Network Working Group J. Yao, Ed.
Internet-Draft X. Lee, Ed.
Expires: July 30, 2006 CNNIC
January 26, 2006
SMTP extension for internationalized email address
draft-yao-ima-smtpext-01.txt
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Abstract
Internationalized eMail Address (IMA) includes two parts, the local
part and the domain part. IMA is different from the
internationalizing domain name (IDN). These are different
infrastructures. IDN can not use the negotiation mechanism. IMA can
use some negotiation mechanisms which provide us a good platform to
solve the problem of IMA. So IMA should be solved in the mail
transport-level using the negotiation mechanism, which is an
architecturally desirable approach. This document specifies the use
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of SMTP extension for IMA delivery. It also mentions the backward
compatible mechanism for downgrade procedure, as specified in an
associated specification. The protocol proposed here is MTA-level
solution which is feasible, architecturally more elegant, and not as
difficult to deploy in relevant communities.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Role of this specification . . . . . . . . . . . . . . . . 3
1.2. Proposal Context . . . . . . . . . . . . . . . . . . . . . 3
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Mail Transport-level Protocol . . . . . . . . . . . . . . . . 4
2.1. Framework for the Internationalization Extension . . . . . 4
2.2. The Address Internationalization Service Extension . . . . 4
2.3. Extended Mailbox Address Syntax . . . . . . . . . . . . . 5
2.4. The ALT-ADDRESS parameter . . . . . . . . . . . . . . . . 6
2.5. Additional ESMTP Changes and Clarifications . . . . . . . 6
2.5.1. The Initial SMTP Exchange . . . . . . . . . . . . . . 6
2.5.2. Trace Fields . . . . . . . . . . . . . . . . . . . . . 7
2.5.3. Mailing List Question . . . . . . . . . . . . . . . . 7
2.5.4. Message Header Label . . . . . . . . . . . . . . . . . 7
3. Potential problems . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Impact to IRI . . . . . . . . . . . . . . . . . . . . . . 7
3.2. POP and IMAP . . . . . . . . . . . . . . . . . . . . . . . 7
4. Implementation Advice . . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Security considerations . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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1. Introduction
1.1. Role of this specification
An overview document [IMA-overview] specifies the requirements for,
and components of, full internationalization of electronic mail.
This document specifies an element of that work, specifically the
definition of an SMTP extension [RFC1869] for IMA transport delivery.
1.2. Proposal Context
In order to use internationalized email addresses, we need to
internationalize both the domain part and the local part of the email
address. Domain part of the email address may be internationalized
through IDNA [RFC3490]. But the local part of the email address
still remains as non-internationalized.
The syntax of Internet email addresses is restricted to a subset of
7-bit ASCII for the domain-part, with a less-restricted subset for
the local-part. These restrictions are specified in RFC 2821
[RFC2821]. To be able to deliver internationalized email through
SMTP servers, we need to upgrade SMTP server to be able to carry IMA.
Since older SMTP servers and the mail-reading clients and other
systems that are downstream from them may not be prepared to handle
these extended addresses, an SMTP extension is specified to identify
and protect the addressing mechanism.
This specification describes a change to the email transport
mechanism that permits IMA in both the envelope and header fields of
messages. The context for the change is described in [IMA-overview]
and the details of the header changes are described in [IMA-
utf8header],
1.3. Terminology
The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED",
and "MAY" in this document are to be interpreted as described in RFC
2119 [RFC2119].
All specialized terms used in this specification are defined in the
IMA overview [IMA-overview] or in [RFC2821] and [RFC2822].
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.
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2. Mail Transport-level Protocol
2.1. Framework for the Internationalization Extension
The following service extension is defined:
1. The name of the SMTP service extension is "Internationalized
Email and Extensions";
2. The EHLO keyword value associated with this extension is
"IEmail";
3. No parameter values are defined for this EHLO keyword value. In
order to permit future (although unanticipated) extensions, the
EHLO response MUST NOT contain any parameters for that keyword.
If a parameter appears, the SMTP client that is conformant to
this version of this specification MUST treat the ESMTP response
as if the IMA keyword did not appear.
4. An optional parameter is added to the SMTP MAIL and RCPT
commands. This parameter is named ALT-ADDRESS. It requires an
argument as a substitute for the internationalized (UTF-8 coded)
address, which is discussed in [IMA-downgrading]. This all-ASCII
address MAY incorporate the IDNA "punycode" form if the domain
name is internationalized. No algorithmic transformation is
specified for the local-part; In the general case, it may
identify a completely separate mailbox from the one identified in
the primary command argument. The domain part of the ALT-ADDRESS
may either be the same one as in the primary address (or its
punycode equivalent) or may be completely different.
