| Email Address Internationalization (EAI) | E.D. Dainow |
| Internet-Draft | Afilias |
| Intended status: Informational | K.F. Fujiwara |
| Expires: March 09, 2013 | JPRS |
| September 7, 2012 |
Guidelines for Internationalized Email Clients
draft-ietf-eai-email-clients-01
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.
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This Internet-Draft will expire on March 09, 2013.
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 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.
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].
[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.
A number of different acronyms are typically used to describe the major functional components of email.
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].
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].
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, 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.
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
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
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 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.
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.
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.
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.
For Case 3, several mechanisms may be required to provide compatibility support. These are outlined in the following sub-sections.
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.
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 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.
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].
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 if so, which addresses caused the problem.
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.
Example 1:
This message can be sent in Internationalized mode.
In Legacy mode the email client would flag Contact2, who does not have an ASCII address.
Example 2:
This message can be sent in either Internationalized or Legacy mode.
Example 3:
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:
This message can be sent in either Internationalized mode or Legacy mode.
Example 5:
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.
In summary, the guidelines outlines in Section 4 and Section 5 will provide the following compatibility solutions:
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.
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.
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 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.
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 ".".
This document does not introduce any security considerations beyond those already covered by the normative references for Email Address Internationalization (EAI).
IANA changes are covered by the normative references for Email Address Internationalization (EAI).