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

Overview and Framework for Internationalized Email


Status of This Memo


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


Copyright Notice


Copyright (C) The IETF Trust (2007).




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


Table of Contents


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

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

为了使用国际化的电子邮件地址,我们需要对电子邮件地址的域部分和本地部分进行国际化。电子邮件地址的域部分已国际化[RFC3490],而本地部分未国际化。如果没有本文档中指定的扩展名,邮箱名称仅限于7位ASCII[RFC2821]的子集。尽管MIME[RFC2045]支持非ASCII数据的传输,但它不提供国际化电子邮件地址的机制。在RFC 2047[RFC2047]中,MIME为一些特定的消息头字段定义了一种编码机制,以容纳非ASCII数据。但是,它不允许使用包含非ASCII字符的电子邮件地址。如果没有此处定义的扩展名或某些等效集,在电子邮件地址的任何部分中包含非ASCII字符的唯一方法是使用RFC 2047编码将它们嵌入到RFC 2822[RFC2822]所称的相关标题的“显示名称”(称为“名称短语”或其他术语)。编码到显示名称中的信息在消息信封中是不可见的,并且出于许多目的,根本不是地址的一部分。

1.1. Role of This Specification
1.1. 本规范的作用

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


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


1.2. Problem Statement
1.2. 问题陈述

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


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


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


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

电子邮件地址的国际化不仅仅是更改SMTP信封的问题;或者修改From、To和Cc头;或者允许升级的邮件用户代理(MUA)解码特殊编码并通过显示本地字符进行响应。要被认为是可用的,地址必须国际化,并在它们出现的所有上下文中一致地处理。这一要求有着深远的影响:补丁和解决方案的集合是不够的。即使它们是足够的,基于解决方案的方法可能会导致应用了不同补丁集和解决方案的各种实现,从而导致用户对什么是实际可用和支持的感到困惑。相反,我们需要建立一个完全国际化的电子邮件环境,重点是允许那些共享一种语言或其他社区的人之间进行有效沟通。这反过来意味着对邮件头环境的更改,以允许在有意义的情况下使用全部Unicode字符,SMTP扩展允许UTF-8[RFC3629]邮件寻址和传递这些扩展头,以及(最后)支持8BITMIME SMTP扩展[RFC1652]的要求因此,所有这些都可以通过邮件系统进行传输,而无需克服标头没有内容传输编码的限制。

1.3. Terminology
1.3. 术语

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


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


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

但是,本文件中使用的术语“最终交付MTA”与RFC 2821中的术语“交付系统”或“最终交付系统”相同。这是SMTP服务器,控制地址的本地部分的格式,并允许检查和解释它们。它接收来自网络的消息,以便发送到邮箱或进行其他本地处理,包括更改信封地址的任何转发或别名,而不是中继。从网络的角度来看,任何本地传递安排(如保存到邮件存储、切换到特定邮件传递程序或代理以及检索邮件的机制)都是在最终传递MTA之后,因此不属于SMTP传输或传递过程的一部分。

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


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


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


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


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


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


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


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

如RFC 2821中所述,由于某种原因无法传递的消息预计会导致通知发送方。这可以通过两种方式之一发生。当SMTP服务器返回指示致命错误的回复代码(“5yz”代码)或持续返回临时故障错误(“4yz”代码)时,会发生一种通常称为“拒绝”的情况。另一种是在SMTP处理过程中接受邮件,然后向发件人生成邮件,通常称为“未送达通知”或“NDN”。目前的做法通常倾向于拒绝NDN,因为生成NDN作为垃圾邮件技术的可能性降低。如果中间MTA接受消息,然后被下一跳服务器拒绝,则后一种情况(NDN)是不可避免的。

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


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

本文件中的关键词“必须”、“应”、“要求”、“应”、“建议”和“可能”应按照RFC 2119[RFC2119]中的说明进行解释。

2. Overview of the Approach
2. 方法概述

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


3. Document Plan
3. 文件计划

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


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

o SMTP扩展。此文档[EAI SMTPext]为RFC 2821中提供的国际化地址提供SMTP扩展。

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

o UTF-8中的电子邮件标题。本文档[EAI-UTF8]实质上更新了RFC 2822,以允许在使用上述SMTP扩展时,电子邮件头中的某些信息直接由UTF-8编码的Unicode字符表示。本文档(可能有一个或多个补充文档)还需要解决与MIME的交互,包括UTF8SMTP与内部MIME头和内容类型之间的关系。

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

o 在传输过程中,从使用SMTP扩展名和UTF-8头的国际化寻址降级为传统电子邮件格式和字符[EAI降级]。无论是在邮件发起点还是在最终传递SMTP服务器成功接收邮件后,降级都涉及不同的限制和可能性;见下文第4.3节和第5节。在这样的最终传递之后发生的处理,特别是和传递到邮箱或消息存储有关的处理,有时称为“消息传递”处理。

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

o 对IMAP协议的扩展,以支持国际化标头[EAI IMAP]。

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

o POP协议的并行扩展[EAI POP]。

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

o 交付通知(DSN)国际化更改的说明[EAI-DSN]。

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

o 这些协议的使用场景[EAI场景]。

4. Overview of Protocol Extensions and Changes
4. 协议扩展和更改概述
4.1. SMTP Extension for Internationalized Email Address
4.1. 国际化电子邮件地址的SMTP扩展

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


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

o 允许在电子邮件地址(本地部分和域名)中使用UTF-8字符串。

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

o 允许在电子邮件头中选择性使用UTF-8字符串(见第4.2节)。

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

o 要求服务器播发8BITMIME扩展[RFC1652],并且客户端支持8位传输,以便可以在不使用特殊内容传输编码的情况下传输头信息。

o Provides information to support downgrading mechanisms.

o 提供支持降级机制的信息。

Some general principles affect the development decisions underlying this work.


