draft-ietf-eai-frmwrk-4952bis-01.txt   draft-ietf-eai-frmwrk-4952bis-02.txt 
Email Address Internationalization J. Klensin Email Address Internationalization J. Klensin
(EAI) (EAI)
Internet-Draft Y. Ko Internet-Draft Y. Ko
Obsoletes: RFC4952 ICU Obsoletes: RFCs 4952, 5504, 5825 ICU
(if approved) July 3, 2010 (if approved) July 12, 2010
Intended status: Informational Intended status: Informational
Expires: January 4, 2011 Expires: January 13, 2011
Overview and Framework for Internationalized Email Overview and Framework for Internationalized Email
draft-ietf-eai-frmwrk-4952bis-01 draft-ietf-eai-frmwrk-4952bis-02
Abstract Abstract
Full use of electronic mail throughout the world requires that, Full use of electronic mail throughout the world requires that,
subject to other constraints, people be able to use close variations subject to other constraints, people be able to use close variations
on their own names, written correctly in their own languages and on their own names, written correctly in their own languages and
scripts, as mailbox names in email addresses. This document scripts, as mailbox names in email addresses. This document
introduces a series of specifications that define mechanisms and introduces a series of specifications that define mechanisms and
protocol extensions needed to fully support internationalized email protocol extensions needed to fully support internationalized email
addresses. These changes include an SMTP extension and extension of addresses. These changes include an SMTP extension and extension of
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 4, 2011. This Internet-Draft will expire on January 13, 2011.
Copyright Notice Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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Without obtaining an adequate license from the person(s) controlling Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other it for publication as an RFC or to translate it into languages other
than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Role of This Specification . . . . . . . . . . . . . . . . . . 4 2. Role of This Specification . . . . . . . . . . . . . . . . . . 5
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5 3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Mail User and Mail Transfer Agents . . . . . . . . . . . . 6 4.1. Mail User and Mail Transfer Agents . . . . . . . . . . . . 6
4.2. Address Character Sets . . . . . . . . . . . . . . . . . . 7 4.2. Address Character Sets . . . . . . . . . . . . . . . . . . 7
4.3. User Types . . . . . . . . . . . . . . . . . . . . . . . . 7 4.3. User Types . . . . . . . . . . . . . . . . . . . . . . . . 7
4.4. Messages . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.4. Messages . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.5. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 8 4.5. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 8
4.6. Undeliverable Messages and Notification . . . . . . . . . 8 4.6. Conventional Message and Internationalized Message . . . . 8
5. Overview of the Approach . . . . . . . . . . . . . . . . . . . 8 4.7. Undeliverable Messages and Notification . . . . . . . . . 8
6. Document Plan . . . . . . . . . . . . . . . . . . . . . . . . 9 5. Overview of the Approach and Document Plan . . . . . . . . . . 9
7. Overview of Protocol Extensions and Changes . . . . . . . . . 9 6. Overview of Protocol Extensions and Changes . . . . . . . . . 9
7.1. SMTP Extension for Internationalized Email Address . . . . 9 6.1. SMTP Extension for Internationalized Email Address . . . . 9
7.2. Transmission of Email Header Fields in UTF-8 Encoding . . 10 6.2. Transmission of Email Header Fields in UTF-8 Encoding . . 10
8. Downgrading before and after SMTP Transactions . . . . . . . . 11 6.3. SMTP Service Extension for DSNs . . . . . . . . . . . . . 11
8.1. Downgrading before or during Message Submission . . . . . 12 7. Downgrading before and after SMTP Transactions . . . . . . . . 11
8.2. Downgrading or Other Processing After Final SMTP 7.1. Downgrading before or during Message Submission . . . . . 12
7.2. Downgrading or Other Processing After Final SMTP
Delivery . . . . . . . . . . . . . . . . . . . . . . . . . 13 Delivery . . . . . . . . . . . . . . . . . . . . . . . . . 13
9. Downgrading in Transit . . . . . . . . . . . . . . . . . . . . 13 8. Downgrading in Transit . . . . . . . . . . . . . . . . . . . . 13
10. User Interface and Configuration Issues . . . . . . . . . . . 13 9. User Interface and Configuration Issues . . . . . . . . . . . 14
10.1. Choices of Mailbox Names and Unicode Normalization . . . . 14 9.1. Choices of Mailbox Names and Unicode Normalization . . . . 14
11. Additional Issues . . . . . . . . . . . . . . . . . . . . . . 15 10. Additional Issues . . . . . . . . . . . . . . . . . . . . . . 15
11.1. Impact on URIs and IRIs . . . . . . . . . . . . . . . . . 15 10.1. Impact on URIs and IRIs . . . . . . . . . . . . . . . . . 16
11.2. Interaction with Delivery Notifications . . . . . . . . . 15 10.2. Use of Email Addresses as Identifiers . . . . . . . . . . 16
11.3. Use of Email Addresses as Identifiers . . . . . . . . . . 16 10.3. Encoded Words, Signed Messages, and Downgrading . . . . . 16
11.4. Encoded Words, Signed Messages, and Downgrading . . . . . 16 10.4. LMTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
11.5. LMTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 10.5. Other Uses of Local Parts . . . . . . . . . . . . . . . . 17
11.6. Other Uses of Local Parts . . . . . . . . . . . . . . . . 16 10.6. Non-Standard Encapsulation Formats . . . . . . . . . . . . 17
11.7. Non-Standard Encapsulation Formats . . . . . . . . . . . . 17 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
12. Experimental Targets . . . . . . . . . . . . . . . . . . . . . 17 12. Security Considerations . . . . . . . . . . . . . . . . . . . 17
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
14. Security Considerations . . . . . . . . . . . . . . . . . . . 17 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19 14.1. Normative References . . . . . . . . . . . . . . . . . . . 19
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19 14.2. Informative References . . . . . . . . . . . . . . . . . . 21
16.1. Normative References . . . . . . . . . . . . . . . . . . . 19 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 24
16.2. Informative References . . . . . . . . . . . . . . . . . . 20 A.1. Changes between -00 and -01 . . . . . . . . . . . . . . . 24
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 22 A.2. Changes between -01 and -02 . . . . . . . . . . . . . . . 24
A.1. Changes between -00 and -01 . . . . . . . . . . . . . . . 23
1. Introduction 1. Introduction
[[anchor1: Note to EAI WG: these two initial drafts are intended to [[anchor1: Note to EAI WG: All comments received on the mailing list
initiate discussion on what should, and should not, be in the about this document have been incorporated into the -02 draft. As
Framework document and how we want those topics covered. As such, it indicated in earlier notes, most placeholders have been removed from
is more of an intermediate draft between RFC 4952 and the first draft it to make the document a WG Last Call candidate. A note was posted
of 4952bis that could be a Last Call candidate. If we are going to on 9 July listing outstanding topics needing discussion in -01
keep the rather aggressive schedule we agreed to in the charter, we (http://www.ietf.org/mail-archive/web/ima/current/msg03259.html) to
need to have enough discussion on critical-path points that a which there was no response before -02 was submitted. It may be
revision suitable (at least) for final review prior to Last Call can useful to refer to that note and the change log below as part of
be posted before the 12 July I-D cutoff. For that to happen, we review of this draft.]]
should have enough discussion to start determining consensus within
the next ten days. So, focused comments and soon, please.]]
In order to use internationalized email addresses, we need to In order to use internationalized email addresses, we need to
internationalize both the domain part and the local part of email internationalize both the domain part and the local part of email
addresses. The domain part of email addresses is already addresses. The domain part of email addresses is already
internationalized [RFC5890], while the local part is not. Without internationalized [RFC5890], while the local part is not.
the extensions specified in this document, the mailbox name is [[anchor2: Note in Draft: RFC 5890, formerly draft-ietf-idnabis-defs,
restricted to a subset of 7-bit ASCII [RFC5321]. Though MIME and the closely-related RFC 5891 - 5894, have been in AUTH48 since
June 6. None of the the author, WG leadership, or ADs are holding it
up. Using the I-D references here is just extra work.]]
Without the extensions specified in this document, the mailbox name
is restricted to a subset of 7-bit ASCII [RFC5321]. Though MIME
[RFC2045] enables the transport of non-ASCII data, it does not [RFC2045] enables the transport of non-ASCII data, it does not
provide a mechanism for internationalized email addresses. In RFC provide a mechanism for internationalized email addresses. In RFC
2047 [RFC2047], MIME defines an encoding mechanism for some specific 2047 [RFC2047], MIME defines an encoding mechanism for some specific
message header fields to accommodate non-ASCII data. However, it message header fields to accommodate non-ASCII data. However, it
does not permit the use of email addresses that include non-ASCII does not permit the use of email addresses that include non-ASCII
characters. Without the extensions defined here, or some equivalent characters. Without the extensions defined here, or some equivalent
set, the only way to incorporate non-ASCII characters in any part of 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 email addresses is to use RFC 2047 coding to embed them in what RFC
5322 [RFC5322] calls the "display name" (known as a "name phrase" or 5322 [RFC5322] calls the "display name" (known as a "name phrase" or
by other terms elsewhere) of the relevant header fields. Information by other terms elsewhere) of the relevant header fields. Information
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2119 [RFC2119]. 2119 [RFC2119].
