Network Working Group J. Yeh, Ed.
Internet-Draft TWNIC
Expires: March 31, 2006 September 27, 2005
Transmission of Email Headers in UTF-8 Encoding
draft-yeh-ima-utf8headers-00.txt
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Copyright (C) The Internet Society (2005).
Abstract
Full internationalization of electronic mail requires not only the
capability to transmit non-ASCII content, to encode selected
information in specific header fields, and to use international
characters in envelope addresses. It also requires being able to
express those addresses and information based on them in mail
headers. This document specifies the use of Unicode encoded in
UTF-8, rather than ASCII, as the base form for Internet email
headers. This form is permitted in transmission only if authorized
by an SMTP extension, as specified in an associated specification.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Role of this specification . . . . . . . . . . . . . . . . 3
1.2. Background and History . . . . . . . . . . . . . . . . . . 3
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Changes to MUAs and to the user's mail environment . . . . . . 4
2.1. Changes to MUA sending . . . . . . . . . . . . . . . . . . 4
2.2. Changes to MUA receiving . . . . . . . . . . . . . . . . . 5
3. Changes to SMTP Servers and Clients . . . . . . . . . . . . . 5
3.1. Impact on Message Headers . . . . . . . . . . . . . . . . 5
3.2. Things not changed from RFC 2822 . . . . . . . . . . . . . 6
3.3. Additional processing rules . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. IANA considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Introduction
1.1. Role of this specification
Full internationalization of electronic mail requires several
capabilities:
o The capability to transmit non-ASCII content, provided for as part
of the basic MIME specification [RFC2045], [RFC2046].
o The capability to encode selected information in specific header
fields, provided for as another part of the MIME specification
[RFC2047].
o The capability to use international characters in envelope
addresses, discussed in [IMA-overview] and specified in [IMA-SMTP-
extension]. And, finally,
o The capability to express those addresses, and information related
to and based on them, in mail headers, defined in this document.
This document specifies the use of Unicode encoded in UTF-8
[RFC3629], rather than ASCII, as the base form for Internet email
headers. This form is permitted in transmission, if and only if
authorized by the SMTP extension specified in [IMA-SMTP-extension].
1.2. Background and History
Mailbox names often represent the names of human users. Many of
these users throughout the world have names that are not normally
represented with just the ASCII repertoire of characters, and would
more the less like to use their real names in their mailbox names.
These users are also likely to use non-ASCII text in their common
names and subjects of email messages, both in what they send and what
they receive. This protocol specifies UTF-8 as the encoding to
represent email header messages.
The traditional format of email messages [RFC2822] only allows ASCII
characters in the headers of messages. This prevents users from
having email addresses that contain non-ASCII characters. It further
forces non-ASCII text in common names, comments, and in free text
(such as in the Subject: field) to be in quoted-printable format
[RFC2047]. This specification describes a change to the email
message format that is connected to the SMTP message transport change
described in the associated specifications [IMA-overview] and [IMA-
SMTP-extension], and that allows non-ASCII characters throughout
email headers. These changes affect SMTP clients, SMTP servers, and
mail user agents (MUAs).
As specified in [IMA-SMTP-extension], an SMTP protocol extension
[RFC2821] is used to prevent the transmission of messages with UTF-8
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headers to systems that cannot handle such messages.
Use this SMTP extension helps prevent against the introduction of
such messages into message stores that might misrepresent or mangle
such messages. It should be noted that using an ESMTP extension does
not prevent against transferring email messages with UTF-8 headers to
other systems that use the email format for messages and that may not
be upgraded, such as the POP and IMAP protocols. Those protocols
will need to be changed in order to handle stored messages that have
UTF-8 headers.
The objective for this protocol is to allow UTF-8 everywhere in the
headers. Issues about how to handle messages that contain UTF-8
headers but are proposed to be delivered to systems that have not
been upgraded to support this capability are discussed elsewhere,
particularly in [IMA-downgrading].
This protocol is workable even if IMA mailbox names are not
presented. For example, the protocol might still be used if just the
subject header has non-ASCII characters, but the protocol MUST be
used if other headers (particularly trace headers such as
"Received:") contain non-ASCII characters.
1.3. Terminology
In this document, headers are "UTF-8 header" if the bodies of headers
contain UTF-8 characters.
Unless otherwise noted, all terms used here are defined in [RFC2821]
or [RFC2822] or in [IMA-overview].
The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED",
and "MAY" in this document are to be interpreted as described in RFC
2119 [RFC2119].
This document is being discussed on the ima mailing list. See
https://www1.ietf.org/mailman/listinfo/ima for information about
subscribing. The list's archive is at
http://www1.ietf.org/mail-archive/web/ima/index.html.
