibility, this document also describes the
X-Archived-At header field, a precursor of the Archived-At header
field. The X-Archived-At header field MAY also be parsed, but SHOULD
not be generated.
The following is the syntax of the X-Archived-At header field in ABNF
according to [RFC4234] (which also defines SP):
obs-archived-at = "X-Archived-At:" SP URI CRLF
The X-Archived-At header field does not allow whitespace inside URI.
3. Implementation and Usage Considerations
3.1. Formats of Archived Message
There is no restriction on the format used to serve the archived
message from the URI in an 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 its 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, an HTTP URI in an
Archived-At header may make it possible 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 (MUA) without loss of
information.
3.2. Implementation Considerations
Mailing list expanders and email archives are often separate pieces
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., a URI based on
the message identifier of the incoming email, and to set up the
archiving system so that it redirects requests for such URIs to the
actual messages. If the email does not contain a message identifier,
a unique identifier can be generated.
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RFC 5064 The Archived-At Message Header Field December 2007
Such a system has been implemented and is in productive use at W3C.
As an example, the URI
"http://www.w3.org/mid/0I5U00G08DFGCR@mailsj-v1.corp.adobe.com",
containing the significant part of the message identifier
"<0I5U00G08DFGCR@mailsj-v1.corp.adobe.com>", is redirected to the URI
of this message in the W3C mailing-list archive at
http://lists.w3.org/Archives/Public/uri/2004Oct/0017.html.
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
http://dev.w3.org/cvsweb/search/bin/msgid-db.pl. These locations may
be subject to change.
When using the message identifier to create an address for the
archived mail, care has to be taken to escape characters in the
message identifier that are not allowed in the URI, or to remove
them, as done above for the "<" and ">" delimiters.
Implementations such as that described above can introduce a security
issue. Somebody might deliberately reuse a message identifier to
break the link to a message. This can be addressed by checking
incoming message identifiers against those of the messages already in
the archive and discarding incoming duplicates, by checking the
content of incoming duplicates and discarding them if they are
significantly different from the first message, by offering multiple
choices in the response to the URI, or by using some authentication
mechanism on incoming messages.
3.3. 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.
A user may want to refer to messages in a non-message context, such
as on a Web page, in an instant message, or in a phone conversation.
In such a case, the user can extract the URI from the Archived-At
header field, avoiding the search for the correct message in the
archive.
A user may want to refer to other messages in a message context.
Referring to a single message is often done by replying to that
message. However, when referring to more than one message, providing
pointers to archived messages is a widespread practice. The
Archived-At header field makes it easier to provide these pointers.
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RFC 5064 The Archived-At Message Header Field December 2007
A user may want to find messages related to a message at hand. The
user may not have received the related messages, and therefore needs
to use an archive. The user may also prefer finding related messages
in the archive rather than in her MUA, because messages in archives
may be linked in ways not provided by the MUA. The Archived-At
header field provides a link to the starting point in the archive
from which 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.
4. Security Considerations
There are many potential security issues when activating and
dereferencing a URI. For more details, including some
countermeasures, 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 a URI
pointing to some malicious content. This can be addressed by using a
secured way of message transmission. Also, somebody may be able to
construct a message that is harmless when received directly, but that
produces problems when accessed via the URI. One reason for this may
be the format used in the archive, where some content was not
adequately escaped. This can be addressed by using adequate
escaping.
The Archived-At header field points to some archived form of the
message itself. This in turn may contain the Archived-At field.
This creates a potential for a denial-of-service attack on the server
pointed to by the URI in the Archived-At header field. The
conditions are that the archived form of the message is downloaded
automatically, and that 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. Second, appropriate
measures for loop detection should be used.
In Section 3.2, an attack is described that may break a URI to a
message by introducing a new message with the same message
identifier. Possible countermeasures are also discussed.
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RFC 5064 The Archived-At Message Header Field December 2007
5. IANA Considerations
5.1. Registration of the Archive-At Header Field
IANA has registered the Archived-At header field in the Message
Header Fields Registry ([RFC3864]) as follows:
Header field name:
Archived-At
Applicable protocol:
mail (RFC 2822) and netnews (RFC 1036)
Status:
standard
Author/Change controller:
IETF
Specification document(s):
RFC 5064
Related information:
none
5.2. Registration of the X-Archived-At Header Field
This section is non-normative (specifically, an implementation that
ignores this section remains compliant with this specification).
IANA has registered the X-Archived-At header field in the Message
Header Fields Registry ([RFC3864]) as follows:
Header field name:
X-Archived-At
Applicable protocol:
mail (RFC 2822) and netnews (RFC 1036)
Status:
deprecated
Author/Change controller:
IETF
Specification document(s):
RFC 5064
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RFC 5064 The Archived-At Message Header Field December 2007
Related information:
none
6. Acknowledgments
The members of the W3C system team, in particular Gerald Oskoboiny,
Olivier Thereaux, Jose Kahan, and Eric Prud'hommeaux, created the
mid-based email archive lookup system and the experimental form of
the Archived-At header. Pete Resnik provided the motivation for
writing this memo. Discussion on the ietf-822@imc.org mailing list,
in particular contributions by Frank Ellermann, Arnt Gulbrandsen,
Graham Klyne, Bruce Lilly, Charles Lindsey, and Keith Moore, led to
further improvements of the proposal. Chris Newman, Chris Lonvick,
Stephane Borzmeyer, Vijay K. Gurbani, and S. Moonesamy provided
additional valuable comments.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822,
April 2001.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[RFC4234] Crocker, D., Ed., 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.
7.2. Informative References
[RFC1036] Horton, M. and R. Adams, "Standard for interchange of
USENET messages", RFC 1036, December 1987.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
Duerst Standards Track [Page 8]
RFC 5064 The Archived-At Message Header Field December 2007
[RFC2369] Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax
for Core Mail List Commands and their Transport through
Message Header Fields", RFC 2369, July 1998.
Author's Address
Martin Duerst (Note: Please write "Duerst" with u-umlaut wherever
possible, for example as "Dürst" in XML and HTML.)
Aoyama Gakuin University
5-10-1 Fuchinobe
Sagamihara, Kanagawa 229-8558
Japan
Phone: +81 42 759 6329
Fax: +81 42 759 6495
EMail: duerst@it.aoyama.ac.jp
URI: http://www.sw.it.aoyama.ac.jp/D%C3%BCrst/
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RFC 5064 The Archived-At Message Header Field December 2007
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Duerst Standards Track [Page 10]