2. Role of This Specification 2. Role of This Specification
This document presents the overview and framework for an approach to This document presents the overview and framework for an approach to
the next stage of email internationalization. This new stage the next stage of email internationalization. This new stage
requires not only internationalization of addresses and header requires not only internationalization of addresses and header
fields, but also associated transport and delivery models. A prior fields, but also associated transport and delivery models. A prior
version of this specification, RFC 4952 [RFC4952], also provided an version of this specification, RFC 4952 [RFC4952], also provided an
introduction to a series of experimental protocols [RFC5335] introduction to a series of experimental protocols [RFC5335]
[RFC5336] [RFC5337] [RFC5504] [RFC5721] [RFC5738] [RFC5825]. [RFC5336] [RFC5337] [RFC5504] [RFC5721] [RFC5738] [RFC5825]. This
[[anchor2: Note in Draft: Is 5825 still relevant, or is a victim of revised form provides overview and conceptual information for the
the "no in-transit downgrade" decision.??]] standards-track successors of a subset of those protocols. Details
This revised document provides overview and conceptual information of the documents and the relationships among them appear in
for the standards-track successors of those protocols. Details of Section 5.
the documents and the relationships among them appear in Section 6.
Taken together, these specifications provide the details for a way to Taken together, these specifications provide the details for a way to
implement and support internationalized email. The document itself implement and support internationalized email. The document itself
describes how the various elements of email internationalization fit describes how the various elements of email internationalization fit
together and the relationships among the [[anchor3: ??? provides a together and the relationships among the primary specifications
roadmap for navigating the]] various documents are involved. associated with message transport, header formats, and handling.
3. Problem Statement 3. Problem Statement
Internationalizing Domain Names in Applications (IDNA) [RFC5890] Internationalizing Domain Names in Applications (IDNA) [RFC5890]
permits internationalized domain names, but deployment has not yet permits internationalized domain names, but deployment has not yet
reached most users. One of the reasons for this is that we do not reached most users. One of the reasons for this is that we do not
yet have fully internationalized naming schemes. Domain names are yet have fully internationalized naming schemes. Domain names are
just one of the various names and identifiers that are required to be just one of the various names and identifiers that are required to be
internationalized. In many contexts, until more of those identifiers internationalized. In many contexts, until more of those identifiers
are internationalized, internationalized domain names alone have are internationalized, internationalized domain names alone have
little value. little value.
Email addresses are prime examples of why it is not good enough to Email addresses are prime examples of why it is not good enough to
just internationalize the domain name. As most of us have learned just internationalize the domain name. As most observers have
from experience, users strongly prefer email addresses that resemble learned from experience, users strongly prefer email addresses that
names or initials to those involving seemingly meaningless strings of resemble names or initials to those involving seemingly meaningless
letters or numbers. Unless the entire email address can use familiar strings of letters or numbers. Unless the entire email address can
characters and formats, users will perceive email as being culturally use familiar characters and formats, users will perceive email as
unfriendly. If the names and initials used in email addresses can be being culturally unfriendly. If the names and initials used in email
expressed in the native languages and writing systems of the users, addresses can be expressed in the native languages and writing
the Internet will be perceived as more natural, especially by those systems of the users, the Internet will be perceived as more natural,
whose native language is not written in a subset of a Roman-derived especially by those whose native language is not written in a subset
script. of a Roman-derived script.
Internationalization of email addresses is not merely a matter of Internationalization of email addresses is not merely a matter of
changing the SMTP envelope; or of modifying the From, To, and Cc changing the SMTP envelope; or of modifying the From, To, and Cc
header fields; or of permitting upgraded Mail User Agents (MUAs) to header fields; or of permitting upgraded Mail User Agents (MUAs) to
decode a special coding and respond by displaying local characters. decode a special coding and respond by displaying local characters.
To be perceived as usable, the addresses must be internationalized To be perceived as usable, the addresses must be internationalized
and handled consistently in all of the contexts in which they occur. and handled consistently in all of the contexts in which they occur.
This requirement has far-reaching implications: collections of This requirement has far-reaching implications: collections of
patches and workarounds are not adequate. Even if they were patches and workarounds are not adequate. Even if they were
adequate, a workaround-based approach may result in an assortment of adequate, a workaround-based approach may result in an assortment of
implementations with different sets of patches and workarounds having implementations with different sets of patches and workarounds having
been applied with consequent user confusion about what is actually been applied with consequent user confusion about what is actually
usable and supported. Instead, we need to build a fully usable and supported. Instead, we need to build a fully
internationalized email environment, focusing on permitting efficient internationalized email environment, focusing on permitting efficient
communication among those who share a language or other community. communication among those who share a language or other community.
That, in turn, implies changes to the mail header environment to That, in turn, implies changes to the mail header environment to
permit the full range of Unicode characters where that makes sense, permit the full range of Unicode characters where that makes sense,
an SMTP Extension to permit UTF-8 [RFC3629] mail addressing and an SMTP Extension to permit UTF-8 [RFC3629] [RFC5198] mail addressing
delivery of those extended header fields, and (finally) a requirement and delivery of those extended header fields, support for
for support of the 8BITMIME SMTP Extension [RFC1652] so that all of internationalized delivery and service notifications [RFC3461]
these can be transported through the mail system without having to [RFC3464], and (finally) a requirement for support of the 8BITMIME
overcome the limitation that header fields do not have content- SMTP Extension [RFC1652] so that all of these can be transported
transfer-encodings. through the mail system without having to overcome the limitation
that header fields do not have content-transfer-encodings.
4. Terminology 4. Terminology
This document assumes a reasonable understanding of the protocols and This document assumes a reasonable understanding of the protocols and
terminology of the core email standards as documented in [RFC5321] terminology of the core email standards as documented in [RFC5321]
and [RFC5322]. and [RFC5322].
4.1. Mail User and Mail Transfer Agents 4.1. Mail User and Mail Transfer Agents
Much of the description in this document depends on the abstractions Much of the description in this document depends on the abstractions
of "Mail Transfer Agent" ("MTA") and "Mail User Agent" ("MUA"). of "Mail Transfer Agent" ("MTA") and "Mail User Agent" ("MUA").
However, it is important to understand that those terms and the However, it is important to understand that those terms and the
underlying concepts postdate the design of the Internet's email underlying concepts postdate the design of the Internet's email
architecture and the application of the "protocols on the wire" architecture and the application of the "protocols on the wire"
principle to it. That email architecture, as it has evolved, and principle to it. That email architecture, as it has evolved, and
that "wire" principle have prevented any strong and standardized that "on the wire" principle have prevented any strong and
distinctions about how MTAs and MUAs interact on a given origin or standardized distinctions about how MTAs and MUAs interact on a given
destination host (or even whether they are separate). origin or destination host (or even whether they are separate).
However, the term "final delivery MTA" is used in this document in a However, the term "final delivery MTA" is used in this document in a
fashion equivalent to the term "delivery system" or "final delivery fashion equivalent to the term "delivery system" or "final delivery
system" of RFC 5321. This is the SMTP server that controls the system" of RFC 5321. This is the SMTP server that controls the
format of the local parts of addresses and is permitted to inspect format of the local parts of addresses and is permitted to inspect
and interpret them. It receives messages from the network for and interpret them. It receives messages from the network for
delivery to mailboxes or for other local processing, including any delivery to mailboxes or for other local processing, including any
forwarding or aliasing that changes envelope addresses, rather than forwarding or aliasing that changes envelope addresses, rather than
relaying. From the perspective of the network, any local delivery relaying. From the perspective of the network, any local delivery
arrangements such as saving to a message store, handoff to specific arrangements such as saving to a message store, handoff to specific
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repertoire [ASCII]; an address is "non-ASCII", or an "i18n-address", repertoire [ASCII]; an address is "non-ASCII", or an "i18n-address",
if any character is not in the ASCII character repertoire. Such if any character is not in the ASCII character repertoire. Such
addresses may be restricted in other ways, but those restrictions are addresses may be restricted in other ways, but those restrictions are
not relevant to this definition. The term "all-ASCII" is also not relevant to this definition. The term "all-ASCII" is also
applied to other protocol elements when the distinction is important, applied to other protocol elements when the distinction is important,
with "non-ASCII" or "internationalized" as its opposite. with "non-ASCII" or "internationalized" as its opposite.
The umbrella term to describe the email address internationalization The umbrella term to describe the email address internationalization
specified by this document and its companion documents is specified by this document and its companion documents is
"UTF8SMTPbis". "UTF8SMTPbis".
[[anchor7: Note in Draft: Keyword to be changed before publication.]] [[anchor5: Note in Draft: Keyword to be changed before publication.]]
For example, an address permitted by this specification is referred For example, an address permitted by this specification is referred
to as a "UTF8SMTPbis (compliant) address". to as a "UTF8SMTPbis (compliant) address".
Please note that, according to the definitions given here, the set of Please note that, according to the definitions given here, the set of
all "all-ASCII" addresses and the set of all "non-ASCII" addresses all "all-ASCII" addresses and the set of all "non-ASCII" addresses
are mutually exclusive. The set of all addresses permitted when are mutually exclusive. The set of all addresses permitted when
UTF8SMTPbis appears is the union of these two sets. UTF8SMTPbis appears is the union of these two sets.
4.3. User Types 4.3. User Types
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An "i18mail user" has one or more non-ASCII email addresses. Such a 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 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 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 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 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 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 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 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 thing as an "i18mail message"; the term applies only to users and
their agents and capabilities. their agents and capabilities. In particular, the use of non-ASCII
message content is an integral part of the MIME specifications
[RFC2045] and does not require these extensions (although it is
compatible with them).
