Network Working Group A. Melnikov (Ed.)
Internet Draft Isode Limited
Intended status: Standards Track June 14, 2007
Expires: December 2007
SMTP Language Extension
draft-melnikov-smtp-lang-07.txt
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
The Simple Mail Transfer Protocol (RFC 2821) allows server responses
to include human-readable text that in many cases needs to be
presented to the user. This document specifies a way for a client to
negotiate which language the server should use when sending
human-readable text. It also extends the UTF-8 Delivery Status
Notifications format to include language field for the human-readable
text.
0. Meta Information on this draft
This information is intended to facilitate discussion.
<<NOTE to RFC Editor: please remove this section>>
The protocol discussed in this document is experimental and subject
to change. Persons planning on either implementing or using this
protocol are STRONGLY URGED to get in touch with the author before
embarking on such a project.
ToDo List:
1). Martin Duerst wrote:
On the other hand, for the LANG parameter for the MAIL
command should allow a language priority list. The reason
for this is that (if my understanding is correct), this
parameter is passed on along the relay chain of SMTP servers,
and is supposed to go back to the original sender, and
using a list increases the chance that there is something
at the relevant server that can be understood by the
originator.
2). Martin Duerst wrote:
For language fallback, I suggest you have a look at
http://www.ietf.org/internet-drafts/draft-ietf-ltru-matching-15.txt
(also IESG-approved). This gives you the (basic) "language-range"
ABNF construct that includes the "*" wildcard.
Also, it seems that the matching going on on the server when the
client issues a LANG command (e.g. in Example 5) is very close to
and can (and should) be described in terms of Section 3.4, Lookup,
of the above draft. The only difference I see is that Section 3.4
requires a solution (maybe default) in all cases, whereas in your
case, the default if no matching language is found is not i-default,
but "no change" or in other words, "previously selected language".
3). Greg Vaudreuil wrote:
The interaction between the LANG verb and the LANG Mail tag is not
specified. The goal of the MAIL FROM tag is to get responses useful
to the message sender. Those responses may come in the form of the
DSN itself (covered) or in the SMTP reply converted to a DSN by the
client SMTP. If the SMTP client requests french SMTP dialogue but
the message sender requests German for an error message, the SMTP
reply code text should be in German, that is, the MAIL-FROM LANG tag
should override the LANG verb for the SMTP responses used to
generate a DSN on the client SMTP side.
4). Chris Newman:
SMTP responses should be allowed to contain multiple languages:
the first is always English, followed by a special delimiter,
followed by text in another language (maybe allow for more than
2 languages). This can be done as a multiline response, e.g.
250-Command has succeeded
250 <delimiter, language mark><text in another language>
<<Followup discussion with Pete: don't specify exact syntax,
but add some text saying that an implementation MAY return
error text in other languages>>
5). Stephane Bortzmeyer wrote:
Otherwise, in the security section, I suggest to include text like:
Languages and language variations such as scripts are often closely
associated with specific social, national, religious or ethnic
affinities. Thus, language tags used in content negotiation, like
other information exchanged on the Internet, might be a source of
concern because they might be used to infer information about the
sender and thus identify potential targets for surveillance.
If, for instance, the same program is both a Web browser and a Mail
User Agent, the fact that the user configured his Web browser to
request pages in a specific language should not automatically imply
that his mail client broadcasts this preference to every Usenet
newsgroup or mailing list.
[Rationale: in countries like Moldavia or the former Yugoslavia,
asking for the cyrillic or the latin script is not innocent and
is often tied to political views.]
Changes since -00
1). Corrected grammar error in LANG command description section
2). Included Mark Crispin's suggestion of allowing the server to
substitute a primary language if the sublanguage asked for is not
available.
3). Added section 5 that describes extended LANG reply
4). Corrected example, more examples
5). Added extension mechanism
6). Specified interaction with RFC-2034 ("SMTP Service Extension for
Returning Enhanced Error Codes")
7). LANG command must always have language-tag as a parameter. Only EHLO
response could be used to examine list of supported languages.