5. No additional SMTP verbs are defined by this extension.
6. Servers offering this extension MUST provide support for, and
announce, the 8BITMIME extension [RFC1652].
2.2. The Address Internationalization Service Extension
An SMTP Server that announces this extension MUST be prepared to
accept a UTF-8 string [RFC3629] in any position in which RFC 2821
specifies that a "mailbox" may appear. That string must be parsed
only as specified in RFC 2821, i.e., by separating the mailbox into
source route, local part and domain part, using only the characters
colon (U+003A), comma (U+002C), and at-sign (U+0040) as specified
there. Once isolated by this parsing process, the local part MUST be
treated as opaque unless the SMTP Server is the final delivery MTA.
Any domain names that are to be looked up in the DNS MUST be
processed into punycode form as specified in IDNA [RFC3490] unless
they are already in that form. Any domain names that are to be
compared to local strings SHOULD be checked for validity and then
MUST be compared as specified in IDNA.
An SMTP Client that receives the IMA extension keyword MAY transmit a
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mailbox name as an internationalized string in UTF-8 form. It MAY
transmit the domain part of that string in either punycode (derived
from the IDNA process) or UTF-8 form. If it sends the domain in
UTF-8 form, the original SMTP client SHOULD first verify that the
string is valid for a domain name according to IDNA rules. As
required by RFC 2821, it MUST not attempt to parse, evaluate, or
transform the local part in any way. If the IMA SMTP extension is
not offered by the Server, the SMTP Client MUST not transmit an
internationalized address. Instead, it MUST either return the
message to the user as undeliverable or replace it with the alternate
ASCII address. If it is replaced, the replacement MUST be either the
ASCII-only address specified with the ALT-ADDRESS parameter or with
an address obtained from another source that conforms to the syntax
rules of RFC 2821.
2.3. Extended Mailbox Address Syntax
RFC 2821, section 4.1.2, defines the syntax of a mailbox as
Mailbox = Local-part "@" Domain
Local-part = Dot-string / Quoted-string
; MAY be case-sensitive
Dot-string = Atom *("." Atom)
Atom = 1*atext
Quoted-string = DQUOTE *qcontent DQUOTE
Domain = (sub-domain 1*("." sub-domain)) / address-literal
sub-domain = Let-dig [Ldh-str]
The key changes made by this specification are, informally, to
o Change the definition of "sub-domain" to permit either the
definition above or a UTF-8 string representing a DNS label that
is conformant with IDNA [RFC3490]. That label MUST NOT contain
the characters "@" or ".", even though those characters can
normally be inserted into a DNS label.
o Change the definition of "Atom" to permit either the definition
above or a UTF-8 string. That string MUST NOT contain any of the
ASCII characters (either graphics or controls) that are not
permitted in "atext"; it is otherwise unrestricted.
According to the description above, define the syntax of an IMA
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mailbox with ABNF [RFC2234] as
Mailbox = Local-part "@" Domain
Local-part = Dot-string / Quoted-string
; MAY be case-sensitive
Dot-string = Atom *("." Atom)
Atom = 1*Ucharacter
Ucharacter = <any UNICODE character,
except ASCII characters that are not permitted in "atext" >
Quoted-string = DQUOTE *qcontent DQUOTE
Domain = (sub-domain 1*("." sub-domain)) / address-literal
sub-domain = Let-dig [Ldh-str] /
<any internationalized domain lable specified by IDNA>
2.4. The ALT-ADDRESS parameter
If the IMA extension is offered, the syntax of the SMTP MAIL and RCPT
commands is extended to support the optional "ALT-ADDRESS" parameter,
which is specified in an associated document [IMA-downgrading].
2.5. Additional ESMTP Changes and Clarifications
The mail transport process involves addresses ("mailboxes") and
domain names in contexts in addition to the MAIL and RCPT commands
and extended alternatives to them. In general, the rule is that,
when RFC 2821 specifies a mailbox, this document expects UTF-8 to be
used for the entire string; when RFC 2821 specifies a domain name,
the name should be in punycode form if its raw form is non-ASCII.
The following subsections list and discuss all of the relevant cases.
Support and use of this extension requires support for 8BITMIME.
2.5.1. The Initial SMTP Exchange
When an SMTP or ESMTP connection is opened, the server sends a
"banner" response consisting of the 220 reply code and some
information. The client then sends the EHLO command. Since the
client cannot know whether the server supports IMA until after it
receives the response from EHLO, any domain names that appear in this
dialogue, or in responses to EHLO, must be in hostname form, i.e.,
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internationalized ones must be in punycode form.