4.4. Messages 4.4. Messages
A "message" is sent from one user (sender) using a particular email A "message" is sent from one user (sender) using a particular email
address to one or more other recipient email addresses (often address to one or more other recipient email addresses (often
referred to just as "users" or "recipient users"). referred to just as "users" or "recipient users").
A conventional message is one that does not use any extension defined
in the SMTP extension document [RFC5336] or in the UTF8header
specification [RFC5335], and is strictly conformant to RFC 5322
[RFC5322].
An internationalized message is a message utilizing one or more of
the extensions defined in this specification or in the UTF8header
specification [RFC5335], so that it is no longer conformant to the
RFC 5322 specification of a message.
4.5. Mailing Lists 4.5. Mailing Lists
A "mailing list" is a mechanism whereby a message may be distributed A "mailing list" is a mechanism whereby a message may be distributed
to multiple recipients by sending it to one recipient address. An to multiple recipients by sending it to one recipient address. An
agent (typically not a human being) at that single address then agent (typically not a human being) at that single address then
causes the message to be redistributed to the target recipients. causes the message to be redistributed to the target recipients.
This agent sets the envelope return address of the redistributed This agent sets the envelope return address of the redistributed
message to a different address from that of the original single message to a different address from that of the original single
recipient message. Using a different envelope return address recipient message. Using a different envelope return address
(reverse-path) causes error (and other automatically generated) (reverse-path) causes error (and other automatically generated)
messages to go to an error handling address. messages to go to an error handling address.
Special provisions for managing mailing lists that might contain non- Special provisions for managing mailing lists that might contain non-
ASCII addresses are discussed in a document that is specific to that ASCII addresses are discussed in a document that is specific to that
topic [EAI-Mailinglist]. topic [EAI-Mailinglist] [RFCNNNNbis-MailingList].
4.6. Undeliverable Messages and Notification 4.6. Conventional Message and Internationalized Message
o A conventional message is one that does not use any extension
defined in the SMTP extension document [RFC5336] or in the
UTF8header specification [RFC5335], and is strictly conformant to
RFC 5322 [RFC5322].
o An internationalized message is a message utilizing one or more of
the extensions defined in this set of specifications, so that it
is no longer conformant to the traditional specification of an
email message or its transport.
4.7. Undeliverable Messages and Notification
As specified in RFC 5321, a message that is undeliverable for some As specified in RFC 5321, a message that is undeliverable for some
reason is expected to result in notification to the sender. This can reason is expected to result in notification to the sender. This can
occur in either of two ways. One, typically called "Rejection", occur in either of two ways. One, typically called "Rejection",
occurs when an SMTP server returns a reply code indicating a fatal occurs when an SMTP server returns a reply code indicating a fatal
error (a "5yz" code) or persistently returns a temporary failure error (a "5yz" code) or persistently returns a temporary failure
error (a "4yz" code). The other involves accepting the message error (a "4yz" code). The other involves accepting the message
during SMTP processing and then generating a message to the sender, during SMTP processing and then generating a message to the sender,
typically known as a "Non-delivery Notification" or "NDN". Current typically known as a "Non-delivery Notification" or "NDN". Current
practice often favors rejection over NDNs because of the reduced practice often favors rejection over NDNs because of the reduced
likelihood that the generation of NDNs will be used as a spamming likelihood that the generation of NDNs will be used as a spamming
technique. The latter, NDN, case is unavoidable if an intermediate technique. The latter, NDN, case is unavoidable if an intermediate
MTA accepts a message that is then rejected by the next-hop server. MTA accepts a message that is then rejected by the next-hop server.
5. Overview of the Approach 5. Overview of the Approach and Document Plan
This set of specifications changes both SMTP and the character This set of specifications changes both SMTP and the character
encoding of email message headers to permit non-ASCII characters to encoding of email message headers to permit non-ASCII characters to
be represented directly. Each important component of the work is be represented directly. Each important component of the work is
described in a separate document. The document set, whose members described in a separate document. The document set, whose members
are described in the next section, also contains informational are described below, also contains informational documents whose
documents whose purpose is to provide implementation suggestions and purpose is to provide implementation suggestions and guidance for the
guidance for the protocols. protocols.
6. Document Plan
In addition to this document, the following documents make up this In addition to this document, the following documents make up this
specification and provide advice and context for it. specification and provide advice and context for it.
[[anchor12: ... Note to WG: if we actually include a list here, the o SMTP extensions. This document [RFC5336bis-SMTP] provides an SMTP
result will be that this document can be approved, but not published,
until those documents on the list are complete. I'm inclined to list
the SMTP extension and headers documents only and hand-wave about the
rest, but we need to discuss. Versions -00 and -01 simply refer to
the current Experimental documents --Editor.]]
o SMTP extensions. This document [RFC5336] provides an SMTP
extension (as provided for in RFC 5321) for internationalized extension (as provided for in RFC 5321) for internationalized
addresses. addresses.
o Email message headers in UTF-8. This document [RFC5335] o Email message headers in UTF-8. This document [RFC5335bis-Hdrs]
essentially updates RFC 5322 to permit some information in email essentially updates RFC 5322 to permit some information in email
message headers to be expressed directly by Unicode characters message headers to be expressed directly by Unicode characters
encoded in UTF-8 when the SMTP extension described above is used. encoded in UTF-8 when the SMTP extension described above is used.
This document, possibly with one or more supplemental ones, will This document, possibly with one or more supplemental ones, will
also need to address the interactions with MIME, including also need to address the interactions with MIME, including
relationships between UTF8SMTPbis and internal MIME headers and relationships between UTF8SMTPbis and internal MIME headers and
content types. content types.
o Extensions to the IMAP protocol to support internationalized o Extensions to delivery status and notification handling to adapt
message headers [RFC5738]. to internationalized addresses [RFC5337bis-DSN].
o Parallel extensions to the POP protocol [RFC5721]. o Extensions to the IMAP protocol to support internationalized
message headers [RFC5738bis-IMAP].
o Description of internationalization changes for delivery o Parallel extensions to the POP protocol [RFC5721]
notifications (DSNs) [RFC5337]. [RFC5721bis-POP3].
7. Overview of Protocol Extensions and Changes 6. Overview of Protocol Extensions and Changes
7.1. SMTP Extension for Internationalized Email Address 6.1. SMTP Extension for Internationalized Email Address
An SMTP extension, "UTF8SMTPbis" is specified as follows: An SMTP extension, "UTF8SMTPbis" is specified as follows:
o Permits the use of UTF-8 strings in email addresses, both local o Permits the use of UTF-8 strings in email addresses, both local
parts and domain names. parts and domain names.
o Permits the selective use of UTF-8 strings in email message o Permits the selective use of UTF-8 strings in email message
headers (see Section 7.2). headers (see Section 6.2).
o Requires that the server advertise the 8BITMIME extension o Requires that the server advertise the 8BITMIME extension
[RFC1652] and that the client support 8-bit transmission so that [RFC1652] and that the client support 8-bit transmission so that
header information can be transmitted without using a special header information can be transmitted without using a special
content-transfer-encoding. content-transfer-encoding.
Some general principles affect the development decisions underlying Some general principles affect the development decisions underlying
this work. this work.
1. Email addresses enter subsystems (such as a user interface) that 1. Email addresses enter subsystems (such as a user interface) that
may perform charset conversions or other encoding changes. When may perform charset conversions or other encoding changes. When
the left hand side of the address includes characters outside the the left hand side of the address includes characters outside the
US-ASCII character repertoire, use of punycode on the right hand US-ASCII character repertoire, use of ASCII-compatible (ACE)
side is discouraged to promote consistent processing of encoding [RFC3492] [RFC5890] on the right hand side is
characters throughout the address. discouraged to promote consistent processing of characters
throughout the address.
2. An SMTP relay must 2. An SMTP relay must
* Either recognize the format explicitly, agreeing to do so via * Either recognize the format explicitly, agreeing to do so via
an ESMTP option, or an ESMTP option, or
* Reject the message or, if necessary, return a non-delivery * Reject the message or, if necessary, return a non-delivery
notification message, so that the sender can make another notification message, so that the sender can make another
plan. plan.
skipping to change at page 10, line 37 skipping to change at page 10, line 42
message generated and sent. message generated and sent.
4. In the interest of interoperability, charsets other than UTF-8 4. In the interest of interoperability, charsets other than UTF-8
are prohibited in mail addresses and message headers being are prohibited in mail addresses and message headers being
transmitted over the Internet. There is no practical way to transmitted over the Internet. There is no practical way to
identify multiple charsets properly with an extension similar to identify multiple charsets properly with an extension similar to
this without introducing great complexity. this without introducing great complexity.
Conformance to the group of standards specified here for email Conformance to the group of standards specified here for email
transport and delivery requires implementation of the SMTP Extension transport and delivery requires implementation of the SMTP Extension
specification, including recognition of the keywords associated with specification and the UTF-8 Header specification. If the system
alternate addresses, and the UTF-8 Header specification. If the implements IMAP or POP, it MUST conform to the i18n IMAP or POP
system implements IMAP or POP, it MUST conform to the i18n IMAP or specifications respectively.
POP specifications respectively.
