Network Working Group J. Klensin

Network Working Group J. Klensin
Request for Comments: 4952
Category: Informational Y. Ko
ICU
July 2007

Overview and Framework for Internationalized Email

Status of This Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The IETF Trust (2007).

Abstract

Full use of electronic mail throughout the world requires that people
be able to use their own names, written correctly in their own
languages and scripts, as mailbox names in email addresses. This
document introduces a series of specifications that define mechanisms
and protocol extensions needed to fully support internationalized
email addresses. These changes include an SMTP extension and
extension of email header syntax to accommodate UTF-8 data. The
document set also includes discussion of key assumptions and issues
in deploying fully internationalized email.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Role of This Specification . . . . . . . . . . . . . . . . 3
1.2. Problem Statement . . . . . . . . . . . . . . . . . . . . 3
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Overview of the Approach . . . . . . . . . . . . . . . . . . . 6
3. Document Plan . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Overview of Protocol Extensions and Changes . . . . . . . . . 7
4.1. SMTP Extension for Internationalized Email Address . . . . 7
4.2. Transmission of Email Header Fields in UTF-8 Encoding . . 8
4.3. Downgrading Mechanism for Backward Compatibility . . . . . 9
5. Downgrading before and after SMTP Transactions . . . . . . . . 10
5.1. Downgrading before or during Message Submission . . . . . 10
5.2. Downgrading or Other Processing After Final SMTP
Delivery . . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Additional Issues . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Impact on URIs and IRIs . . . . . . . . . . . . . . . . . 11
6.2. Interaction with Delivery Notifications . . . . . . . . . 12
6.3. Use of Email Addresses as Identifiers . . . . . . . . . . 12
6.4. Encoded Words, Signed Messages, and Downgrading . . . . . 12
6.5. Other Uses of Local Parts . . . . . . . . . . . . . . . . 13
6.6. Non-Standard Encapsulation Formats . . . . . . . . . . . . 13
7. Experimental Targets . . . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9. Security Considerations . . . . . . . . . . . . . . . . . . . 14
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . . 16

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1. Introduction

In order to use internationalized email addresses, we need to
internationalize both the domain part and the local part of email
addresses. The domain part of email addresses is already
internationalized [RFC3490], while the local part is not. Without
the extensions specified in this document, the mailbox name is
restricted to a subset of 7-bit ASCII [RFC2821]. Though MIME
[RFC2045] enables the transport of non-ASCII data, it does not
provide a mechanism for internationalized email addresses. In RFC
2047 [RFC2047], MIME defines an encoding mechanism for some specific
message header fields to accommodate non-ASCII data. However, it
does not permit the use of email addresses that include non-ASCII
characters. Without the extensions defined here, or some equivalent
set, the only way to incorporate non-ASCII characters in any part of
email addresses is to use RFC 2047 coding to embed them in what RFC
2822 [RFC2822] calls the "display name" (known as a "name phrase" or
by other terms elsewhere) of the relevant headers. Information coded
into the display name is invisible in the message envelope and, for
many purposes, is not part of the address at all.

1.1. Role of This Specification

This document presents the overview and framework for an approach to
the next stage of email internationalization. This new stage
requires not only internationalization of addresses and headers, but
also associated transport and delivery models.

This document provides the framework for a series of experimental
specifications that, together, provide the details for a way to
implement and support internationalized email. The document itself
describes how the various elements of email internationalization fit
together and how the relationships among the various documents are
involved.

1.2. Problem Statement

Internationalizing Domain Names in Applications (IDNA) [RFC3490]
permits internationalized domain names, but deployment has not yet
reached most users. One of the reasons for this is that we do not
yet have fully internationalized naming schemes. Domain names are
just one of the various names and identifiers that are required to be
internationalized. In many contexts, until more of those identifiers
are internationalized, internationalized domain names alone have
little value.