Changes since -01
1). Corrected ABNF for CR
2). Updated Copyright section
3). Other minor bugfixes
Changes since -02
1). Extended DSN format to include language tag
2). Fixed few typos.
Changes since -03
1). Changed DSN format to include language tag and translation of text
part of diagnostic-code-field. Don't use diagnostic-code-field for
a non English text.
2). Added LANG parameter to MAIL FROM.
Changes since -04
1). Updated boilerplate.
2). Updated references. Split references into Normative and Informative.
3). Updated ABNF (ABNF for UTF-8 responses, allow for multiple language
tags)
4). Clarified that LANGUAGE extension is applicable to both MTAs and
MSAs.
5). Added '*' language (to match EAI POP3 draft).
6). Fixed EHLO capabilities in examples to match HELP output.
7). Added new section on line length limit.
8). Copied Security Considerations from EAI POP3 document.
9). Removed any mentioning of extensions to the LANG command.
10). Made extended data prefix optional (currently it is only used
in response to the LANG command)
11). Many minor editorial changes.
Changes since -05
1). Updated boilerplate.
2). Replaced reference to RFC 3066 with draft-ietf-ltru-registry,
removed some text as a result of this change.
3). Clarified that if the receiving MTA doesn't support the LANGUAGE
extension, then the LANG parameter must be silently ignored.
4). Updated DSN format ABNF as per comment from Harald Alvestrand.
Open issues
0). Open issues are enclosed in <<>>.
1. Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively. If such lines are wrapped without a new "C:" or
"S:" label, then the wrapping is for editorial clarity and is not
part of the command.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [KEYWORDS].
2. Framework for the Language SMTP service extension
The Language SMTP service extension uses the SMTP service extension
mechanism described in [ESMTP]. The following SMTP service extension
is therefore defined:
(1) The name of the SMTP service extension is "Language". This
extension is applicable to regular SMTP [ESMTP], Message
Submission Protocol [SUBMIT] and LMTP [LMTP].
(2) The EHLO keyword value associated with this service extension is
"LANGUAGE".
(3) The LANGUAGE EHLO keyword will have zero or more space separated
arguments, each containing a name of supported language tags.
If no arguments are specified, this means that server is
unable to enumerate the list of languages it supports.
(4) A new SMTP verb "LANG" is defined by this document.
(5) One optional parameter is added to the MAIL command:
An optional parameter for the MAIL command, using the
esmtp-keyword "LANG", (used to propagate a language that should be
used in human readable part and/or localized-diagnostic-text-field
field of "message/delivery-status" part (see section 6.) of a
delivery status notification for the message), is defined in
section 7.
3. LANG Command
LANG 1*(SP language-tag)
Arguments:
one or more language tag as defined in [LANG-TAGS].
Restrictions:
The LANG command is permitted throughout a mail connection.
Reply Codes:
Success:
250 LANG command completed successfully
Error:
504 None of the specified languages is supported
421 <domain> Service not available, closing transmission
channel
Discussion:
The LANG command requests that human-readable text emitted by
the server be localized to one of the language specified in the
argument. If multiple language tags are specified, they are
specified in the decreasing order of preference. This is called
"the language priority list" in [LANG-MATCH].
If the SMTP client is an MTA, the LANG command can be used to
return potential error messages in the language requested by
the original sender.
If a sublanguage was asked for and not available but the
primary language is available, the server SHOULD switch to the
primary language and MUST use an extended LANG reply containing
the identifier of the primary language it switched to as
described in section 5.
<<Should the fallback to primary languages be removed?>>
<<Need to describe that the server first picks a language from
the list, if it can't find any, it should try to use
sublanguages for the specified languages>>
Any server that supports this extension MUST support the
language "i-default". It SHOULD <<MUST?>> use the language
"i-default" as described in [CHARSET-POLICY] as its default
language until another supported language is negotiated by the
client. If a server is able to enumerate supported languages
it MUST include "i-default" in the EHLO response. Otherwise it
MUST NOT return any language in the LANGUAGE EHLO response.
The special "*" language range argument indicates a request to
use a language designated as preferred by the server
administrator. The preferred language MAY vary based on the
currently authenticated user.