7.2. Transmission of Email Header Fields in UTF-8 Encoding 6.2. Transmission of Email Header Fields in UTF-8 Encoding
There are many places in MUAs or in a user presentation in which There are many places in MUAs or in a user presentation in which
email addresses or domain names appear. Examples include the email addresses or domain names appear. Examples include the
conventional From, To, or Cc header fields; Message-ID and conventional From, To, or Cc header fields; Message-ID and
In-Reply-To header fields that normally contain domain names (but In-Reply-To header fields that normally contain domain names (but
that may be a special case); and in message bodies. Each of these that may be a special case); and in message bodies. Each of these
must be examined from an internationalization perspective. The user must be examined from an internationalization perspective. The user
will expect to see mailbox and domain names in local characters, and will expect to see mailbox and domain names in local characters, and
to see them consistently. If non-obvious encodings, such as to see them consistently. If non-obvious encodings, such as
protocol-specific ASCII-Compatible Encoding (ACE) variants, are used, protocol-specific ASCII-Compatible Encoding (ACE) variants, are used,
skipping to change at page 11, line 17 skipping to change at page 11, line 20
Similarly, if different codings are used for mail transport and Similarly, if different codings are used for mail transport and
message bodies, the user is particularly likely to be surprised, if message bodies, the user is particularly likely to be surprised, if
only as a consequence of the long-established "things leak" only as a consequence of the long-established "things leak"
principle. The only practical way to avoid these sources of principle. The only practical way to avoid these sources of
discomfort, in both the medium and the longer term, is to have the 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 encodings used in transport be as similar to the encodings used in
message headers and message bodies as possible. message headers and message bodies as possible.
When email local parts are internationalized, it seems clear that When email local parts are internationalized, it seems clear that
they should be accompanied by arrangements for the message headers to they should be accompanied by arrangements for the message headers to
be in the fully internationalized form. That form should use UTF-8 be in the fully internationalized form. That form should presumably
rather than ASCII as the base character set for the contents of use UTF-8 rather than ASCII as the base character set for the
header fields (protocol elements such as the header field names contents of header fields (protocol elements such as the header field
themselves will remain entirely in ASCII). For transition purposes names themselves are unchanged and remain entirely in ASCII). For
and compatibility with legacy systems, this can done by extending the transition purposes and compatibility with legacy systems, this can
encoding models of [RFC2045] and [RFC2231]. However, the target is done by extending the traditional MIME encoding models for non-ASCII
fully internationalized message headers, as discussed in [RFC5335] characters in headers [RFC2045] [RFC2231]. However, the target is
and not an extended and painful transition. fully internationalized message headers, as discussed in
[RFC5335bis-Hdrs] and not an extended and painful transition.
8. Downgrading before and after SMTP Transactions 6.3. SMTP Service Extension for DSNs
The existing Draft Standard Delivery status notifications (DSNs)
specification [RFC3461] is limited to ASCII text in the machine
readable portions of the protocol. "International Delivery and
Disposition Notifications" [RFC5337bis-DSN] adds a new address type
for international email addresses so an original recipient address
with non-ASCII characters can be correctly preserved even after
downgrading. If an SMTP server advertises both the UTF8SMTPbis and
the DSN extension, that server MUST implement internationalized DSNs
including support for the ORCPT parameter specified in RFC 3461
[RFC3461].
7. Downgrading before and after SMTP Transactions
An important issue with these extensions is how to handle An important issue with these extensions is how to handle
interactions between systems that support non-ASCII addresses and interactions between systems that support non-ASCII addresses and
legacy systems that expect ASCII. There is, of course, no problem legacy systems that expect ASCII. There is, of course, no problem
with ASCII-only systems sending to those that can handle with ASCII-only systems sending to those that can handle
internationalized forms because the ASCII forms are just a proper internationalized forms because the ASCII forms are just a proper
subset. But, when systems that support these extensions send mail, subset. But, when systems that support these extensions send mail,
they may include non-ASCII addresses for senders, receivers, or both they may include non-ASCII addresses for senders, receivers, or both
and might also provide non-ASCII header information other than and might also provide non-ASCII header information other than
addresses. If the extension is not supported by the first-hop system addresses. If the extension is not supported by the first-hop system
skipping to change at page 12, line 6 skipping to change at page 12, line 23
addresses available for all senders and recipients. Mechanisms by addresses available for all senders and recipients. Mechanisms by
which such addresses may be found or identified are outside the scope which such addresses may be found or identified are outside the scope
of these specifications as are decisions about the design of of these specifications as are decisions about the design of
originating systems such as whether any required transformations are originating systems such as whether any required transformations are
made by the user, the originating MUA, or the Submission server. made by the user, the originating MUA, or the Submission server.
A somewhat more complex situation arises when the first-hop system A somewhat more complex situation arises when the first-hop system
supports these extensions but some subsequent server in the SMTP supports these extensions but some subsequent server in the SMTP
transmission chain does not. It is important to note that most cases transmission chain does not. It is important to note that most cases
of that situation will be the result of configuration errors: of that situation will be the result of configuration errors:
especially if it hosts non-ASCII addresses, a final delivery server especially if it hosts non-ASCII addresses, a final delivery MTA that
that accepts these extensions should not be configured with lower- accepts these extensions should not be configured with lower-
preference MX hosts that do not. While the experiments that preceded preference MX hosts that do not. While the experiments that preceded
these specifications included a mechanism for passing backup ASCII these specifications included a mechanism for passing backup ASCII
addresses to intermediate relay systems and having those systems addresses to intermediate relay systems and having those systems
alter the relevant message header fields and substitute the alter the relevant message header fields and substitute the addresses
addresses, the requirements and long-term implications of that system [RFC5504], the requirements and long-term implications of that system
proved too complex to be satisfactory. Consequently, if an proved too complex to be satisfactory. Consequently, if an
intermediate SMTP relay that is transmitting a message that requires intermediate SMTP relay that is transmitting a message that requires
these extensions and discovers that the next system in the chain does these extensions and discovers that the next system in the chain does
not support them, it will have little choice other than to reject or not support them, it will have little choice other than to reject or
return the message. return the message.
As discussed above, downgrading to an ASCII-only form may occur As discussed above, downgrading to an ASCII-only form may occur
before or during the initial message submission. It might also occur before or during the initial message submission. It might also occur
after the delivery to the final delivery MTA in order to accommodate after the delivery to the final delivery MTA in order to accommodate
messages stores or IMAP or POP servers or clients that have different messages stores or IMAP or POP servers or clients that have different
capabilities than the delivery MTA. These two cases are discussed in capabilities than the delivery MTA. These two cases are discussed in
the subsections below. the subsections below.
8.1. Downgrading before or during Message Submission 7.1. Downgrading before or during Message Submission
Perhaps obviously, the most convenient time to find an ASCII address Perhaps obviously, the most convenient time to find an ASCII address
corresponding to an internationalized address is at the originating corresponding to an internationalized address is at the originating
MUA. This can occur either before the message is sent or after the MUA. This can occur either before the message is sent or after the
internationalized form of the message is rejected. It is also the internationalized form of the message is rejected. It is also the
most convenient time to convert a message from the internationalized most convenient time to convert a message from the internationalized
form into conventional ASCII form or to generate a non-delivery form into conventional ASCII form or to generate a non-delivery
message to the sender if either is necessary. At that point, the message to the sender if either is necessary. At that point, the
user has a full range of choices available, including contacting the user has a full range of choices available, including contacting the
intended recipient out of band for an alternate address, consulting intended recipient out of band for an alternate address, consulting
skipping to change at page 13, line 5 skipping to change at page 13, line 20
of a user of at least moderate intelligence who wishes to communicate of a user of at least moderate intelligence who wishes to communicate
with another such user. with another such user.
In this context, one can easily imagine modifications to message In this context, one can easily imagine modifications to message
submission servers (as described in [RFC4409]) so that they would submission servers (as described in [RFC4409]) so that they would
perform downgrading, or perhaps even upgrading, operations, receiving perform downgrading, or perhaps even upgrading, operations, receiving
messages with one or more of the internationalization extensions messages with one or more of the internationalization extensions
discussed here and adapting the outgoing message, as needed, to discussed here and adapting the outgoing message, as needed, to
respond to the delivery or next-hop environment it encounters. respond to the delivery or next-hop environment it encounters.
8.2. Downgrading or Other Processing After Final SMTP Delivery 7.2. Downgrading or Other Processing After Final SMTP Delivery
When an email message is received by a final delivery SMTP server, it When an email message is received by a final delivery MTA, it is
is usually stored in some form. Then it is retrieved either by usually stored in some form. Then it is retrieved either by software
software that reads the stored form directly or by client software that reads the stored form directly or by client software via some
via some email retrieval mechanisms such as POP or IMAP. email retrieval mechanisms such as POP or IMAP.
The SMTP extension described in Section 7.1 provides protection only The SMTP extension described in Section 6.1 provides protection only
in transport. It does not prevent MUAs and email retrieval in transport. It does not prevent MUAs and email retrieval
mechanisms that have not been upgraded to understand mechanisms that have not been upgraded to understand
internationalized addresses and UTF-8 message headers from accessing internationalized addresses and UTF-8 message headers from accessing
stored internationalized emails. stored internationalized emails.
Since the final delivery SMTP server (or, to be more specific, its Since the final delivery MTA (or, to be more specific, its
corresponding mail storage agent) cannot safely assume that agents corresponding mail storage agent) cannot safely assume that agents
accessing email storage will always be capable of handling the accessing email storage will always be capable of handling the
extensions proposed here, it MAY either downgrade internationalized extensions proposed here, it MAY either downgrade internationalized
emails or specially identify messages that utilize these extensions, emails or specially identify messages that utilize these extensions,
or both. If this is done, the final delivery SMTP server SHOULD or both. If this is done, the final delivery MTA SHOULD include a
include a mechanism to preserve or recover the original mechanism to preserve or recover the original internationalized forms
internationalized forms without information loss to support access by without information loss to support access by UTF8SMTPbis-aware
UTF8SMTPbis-aware agents. agents.