Email addresses are prime examples of why it is not good enough to
just internationalize the domain name. As most of us have learned

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from experience, users strongly prefer email addresses that resemble
names or initials to those involving seemingly meaningless strings of
letters or numbers. Unless the entire email address can use familiar
characters and formats, users will perceive email as being culturally
unfriendly. If the names and initials used in email addresses can be
expressed in the native languages and writing systems of the users,
the Internet will be perceived as more natural, especially by those
whose native language is not written in a subset of a Roman-derived
script.

Internationalization of email addresses is not merely a matter of
changing the SMTP envelope; or of modifying the From, To, and Cc
headers; or of permitting upgraded Mail User Agents (MUAs) to decode
a special coding and respond by displaying local characters. To be
perceived as usable, the addresses must be internationalized and
handled consistently in all of the contexts in which they occur.
This requirement has far-reaching implications: collections of
patches and workarounds are not adequate. Even if they were
adequate, a workaround-based approach may result in an assortment of
implementations with different sets of patches and workarounds having
been applied with consequent user confusion about what is actually
usable and supported. Instead, we need to build a fully
internationalized email environment, focusing on permitting efficient
communication among those who share a language or other community.
That, in turn, implies changes to the mail header environment to
permit the full range of Unicode characters where that makes sense,
an SMTP Extension to permit UTF-8 [RFC3629] mail addressing and
delivery of those extended headers, and (finally) a requirement for
support of the 8BITMIME SMTP extension [RFC1652] so that all of these
can be transported through the mail system without having to overcome
the limitation that headers do not have content-transfer-encodings.

1.3. Terminology

This document assumes a reasonable understanding of the protocols and
terminology of the core email standards as documented in [RFC2821]
and [RFC2822].

Much of the description in this document depends on the abstractions
of "Mail Transfer Agent" ("MTA") and "Mail User Agent" ("MUA").
However, it is important to understand that those terms and the
underlying concepts postdate the design of the Internet's email
architecture and the application of the "protocols on the wire"
principle to it. That email architecture, as it has evolved, and the
"wire" principle have prevented any strong and standardized
distinctions about how MTAs and MUAs interact on a given origin or
destination host (or even whether they are separate).

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However, the term "final delivery MTA" is used in this document in a
fashion equivalent to the term "delivery system" or "final delivery
system" of RFC 2821. This is the SMTP server that controls the
format of the local parts of addresses and is permitted to inspect
and interpret them. It receives messages from the network for
delivery to mailboxes or for other local processing, including any
forwarding or aliasing that changes envelope addresses, rather than
relaying. From the perspective of the network, any local delivery
arrangements such as saving to a message store, handoff to specific
message delivery programs or agents, and mechanisms for retrieving
messages are all "behind" the final delivery MTA and hence are not
part of the SMTP transport or delivery process.

In this document, an address is "all-ASCII", or just an "ASCII
address", if every character in the address is in the ASCII character
repertoire [ASCII]; an address is "non-ASCII", or an "i18n-address",
if any character is not in the ASCII character repertoire. Such
addresses may be restricted in other ways, but those restrictions are
not relevant to this definition. The term "all-ASCII" is also
applied to other protocol elements when the distinction is important,
with "non-ASCII" or "internationalized" as its opposite.

The umbrella term to describe the email address internationalization
specified by this document and its companion documents is "UTF8SMTP".
For example, an address permitted by this specification is referred
to as a "UTF8SMTP (compliant) address".

Please note that, according to the definitions given here, the set of
all "all-ASCII" addresses and the set of all "non-ASCII" addresses
are mutually exclusive. The set of all UTF8SMTP addresses is the
union of these two sets.

An "ASCII user" (i) exclusively uses email addresses that contain
ASCII characters only, and (ii) cannot generate recipient addresses
that contain non-ASCII characters.

An "i18mail user" has one or more non-ASCII email addresses. Such a
user may have ASCII addresses too; if the user has more than one
email account and a corresponding address, or more than one alias for
the same address, he or she has some method to choose which address
to use on outgoing email. Note that under this definition, it is not
possible to tell from an ASCII address if the owner of that address
is an i18mail user or not. (A non-ASCII address implies a belief
that the owner of that address is an i18mail user.) There is no such
thing as an "i18mail message"; the term applies only to users and
their agents and capabilities.