If the server can't find any language (or sublanguage) that it
supports, the server returns the 504 reply code. Servers
supporting the Enhanced Error Codes extension [RFC-2034]
SHOULD use the 5.3.3 "System not capable of selected features"
[ENHANCED-SC] error code in this case.
If the command succeeds, the server will return human-readable
responses in the specified language starting with the
successful 250 response to the LANG command. These responses
will be in UTF-8 [UTF-8]. In particular, LANG command MAY
affect the result of a HELP command. The successful 250
response to the LANG command MUST use the extended reply as
described in section 5. This reply will communicate the
selected language to the client.
If the command fails, the server will continue to return human-
readable responses in the language it was previously using.
Example 1:
< The server defaults to using responses in "i-default" language
until the user explicitly changes the language. >
S: 220 smtp.example.com ESMTP server ready
C: EHLO main.example.com
S: 250-smtp.example.com
S: 250-AUTH CRAM-MD5 DIGEST-MD5
S: 250-EXPN
S: 250-VRFY
S: 250-DSN
S: 250 LANGUAGE EN FR RU i-default
C: HELP
S: 214-This is Bukamail version X.X.X
S: 214-Topics:
S: 214- HELO EHLO MAIL RCPT DATA
S: 214- RSET NOOP QUIT HELP VRFY
S: 214- EXPN VERB AUTH DSN
S: 214-For more info use "HELP <topic>".
S: 214 End of HELP info
< Once the client changes the language, all responses will be in
that language starting with 250 response to the LANG command.
Note that in the following examples accented characters
are not shown, as they are not allowed in RFCs. However the
accented characters, encoded in UTF-8, would be present in
the actual protocol exchange. >
C: LANG FR
S: 250 La commande LANG a ete executee avec succes
C: HELP
S: 214-Ici le programme Bukamail version X.X.X
S: 214-Topics:
S: 214- HELO EHLO MAIL RCPT DATA
S: 214- RSET NOOP QUIT HELP VRFY
S: 214- EXPN VERB AUTH DSN
S: 214-Pour obtenir de l'information supplementaire, utilisez
"HELP <topic>".
S: 214 Fin de l'information
< If a server does not support the requested language, responses
will continue to be returned in the current language the
server is using. >
C: LANG DE
S: 504 Cette langue n'est pas acceptee
Example 2:
< The client tries to select a language that is not supported >
C: LANG be-Latn
S: 504 The specified language is not supported.
Example 3:
< The client tries to use a LANG extension not supported by
the server >
C: LANG i-default (blah blah)
S: 504 LANG extension blah is not recognized.
4. SMTP Line length limit
This extension doesn't affect the maximim text line length as defined
by [ESMTP] and extended by other ESMTP extensions. However ESMTP
maximum line length is defined in octets and *not* Unicode
characters. If a server implementation truncates (or splits to create
multiline response) UTF-8 response text to conform to such ESMTP
limits, it MUST truncate the response at the Unicode character
boundary. (E.g. if a response encoded in UTF-8 would be 1002 octets
including the terminating CRLF and the last Unicode character before
CRLF is represented as 3 UTF-8 octets, the response must be truncated
after 997 octets, with the terminating CRLF added after that).
5. "LANG" extended reply
Extended reply is the reply that contains additional information in
the text part. Extended reply allows to pass additional information
from server to client. Client may choose to ignore additional
information in an extended reply. Thus client that doesn't recognize
an extended reply would treat it as a regular SMTP reply.
Example 4:
< The client tries to select the language, but it is
unavailable. However primary language is available>
C: LANG FR-ca
S: 250 [LANG FR]La commande LANG a ete executee avec succes
Client that supports LANGUAGE extension MUST recognize Enhanced Error
Codes defined in [RFC-2034]. When server supports both LANGUAGE and
ENHANCEDSTATUSCODES extensions, Extended reply data MUST follow any
Enhanced Error Code in reply.