9. Downgrading in Transit 8. Downgrading in Transit
[[anchor16: Note in Draft and Question for the WG: We could discuss The base SMTP specification (Section 2.3.11 of RFC 5321 [RFC5321])
the various issues with in-transit downgrading including the states that "due to a long history of problems when intermediate
complexities of carrying backup addresses, the problems that hosts have attempted to optimize transport by modifying them, the
motivated the "don't mess with addresses in transit" (paraphrased, local-part MUST be interpreted and assigned semantics only by the
obviously) rule in RFC 5321 and friends, and so on. Or we could omit host specified in the domain part of the address". This is not a new
it (and this section). Pragmatically, I think it would take us some requirement; equivalent statements appeared in specifications in 2001
time to reach consensus on what, exactly, should be said and that [RFC2821] and even in 1989 [RFC1123].
might delay progress. But input is clearly needed -- if it is not
received before we prepared -02, this section will simply be
dropped.]]
10. User Interface and Configuration Issues Adherence to this rule means that a downgrade mechanism that
transforms the local-part of an email address cannot be done in
transit. It can only be done at the endpoints, namely by the MUA or
submission server or by the final delivery MTA.
One of the reasons for this rule has to do with legacy email systems
that use source routing in the local-part of the address field.
Transforming the email address destroys such routing information.
There is no way a server other than the final delivery server can
know, for example, whether the local-part of user&foo@example.com is
a route ("user" is reached via "foo") or simply a local address.
9. User Interface and Configuration Issues
Internationalization of addresses and message headers, especially in Internationalization of addresses and message headers, especially in
combination with variations on character coding that are inherent to combination with variations on character coding that are inherent to
Unicode, may make careful choices of addresses and careful Unicode, may make careful choices of addresses and careful
configuration of servers and DNS records even more important than configuration of servers and DNS records even more important than
they are for traditional Internet email. It is likely that, as they are for traditional Internet email. It is likely that, as
experience develops with the use of these protocols, it will be experience develops with the use of these protocols, it will be
desirable to produce one or more additional documents that offer desirable to produce one or more additional documents that offer
guidance for configuration and interfaces. A document that discusses guidance for configuration and interfaces. A document that discusses
issues with mail user agents (MUAs), especially with regard to issues with mail user agents (MUAs), especially with regard to
downgrading, is expected to be developed in the EAI Working Group. downgrading, is expected to be developed in the EAI Working Group.
[[anchor15: Note in Draft: What do we want to do about this?]]
The subsections below address some other issues. The subsections below address some other issues.
10.1. Choices of Mailbox Names and Unicode Normalization 9.1. Choices of Mailbox Names and Unicode Normalization
It has long been the case the email syntax permits choices about It has long been the case that the email syntax permits choices about
mailbox names that that are unwise in practice if one actually mailbox names that are unwise in practice if one actually intends the
intends the mailboxes to be accessible to a broad range of senders. mailboxes to be accessible to a broad range of senders. The most-
The most-often-cited examples involve the use of case-sensitivity and often-cited examples involve the use of case-sensitivity and tricky
tricky quoting of embedded characters in mailbox local parts. While quoting of embedded characters in mailbox local parts. While these
these are permitted by the protocols and servers are expected to are permitted by the protocols and servers are expected to support
support them and there are special cases where they can provide them and there are special cases where they can provide value, taking
value, taking advantage of those features is almost always bad advantage of those features is almost always bad practice unless the
practice. intent is to create some form of security by obscurity.
In the absence of this extension, SMTP clients and servers are In the absence of these extensions, SMTP clients and servers are
constrained to using only those addresses permitted by RFC 5321. The constrained to using only those addresses permitted by RFC 5321. The
local parts of those addresses MAY be made up of any ASCII characters local parts of those addresses MAY be made up of any ASCII characters
except the control characters that 5321 prohibits, although some of except the control characters that 5321 prohibits, although some of
them MUST be quoted as specified there. It is notable in an them MUST be quoted as specified there. It is notable in an
internationalization context that there is a long history on some internationalization context that there is a long history on some
systems of using overstruck ASCII characters (a character, a systems of using overstruck ASCII characters (a character, a
backspace, and another character) within a quoted string to backspace, and another character) within a quoted string to
approximate non-ASCII characters. This form of internationalization approximate non-ASCII characters. This form of internationalization
was permitted by RFC 821 but is prohibited by RFC 5321 because it was permitted by RFC 821 but is prohibited by RFC 5321 because it
requires a backspace character (a prohibited C0 control). The requires a backspace character (a prohibited C0 control). The
practice SHOULD be phased out as this extension becomes widely practice SHOULD be phased out as this extension becomes widely
deployed but backward-compatibility considerations may require that deployed but backward-compatibility considerations may require that
it continue to be recognized. it continue to be recognized.
For the particular case of EAI mailbox names, special attention must For the particular case of EAI mailbox names, special attention must
be paid to Unicode normalization, in part because Unicode strings may be paid to Unicode normalization [Unicode-UAX15], in part because
be normalized by other processes independent of what a mail protocol Unicode strings may be normalized by other processes independent of
specifies (this is exactly analogous to what may happen with quoting what a mail protocol specifies (this is exactly analogous to what may
and dequoting in traditional addresses). Consequently, the following happen with quoting and dequoting in traditional addresses).
principles are offered as advice to those who are selecting names for Consequently, the following principles are offered as advice to those
mailboxes: who are selecting names for mailboxes:
o In general, it is wise for servers to provide addresses only in o In general, it is wise to support addresses in Normalized form,
Normalized form and to normalize strings on receipt, using either using either Normalization Form NFC and, except in unusual
Normalization Form NFC and, except in unusual circumstances, NFKC. circumstances, NFKC.
[[anchor19: Note in Draft: "Normalize on receipt" is consistent
with the recommendations in draft-iab-i18n-encoding. The issue
with NFKC is that some of the characters mapped out may be
significant, especially in personal names. Anyone with objections
should speak up. Soon.]]
o It may be wise to support other forms of the same local-part o It may be wise to support other forms of the same local-part
string, either as aliases or by normalization of strings reaching string, either as aliases or by normalization of strings reaching
the delivery server, in the event that the sender does not send the delivery server, in the event that the sender does not send
the strings in normalized form. the strings in normalized form.
o Stated differently and in more specific terms, the rules of the o Stated differently and in more specific terms, the rules of the
protocol for local-part strings essentially provide that: protocol for local-part strings essentially provide that:
* Unnormalized strings are valid, but sufficiently bad practice * Unnormalized strings are valid, but sufficiently bad practice
that they may not work reliably on a global basis. that they may not work reliably on a global basis.
* C0 (and presumably C1) controls (see The Unicode Standard) are * C0 (and presumably C1) controls (see The Unicode Standard
prohibited, the first in RFC 5321 and the second by an obvious [Unicode52]) are prohibited, the first in RFC 5321 and the
extension from it. second by an obvious extension from it [RFC5198].
* Other kinds of punctuation, spaces, etc., are risky practice. * Other kinds of punctuation, spaces, etc., are risky practice.
Perhaps they will work, and SMTP receiver code is required to Perhaps they will work, and SMTP receiver code is required to
handle them, but creating dependencies on them in mailbox names handle them, but creating dependencies on them in mailbox names
that are chosen is usually a bad practice and may lead to that are chosen is usually a bad practice and may lead to
interoperability problems. interoperability problems.
11. Additional Issues 10. Additional Issues
This section identifies issues that are not covered, or not covered This section identifies issues that are not covered, or not covered
comprehensively, as part of this set of specifications, but that will comprehensively, as part of this set of specifications, but that will
require ongoing review as part of deployment of email address and require ongoing review as part of deployment of email address and
header internationalization. header internationalization.
11.1. Impact on URIs and IRIs 10.1. 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.
In particular, providing an alternate address as part of a mailto:
URI may require some fairly careful work on the syntax of that URI.
11.2. Interaction with Delivery Notifications
The advent of UTF8SMTPbis will make necessary consideration of the The mailto: schema [RFC2368] and discussed in the Internationalized
interaction with delivery notification mechanisms, including the Resource Identifier (IRI) specification [RFC3987] may need to be
ASCII-only SMTP extension for requesting delivery notifications modified when this work is completed and standardized.
(DSNs) [RFC3461], and the format of delivery notifications [RFC3464].
A new document, "International Delivery and Disposition
Notifications" [RFC5337] adds a new address type for international
email addresses so an original recipient address with non-ASCII
characters can be correctly preserved even after downgrading. If an
SMTP server advertises both the UTF8SMTPbis and the DSN extension,
that server MUST implement internationalized DSNs, including support
for the ORCPT parameter.
11.3. Use of Email Addresses as Identifiers 10.2. Use of Email Addresses as Identifiers
There are a number of places in contemporary Internet usage in which There are a number of places in contemporary Internet usage in which
email addresses are used as identifiers for individuals, including as email addresses are used as identifiers for individuals, including as
identifiers to Web servers supporting some electronic commerce sites. identifiers to Web servers supporting some electronic commerce sites
These documents do not address those uses, but it is reasonable to and in some X.509 certificates [RFC5280]. These documents do not
expect that some difficulties will be encountered when address those uses, but it is reasonable to expect that some
internationalized addresses are first used in those contexts, many of difficulties will be encountered when internationalized addresses are
which cannot even handle the full range of addresses permitted today. first used in those contexts, many of which cannot even handle the
full range of addresses permitted today.