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A "message" is sent from one user (sender) using a particular email
address to one or more other recipient email addresses (often
referred to just as "users" or "recipient users").

A "mailing list" is a mechanism whereby a message may be distributed
to multiple recipients by sending it to one recipient address. An
agent (typically not a human being) at that single address then
causes the message to be redistributed to the target recipients.
This agent sets the envelope return address of the redistributed
message to a different address from that of the original single
recipient message. Using a different envelope return address
(reverse-path) causes error (and other automatically generated)
messages to go to an error handling address.

As specified in RFC 2821, a message that is undeliverable for some
reason is expected to result in notification to the sender. This can
occur in either of two ways. One, typically called "Rejection",
occurs when an SMTP server returns a reply code indicating a fatal
error (a "5yz" code) or persistently returns a temporary failure
error (a "4yz" code). The other involves accepting the message
during SMTP processing and then generating a message to the sender,
typically known as a "Non-delivery Notification" or "NDN". Current
practice often favors rejection over NDNs because of the reduced
likelihood that the generation of NDNs will be used as a spamming
technique. The latter, NDN, case is unavoidable if an intermediate
MTA accepts a message that is then rejected by the next-hop server.

The pronouns "he" and "she" are used interchangeably to indicate a
human of indeterminate gender.

The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED",
and "MAY" in this document are to be interpreted as described in RFC
2119 [RFC2119].

2. Overview of the Approach

This set of specifications changes both SMTP and the format of email
headers to permit non-ASCII characters to be represented directly.
Each important component of the work is described in a separate
document. The document set, whose members are described in the next
section, also contains informational documents whose purpose is to
provide implementation suggestions and guidance for the protocols.

3. Document Plan

In addition to this document, the following documents make up this
specification and provide advice and context for it.

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o SMTP extensions. This document [EAI-SMTPext] provides an SMTP
extension for internationalized addresses, as provided for in RFC
2821.

o Email headers in UTF-8. This document [EAI-UTF8] essentially
updates RFC 2822 to permit some information in email headers to be
expressed directly by Unicode characters encoded in UTF-8 when the
SMTP extension described above is used. This document, possibly
with one or more supplemental ones, will also need to address the
interactions with MIME, including relationships between UTF8SMTP
and internal MIME headers and content types.

o In-transit downgrading from internationalized addressing with the
SMTP extension and UTF-8 headers to traditional email formats and
characters [EAI-downgrade]. Downgrading either at the point of
message origination or after the mail has successfully been
received by a final delivery SMTP server involve different
constraints and possibilities; see Section 4.3 and Section 5,
below. Processing that occurs after such final delivery,
particularly processing that is involved with the delivery to a
mailbox or message store, is sometimes called "Message Delivery"
processing.

o Extensions to the IMAP protocol to support internationalized
headers [EAI-imap].

o Parallel extensions to the POP protocol [EAI-pop].

o Description of internationalization changes for delivery
notifications (DSNs) [EAI-DSN].

o Scenarios for the use of these protocols [EAI-scenarios].

4. Overview of Protocol Extensions and Changes

4.1. SMTP Extension for Internationalized Email Address

An SMTP extension, "UTF8SMTP" is specified as follows:

o Permits the use of UTF-8 strings in email addresses, both local
parts and domain names.

o Permits the selective use of UTF-8 strings in email headers (see
Section 4.2).

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o Requires that the server advertise the 8BITMIME extension
[RFC1652] and that the client support 8-bit transmission so that
header information can be transmitted without using special
content-transfer-encoding.

o Provides information to support downgrading mechanisms.

Some general principles affect the development decisions underlying
this work.

