Network Working Group J. Yeh, Ed.
Internet-Draft TWNIC
Intended status: Informational August 7, 2006
Expires: February 8, 2007
Internationalized Email Headers
draft-ietf-eai-utf8headers-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
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 non-ASCII
characters in envelope addresses. It also requires being able to
express those addresses and information based on them in mail header
fields. This document specifies the use of Unicode encoded in UTF-8,
rather than ASCII, as the base form for Internet email header field
bodies. This form is permitted in transmission only if authorized by
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an SMTP extension, as specified in an associated specification.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Role of this specification . . . . . . . . . . . . . . . . 3
2. Background and History . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Pre-requirement . . . . . . . . . . . . . . . . . . . . . . . 4
5. Identification of internationalized email . . . . . . . . . . 5
6. Changes on Message Header Fields . . . . . . . . . . . . . . . 6
6.1. UTF8 Syntax . . . . . . . . . . . . . . . . . . . . . . . 6
6.2. Syntax extend from RFC 2822 . . . . . . . . . . . . . . . 7
6.3. Change on addr-spec syntax . . . . . . . . . . . . . . . . 8
6.4. Trace field syntax . . . . . . . . . . . . . . . . . . . . 9
7. Additional issue . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Mailing list header fields . . . . . . . . . . . . . . . . 9
7.2. MIME headers . . . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
11. Edit history . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. draft-ietf-eai-utf8header-00 . . . . . . . . . . . . . . . 11
11.2. draft-yeh-ima-utf8header-01 . . . . . . . . . . . . . . . 11
11.3. draft-yeh-ima-utf8header-00 . . . . . . . . . . . . . . . 11
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
12.1. Normative References . . . . . . . . . . . . . . . . . . . 11
12.2. Informative References . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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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 [EAI-overview] and specified in
[EAI-SMTP-extension]. And, finally,
o The capability to express those addresses, and information related
to and based on them, in mail header fields, 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
header fields. This form is permitted in transmission, if authorized
by the SMTP extension specified in [EAI-SMTP-extension].
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 or 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 field bodies.
The traditional format of email messages [RFC2822] only allows ASCII
characters in the header fields 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 MIME 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 [EAI-overview] and
[EAI-SMTP-extension], and that allows non-ASCII characters throughout
email header fields. These changes affect SMTP clients, SMTP
servers, mail user agents (MUAs), list expanders and and gateways to
other media.
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As specified in [EAI-SMTP-extension], an SMTP protocol extension
"UTF8SMTP" is used to prevent the transmission of messages with UTF-8
header fields to systems that cannot handle such messages.
Use of 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 header
fields 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 also need to be changed in order to handle stored messages
that have UTF-8 header fields.
The objective for this protocol is to allow UTF-8 in email header
fields. Issues about how to handle messages that contain UTF-8
header fields but are proposed to be delivered to systems that have
not been upgraded to support this capability are discussed elsewhere,
particularly in [EAI-downgrading].
3. Terminology
In this document, header fields are "UTF-8 header" if the bodies of
those headers contain UTF-8 characters.
Unless otherwise noted, all terms used here are defined in [RFC2821]
or [RFC2822] or in [EAI-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.
4. Pre-requirement
The use of UTF-8 header fields is dependent on the use of an SMTP
extension named "UTF8SMTP".
That protocol is defined in [EAI-SMTP-extension]. If that extension
is not supported, UTF-8 header fields MUST NOT be transmitted by
SMTP.
Sending MUAs that follow this protocol MUST create all header fields
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encoded in UTF-8. No other direct encodings (like Big-5) are
allowed. Though there's nothing bad to use [RFC2047], but it is not
recommended in this document.
5. Identification of internationalized email
When a SMTP client tries to send a mail to a SMTP server that does
not support EAI, the client should know whether the message requires
the support for EAI or not. In addition to this, identification of
internationalized email is also required when a message is stored and
resent. Checking the presence of UTF-8 characters in the header
whenever such an identification is required may also achieve the
goal. However, this type of repeated processing wastes time and
processing power of involved systems. It is nice to have a mechanism
(such as self-label) or some indicator to identify whether the
message is new format(i.e. EAI compliant) or old one (i.e. RFC 2822
compliant).
To be able to do so, sending MUA MUST insert a new header field to
identify the presence of i18n information (particularly UTF-8
headers) in the message. The new header specified as "UTF8SMTP", and
elements of the header is the version number of i18n email. The i18n
header field syntax specified as below:
header-content = "UTF8SMTP:" [FWS] content-code [FWS]
*( ";" parameter ) CRLF
content-code = "UTF8" / "ASCII" / "Downgraded"
parameter = "Header-Language:" [FWS] Language-List
Language-List = Language-Tag [FWS]
*("," [FWS] Language-Tag [FWS])
The Language-Tag token is the language identifier described in
[RFC3066] (ex: zh-TW).