11.4. Encoded Words, Signed Messages, and Downgrading 10.3. Encoded Words, Signed Messages, and Downgrading
One particular characteristic of the email format is its persistency: One particular characteristic of the email format is its persistency:
MUAs are expected to handle messages that were originally sent MUAs are expected to handle messages that were originally sent
decades ago and not just those delivered seconds ago. As such, MUAs decades ago and not just those delivered seconds ago. As such, MUAs
and mail filtering software, such as that specified in Sieve and mail filtering software, such as that specified in Sieve
[RFC5228], will need to continue to accept and decode header fields [RFC5228], will need to continue to accept and decode header fields
that use the "encoded word" mechanism [RFC2047] to accommodate non- that use the "encoded word" mechanism [RFC2047] to accommodate non-
ASCII characters in some header fields. While extensions to both ASCII characters in some header fields. While extensions to both
POP3 and IMAP have been proposed to enable automatic EAI-upgrade -- POP3 [RFC1939] and IMAP [RFC3501] have been defined that include
including RFC 2047 decoding -- of messages by the POP3 or IMAP automatic upgrading of messages that carry non-ASCII information in
server, there are message structures and MIME content-types for which encoded form -- including RFC 2047 decoding -- of messages by the
that cannot be done or where the change would have unacceptable side POP3 [RFC5721bis-POP3] or IMAP [RFC5738bis-IMAP] server, there are
effects. 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 For example, message parts that are cryptographically signed, using
e.g., S/MIME [RFC3851] or Pretty Good Privacy (PGP) [RFC3156], cannot 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 be upgraded from the RFC 2047 form to normal UTF-8 characters without
breaking the signature. Similarly, message parts that are encrypted breaking the signature. Similarly, message parts that are encrypted
may contain, when decrypted, header fields that use the RFC 2047 may contain, when decrypted, header fields that use the RFC 2047
encoding; such messages cannot be 'fully' upgraded without access to encoding; such messages cannot be 'fully' upgraded without access to
cryptographic keys. cryptographic keys.
11.5. LMTP Similar issues may arise if messages are signed and then subsequently
downgraded, e.g., as discussed in Section 7.1, 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.
LMTP [RFC2033] may be used as the final delivery agent. In such 10.4. LMTP
cases, LMTP may be arranged to deliver the mail to the mail store.
The mail store may not have UTF8SMTPbis capability. LMTP need to be
updated to deal with these situations.
11.6. Other Uses of Local Parts LMTP [RFC2033] may be used as part of the final delivery agent. In
such cases, LMTP may be arranged to deliver the mail to the mail
store. The mail store may not have UTF8SMTPbis capability. LMTP may
need to be updated to deal with these situations.
10.5. Other Uses of Local Parts
Local parts are sometimes used to construct domain labels, e.g., the Local parts are sometimes used to construct domain labels, e.g., the
local part "user" in the address user@domain.example could be local part "user" in the address user@domain.example could be
converted into a vanity host user.domain.example with its Web space converted into a vanity host user.domain.example with its Web space
at <http://user.domain.example> and the catchall addresses at <http://user.domain.example> and the catchall addresses
any.thing.goes@user.domain.example. any.thing.goes@user.domain.example.
Such schemes are obviously limited by, among other things, the SMTP Such schemes are obviously limited by, among other things, the SMTP
rules for domain names, and will not work without further rules for domain names, and will not work without further
restrictions for other local parts such as the <utf8-local-part> restrictions for other local parts such as the <utf8-local-part>
specified in [RFC5335]. Whether this issue is relevant to these specified in [RFC5335bis-Hdrs]. Whether those limitations are
specifications is an open question. It may be simply another case of relevant to these specifications is an open question. It may be
the considerable flexibility accorded to delivery MTAs in determining simply another case of the considerable flexibility accorded to
the mailbox names they will accept and how they are interpreted. delivery MTAs in determining the mailbox names they will accept and
how they are interpreted.
11.7. Non-Standard Encapsulation Formats 10.6. Non-Standard Encapsulation Formats
Some applications use formats similar to the application/mbox format Some applications use formats similar to the application/mbox format
defined in [RFC4155] instead of the message/digest RFC 2046, Section defined in [RFC4155] instead of the message/digest form described in
5.1.5 [RFC2046] form to transfer multiple messages as single units. RFC 2046, Section 5.1.5 [RFC2046] to transfer multiple messages as
Insofar as such applications assume that all stored messages use the single units. Insofar as such applications assume that all stored
message/rfc822 RFC 2046, Section 5.2.1 [RFC2046] format with US-ASCII messages use the message/rfc822 format described in RFC 2046, Section
message headers, they are not ready for the extensions specified in 5.2.1 [RFC2046] with US-ASCII message headers, they are not ready for
this series of documents and special measures may be needed to the extensions specified in this series of documents and special
properly detect and process them. measures may be needed to properly detect and process them.
12. Experimental Targets
[[anchor26: Note in draft: this section is left in this draft for
convenience in review. It will be removed with -02.]]
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.
13. IANA Considerations 11. IANA Considerations
This overview description and framework document does not contemplate This overview description and framework document does not contemplate
any IANA registrations or other actions. Some of the documents in any IANA registrations or other actions. Some of the documents in
the group have their own IANA considerations sections and the group have their own IANA considerations sections and
requirements. requirements.
14. Security Considerations 12. Security Considerations
Any expansion of permitted characters and encoding forms in email Any expansion of permitted characters and encoding forms in email
addresses raises some risks. There have been discussions on so addresses raises some risks. There have been discussions on so
called "IDN-spoofing" or "IDN homograph attacks". These attacks called "IDN-spoofing" or "IDN homograph attacks". These attacks
allow an attacker (or "phisher") to spoof the domain or URLs of allow an attacker (or "phisher") to spoof the domain or URLs of
businesses. The same kind of attack is also possible on the local businesses. The same kind of attack is also possible on the local
part of internationalized email addresses. It should be noted that part of internationalized email addresses. It should be noted that
the proposed fix involving forcing all displayed elements into the proposed fix involving forcing all displayed elements into
normalized lower-case works for domain names in URLs, but not email normalized lower-case works for domain names in URLs, but not email
local parts since those are case sensitive. local parts since those are case sensitive.
skipping to change at page 18, line 24 skipping to change at page 18, line 27
conventions. They are increased with very large mail systems in conventions. They are increased with very large mail systems in
which users can freely select their own addresses. which users can freely select their own addresses.
The internationalization of email addresses and message headers must The internationalization of email addresses and message headers must
not leave the Internet less secure than it is without the required not leave the Internet less secure than it is without the required
extensions. The requirements and mechanisms documented in this set extensions. The requirements and mechanisms documented in this set
of specifications do not, in general, raise any new security issues. of specifications do not, in general, raise any new security issues.
They do require a review of issues associated with confusable They do require a review of issues associated with confusable
characters -- a topic that is being explored thoroughly elsewhere characters -- a topic that is being explored thoroughly elsewhere
(see, e.g., [RFC4690]) -- and, potentially, some issues with UTF-8 (see, e.g., RFC 4690 [RFC4690]) -- and, potentially, some issues with
normalization, discussed in [RFC3629], and other transformations. UTF-8 normalization, discussed in RFC 3629 [RFC3629], and other
Normalization and other issues associated with transformations and transformations. Normalization and other issues associated with
standard forms are also part of the subject of ongoing work discussed transformations and standard forms are also part of the subject of
in [RFC5198], in [RFC5893] and elsewhere. work described elsewhere [RFC5198] [RFC5893] [IAB-idn-encoding].
Some issues specifically related to internationalized addresses and Some issues specifically related to internationalized addresses and
message headers are discussed in more detail in the other documents message headers are discussed in more detail in the other documents
in this set. However, in particular, caution should be taken that in this set. However, in particular, caution should be taken that
any "downgrading" mechanism, or use of downgraded addresses, does not any "downgrading" mechanism, or use of downgraded addresses, does not
inappropriately assume authenticated bindings between the inappropriately assume authenticated bindings between the
internationalized and ASCII addresses. Expecting and most or all internationalized and ASCII addresses. Expecting and most or all
such transformations prior to final delivery be done by systems that such transformations prior to final delivery be done by systems that
are presumed to be under the administrative control of the sending are presumed to be under the administrative control of the sending
user ameliorates the potential problem somewhat as compared to what user ameliorates the potential problem somewhat as compared to what
it would be if the relationships were changed in transit. it would be if the relationships were changed in transit.
The new UTF-8 header and message formats might also raise, or The new UTF-8 header and message formats might also raise, or
aggravate, another known issue. If the model creates new forms of an aggravate, another known issue. If the model creates new forms of an
'invalid' or 'malformed' message, then a new email attack is created: 'invalid' or 'malformed' message, then a new email attack is created:
in an effort to be robust, some or most agents will accept such 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 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 interprets such a message differently than the MUA used by the
be possible to create a message that appears acceptable under the recipient, then it may be possible to create a message that appears
filter's interpretation but should be rejected under the acceptable under the filter's interpretation but should be rejected
interpretation given to it by the final MUA. Such attacks already under the interpretation given to it by that MUA. Such attacks
exist for existing messages and encoding layers, e.g., invalid MIME already exist for existing messages and encoding layers, e.g.,
syntax, invalid HTML markup, and invalid coding of particular image invalid MIME syntax, invalid HTML markup, and invalid coding of
types. particular image types.