1. Email addresses enter subsystems (such as a user interface) that
may perform charset conversions or other encoding changes. When
the left hand side of the address includes characters outside the
US-ASCII character repertoire, use of punycode on the right hand
side is discouraged to promote consistent processing of
characters throughout the address.

2. An SMTP relay must

* Either recognize the format explicitly, agreeing to do so via
an ESMTP option,

* Select and use an ASCII-only address, downgrading other
information as needed (see Section 4.3), or

* Reject the message or, if necessary, return a non-delivery
notification message, so that the sender can make another
plan.

If the message cannot be forwarded because the next-hop system
cannot accept the extension and insufficient information is
available to reliably downgrade it, it MUST be rejected or a non-
delivery message generated and sent.

3. In the interest of interoperability, charsets other than UTF-8
are prohibited in mail addresses and headers. There is no
practical way to identify them properly with an extension similar
to this without introducing great complexity.

Conformance to the group of standards specified here for email
transport and delivery requires implementation of the SMTP Extension
specification, including recognition of the keywords associated with
alternate addresses, and the UTF-8 Header specification. Support for
downgrading is not required, but, if implemented, MUST be implemented
as specified. Similarly, if the system implements IMAP or POP, it
MUST conform to the i18n IMAP or POP specifications respectively.

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4.2. Transmission of Email Header Fields in UTF-8 Encoding

There are many places in MUAs or in a user presentation in which
email addresses or domain names appear. Examples include the
conventional From, To, or Cc header fields; Message-ID and
In-Reply-To header fields that normally contain domain names (but
that may be a special case); and in message bodies. Each of these
must be examined from an internationalization perspective. The user
will expect to see mailbox and domain names in local characters, and
to see them consistently. If non-obvious encodings, such as
protocol-specific ASCII-Compatible Encoding (ACE) variants, are used,
the user will inevitably, if only occasionally, see them rather than
"native" characters and will find that discomfiting or astonishing.
Similarly, if different codings are used for mail transport and
message bodies, the user is particularly likely to be surprised, if
only as a consequence of the long-established "things leak"
principle. The only practical way to avoid these sources of
discomfort, in both the medium and the longer term, is to have the
encodings used in transport be as similar to the encodings used in
message headers and message bodies as possible.

When email local parts are internationalized, it seems clear that
they should be accompanied by arrangements for the email headers to
be in the fully internationalized form. That form should presumably
use UTF-8 rather than ASCII as the base character set for the
contents of header fields (protocol elements such as the header field
names themselves will remain entirely in ASCII). For transition
purposes and compatibility with legacy systems, this can done by
extending the encoding models of [RFC2045] and [RFC2231]. However,
our target should be fully internationalized headers, as discussed in
[EAI-UTF8].

4.3. Downgrading Mechanism for Backward Compatibility

As with any use of the SMTP extension mechanism, there is always the
possibility of a client that requires the feature encountering a
server that does not support the required feature. In the case of
email address and header internationalization, the risk should be
minimized by the fact that the selection of submission servers are
presumably under the control of the sender's client and the selection
of potential intermediate relays is under the control of the
administration of the final delivery server.

For situations in which a client that needs to use UTF8SMTP
encounters a server that does not support the extension UTF8SMTP,
there are two possibilities:

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o Reject the message or generate and send a non-delivery message,
requiring the sender to resubmit it with traditional-format
addresses and headers.

o Figure out a way to downgrade the envelope or message body in
transit. Especially when internationalized addresses are
involved, downgrading will require that all-ASCII addresses be
obtained from some source. An optional extension parameter is
provided as a way of transmitting an alternate address. Downgrade
issues and a specification are discussed in [EAI-downgrade].

(The client can also try an alternate next-hop host or requeue the
message and try later, on the assumption that the lack of UTF8SMTP is
a transient failure; since this ultimately resolves to success or
failure, it doesn't change the discussion here.)

The first of these two options, that of rejecting or returning the
message to the sender MAY always be chosen.

