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).
AbstractFull 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.
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
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. 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
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. 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).
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.
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].
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].
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.
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.
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. RFC2368] and discussed in the Internationalized Resource Identifier (IRI) specification [RFC3987] may need to be modified when this work is completed and standardized.
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]. 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
either the primary or MIME bodypart headers. When signatures are present, downgrading must be performed with extreme care if at all. 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. 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.
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
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. 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
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. [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. [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.
[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.
[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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