[note: Thought the "parameter" token is currently used only for the
header-language, but can be extended in future use.]
While we can't require ordering of headers, it would be good to have
it appear as near the top of the headers as possible. It would also
be good to be able to guarantee that it will be there when the
message is dropped into a mail store. Thus, when a i18n email is
delivered.
o The "UTF8SMTP" header field MUST be inserted by the originating
MUA.
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o The "UTF8SMTP" header field MUST be inserted, along with Return-
path, by the final delivery MTA if not presented.
o Whenever the email need to downgrade, the content-code of the
"UTF8SMTP" MUST change to "Downgraded", the detail of downgrade
process will describe in [EAI-downgrading].
o If more than one "UTF8SMTP" header is presented, MTA SHOULD simply
ignore any excess ones.
6. Changes on Message Header Fields
SMTP client can send header fields in UTF-8 format, if the UTF8SMTP
extension advertised by SMTP server.
This protocol does NOT change the definition of header field names.
That is, only the bodies of header fields are allowed to have UTF-8
characters; the rules in RFC 2822 for header names are not changed.
To be able to do so, the header definition in RFC 2822 must extended
to support new format. That following ABNF is defined to substitute
those definition in RFC 2822.
For those tokens not referred in this section remains as the original
definition in RFC 2822.
6.1. UTF8 Syntax
The use of UTF8 characters are defined as following.
UTF8-xtra-char = UTF8-2 / UTF8-3 / UTF8-4
UTF8-2 = %xC2-DF UTF8-tail
UTF8-3 = %xE0 %xA0-BF UTF8-tail /
%xE1-EC 2(UTF8-tail) /
%xED %x80-9F UTF8-tail /
%xEE-EF 2(UTF8-tail)
UTF8-4 = %xF0 %x90-BF 2(UTF8-tail) /
%xF1-F7 3(UTF8-tail)
UTF8-tail = %x80-BF
These are taken from FRC 3629, but keep in this document for
convenient reason.
[Note in draft: Weather normalizing is needed or not will be place in
here.]
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6.2. Syntax extend from RFC 2822
The following rules are intended to extend the corresponding rules in
RFC 2822 to allow UTF8 characters.
ctext = NO-WS-CTL / ; all of <text> except
%d33-39 / ; SP, HTAB, "(", ")"
%d42-91 / ; and "\"
%d93-126 /
UTF8-xtra-char
qtext = NO-WS-CTL / ; all of <text> except
%d33 / ; The rest of the US-ASCII
%d35-91 / ; characters not including "\"
%d93-126 / ; or the quote character
UTF8-xtra-char
text = %d1-9 / ; all UTF-8 characters except
%d11-12 / ; US-ASCII NUL, CR and LF
%d14-127 /
UTF8-xtra-char
utext = NO-WS-CTL / ; Non white space controls
%d33-126 / ; The rest of US-ASCII
UTF8-xtra-char
This means that all the RFC 2822 constructs that build upon these
will permit UTF-8 characters, including comments and quoted strings.
Besides, in order to allow UTF8 characters in <addr-spec> we have to
change the syntax of <atext>. However, it will also lead <mesg-id>
to allow UTF8 characters, which is not allowed due to the limitation
describe in Section 6.4. So <utf8-atext> is added to meet this
requirement.
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utf8-atext = ALPHA / DIGIT /
"!" / "#" / ; Any character except
"$" / "%" / ; controls, SP, and specials.
"&" / "'" / ; Used for atoms
"*" / "+" /
"-" / "/" /
"=" / "?" /
"^" / "_" /
"`" / "{" /
"|" / "}" /
"~" /
UTF8-xtra-char
utf8-atom = [CFWS] 1*utf8-atext [CFWS]
utf8-dot-atom = [CFWS] utf8-dot-atom-text [CFWS]
utf8-dot-atom-text = 1*utf8-atext *("." 1*utf8-atext)
[NOTE IN DRAFT: If any header needs to be restricted to disallow
this, please raise the issue on the mailing list.]
Note, however, that this does not remove any constraint on the
character set of protocol elements; for instance, all the allowed
values for timezone in the Date: headers are still expressed in
ASCII.