If a UTF8SMTP capable client is sending a message that does not
require the extended capabilities, it SHOULD send the message whether
or not the server announces support for the extension. In other
words, both the addresses in the envelope and the entire set of
headers of the message are entirely in ASCII (perhaps including
encoded words in the headers). In that case, the client SHOULD send
the message whether or not the server announces the capability
specified here.

5. Downgrading before and after SMTP Transactions

In addition to the in-transit downgrades discussed above, downgrading
may also occur before or during the initial message submission or
after the delivery to the final delivery MTA. Because these cases
have a different set of available information from in-transit cases,
the constraints and opportunities may be somewhat different too.
These two cases are discussed in the subsections below.

5.1. Downgrading before or during Message Submission

Perhaps obviously, the most convenient time to find an ASCII address
corresponding to an internationalized address is at the originating
MUA. This can occur either before the message is sent or after the
internationalized form of the message is rejected. It is also the
most convenient time to convert a message from the internationalized
form into conventional ASCII form or to generate a non-delivery
message to the sender if either is necessary. At that point, the
user has a full range of choices available, including contacting the
intended recipient out of band for an alternate address, consulting

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appropriate directories, arranging for translation of both addresses
and message content into a different language, and so on. While it
is natural to think of message downgrading as optimally being a
fully-automated process, we should not underestimate the capabilities
of a user of at least moderate intelligence who wishes to communicate
with another such user.

In this context, one can easily imagine modifications to message
submission servers (as described in [RFC4409]) so that they would
perform downgrading, or perhaps even upgrading, operations, receiving
messages with one or more of the internationalization extensions
discussed here and adapting the outgoing message, as needed, to
respond to the delivery or next-hop environment it encounters.

5.2. Downgrading or Other Processing After Final SMTP Delivery

When an email message is received by a final delivery SMTP server, it
is usually stored in some form. Then it is retrieved either by
software that reads the stored form directly or by client software
via some email retrieval mechanisms such as POP or IMAP.

The SMTP extension described in Section 4.1 provides protection only
in transport. It does not prevent MUAs and email retrieval
mechanisms that have not been upgraded to understand
internationalized addresses and UTF-8 headers from accessing stored
internationalized emails.

Since the final delivery SMTP server (or, to be more specific, its
corresponding mail storage agent) cannot safely assume that agents
accessing email storage will always be capable of handling the
extensions proposed here, it MAY either downgrade internationalized
emails or specially identify messages that utilize these extensions,
or both. If this is done, the final delivery SMTP server SHOULD
include a mechanism to preserve or recover the original
internationalized forms without information loss to support access by
UTF8SMTP-aware agents.

6. Additional Issues

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
deployment of email address and header internationalization.

6.1. Impact on URIs and IRIs

The mailto: schema defined in [RFC2368] and discussed in the
Internationalized Resource Identifier (IRI) specification [RFC3987]
may need to be modified when this work is completed and standardized.

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6.2. Interaction with Delivery Notifications

The advent of UTF8SMTP will make necessary consideration of the
interaction with delivery notification mechanisms, including the SMTP
extension for requesting delivery notifications [RFC3461], and the
format of delivery notifications [RFC3464]. These issues are
discussed in a forthcoming document that will update those RFCs as
needed [EAI-DSN].

6.3. Use of Email Addresses as Identifiers

There are a number of places in contemporary Internet usage in which
email addresses are used as identifiers for individuals, including as
identifiers to Web servers supporting some electronic commerce sites.
These documents do not address those uses, but it is reasonable to
expect that some difficulties will be encountered when
internationalized addresses are first used in those contexts, many of
which cannot even handle the full range of addresses permitted today.

6.4. Encoded Words, Signed Messages, and Downgrading

One particular characteristic of the email format is its persistency:
MUAs are expected to handle messages that were originally sent
decades ago and not just those delivered seconds ago. As such, MUAs
and mail filtering software, such as that specified in Sieve
[RFC3028], will need to continue to accept and decode header fields
that use the "encoded word" mechanism [RFC2047] to accommodate
non-ASCII characters in some header fields. While extensions to both
POP3 and IMAP have been proposed to enable automatic EAI-upgrade --
including RFC 2047 decoding -- of messages by the POP3 or IMAP
server, there are message structures and MIME content-types for which
that cannot be done or where the change would have unacceptable side
effects.