In addition, email addresses are used in many contexts other than In addition, email addresses are used in many contexts other than
sending mail, such as for identifiers under various circumstances sending mail, such as for identifiers under various circumstances
(see Section 11.3). Each of those contexts will need to be (see Section 10.2). Each of those contexts will need to be
evaluated, in turn, to determine whether the use of non-ASCII forms evaluated, in turn, to determine whether the use of non-ASCII forms
is appropriate and what particular issues they raise. is appropriate and what particular issues they raise.
This work will clearly affect any systems or mechanisms that are This work will clearly affect any systems or mechanisms that are
dependent on digital signatures or similar integrity protection for dependent on digital signatures or similar integrity protection for
email message headers (see also the discussion in Section 11.4). email message headers (see also the discussion in Section 10.3).
Many conventional uses of PGP and S/MIME are not affected since they Many conventional uses of PGP and S/MIME are not affected since they
are used to sign body parts but not message headers. On the other are used to sign body parts but not message headers. On the other
hand, the developing work on domain keys identified mail (DKIM hand, the developing work on domain keys identified mail (DKIM)
[RFC5863]) will eventually need to consider this work and vice versa: [RFC5863] will eventually need to consider this work and vice versa:
while this specification does not address or solve the issues raised while this specification does not address or solve the issues raised
by DKIM and other signed header mechanisms, the issues will have to by DKIM and other signed header mechanisms, the issues will have to
be coordinated and resolved eventually if the two sets of protocols be coordinated and resolved eventually if the two sets of protocols
are to co-exist. In addition, to the degree to which email addresses are to co-exist. In addition, to the degree to which email addresses
appear in PKI (Public Key Infrastructure) certificates, standards appear in PKI (Public Key Infrastructure) certificates, standards
addressing such certificates will need to be upgraded to address addressing such certificates will need to be upgraded to address
these internationalized addresses. Those upgrades will need to these internationalized addresses. Those upgrades will need to
address questions of spoofing by look-alikes of the addresses address questions of spoofing by look-alikes of the addresses
themselves. themselves.
15. Acknowledgements 13. Acknowledgments
This document is an update to, and derived from, RFC 4952. This This document is an update to, and derived from, RFC 4952. This
document would have been impossible without the work and document would have been impossible without the work and
contributions acknowledged in it. The present document benefited contributions acknowledged in it. The present document benefited
significantly from discussions in the EAI WG and elsewhere after RFC significantly from discussions in the EAI WG and elsewhere after RFC
4952 was published, especially discussions about the experimental 4952 was published, especially discussions about the experimental
versions of other documents in the internationalized email versions of other documents in the internationalized email
collection, and from RFC errata on RFC 4952 itself. collection, and from RFC errata on RFC 4952 itself.
16. References Special thanks are due to Ernie Dainow for careful reviews and
suggested text in this version.
16.1. Normative References 14. References
[ASCII] American National Standards Institute (formerly 14.1. Normative References
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 [ASCII] American National Standards Institute
versions with slight modifications, but the 1968 (formerly United States of America
version remains definitive for the Internet. Standards Institute), "USA Code for
Information Interchange", ANSI X3.4-1968,
1968.
[RFC1652] Klensin, J., Freed, N., Rose, M., Stefferud, E., ANSI X3.4-1968 has been replaced by newer
and D. Crocker, "SMTP Service Extension for versions with slight modifications, but the
8bit-MIMEtransport", RFC 1652, July 1994. 1968 version remains definitive for the
Internet.
[RFC2119] Bradner, S., "Key words for use in RFCs to [RFC1652] Klensin, J., Freed, N., Rose, M.,
Indicate Requirement Levels'", RFC 2119, BCP 14, Stefferud, E., and D. Crocker, "SMTP
March 1997. Service Extension for 8bit-MIMEtransport",
RFC 1652, July 1994.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of [RFC2119] Bradner, S., "Key words for use in RFCs to
ISO 10646", STD 63, RFC 3629, November 2003. Indicate Requirement Levels'", RFC 2119,
BCP 14, March 1997.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", [RFC3629] Yergeau, F., "UTF-8, a transformation
RFC 5321, October 2008. format of ISO 10646", STD 63, RFC 3629,
November 2003.
[RFC5890] Klensin, J., "Internationalized Domain Names for [RFC5321] Klensin, J., "Simple Mail Transfer
Applications (IDNA): Definitions and Document Protocol", RFC 5321, October 2008.
Framework", RFC 5890, June 2010.
16.2. Informative References [RFC5322] Resnick, P., Ed., "Internet Message
Format", RFC 5322, October 2008.
[EAI-Mailinglist] Gellens, R., "Mailing Lists and [RFC5335bis-Hdrs] Yang, A. and S. Steele, "Internationalized
Internationalized Email Addresses", June 2010, < Email Headers", July 2010, <https://
https://datatracker.ietf.org/doc/ datatracker.ietf.org/doc/
draft-ietf-eai-mailinglist/>. draft-ietf-eai-rfc5335bis/>.
[Hoffman-IMAA] Hoffman, P. and A. Costello, "Internationalizing [RFC5336bis-SMTP] Yao, J. and W. Mao, "SMTP Extension for
Mail Addresses in Applications (IMAA)", Work Internationalized Email Address",
in Progress, October 2003. June 2010, <https://datatracker.ietf.org/
doc/draft-ietf-eai-rfc5336bis/>.
[JET-IMA] Yao, J. and J. Yeh, "Internationalized eMail [RFC5337bis-DSN] Not yet posted?, "Internationalized
Address (IMA)", Work in Progress, June 2005. Delivery Status and Disposition
Notifications", Unwritten waiting for I-D,
2010.
[Klensin-emailaddr] Klensin, J., "Internationalization of Email [RFC5721bis-POP3] Not yet posted?, "POP3 Support for UTF-8",
Addresses", Work in Progress, July 2005. Unwritten waiting for I-D, 2010.
[RFC2033] Myers, J., "Local Mail Transfer Protocol", [RFC5738bis-IMAP] Not yet posted?, "IMAP Support for UTF-8",
RFC 2033, October 1996. Unwritten waiting for I-D, 2010.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose [RFC5890] Klensin, J., "Internationalized Domain
Internet Mail Extensions (MIME) Part One: Format Names for Applications (IDNA): Definitions
of Internet Message Bodies", RFC 2045, and Document Framework", RFC 5890,
November 1996. June 2010.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose [RFCNNNNbis-MailingList] Not yet posted?, "Mailing Lists and
Internet Mail Extensions (MIME) Part Two: Media Internationalized Email Addresses", First
Types", RFC 2046, November 1996. Version still not in RFC Editor queue https
://datatracker.ietf.org/doc/
draft-ietf-eai-mailinglist/,
Unwritten waiting for I-D, 2010.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail 14.2. Informative References
Extensions) Part Three: Message Header
Extensions for Non-ASCII Text", RFC 2047,
November 1996.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value [EAI-Mailinglist] Gellens, R., "Mailing Lists and
and Encoded Word Extensions: Internationalized Email Addresses",
Character Sets, Languages, and Continuations", March 2010, <https://datatracker.ietf.org/
RFC 2231, November 1997. doc/draft-ietf-eai-mailinglist/>.
[RFC2368] Hoffman, P., Masinter, L., and J. Zawinski, "The [IAB-idn-encoding] Thaler, D., Klensin, J., and S. Cheshire,
mailto URL scheme", RFC 2368, July 1998. "IAB Thoughts on Encodings for
Internationalized Domain Names", 2010, <htt
ps://datatracker.ietf.org/doc/
draft-iab-idn-encoding/>.
[RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. [RFC1123] Braden, R., "Requirements for Internet
Roessler, "MIME Security with OpenPGP", Hosts - Application and Support", STD 3,
RFC 3156, August 2001. RFC 1123, October 1989.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) [RFC1939] Myers, J. and M. Rose, "Post Office
Service Extension for Delivery Status Protocol - Version 3", STD 53, RFC 1939,
Notifications (DSNs)", RFC 3461, January 2003. May 1996.
[RFC3464] Moore, K. and G. Vaudreuil, "An Extensible [RFC2033] Myers, J., "Local Mail Transfer Protocol",
Message Format for Delivery Status RFC 2033, October 1996.
Notifications", RFC 3464, January 2003.
[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail [RFC2045] Freed, N. and N. Borenstein, "Multipurpose
Extensions (S/MIME) Version 3.1 Message Internet Mail Extensions (MIME) Part One:
Specification", RFC 3851, July 2004. Format of Internet Message Bodies",
RFC 2045, November 1996.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized [RFC2046] Freed, N. and N. Borenstein, "Multipurpose
Resource Identifiers (IRIs)", RFC 3987, Internet Mail Extensions (MIME) Part Two:
January 2005. Media Types", RFC 2046, November 1996.
[RFC4155] Hall, E., "The application/mbox Media Type", [RFC2047] Moore, K., "MIME (Multipurpose Internet
RFC 4155, September 2005. Mail Extensions) Part Three: Message Header
Extensions for Non-ASCII Text", RFC 2047,
November 1996.
[RFC4409] Gellens, R. and J. Klensin, "Message Submission [RFC2231] Freed, N. and K. Moore, "MIME Parameter
for Mail", RFC 4409, April 2006. Value and Encoded Word Extensions: Characte
r Sets, Languages, and Continuations",
RFC 2231, November 1997.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, [RFC2368] Hoffman, P., Masinter, L., and J. Zawinski,
"Review and Recommendations for "The mailto URL scheme", RFC 2368,
Internationalized Domain Names (IDNs)", July 1998.