Example 5:
< The server supports both LANGUAGE and ENHANCEDSTATUSCODES>
S: 220 smtp.example.com ESMTP server ready
C: EHLO main.example.com
S: 250-smtp.example.com
S: 250-LANGUAGE EN FR RU i-default
S: 250 ENHANCEDSTATUSCODES
C: LANG FR-ca
S: 250 2.0.0 [LANG FR]La Language commande a ete execute avec
success
6. The LANG parameter of the ESMTP MAIL command
Then LANG esmtp-keyword on the extended MAIL command specifies what
language should be used in human readable part and/or
localized-diagnostic-text-field field of "message/delivery-status"
part (see section 7.) of a delivery status notification for the
message.
If the LANG esmtp-keyword is used, it MUST have an associated
esmtp-value. The ABNF for the LANG parameter is:
lang-parameter = "LANG=" language-tag
If the message is relayed to another SMTP server that supports
LANGUAGE ESMTP extension, the MTA acting as the client MUST check if
the receiving MTA lists the language specified in lang-param
("requested language") in the list of supported language tags in
LANGUAGE EHLO response. If the receiving MTA either lists the
requested language or doesn't list any language tag (i.e. the
receiving MTA is unable to list languages it supports) the sender
MUST issue LANG command for the requested language. After that,
regardless of the result of LANG command, the client MTA MUST
specify LANG parameter in MAIL command.
The receiving MTA SHOULD use the language specified in LANG parameter
if it has to generates a DSN for the message. Human readable part in
generated DSN SHOULD contain the description of the event in both
i-default and the requested language. If the receiving MTA doesn't
support the requested language, it MUST act as if the client didn't
specify any LANG parameter in the MAIL command.
If the receiving MTA doesn't support the LANGUAGE extension, the
sending MTA MUST behave as if the LANG parameter was not specified,
i.e. the LANG parameter MUST be silently dropped.
Example 6:
< Relaying of the message >
S: 220 smtp.example.com ESMTP server ready
C: EHLO main.example.com
S: 250-smtp.example.com
S: 250-DSN
S: 250-8BITMIME
S: 250 LANGUAGE
C: LANG RU
S: 504 Unsupported language
C: MAIL FROM:<Katerina@example.ru> LANG=ru
S: 250 <Katerina@example.ru> sender ok
C: DATA
S: 354 okay, send message
C: (message goes here)
C: .
S: 250 message accepted
C: QUIT
S: 221 goodbye
7. Delivery status notifications and extension
This section updates [EAI-DSN].
The format of International delivery status notifications
(message/utf-8-delivery-status content type) is specified in
[EAI-DSN]. This memo extends the per-recipient-fields of [EAI-DSN]
[DSN] to include one new field, Localized-Diagnostic, that is
equivalent to text part of Diagnostic-Code but includes a language
tag and contains text in the specified language. In the Augmented
BNF [ABNF], per-recipient-fields is therefore extended as follows:
per-recipient-fields =
[ original-recipient-field CRLF ]
final-recipient-field CRLF
action-field CRLF
status-field CRLF
[ remote-mta-field CRLF ]
[ *(localized-diagnostic-text-field CRLF)
diagnostic-code-field CRLF ]
[ last-attempt-date-field CRLF ]
[ will-retry-until-field CRLF ]
*( extension-field CRLF )
<<Should the localized-diagnostic-text-field be moved to the very
end, in order to improve interoperability?>>
localized-diagnostic-text-field = "Localized-Diagnostic" ":"
language ";" *utf8-text
where language is a language tag as described in [LANG-TAGS] and
ABNF for utf8-text is specified in Section 8 of this document.
An SMTP server that supports both DSN and LANGUAGE extensions SHOULD
include localized-diagnostic-text-field. Note that multiple
Localized-Diagnostic DSN fields are allowed, but they MUST use
different language tags. diagnostic-code-field MUST NOT contain
text in any language other than English.
8. Formal Syntax
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in [ABNF]. Non-terminals referenced but not
defined below are as defined by [ESMTP] or [ABNF].
Except as noted otherwise, all alphabetic characters are
case-insensitive. The use of upper or lower case characters to
define token strings is for editorial clarity only. Implementations
MUST accept these strings in a case-insensitive fashion.