For example, message parts that are cryptographically signed, using
e.g., S/MIME [RFC3851] or Pretty Good Privacy (PGP) [RFC3156], cannot
be upgraded from the RFC 2047 form to normal UTF-8 characters without
breaking the signature. Similarly, message parts that are encrypted
may contain, when decrypted, header fields that use the RFC 2047
encoding; such messages cannot be 'fully' upgraded without access to
cryptographic keys.

Similar issues may arise if signed messages are downgraded in transit
[EAI-downgrade] and then an attempt is made to upgrade them to the
original form and then verify the signatures. Even the very subtle
changes that may result from algorithms to downgrade and then upgrade
again may be sufficient to invalidate the signatures if they impact

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either the primary or MIME bodypart headers. When signatures are
present, downgrading must be performed with extreme care if at all.

6.5. Other Uses of Local Parts

Local parts are sometimes used to construct domain labels, e.g., the
local part "user" in the address [email protected] could be
converted into a vanity host user.domain.example with its Web space
at <http://user.domain.example> and the catchall addresses
[email protected].

Such schemes are obviously limited by, among other things, the SMTP
rules for domain names, and will not work without further
restrictions for other local parts such as the <utf8-local-part>
specified in [EAI-UTF8]. Whether this issue is relevant to these
specifications is an open question. It may be simply another case of
the considerable flexibility accorded to delivery MTAs in determining
the mailbox names they will accept and how they are interpreted.

6.6. Non-Standard Encapsulation Formats

Some applications use formats similar to the application/mbox format
defined in [RFC4155] instead of the message/digest RFC 2046, Section
5.1.5 [RFC2046] form to transfer multiple messages as single units.
Insofar as such applications assume that all stored messages use the
message/rfc822 RFC 2046, Section 5.2.1 [RFC2046] format with US-ASCII
headers, they are not ready for the extensions specified in this
series of documents and special measures may be needed to properly
detect and process them.

7. Experimental Targets

In addition to the simple question of whether the model outlined here
can be made to work in a satisfactory way for upgraded systems and
provide adequate protection for un-upgraded ones, we expect that
actually working with the systems will provide answers to two
additional questions: what restrictions such as character lists or
normalization should be placed, if any, on the characters that are
permitted to be used in address local-parts and how useful, in
practice, will downgrading turn out to be given whatever restrictions
and constraints that must be placed upon it.

8. IANA Considerations

This overview description and framework document does not contemplate
any IANA registrations or other actions. Some of the documents in
the group have their own IANA considerations sections and
requirements.

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9. Security Considerations

Any expansion of permitted characters and encoding forms in email
addresses raises some risks. There have been discussions on so
called "IDN-spoofing" or "IDN homograph attacks". These attacks
allow an attacker (or "phisher") to spoof the domain or URLs of
businesses. The same kind of attack is also possible on the local
part of internationalized email addresses. It should be noted that
the proposed fix involving forcing all displayed elements into
normalized lower-case works for domain names in URLs, but not email
local parts since those are case sensitive.

Since email addresses are often transcribed from business cards and
notes on paper, they are subject to problems arising from confusable
characters (see [RFC4690]). These problems are somewhat reduced if
the domain associated with the mailbox is unambiguous and supports a
relatively small number of mailboxes whose names follow local system
conventions. They are increased with very large mail systems in
which users can freely select their own addresses.

