Internet DRAFT - draft-ietf-eai-email-clients
draft-ietf-eai-email-clients
Email Address Internationalization E. Dainow
(EAI) Afilias
Internet-Draft K. Fujiwara
Intended status: Informational JPRS
Expires: March 11, 2013 September 7, 2012
Guidelines for Internationalized Email Clients
draft-ietf-eai-email-clients-01
Abstract
This document provides some guidelines for email clients that support
Email Address Internationalization (EAI) as outlined in [RFC6530]. A
number of interoperability cases between different versions of email
components are reviewed. Recommendations are made to improve
interoperability and usability and to minimize discrepancies between
the display of composed and received email in different language
environments.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on March 11, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Conventions used in this document . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Interoperability . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Interoperability Scenarios . . . . . . . . . . . . . . . . 5
5. Compatibility Support . . . . . . . . . . . . . . . . . . . . 6
5.1. Address Book . . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Message Mode . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. Message Format . . . . . . . . . . . . . . . . . . . . . . 8
5.4. Error Handling . . . . . . . . . . . . . . . . . . . . . . 8
5.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.6. Limitations . . . . . . . . . . . . . . . . . . . . . . . 10
6. Mailbox Integration . . . . . . . . . . . . . . . . . . . . . 12
7. Character Encoding . . . . . . . . . . . . . . . . . . . . . . 12
8. Normalization . . . . . . . . . . . . . . . . . . . . . . . . 13
9. Security Considerations . . . . . . . . . . . . . . . . . . . 13
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
12. Normative References . . . . . . . . . . . . . . . . . . . . . 14
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1. Conventions used in this document
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 [RFC2119].
2. Introduction
[RFC6530] Overview and Framework for Internationalized Email
describes changes to electronic mail (email) to fully support
internationalized characters. The fundamental change is to remove
the ASCII only restriction on email addresses and allow them to
contain UTF-8 characters. Additional documents provide detailed
specifications for the extensions required to email headers [RFC6532]
and to the protocols SMTP [RFC6531], POP [I-D.ietf-eai-rfc5721bis]
and IMAP [I-D.ietf-eai-5738bis].
This document provides guidelines for email clients that support
these specifications for Email Address Internationalization (EAI).
It does not introduce any protocol extensions that are not defined in
the above documents. It highlights the extensions that are important
to the design and implementation of email clients and makes a number
of recommendations intended to improve interoperability and
usability.
3. Terminology
A number of different acronyms are typically used to describe the
major functional components of email.
Mail User Agent (MUA)
Message Submission Agent (MSA)
Message Transfer Agent (MTA)
Message Delivery Agent (MDA)
Message Store (MS)
The architecture of modern email systems can range from simple, with
all components running on one server, to very complex, with
components being distributed across multiple, geographically
dispersed machines. Nevertheless, the above terminology is generally
sufficient to represent different architectures from a functional
point of view. For a comprehensive description of email architecture
see [RFC5598].
sender -> MUA -> MSA -> MTA
...
MTA -> MDA -> MS -> PIF -> MUA -> recipient
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In this context, an "Email Client" is an MUA that has an interface to
an MSA to send email and an interface to the MS to retrieve email.
The interface to retrieve mail (PIF) is a POP or IMAP server or
direct access to the File system. The MUA also provides a User
Interface (UI) that allows an end user to read (display) and write
(compose) their email.
A common email architecture includes the MSA function within the MTA.
An improved architecture that better addresses security concerns is a
separate MSA component as shown here [RFC6409], [RFC5068].
"SMTPUTF8" is used to indicate email address internationalization as
specified by [RFC6530] and related documents.
"ASCII" refers to the strict 7-bit ASCII character set
[ANSI.X3-4.1968].
"UTF-8", Unicode Transformation Format/8-bit is a character encoding
scheme that can represent any character in the Unicode standard
[RFC3629]. It contains ASCII as a subset.
"message/global" is an email message that contains UTF-8 characters
beyond 7-bit ASCII in message headers and/or body parts [RFC6532].
"message/rfc822" is an email message that contains only 7 bit ASCII
and does not use any SMTPUTF8 extensions. Note that the original
message (as composed by the user) may contain non-ASCII characters
that have been encoded into ASCII using IDNA [RFC5890], MIME body
encoding [RFC2045] or MIME header encoding [RFC2047].
