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RFC 4952

Overview and Framework for Internationalized Email

Pages: 20
Obsoleted by:  6530
Updated by:  5336

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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 user@domain.example could be converted into a vanity host user.domain.example with its Web space at <http://user.domain.example> and the catchall addresses any.thing.goes@user.domain.example. 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: john-ietf@jck.com YangWoo Ko ICU 119 Munjiro Yuseong-gu, Daejeon 305-732 Republic of Korea EMail: yw@mrko.pe.kr
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