The internationalization of email addresses and headers must not
leave the Internet less secure than it is without the required
extensions. The requirements and mechanisms documented in this set
of specifications do not, in general, raise any new security issues.
They do require a review of issues associated with confusable
characters -- a topic that is being explored thoroughly elsewhere
(see, e.g., [RFC4690]) -- and, potentially, some issues with UTF-8
normalization, discussed in [RFC3629], and other transformations.
Normalization and other issues associated with transformations and
standard forms are also part of the subject of ongoing work discussed
in [Net-Unicode], in [IDNAbis-BIDI] and elsewhere. Some issues
specifically related to internationalized addresses and headers are
discussed in more detail in the other documents in this set.
However, in particular, caution should be taken that any
"downgrading" mechanism, or use of downgraded addresses, does not
inappropriately assume authenticated bindings between the
internationalized and ASCII addresses.

The new UTF-8 header and message formats might also raise, or
aggravate, another known issue. If the model creates new forms of an
'invalid' or 'malformed' message, then a new email attack is created:
in an effort to be robust, some or most agents will accept such
message and interpret them as if they were well-formed. If a filter
interprets such a message differently than the final MUA, then it may
be possible to create a message that appears acceptable under the
filter's interpretation but should be rejected under the
interpretation given to it by the final MUA. Such attacks already
exist for existing messages and encoding layers, e.g., invalid MIME

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syntax, invalid HTML markup, and invalid coding of particular image
types.

Models for the "downgrading" of messages or addresses from UTF-8 form
to some ASCII form, including those described in [EAI-downgrade],
pose another special problem and risk. Any system that transforms
one address or set of mail header fields into another becomes a point
at which spoofing attacks can occur and those who wish to spoof
messages might be able to do so by imitating a message downgraded
from one with a legitimate original address.

In addition, email addresses are used in many contexts other than
sending mail, such as for identifiers under various circumstances
(see Section 6.3). Each of those contexts will need to be evaluated,
in turn, to determine whether the use of non-ASCII forms is
appropriate and what particular issues they raise.

This work will clearly impact any systems or mechanisms that are
dependent on digital signatures or similar integrity protection for
mail headers (see also the discussion in Section 6.4). Many
conventional uses of PGP and S/MIME are not affected since they are
used to sign body parts but not headers. On the other hand, the
developing work on domain keys identified mail (DKIM [DKIM-Charter])
will eventually need to consider this work and vice versa: while this
experiment does not propose to address or solve the issues raised by
DKIM and other signed header mechanisms, the issues will have to be
coordinated and resolved eventually if the two sets of protocols are
to co-exist. In addition, to the degree to which email addresses
appear in PKI (Public Key Infrastructure) certificates, standards
addressing such certificates will need to be upgraded to address
these internationalized addresses. Those upgrades will need to
address questions of spoofing by look-alikes of the addresses
themselves.

10. Acknowledgements

This document, and the related ones, were originally derived from
documents by John Klensin and the JET group [Klensin-emailaddr],
[JET-IMA]. The work drew inspiration from discussions on the "IMAA"
mailing list, sponsored by the Internet Mail Consortium and
especially from an early document by Paul Hoffman and Adam Costello
[Hoffman-IMAA] that attempted to define an MUA-only solution to the
address internationalization problem.

More recent documents have benefited from considerable discussion
within the IETF EAI Working Group and especially from suggestions and
text provided by Martin Duerst, Frank Ellermann, Philip Guenther,
Kari Hurtta, and Alexey Melnikov, and from extended discussions among

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the editors and authors of the core documents cited in Section 3:
Harald Alvestrand, Kazunori Fujiwara, Chris Newman, Pete Resnick,
Jiankang Yao, Jeff Yeh, and Yoshiro Yoneya.

Additional comments received during IETF Last Call, including those
from Paul Hoffman and Robert Sparks, were helpful in making the
document more clear and comprehensive.

11. References

11.1. 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.

[RFC1652] Klensin, J., Freed, N., Rose, M., Stefferud, E.,
and D. Crocker, "SMTP Service Extension for
8bit-MIMEtransport", RFC 1652, July 1994.

[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels'", RFC 2119, BCP 14,
March 1997.

[RFC2821] Klensin, J., "Simple Mail Transfer Protocol",
RFC 2821, April 2001.