4. Interoperability
Internationalized Email is not compatible with legacy email systems,
those based on prior Internet email standards [RFC5321], [RFC5322].
Non-ASCII email addresses cannot be submitted in legacy SMTP commands
like MAIL FROM or RCPT TO. In addition the Internationalized Email
standard does not include a method to "downgrade" message/global to
message/rfc822.
An Internationalized message cannot be transmitted via SMTP if the
receiving MTA does not announce SMTPUTF8 in response to EHLO. There
are two failure cases that an email client may have to handle
described in Section 3.2 of [RFC6531].
a) If the client is submitting a message to an MSA that does not
support SMTPUTF8, the message will be rejected.
b) If the MSA does support SMTPUTF8 but a downstream MTA does not,
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then the mail will bounce. That is, a delivery status notification
(DSN) that the mail could not be delivered will be sent back to the
sender.
Incompatibility between Internationalized email and legacy systems is
expected to be important initially during a transition period but
less important over time as more email systems upgrade to support the
SMTPUTF8 extensions. To the extent that this incompatibility is
deemed important at the time an implementation is undertaken, the
email client should provide methods to prevent or at least minimize
these failures.
4.1. Interoperability Scenarios
The following scenarios cover the different cases of sending mail
from an Internationalized server to a legacy server.
'I' indicates an Internationalized address (a non-ASCII address on an
Internationalized mail server).
'IA' indicates an ASCII address on an Internationalized server.
'LA' indicates an address on a Legacy mail server, which must be
ASCII.
Case 1. The simple compatibility case
From: IA1 (or LA1)
To: LA2
Subject: ...
Body ...
The message will be successfully sent as long as the email client
sends message/rfc822 rather than message/global.
Case 2. The simple incompatibility case
From: I1
To: LA2
Subject: ...
Body ...
The message will be rejected by the MSA or will bounce from a
downstream SMTP server.
If user I1 also has an ASCII email address IA1 or LA1, there may be a
simple workaround. If the email client supports multiple email
accounts, the user just has to switch the From address to an ASCII
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address and it becomes Case 1.
Case 3. The general incompatibility cases
The general case is a mix of Internationalized and legacy addresses.
While many combinations are possible, the two cases below essentially
cover all possibilities.
From: I1
To: LA2
Cc: I3
The message will be sent to I3 but it will bounce from LA2.
Switching the From address to an ASCII address as in Case 2 is not a
solution, as the following case demonstrates.
From: IA1 (or LA1)
To: LA2
Cc: I3
This message will bounce from LA2 since the address in the Cc header
cannot be transmitted to a legacy server.
In these cases, users will likely send the message twice in order to
reach all intended recipients. First, to the original list and then
using an ASCII address to the bounced recipients.
If users know beforehand which addresses are on legacy servers, they
can avoid bounced messages by removing those addresses, but they
still have to send a second email to reach recipients that were
removed.
5. Compatibility Support
An email client can provide support to minimize the incompatibility
problems outlined in Section 4. There may be several ways to do
this. Following are guidelines on some of the ways that this can be
accomplished.
At the very least, to provide basic compatibility between
Internationalized and legacy systems, if all email addresses in the
SMTP envelope and the message headers are ASCII, then a message/
rfc822 should be sent (Case 1 above).
For Case 2, the email client should support multiple email accounts
and allow the user to switch the From address at any time during
composition of the message.
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For Case 3, several mechanisms may be required to provide
compatibility support. These are outlined in the following sub-
sections.
5.1. Address Book
Each contact in the address book should be able to have several email
addresses, each of which is configured to be either an
Internationalized or a Legacy address.
The user may not necessarily know if an ASCII address they enter in
their address book is on a legacy server or not. If it is configured
as an Internationalized address and that turns out to be wrong, then
email sent to that contact may bounce. The user can then re-
configure the address as Legacy so the email client can provide
warnings of a possible bounce on subsequent messages.
5.2. Message Mode
Message composition should have "Message Mode" option to specify
"Internationalized Mode" or "Legacy Mode".
If the type of each address in the headers does not conform to the
message mode, then the user is given a warning about those addresses
that don't match the mode. In a graphical user interface this might
be done by setting such addresses to a different color such as red.