RFC 4690, September 2006.
[RFC4952] Klensin, J. and Y. Ko, "Overview and Framework [RFC2821] Klensin, J., "Simple Mail Transfer
for Internationalized Email", RFC 4952, Protocol", RFC 2821, April 2001.
July 2007.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format [RFC3156] Elkins, M., Del Torto, D., Levien, R., and
for Network Interchange", RFC 5198, March 2008. T. Roessler, "MIME Security with OpenPGP",
RFC 3156, August 2001.
[RFC5228] Guenther, P. and T. Showalter, "Sieve: An Email [RFC3461] Moore, K., "Simple Mail Transfer Protocol
Filtering Language", RFC 5228, January 2008. (SMTP) Service Extension for Delivery
Status Notifications (DSNs)", RFC 3461,
January 2003.
[RFC5322] Resnick, P., Ed., "Internet Message Format", [RFC3464] Moore, K. and G. Vaudreuil, "An Extensible
RFC 5322, October 2008. Message Format for Delivery Status
Notifications", RFC 3464, January 2003.
[RFC5335] Abel, Y., "Internationalized Email Headers", [RFC3492] Costello, A., "Punycode: A Bootstring
RFC 5335, September 2008. encoding of Unicode for Internationalized
Domain Names in Applications (IDNA)",
RFC 3492, March 2003.
[RFC5336] Yao, J. and W. Mao, "SMTP Extension for [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS
Internationalized Email Addresses", RFC 5336, PROTOCOL - VERSION 4rev1", RFC 3501,
September 2008. March 2003.
[RFC5337] Newman, C. and A. Melnikov, "Internationalized [RFC3851] Ramsdell, B., "Secure/Multipurpose Internet
Delivery Status and Disposition Notifications", Mail Extensions (S/MIME) Version 3.1
RFC 5337, September 2008. Message Specification", RFC 3851,
July 2004.
[RFC5504] Fujiwara, K. and Y. Yoneya, "Downgrading [RFC3987] Duerst, M. and M. Suignard,
Mechanism for Email Address "Internationalized Resource Identifiers
Internationalization", RFC 5504, March 2009. (IRIs)", RFC 3987, January 2005.
[RFC5721] Gellens, R. and C. Newman, "POP3 Support for [RFC4155] Hall, E., "The application/mbox Media
UTF-8", RFC 5721, February 2010. Type", RFC 4155, September 2005.
[RFC5738] Resnick, P. and C. Newman, "IMAP Support for [RFC4409] Gellens, R. and J. Klensin, "Message
UTF-8", RFC 5738, March 2010. Submission for Mail", RFC 4409, April 2006.
[RFC5825] Fujiwara, K. and B. Leiba, "Displaying [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and
Downgraded Messages for Email Address IAB, "Review and Recommendations for
Internationalization", RFC 5825, April 2010. Internationalized Domain Names (IDNs)",
RFC 4690, September 2006.
[RFC5863] Hansen, T., Siegel, E., Hallam-Baker, P., and D. [RFC4952] Klensin, J. and Y. Ko, "Overview and
Crocker, "DomainKeys Identified Mail (DKIM) Framework for Internationalized Email",
Development, Deployment, and Operations", RFC 4952, July 2007.
RFC 5863, May 2010.
[RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left [RFC5198] Klensin, J. and M. Padlipsky, "Unicode
Scripts for Internationalized Domain Names for Format for Network Interchange", RFC 5198,
Applications (IDNA)", RFC 5893, June 2010. March 2008.
[RFC5228] Guenther, P. and T. Showalter, "Sieve: An
Email Filtering Language", RFC 5228,
January 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S.,
Boeyen, S., Housley, R., and W. Polk,
"Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5335] Abel, Y., "Internationalized Email
Headers", RFC 5335, September 2008.
[RFC5336] Yao, J. and W. Mao, "SMTP Extension for
Internationalized Email Addresses",
RFC 5336, September 2008.
[RFC5337] Newman, C. and A. Melnikov,
"Internationalized Delivery Status and
Disposition Notifications", RFC 5337,
September 2008.
[RFC5504] Fujiwara, K. and Y. Yoneya, "Downgrading
Mechanism for Email Address
Internationalization", RFC 5504,
March 2009.
[RFC5721] Gellens, R. and C. Newman, "POP3 Support
for UTF-8", RFC 5721, February 2010.
[RFC5738] Resnick, P. and C. Newman, "IMAP Support
for UTF-8", RFC 5738, March 2010.
[RFC5825] Fujiwara, K. and B. Leiba, "Displaying
Downgraded Messages for Email Address
Internationalization", RFC 5825,
April 2010.
[RFC5863] Hansen, T., Siegel, E., Hallam-Baker, P.,
and D. Crocker, "DomainKeys Identified Mail
(DKIM) Development, Deployment, and
Operations", RFC 5863, May 2010.
[RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left
Scripts for Internationalized Domain Names
for Applications (IDNA)", RFC 5893,
June 2010.
[Unicode-UAX15] The Unicode Consortium, "Unicode Standard
Annex #15: Unicode Normalization Forms",
March 2008,
<http://www.unicode.org/reports/tr15/>.
[Unicode52] The Unicode Consortium. The Unicode
Standard, Version 5.2.0, defined by:, "The
Unicode Standard, Version 5.2.0", (Mountain
View, CA: The Unicode Consortium,
2009. ISBN 978-1-936213-00-9)., <http://
www.unicode.org/versions/Unicode5.2.0/>.
Appendix A. Change Log Appendix A. Change Log
[[RFC Editor: Please remove this section prior to publication.]] [[RFC Editor: Please remove this section prior to publication.]]
A.1. Changes between -00 and -01 A.1. Changes between -00 and -01
o Because there has been no feedback on the mailing list, updated o Because there has been no feedback on the mailing list, updated
the various questions to refer to this version as well. the various questions to refer to this version as well.
o Reflected RFC Editor erratum #1507 by correcting terminology for o Reflected RFC Editor erratum #1507 by correcting terminology for
headers and header fields and distinguishing between "message headers and header fields and distinguishing between "message
headers" and different sorts of headers (e.g., the MIME ones). headers" and different sorts of headers (e.g., the MIME ones).
o Merged earlier sections 4.4 and 4.6 into an expanded Section 4.4. A.2. Changes between -01 and -02
o Merged earlier Section 11.6 into Section 11.2 and eliminated the Note that section numbers in the list that follows may refer to -01
note in draft. and not -02.
o Eliminated former last paragraph of Section 11.4 as an artifact of o Discussion of RFC 5825 ("downgraded display") has been removed per
in-transit downgrading. the earlier note and on-list discussion. Any needed discussion
about reconstructed messages will need to appear in the IMAP and
POP documents. However, the introductory material has been
reworded to permit keeping 5504 and 5825 on the list there,
without which the back chain would not be complete. For
consistency with this change, 5504 and 5825 have been added to the
"Obsoletes" list (as far as I know, an Informational spec can
obsolete or update Experimental ones, so no downref problem here
--JcK).
o Updated a few references. o Reference to alternate addresses dropped from (former) Section
7.1.
o Reference to RFC 5504 added to (former) Section 8 for
completeness.
o Ernie's draft comments added (with some minor edits) to replace
the placeholder in (former) Section 9 ("Downgrading in Transit").
It is intended to capture at least an introduction the earlier
discussions of algorithmic downgrading generally and ACE/Punycode
transformations in particular. Anyone who is unhappy with it
should say so and propose alternate text. RSN.
o In the interest of clarity and consistency with the terminology in
Section 4.1, all uses of "final delivery SMTP server" and "final
delivery server" have been changed to "final delivery MTA".
o Placeholder at the end of Section 2 has been removed and the text
revised to promise less. The "Document Plan" (Section 5) has been
revised accordingly. We need to discuss this at IETF 78 if not
sooner.
o Sections 5 and 6 have been collapsed into one -- there wasn't
enough left in the former Section 5 to justify a separate section.
o Former Section 11.1 has been dropped and the DSN document moved up
into the "Document Plan" as suggested earlier.
o Section 12, "Experimental Targets", has been removed.
o Updated references for the new version EAI documents and added
placeholders for all of the known remaining drafts that will
become part of the core EAI series but that have not been written.
o Inserted an additional clarification about the relationship of
these extensions to non-ASCII messages.
o Changed some normative/informative reference classifications based
on review of the new text.
o Removed references to the pre-EAI documents that were cited for
historical context in 4952.
o Got rid of a remaining pointer to address downgrading in the
discussion of an updated MAILTO URI.
o Minor additional editorial cleanups and tuning.
Authors' Addresses Authors' Addresses
John C Klensin John C KLENSIN
1770 Massachusetts Ave, #322 1770 Massachusetts Ave, #322
Cambridge, MA 02140 Cambridge, MA 02140
USA USA
Phone: +1 617 491 5735 Phone: +1 617 491 5735
EMail: john-ietf@jck.com EMail: john-ietf@jck.com
YangWoo Ko YangWoo KO
ICU ICU
119 Munjiro 119 Munjiro
Yuseong-gu, Daejeon 305-732 Yuseong-gu, Daejeon 305-732
Republic of Korea Republic of Korea
EMail: yw@mrko.pe.kr EMail: yw@mrko.pe.kr
 End of changes. 121 change blocks. 
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