Internet DRAFT - draft-ietf-eai-simpledowngrade
draft-ietf-eai-simpledowngrade
Network Working Group Arnt Gulbrandsen
Internet-Draft August 2012
Intended Status: Proposed Standard
Updates: 3501
Simplified POP/IMAP Downgrading for Internationalized Email
draft-ietf-eai-simpledowngrade-07.txt
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Abstract
This document specifies a method for IMAP and POP servers to serve
internationalized messages to conventional clients. The specification
is simple, easy to implement and provides only rudimentary results.
1. Overview
It may happen that a conventional IMAP or POP client opens a mailbox
containing internationalized messages, or even attempt to read
internationalized messages, for instance when a user has both
internationalized and conventional MUAs.
Some operations cannot be performed by conventional clients. Most
importantly, an internationalized message usually contains at least
one internationalized address, so address-based operations are only
rarely possible. This includes displaying the addresses, replying,
and most types of address-based signature or security processing.
Still, the sender's name, the message subject, body text and
attachments can easily be displayed, so a helpful IMAP/POP server may
prefer to provide access to what it can rather than hide the message
entirely.
This document specifies a way to present such messages to the client.
It values simplicity of implementation over fidelity of
representation, since implementing a high-fidelity downgrade
algorithm is likely more work than implementing proper support for
[RFC5721] and/or [RFC5738].
The server is assumed to be internationalized internally, and to
store messages internationalized messages natively. When it needs to
present an internationalized message to a conventional client, it
synthesizes a conventional message containing most of the information
and presents that (the "synthetic message").
2. Information preserved and lost
The synthetic message is intended to convey the most important
information to the user. Where information is lost, the user should
see the message as incomplete rather than modified.
The synthetic message is not intended to convey any information to
the client software that would require or enable it to apply special
handling to the message. Client authors who wish to handle
internationalized messages are encouraged to implement [RFC5738].
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Upper case in examples represents non-ASCII. example.com is a plain
domain, EXAMPLE.com represents a non-ASCII domain in the .com top-
level domain.
2.1 Email addresses
Each internationalized email address in the header fields listed
below is replaced with an invalid email address whose display-name
tells the user what happened.
The format of the display-name is explicitly unspecified. Anything
which tells the user what happened is good. Anything which produces
an email address which might belong to someone else is bad.
Given an internationalized address "Fred Foo <fred@EXAMPLE.com>", an
implementation may choose to render it e.g. as these examples:
"fred@EXAMPLE.com" <invalid@internationalized-address.invalid>
Fred Foo <invalid@internationalized.invalid>
internationalized-address:;
fred:;
(The .invalid top-level domain is reserved by [RFC2606], therefore
the first two examples are syntactically valid, but will never belong
to anyone. Note that the display-name often will need [RFC2047]
encoding.)
The affected header fields are Bcc, Cc, From, Reply-To, Resent-Bcc,
Resent-Cc, Resent-From, Resent-Sender, Resent-To, Return-Path, Sender
and To. Any addresses present in other header fields, such as
Received, are not regarded as addresses by this specification.
2.2 MIME parameters
Any MIME parameter [RFC2045] (whether in the message header or a
bodypart header) which cannot be presented as-is to the client is
silently excised.
Given a field such as
Content-Disposition: attachment; filename=FOO
the field is presented as
Content-Disposition: attachment
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2.3 "Subject"
If the Subject field cannot be presented as-is, the server presents a
representation encoded as specified in [RFC2047].
2.4 Remaining header fields
Any header field which cannot be presented to the client even after
the modifications in sections 2.1-2.3 is silently excised.
3. IMAP-specific details
IMAP allows clients to retrieve the message size without downloading
it, using RFC822.SIZE, BODY.SIZE[] and so on. [RFC3501] requires that
the returned size be exact.
This specification relaxes that requirement: When a conventional
client requests size information for a message, the IMAP server is
permitted to return size information for the internationalized
message, even though the synthetic message's size differs.
When an IMAP server carries out downgrading as part of generating
FETCH responses, it reports which messages were synthesised using a
response code and attendant UID set. This can be helpful to humans
debugging the server and/or client.
C: a UID FETCH 1:* BODY.PEEK[HEADER.FIELDS(To From Cc)]
S: 1 FETCH (UID 65 [...]
S: 2 FETCH (UID 70 [...]
S: a OK [DOWNGRADED 70,105,108,109] Done
The message-set argument to DOWNGRADED contains UIDs.