The user would typically first change the message mode to see if the
warnings disappear.
When the mode is switched, the email client switches addresses in
message header fields to match the mode, selecting from the list of
addresses in each contact.
There are cases where both modes provide warnings (see Example 5
below). In these cases, the user can remove the addresses that don't
conform to the mode.
For Internationalized mode, the user has an additional option to send
the message anyway, without removing flagged addresses. They would
have to handle bounced messages from Legacy servers later. The
option to send anyway cannot be provided in Legacy mode, as it is not
possible to compose a message/rfc822 if any sender or recipient
address is not ASCII.
Where both modes provide warnings, users will likely want to send the
message in each mode in order to reach all recipients. The email
client should make it easy to do this. There are many possible
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designs to accomplish this. The following is one example.
An option is provided when composing email to add a second message
header section in the other mode that allows the user to move
addresses between sections. This is in addition to making individual
changes to address headers as in normal email composition. The
Subject and Body are common so the user can compose a single message
but have it sent in the two different modes to different recipients.
Following is an example of this for Case 3 above.
---------------------------------------------
Legacy Internationalized
From: IA1 From: I1
To: LA2 <---> To:
Cc: <---> Cc: I3
---------------------------------------------
Subject: ...
Body: ...
---------------------------------------------
5.3. Message Format
In Internationalized Mode, mail should be sent as message/global.
The aim of Internationalized Email is 8 bit clean messages using
UTF-8 encoding to represent Unicode characters in header fields and
the message body.
In Legacy Mode, mail must be sent as message/rfc822. This may
include non-ASCII characters that are encoded into ASCII using MIME
body encoding [RFC2045] or MIME header encoding [RFC2047]. Any
encoding should be based on UTF-8. In the interest of
interoperability, charsets other than UTF-8 are prohibited in mail
addresses and message headers described in Section 7.1 of [RFC6530].
5.4. Error Handling
If a message is rejected by the MSA with a response code that
indicates incompatibility with legacy email described in Section 3.2
of [RFC6531], the compose window should be kept open so that the user
can make changes and retry. The email client should provide guidance
to the user about switching the Message Mode, reconfiguring the type
of an address in the address book or adding an ASCII legacy address
for a contact in the address book.
Similarly, if a message bounces, the email client could parse the
delivery status notifications and message disposition notifications
[RFC6533] to determine if the failure was a compatibility problem and
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if so, which addresses caused the problem.
5.5. Examples
The following examples illustrate most of the different possible
cases.
Suppose the user (Sender) has set up the following email account
containing two email addresses, an Internationalized address and an
ASCII address on an Internationalized server.
Sender: I0, IA0
Examples are not provided for the following cases:
a) Sender: I0, LA0
If the Sender has both Internationalized and Legacy addresses, then
this is equivalent to the above.
b) Sender: I0
If the Sender has only Internationalized addresses, then it cannot
send Legacy messages. The email client cannot provide an option to
switch the Message Mode to Legacy.
c) Sender: LA0
If the Sender has only accounts on Legacy servers, then it cannot
send Internationalized messages. The email client cannot provide an
option to switch the Message Mode to Internationalized.
The address book has the following contacts with email addresses.
Contact1: I1, IA1
Contact2: I2
Contact3: IA3
Contact4: LA4
Example 1:
From: Sender
To: Contact1
CC: Contact2
This message can be sent in Internationalized mode.
In Legacy mode the email client would flag Contact2, who does not
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have an ASCII address.
Example 2:
From: Sender
To: Contact1
CC: Contact3
This message can be sent in either Internationalized or Legacy mode.
Example 3:
From: Sender
To: Contact1
CC: Contact4
This message cannot be sent in Internationalized mode. Contact4
would be flagged since it is not on an Internationalized server.
This message can be sent in Legacy mode.
Example 4:
From: Sender
To: Contact2
CC: Contact3
This message can be sent in either Internationalized mode or Legacy
mode.
Example 5:
From: Sender
To: Contact2
CC: Contact4
This message cannot be sent in either mode.
Internationalized mode would flag Contact4 which is on a Legacy
server. The user can remove Contact4 or use the send anyway option.
Legacy mode would flag Contact2 who does not have an ASCII address.