[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications
(IDNA)", RFC 3490, March 2003.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of
ISO 10646", STD 63, RFC 3629, November 2003.

11.2. Informative References

[DKIM-Charter] IETF, "Domain Keys Identified Mail (dkim)",
October 2006, <http://www.ietf.org/
html.charters/dkim-charter.html>.

[EAI-DSN] Newman, C., "UTF-8 Delivery and Disposition
Notification", Work in Progress, January 2007.

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[EAI-SMTPext] Yao, J., Ed. and W. Mao, Ed., "SMTP extension
for internationalized email address", Work
in Progress, June 2007.

[EAI-UTF8] Yeh, J., "Internationalized Email Headers", Work
in Progress, April 2007.

[EAI-downgrade] Yoneya, Y., Ed. and K. Fujiwara, Ed.,
"Downgrading mechanism for Internationalized
eMail Address (IMA)", Work in Progress,
March 2007.

[EAI-imap] Resnick, P. and C. Newman, "IMAP Support for
UTF-8", Work in Progress, March 2007.

[EAI-pop] Newman, C., "POP3 Support for UTF-8", Work
in Progress, January 2007.

[EAI-scenarios] Alvestrand, H., "UTF-8 Mail: Scenarios", Work
in Progress, February 2007.

[Hoffman-IMAA] Hoffman, P. and A. Costello, "Internationalizing
Mail Addresses in Applications (IMAA)", Work
in Progress, October 2003.

[IDNAbis-BIDI] Alvestrand, H. and C. Karp, "An IDNA problem in
right-to-left scripts", Work in Progress,
October 2006.

[JET-IMA] Yao, J. and J. Yeh, "Internationalized eMail
Address (IMA)", Work in Progress, June 2005.

[Klensin-emailaddr] Klensin, J., "Internationalization of Email
Addresses", Work in Progress, July 2005.

[Net-Unicode] Klensin, J. and M. Padlipsky, "Unicode Format
for Network Interchange", Work in Progress,
March 2007.

[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.

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[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail
Extensions) Part Three: Message Header
Extensions for Non-ASCII Text", RFC 2047,
November 1996.

[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value
and Encoded Word Extensions:
Character Sets, Languages, and Continuations",
RFC 2231, November 1997.

[RFC2368] Hoffman, P., Masinter, L., and J. Zawinski, "The
mailto URL scheme", RFC 2368, July 1998.

[RFC2822] Resnick, P., "Internet Message Format",
RFC 2822, April 2001.

[RFC3028] Showalter, T., "Sieve: A Mail Filtering
Language", RFC 3028, January 2001.

[RFC3156] Elkins, M., Del Torto, D., Levien, R., and T.
Roessler, "MIME Security with OpenPGP",
RFC 3156, August 2001.

[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP)
Service Extension for Delivery Status
Notifications (DSNs)", RFC 3461, January 2003.

[RFC3464] Moore, K. and G. Vaudreuil, "An Extensible
Message Format for Delivery Status
Notifications", RFC 3464, January 2003.

[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message
Specification", RFC 3851, July 2004.

[RFC3987] Duerst, M. and M. Suignard, "Internationalized
Resource Identifiers (IRIs)", RFC 3987,
January 2005.

[RFC4155] Hall, E., "The application/mbox Media Type",
RFC 4155, September 2005.

[RFC4409] Gellens, R. and J. Klensin, "Message Submission
for Mail", RFC 4409, April 2006.

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[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB,
"Review and Recommendations for
Internationalized Domain Names (IDNs)",
RFC 4690, September 2006.

Authors' Addresses

John C Klensin
1770 Massachusetts Ave, #322
Cambridge, MA 02140
USA

Phone: +1 617 491 5735
EMail: [email protected]

YangWoo Ko
ICU
119 Munjiro
Yuseong-gu, Daejeon 305-732
Republic of Korea

EMail: [email protected]

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Full Copyright Statement

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