2. Changes to MUAs and to the user's mail environment
2.1. Changes to MUA sending
Sending MUAs that follow this protocol MUST create all headers
encoded in UTF-8. No other direct encodings are allowed. MUAs MAY
continue to use quoted-printable text to specify some text in other
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encodings; however this is not recommended because it is likely that
this will not interoperate well with MUAs that follow this
specification.
2.2. Changes to MUA receiving
Receiving MUAs that follow this protocol MUST able to handle email
headers encoded in UTF-8. Which means that the email fetching
prototol such as POP3 or IMAP MAY need to be updated.
3. Changes to SMTP Servers and Clients
The use of UTF-8 headers is dependent on the use of an SMTP extension
named "IMA".
That protocol is defined in [IMA-SMTP-extension]. If that extension
is not supported, UTF-8 headers MUST NOT be transmitted.
3.1. Impact on Message Headers
If an SMTP server advertises the IMA extension, an SMTP client that
supports this protocol SHOULD send message headers as described in
this document.
The final delivery SMTP server is responsible for knowing whether the
message store can handle UTF-8 headers or not. A terminal SMTP
server MUST NOT advertise the IMA extension if the message store
cannot handle UTF-8 headers.
If an SMTP client see the IMA extension advertised by an SMTP server,
the SMTP client MUST send all header message in UTF-8. However, the
Message-ID is the unique identifier of a single email. In order to
maintain the identity, message identifiers of the Message-ID fields
MUST be created in all ASCII. Also when In-Reply-To or Reference are
presented in email header, the Message-ID in these header fields MUST
be created in all ASCII.
If an SMTP client does not see the IMA extension advertised by an
SMTP server, the SMTP client MAY either
o Downgrade the non-ASCII contents of all header bodies before
continuing to send the message, as described in [IMA-downgrading].
The SMTP client SHOULD send the message with the downgraded header
bodies as a normal message.
o Reject the message with a reply code of 558. If any header body
cannot be downgraded, this second option MUST be chosen.
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3.2. Things not changed from RFC 2822
Note that this protocol does change the definition of header field
names. That is, only the bodies of headers are allowed to have non-
ASCII characters; the rules in RFC 2822 for header names are not
changed.
Similarly, this protocol does not change the date and time
specification in RFC 2822.
3.3. Additional processing rules
In order to make mail retrieval easier, final delivery SMTP servers
SHOULD write messages addressed to either the non-ASCII address or
the all-ASCII address into the same mailbox. However, given that
this is quite different than common practice today, the ramifications
for doing this should be studied carefully before this is
implemented.
4. Security Considerations
If a user has a non-ASCII mailbox address and a all-ASCII mailbox
address, a digital certificate that identifies that user SHOULD have
both addresses in the identity. Having multiple email addresses as
identities in a single certificate is already supported in PKIX and
OpenPGP.
Because UTF-8 often requires several octets to encode a single
character, internationalized local parts may cause mail addresses to
become longer. Then may possibly make it harder to keep lines in a
header under 78 characters. Lines that are longer than 78 characters
(which is a SHOULD specification, not a MUST specification, in RFC
2822) could possibly cause mail user agents to fail in ways that
affect security.
5. IANA considerations
The ESMTP extension needed to support this specification is specified
in [IMA-SMTP-extension]. This specification does not require any
additional IANA actions in that regard.
6. Acknowledgements
This document was created by incorporating a good deal of material
from an old Internet Draft by Paul Hoffman [Hoffman-utf8-headers].
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While many of the concepts and details have changed, the
contributions from that draft are greatly appreciated.
Most of the content of this document is provided by John C Klensin.
Also some significant comments and suggestions were received from
Yangwoo KO, Yoshiro YONEYA, and other members of the JET team and
were incorporated into the document. The editor is much great thanks
to their contribution sincerely.
7. References
7.1. Normative References
[ASCII] American National Standards Institute (formerly United
States of America Standards Institute), "USA Code for
Information Interchange", ANSI X3.4-1968, 1968.
ANSI X3.4-1968 has been replaced by newer versions with
slight modifications, but the 1968 version remains
definitive for the Internet.
[IMA-SMTP-extension]
Yao, J., Ed., "SMTP extension for internationalized email
address", draft-yao-ima-smtpext-00.txt (work in progress),
September 2005.
[IMA-overview]
"Overview and Framework of Internationalized Email Address
Delivery", 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
7.2. Informative References
[]
Hoffman, P., "SMTP Service Extensions or Transmission of
Headers in UTF-8 Encoding",
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draft-hoffman-utf8headers-00.txt (work in progress),
December 2003.
[IMA-downgrading]
"whatever we call the downgrading document", 2005.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
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Author's Address
Jeff Yeh (editor)
TWNIC
4F-2, No. 9, Sec 2, Roosvelt Rd.
Taipei, 100
Taiwan
Phone: +886 2 23411313 ext 506
Email: jeff@twnic.net.tw
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