The user would have to remove Contact2 in order to send this message.
5.6. Limitations
In summary, the guidelines outlines in Section 4 and Section 5 will
provide the following compatibility solutions:
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1. When there is an ASCII address for all contacts in the message,
then a single legacy compatible message can be sent to all
recipients.
2. When some contacts in the message do not have an ASCII address
and some have only ASCII addresses on legacy servers, then the
message can be split into two. One message is sent as an
Internationalized message to recipients on Internationalized servers.
The other is sent as a legacy compatible message to recipients on
legacy servers.
These guidelines have a number of limitations.
a) Unknown Address Types
Message Mode is effective only if users are fairly disciplined about
keeping addresses in their address book and configuring the type
correctly as Internationalized or Legacy.
When replying to an email, the message may have addresses that are
not in the address book. The user may also enter addresses directly
during message composition that are not in the address book.
The email client may determine by inspection that some addresses are
Internationalized. If an address contains any non-ASCII character,
then it must be Internationalized. However, an ASCII address may be
on either an Internationalized server or a Legacy server and there is
no way software can determine this automatically.
In such cases, it may be useful for the email client to flag unknown
address types in a message so that the user is not lead to believe
that the message will not bounce just because there were no
incompatibility warnings.
b) Address Removal
When email addresses are removed from a message to meet compatibility
requirements, recipients do not see everyone who was intended to be
part of the conversation. The email client can provide the address
of removed recipients by using an empty group. This technique is
described in Section 3.1.8 of [I-D.ietf-eai-popimap-downgrade].
This is not an ideal solution, since replies to the message will not
reach everyone intended. But at least it provides the necessary
contact information to recipients who may be able to use other
methods to reply to all intended.
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6. Mailbox Integration
If more than one email address is used for the sender user, emails
may arrive at different email accounts. There are several ways to
provide mailbox integration so the user is able to view all mail in
one location, such as a single 'Inbox' folder.
If integration is done on the server, through the use of aliases,
then the email client does not need to do anything. All mail will be
received at the client from one address.
The email client should provide mailbox integration for cases where
server side integration is not available and for more flexibility on
the part of the user. Many email clients already provide a
convenient way to manage multiple email accounts.
An option to view all mail from a group of accounts in one integrated
folder should also be provided.
7. Character Encoding
Email message bodies may be composed and displayed using many
different character encoding schemes. Numerous character encodings
have been developed over time in order to best represent different
language scripts. In recent years there has been a trend to prefer
Unicode as a "universal" character set and UTF-8 as the preferred
encoding method.
A good general principle to follow is to minimize character
conversions. This will reduce the chance that the received message
is displayed differently from how it was composed. Displaying
received mail SHOULD use the character encoding of the received mail.
Since older MUAs may not be able to parse UTF-8, the MUA SHOULD try
to reply to mail using the character encoding of the received mail.
This may not be possible if the sender adds new characters that
cannot be encoded in the original encoding. For example, if the
received message is encoded in ISO-2022-JP and characters in ISO-
8859-1 are added to the message, the text cannot be carried in ISO-
2022-JP and conversion to UTF-8 may be the best solution.
For new mail, A SMTPUTF8 compliant MUA SHOULD use UTF-8 as the
default encoding if the message type is global or if the envelope
contains non-ASCII addresses. If email clients utilize this default,
character conversions will be minimized and there will be less chance
that someone will receive mail in an unrecognized encoding.
If the message type is rfc822, other considerations may apply, such
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as using the system locale/language.
Notwithstanding the above, there may be cases where the default does
not work well. There SHOULD be options for the user to reset the
default character encoding. There SHOULD also be options to change
the encoding when reading or writing individual email messages.
8. Normalization
Different sequences of UTF-8 characters may represent the same thing.
Normalization is a process that converts all canonically equivalent
sequences to a single unique form.
Normalization of email headers is specified in Section 3.1 of
[RFC6532]. The MUA SHOULD normalize all email addresses in the
envelope and message headers.
For message bodies that contain UTF-8 characters (message/global),
the "Net-Unicode" standardized text transmission format specified in
[RFC5198] SHOULD be followed. It covers both normalization and
control characters that may affect display of text.
If the MUA saves email addresses (such as in an address book), they
SHOULD be stored in normalized form.