2.5.2. Trace Fields
Internationalized domain names in Received fields should be
transmitted in punycode form. Addresses in "for" clauses need
further examination and might be treated differently depending on
[IMA-utf8header]. The reasoning in the introductory portion of [IMA-
overview] strongly suggests that these addresses be in UTF-8 form,
rather than some specialized encoding.
2.5.3. Mailing List Question
How a mixture of traditional and internationalized addresses on a
mailing list will impact message flows, error reports, and delivery
notifications in all plausible combinations of servers and addresses,
including internationalized and traditional reverse paths, when some
of the involved SMTP servers does not support IMA capability. This
is an issue, which we can delve into in detail in the future proposed
IEE working group. We will proposed the detail solution to it in
another document, and do some experiments to find the best solution
to it. Since SMTP has the negotiable mechanism, any problem will
become easier after negotiation.
2.5.4. Message Header Label
There is a hot discussion about message header label when SMTP
messages are transmitted on wire. How to identify them and
distinguish them from the normal message. Many referred the famous
"MIME-Version:1.0" as the example. In order to get the robustness in
the absence of context, it is a issue we should consider that whether
or not we need a mechanism(such as self-label) or some indicator to
distinguish or recognize whether the message is a "stored" message be
of new format(i.e. IMA compliant) or old one (i.e. RFC 822
compliant). More detailed discussion is needed after the future
proposed IEE working group is formed.
3. Potential problems
3.1. Impact to IRI
The mailto: schema in IRI [RFC3987] may need to be modified when IMA
is standardized.
3.2. POP and IMAP
While SMTP mainly takes care of the transportation of messages and
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the header fields on wire, POP essentially handles the retrieval of
mail objects from the server by a client. In order to use
internationalized user names based on IMA for the retrieval of
messages from a mail server using the POP protocol, a new capability
should be introduced following the POP3 extension mechanism
[RFC2449].
IMAP [RFC3501] uses the traditional user name which is based on
ASCII. IMAP should be updated to support the internationalized user
names based on IMA for the retrieval of messages from a mail server.
4. Implementation Advice
In the absence of this extension, SMTP clients and servers are
constrained to using only those addresses permitted by RFC 2821. The
local parts of those addresses may be made up of any ASCII
characters, although certain of them must be quoted as specified
there. It is notable in an internationalization context that there
is a long history on some systems of using overstruck ASCII
characters (a character, a backspace, and another character) within a
quoted string to approximate non-ASCII characters. This form of
internationalization should be phased out as this extension becomes
widely deployed but backward-compatibility considerations require
that it continue to be supported.
5. IANA Considerations
IANA is requested to add "IEmail" to the SMTP extensions registry
with the entry pointing to this specification for its definition.
6. Security considerations
See the extended security considerations discussion in [IMA-overview]
7. Acknowledgements
Much of the text in the initial version of this document was derived
or copied from [Klensin-emailaddr] with the permission of the author.
Significant comments and suggestions were received from Nai-Wen Hsu,
Yangwoo KO, Yoshiro YONEYA, and other members of the JET team and
were incorporated into the document. Special thanks to those
contributors for this version of document, those includes (but not
limited to) John C Klensin, Charles Lindsey, Dave Crocker, Harald
Tveit Alvestrand, Martin Duerst, Edmon Chung
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8. References
8.1. 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-overview]
Klensin, J. and Y. Ko, "Overview and Framework of
Internationalized Email Address Delivery",
draft-klensin-ima-framework-00 (work in progress),
October 2005.
[]
Klensin, J. and J. Yeh, "Transmission of Email Headers in
UTF-8 Encoding", draft-yeh-utf8headers-00 (work in
progress), October 2005.
[RFC1652] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
RFC 1652, July 1994.
[RFC1869] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
November 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 10, RFC 2234, November 1997.
[RFC2449] Gellens, R., Newman, C., and L. Lundblade, "POP3 Extension
Mechanism", RFC 2449, November 1998.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
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RFC 3490, March 2003.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, November 2003.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
8.2. Informative References
[IMA-downgrading]
YONEYA, Y. and K. Fujiwara, "Downgrade Mechanism for
Internationalized Email Address (IMA)",
draft-yoneya-ima-downgrade-00 (work in progress),
October 2005.
[Klensin-emailaddr]
Klensin, J., "Internationalization of Email Addresses",
draft-klensin-emailaddr-i18n-03 (work in progress),
July 2005.
[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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Authors' Addresses
Jiankang YAO (editor)
CNNIC
No.4 South 4th Street, Zhongguancun
Beijing
Phone: +86 10 58813007
Email: yaojk@cnnic.cn
Xiaodong LEE (editor)
CNNIC
No.4 South 4th Street, Zhongguancun
Beijing
Phone: +86 10 58813020
Email: lee@cnnic.cn
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