LANG-Command = "LANG" 1*(SP language-tag) CRLF
LANGUAGE-List = "LANGUAGE" *(SP <language-tag>) CRLF
; Note 1: When "i-default" is used, all responses MUST be
; entirely in ASCII.
;
; Note 2: See [LANG-TAGS] for the list of allowed
; language tags.
language-tag = <language-tag> as defined in [LANG-TAGS]
Reply-line |= Lang-Reply-line
; Reply-line is defined in [ESMTP].
; See section 5 for description of Lang-Reply-line
Lang-Reply-line = Reply-code [ SP ext-text ] CRLF
; Reply line for a LANG command
ext-text = [ext-data] utf8-text
utf8-text = 1*utf8-no-cr-lf
utf8-no-cr-lf = %d1-9 / ; Any Unicode character except for NUL,
%d11 / ; CR and LF
%d12 /
%d14-127 /
UTFMB
UTFMB = UTF2 / UTF3 / UTF4
UTF0 = %x80-BF
UTF2 = %xC2-DF UTF0
UTF3 = %xE0 %xA0-BF UTF0 / %xE1-EC 2(UTF0) /
%xED %x80-9F UTF0 / %xEE-EF 2(UTF0)
UTF4 = %xF0 %x90-BF 2(UTF0) / %xF1-F3 3(UTF0) /
%xF4 %x80-8F 2(UTF0)
ext-data = "[" ext-name SP ext-value "]"
; Note 1: In the case of multiline response the same ext-data
; SHOULD appear on every line.
;
; Note 2: In case when server also supports "SMTP Service
; Extension for Returning Enhanced Error Codes" [RFC-2034],
; ext-data MUST follow any Enhanced Error Code.
ext-name = "LANG"
ext-value = Primary-tag
; Primary tag as defined by [LANG-TAGS]
9. Security Considerations
It is possible for a man-in-the-middle attacker to insert a LANG
command in the command stream thus making protocol-level diagnostic
responses unintelligible to the user. A mechanism to integrity
protect the session, such as TLS [TLS] can be used to defeat such
attacks.
10. References
10.1. Normative References
[ESMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[SUBMIT] Gellens, R. and J. Klensin, "Message Submission for
Mail", RFC 4409, April 2006.
[LMTP] Myers, J., "Local Mail Transfer Protocol", Carnegie-Mellon
University, RFC 2033, October 1996.
[LANG-TAGS] Phillips, A. and M. Davis, "Tags for Identifying
Languages", RFC 4646, September 2006.
[LANG-MATCH] Phillips, A. and M. Davis, "Matching of Language Tags",
RFC 4647, September 2006.
[CHARSET-POLICY] Alvestrand, H., "IETF Policy on Character Sets and
Languages", RFC 2277, January 1998.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[RFC-2034] Freed, N., "SMTP Service Extension for Returning Enhanced
Error Codes", RFC 2034, Innosoft, October 1996.
[ENHANCED-SC] Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC 3463, January 2003.
[EAI-DSN] Newman, C., "International Delivery and Disposition
Notifications", work in progress, draft-ietf-eai-dsn-XX.txt
[DSN] Moore , K., Vaudreuil, G., "An Extensible Message Format for
Delivery Status Notifications", University of Tennessee, Lucent
Technologies, RFC 3464, January 2003.
10.2. Informative References
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.1", RFC 4346, April 2006.
11. Acknowledgments
This document is based on the early version of
draft-gahrns-imap-language-xx.txt. Thus the work of Andrew McCown is
appreciated.
Many thanks to the following people who gave feedback on the
document: Chris Newman, Brad Knowles, Paul Hoffman, Harald
Alvestrand, Martin Duerst, Greg Vaudreuil and Stephane Bortzmeyer.
12. Author's Address
Mike Gahrns
Microsoft
One Microsoft Way
Redmond, WA, 98072
Phone: (425) 936-9833
Email: mikega@microsoft.com
Alexey Melnikov (Editor)
Isode Limited
5 Castle Business Village
36 Station Road
Hampton, Middlesex
TW12 2BX,
United Kingdom
EMail: Alexey.Melnikov@isode.com
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgment
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).