Other normalizations may be needed in specific language environments.
For example, in the Japanese environment, special considerations are
needed for the "@" and "." symbols. Most Japanese input methods
convert "@" to FULLWIDTH COMMERCIAL AT (U+FF20) and "." to either
IDEOGRAPHIC FULL STOP (U+3002) or FILLWIDTH FULL STOP (U+FF0E). In
email addresses, "@" is needed to separate the local name from the
domain name and "." to separate domain name labels. Normalization is
necessary to replace FULLWIDTH COMMERCIAL AT (U+FF20) with ASCII "@",
IDEOGRAPHIC FULL STOP (U+3002) with ASCII "." and FILLWIDTH FULL STOP
(U+FF0E) with ASCII ".".
9. Security Considerations
This document does not introduce any security considerations beyond
those already covered by the normative references for Email Address
Internationalization (EAI).
10. IANA Considerations
IANA changes are covered by the normative references for Email
Address Internationalization (EAI).
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11. Acknowledgments
12. Normative References
[ANSI.X3-4.1968] American National Standards
Institute, "USA Code for
Information Interchange",
ANSI X3.4, 1968.
[I-D.ietf-eai-5738bis] Resnick, P., Newman, C., and S.
Shen, "IMAP Support for UTF-8",
draft-ietf-eai-5738bis-09 (work in
progress), August 2012.
[I-D.ietf-eai-popimap-downgrade] Fujiwara, K., "Post-delivery
Message Downgrading for
Internationalized Email Messages",
draft-ietf-eai-popimap-downgrade-07
(work in progress), August 2012.
[I-D.ietf-eai-rfc5721bis] Gellens, R., Newman, C., Yao, J.,
and K. Fujiwara, "POP3 Support for
UTF-8",
draft-ietf-eai-rfc5721bis-07 (work
in progress), July 2012.
[RFC2045] Freed, N. and N. Borenstein,
"Multipurpose Internet Mail
Extensions (MIME) Part One: Format
of Internet Message Bodies",
RFC 2045, November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose
Internet Mail Extensions) Part
Three: Message Header Extensions
for Non-ASCII Text", RFC 2047,
November 1996.
[RFC2119] Bradner, S., "Key words for use in
RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119,
March 1997.
[RFC3629] Yergeau, F., "UTF-8, a
transformation format of ISO
10646", STD 63, RFC 3629,
November 2003.
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[RFC5068] Hutzler, C., Crocker, D., Resnick,
P., Allman, E., and T. Finch,
"Email Submission Operations:
Access and Accountability
Requirements", BCP 134, RFC 5068,
November 2007.
[RFC5198] Klensin, J. and M. Padlipsky,
"Unicode Format for Network
Interchange", RFC 5198, March 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer
Protocol", RFC 5321, October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message
Format", RFC 5322, October 2008.
[RFC5598] Crocker, D., "Internet Mail
Architecture", RFC 5598, July 2009.
[RFC5890] Klensin, J., "Internationalized
Domain Names for Applications
(IDNA): Definitions and Document
Framework", RFC 5890, August 2010.
[RFC6409] Gellens, R. and J. Klensin,
"Message Submission for Mail",
STD 72, RFC 6409, November 2011.
[RFC6530] Klensin, J. and Y. Ko, "Overview
and Framework for Internationalized
Email", RFC 6530, February 2012.
[RFC6531] Yao, J. and W. Mao, "SMTP Extension
for Internationalized Email",
RFC 6531, February 2012.
[RFC6532] Yang, A., Steele, S., and N. Freed,
"Internationalized Email Headers",
RFC 6532, February 2012.
[RFC6533] Hansen, T., Newman, C., and A.
Melnikov, "Internationalized
Delivery Status and Disposition
Notifications", RFC 6533,
February 2012.
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Authors' Addresses
Ernie Dainow
Afilias Canada
4141 Yonge Street
Toronto, Ontario M2P 2A8
Canada
Phone:
EMail: edainow@afilias.info
Kazunori Fujiwara
Japan Registry Services Co., Ltd.
Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
Chiyoda-ku, Tokyo 101-0065
Japan
Phone: +81 3 5215 8451
EMail: fujiwara@jprs.co.jp
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