6.3. Change on addr-spec syntax
In this specification, internationalized email address will be
presented in UTF-8. Thus, all header fields involving <mailbox>es
may be different from traditional ones. There might be EAI unaware
MTAs in the mail routing path. In that case, MTA may bounce the
message with reply code 550, or downgrade the non-ASCII contents of
all header bodies before continuing to send the message. The
downgrade process involve with a new ALT-ADDRESS parameter. When
downgrade occurs, the ALT-ADDRESS will be used for mail delivery
instead of the internationalized email address, the detail is
described in [EAI-downgrading].
angle-addr = [CFWS] "<" utf8-addr-spec [alt-address] ">" [CFWS]
utf8-addr-spec = utf8-local-part "@" utf8-domain
utf8-local-part= utf8-dot-atom / quoted-string / obs-local-part
utf8-domain = utf8-dot-atom / domain-literal / obs-domain
alt-address = "{" [CFWS] addr-spec [CFWS] "}"
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Below list a few possible <mailbox> representation as example.
"DISPLAY_NAME" <ASCII@ASCII>
; tradition mailbox format
"DISPLAY_NAME" <non-ASCII@non-ASCII>
; EAI but no ALT-ADDRESS parameter provided,
; message will bounce if UTF8SMTP extension is not supported
"DISPLAY_NAME" <non-ASCII@non-ASCII {ASCII@ASCII}>
; UTF8SMTP with ALT-ADDRESS parameter provided,
; ALT-ADDRESS can be used if downgrade is necessary
6.4. Trace field syntax
Internationalized domain names in Received fields must be transmitted
in punycode form. "For" fields containing internationalized
addresses are prohibited, since subsequent downgrading would force
violating rules in RFC 2821 prohibiting altering existing Received
fields. With these two restrictions, there should be no need for
UTF-8 information in Received fields and such information is
prohibited to preserve the integrity of those fields. More
generally, UTF-8 information of any sort MUST NOT appear in Received
fields, even in comments within those fields.
7. Additional issue
This section identifies issues that are not covered as part of this
set of specifications, but that will need to be considered as part of
EAI deployment.
7.1. Mailing list header fields
All mailing list and mail redistribution related header fields may
need further discuss.
7.2. MIME headers
MIME header bodies (parameter <value> in [RFC2231]) need to allow
UTF8 characters in conformance with this specification.
8. Security Considerations
If a user has a non-ASCII mailbox address and an ASCII mailbox
address, a digital certificate that identifies that user may have
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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. As specified in RFC 2822, each line of characters
MUST be no more 998 octets, excluding the CRLF.
In this specification, a user could provide a ASCII alternative
address for a non-ASCII address. However, it is possible these two
address going to different mailbox, or even different person. This
might not be the protocol problem, but user's personal choice (or
administration policy).
9. IANA considerations
IANA is requested to add the "UTF8SMTP" new header to the registry
with the entry pointing to this specification for its definition.
For those headers that modified in this document need to have their
registrations modified, so as to refer to the specification in
addition to their current definitions.
10. Acknowledgements
This document was created by incorporating a good deal of material
from an old Internet Draft by Paul Hoffman [Hoffman-utf8-headers].
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
Charles H. Lindsey, Chris Newman, 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.
11. Edit history
This section is used for tracking the update of this document. Will
be removed after finalize.
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11.1. draft-ietf-eai-utf8header-00
1. ABNF revise.
2. Terminology sync with overview document.
3. addr-spec change, put ALT-ADDRESS inside "<" and ">" quote with
"{" and "}".
4. add IANA considerations to register the new 2822 header
"UTF8SMTP".
5. add Security considerations about relation of EAI address to ALT-
ADDRESS.
11.2. draft-yeh-ima-utf8header-01
1. ABNF added.
2. Editrial changes.
3. Sent it as WG document.
11.3. draft-yeh-ima-utf8header-00
1. Section re-arranged.
2. Remove contains are not below to this document to.
12. References
12.1. 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.
[EAI-SMTP-extension]
Yao, J., Ed. and Wei. Mao, "SMTP extension for
internationalized email address",
draft-ietf-eai-smtpext-02.txt (work in progress),
July 2006.
[EAI-overview]
Klensin, J. and Y. Ko, "Overview and Framework of
Internationalized Email Address Delivery",
draft-ietf-eai-framework-01.txt (work in progress),
June 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions: Character Sets, Languages, and
Continuations", RFC 2231, November 1997.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
12.2. Informative References
[EAI-downgrading]
YONEYA, Yoshiro., Ed. and Kazunori. Fujiwara, Ed.,
"Downgrading mechanism for Internationalized eMail Address
(IMA)", draft-ietf-eai-downgrade-01.txt (work in
progress), June 2006.
[]
Hoffman, P., "SMTP Service Extensions or Transmission of
Headers in UTF-8 Encoding",
draft-hoffman-utf8headers-00.txt (work in progress),
December 2003.
[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.
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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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