Note that DOWNGRADED does not necessarily mention all the
internationalized messages in the mailbox. In the example above, we
know that UID 65 does not contain internationalized addresses in
From, To and Cc. It may contain an internationalized Subject, etc.
4. POP-specific details
The number of lines specified in the TOP command (see [RFC1939])
refers to the synthetic message. The message size reported by e.g.
LIST may refer to either the internationalized or the synthetic
message.
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5. Security Considerations
If the internationalized message uses any sort of signature, the
synthetic message's signature almost certainly is invalid. This is a
necessary limitation of displaying internationalized messages in
conventional clients, since the client does not support
internationalized addresses.
If any excised information is significant, then that information does
not arrive at the recipient. Notably, the Message-Id, In-Reference-To
and References fields may be excised, which might cause a lack of
context when the recipient reads the message.
Some POP or IMAP clients, such as Fetchmail, download messages and
delete the version on the server. This may lead to permanent loss of
information when the only remaining version of a message is the
synthetic message.
Other clients cache messages for a very long time, even across client
upgrades, such as the stock Android client. When such a client is
internationalized, care must be taken so that it will not use an old
synthetic message from its cache rather than retrieve the real
message from the server.
6. Acknowledgements
Claudio Allocchio, Ned Freed, Kazunori Fujiwara, Ted Hardie, John
Klensin, Barry Leiba, John Levine, Alexey Melnikov, Chris Newman,
Joseph Yee and the originator of rule 12 in [RFC1925] helped with
this document.
7. IANA Considerations
The IANA is requested to add DOWNGRADED to the IMAP Response Code
registry.
8. Normative References
[RFC1939] Myers, J and M. Rose, "Post Office Protocol - Version 3",
RFC 1939, May 1996.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
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[RFC2047] Moore, "MIME (Multipurpose Internet Mail Extensions) Part
Three: Message Header Extensions for Non-ASCII Text", RFC
2047, November 1996.
[RFC2606] Eastlake, D. and A. Panitz, "Reserved Top Level DNS
Names", BCP 32, RFC 2606, June 1999.
[RFC3501] Crispin, "Internet Message Access Protocol - Version
4rev1", RFC 3501, June 2003.
9. Informative References
[RFC1925] Callon, R., "Fundamental Truths of Networking", RFC 1925,
April 1996.
[RFC5721] Gellens, R., and C. Newman, "POP3 Support for UTF-8", RFC
5721, February 2010.
[RFC5738] Resnick, P. and C. Newman, "IMAP Support for UTF-8", RFC
5738, March 2010.
10. Author's Address
Arnt Gulbrandsen
Schweppermannstr. 8
D-81671 Muenchen
Germany
Fax: +49 89 4502 9758
Email: arnt@gulbrandsen.priv.no
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(RFC Editor: Please delete everything after this point)
Open Issues
Should Kazunori Fujiwara's downgrade document also mention
DOWNGRADED?
RFC Editor: IF 5721 and/or 5738 have been superseded by new RFCs at
this time, please change the references to those RFCs throughout this
document. Well, except in the previous sentence. I'm such a pedant.
RFC Editor: I do not know the difference between that and which. Will
and shall outnumber me too. Please fix all that. Thank you.
Changes since -00
Added a rule to handle Subject
Removed the sentence about unknown:;
Terminology fixes
Changes since -01
Nits from Joseph Yee.
Clarified the address rendering and added non-.invalid examples,
based on suggestions from Kazunori Fujiwara.
Many changes from Barry Leiba: Simplified and better terminology,
reformatted examples, more references, etc.
Specified POP TOP. A bit of a no-op specification.
Mention BODY.SIZE[] as well as RFC822.SIZE. Wave hands so
BODY.SIZE[1] sneaks past.
http://rant.gulbrandsen.priv.no/good-bad-rfc fwiw
Changes since -02
Added the DOWNGRADED response code, since both Barry and Alexey wants
it.
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Changes since -03
Added/changed text in response to appsdir reviews from Ted Hardie and
Claudio Allocchio.
Changes since -04
Closed two open issues; the interest in them was clearly negligible.
"Updates: 3501" because of the SIZE relaxation.
Security considerations about download-and-delete and long-term
caching.
Bring on the WGLC!
Changes since -05
Text changes from John Klensin
Changes since -06
Text changes from Barry Leiba. I hate case sensitivity in human
language, but right now I need to pack my suitcases, not argue.
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