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

1. 电子邮件地址输入可能执行字符集转换或其他编码更改的子系统(如用户界面)。当地址的左侧包含US-ASCII字符表以外的字符时,不鼓励在右侧使用punycode,以促进整个地址中字符的一致处理。

2. An SMTP relay must

2. 必须使用SMTP中继

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

* 明确识别格式,同意通过ESMTP选项进行识别,

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

* 选择并使用仅ASCII地址,根据需要降级其他信息(见第4.3节),或

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

* 拒绝邮件,或在必要时返回未送达通知邮件,以便发件人可以制定其他计划。

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


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

3. 为了实现互操作性,邮件地址和邮件头中禁止使用UTF-8以外的字符集。如果不引入极大的复杂性,那么使用类似于此的扩展来正确识别它们是没有实际方法的。

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

要符合此处为电子邮件传输和传递指定的一组标准,需要实施SMTP扩展规范,包括识别与备用地址相关的关键字,以及UTF-8标头规范。不需要支持降级,但如果实施,则必须按照规定实施。同样,如果系统实现IMAP或POP,则必须分别符合i18n IMAP或POP规范。

4.2. Transmission of Email Header Fields in UTF-8 Encoding
4.2. UTF-8编码中电子邮件头字段的传输

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


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


4.3. Downgrading Mechanism for Backward Compatibility
4.3. 用于向后兼容性的降级机制

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


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


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

o 拒绝邮件或生成并发送未送达邮件,要求发件人使用传统格式的地址和邮件头重新提交邮件。

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

o 找出一种在传输过程中降级信封或邮件正文的方法。特别是当涉及国际化地址时,降级将要求从某个来源获取所有ASCII地址。提供可选扩展参数作为传输备用地址的方式。降级问题和规范在[EAI降级]中讨论。

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


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


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


5. Downgrading before and after SMTP Transactions
5. SMTP事务前后降级

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


5.1. Downgrading before or during Message Submission
5.1. 在邮件提交之前或提交期间降级

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


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


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


5.2. Downgrading or Other Processing After Final SMTP Delivery
5.2. 最终SMTP传递后降级或其他处理

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


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


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


6. Additional Issues
6. 其他问题

This section identifies issues that are not covered as part of this set of specifications, but that will need to be considered as part of deployment of email address and header internationalization.


6.1. Impact on URIs and IRIs
6.1. 对URI和IRIs的影响

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.


6.2. Interaction with Delivery Notifications
6.2. 与交付通知的交互

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


6.3. Use of Email Addresses as Identifiers
6.3. 使用电子邮件地址作为标识符

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


6.4. Encoded Words, Signed Messages, and Downgrading
6.4. 编码字、签名消息和降级

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

电子邮件格式的一个特殊特征是它的持久性:MUA被期望处理几十年前最初发送的消息,而不仅仅是几秒钟前发送的消息。因此,MUA和邮件过滤软件(如筛[RFC3028]中指定的)将需要继续接受和解码使用“编码字”机制[RFC2047]的标题字段,以在某些标题字段中容纳非ASCII字符。虽然已经提出了对POP3和IMAP的扩展,以支持POP3或IMAP服务器对消息进行自动EAI升级(包括RFC 2047解码),但仍存在无法完成的消息结构和MIME内容类型,或者更改会产生不可接受的副作用。

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

例如,使用S/MIME[RFC3851]或Pretty Good Privacy(PGP)[RFC3156]等加密签名的消息部分无法在不破坏签名的情况下从RFC 2047表单升级为正常UTF-8字符。类似地,加密的消息部分在解密时可包含使用RFC 2047编码的报头字段;如果不访问加密密钥,则无法“完全”升级此类消息。

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


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

主标题或MIME bodypart标题。如果存在签名,则必须非常小心地进行降级。

6.5. Other Uses of Local Parts
6.5. 本地零件的其他用途

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


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

除其他外,此类方案显然受到域名SMTP规则的限制,如果没有对其他本地部分(如[EAI-utf8]中指定的<utf8 local part>)的进一步限制,这些方案将无法工作。这个问题是否与这些规范相关是一个悬而未决的问题。这可能只是传递MTA在确定他们将接受的邮箱名称以及如何解释这些名称时具有相当大的灵活性的另一个例子。

6.6. Non-Standard Encapsulation Formats
6.6. 非标准封装格式

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

一些应用程序使用与[RFC4155]中定义的应用程序/mbox格式类似的格式,而不是第5.1.5节[RFC2046]表单中的消息/摘要RFC 2046,将多条消息作为单个单元传输。如果此类应用程序假定所有存储的消息都使用带有US-ASCII头的message/rfc822 RFC 2046第5.2.1节[RFC2046]格式,则这些应用程序尚未准备好进行本系列文档中指定的扩展,可能需要采取特殊措施来正确检测和处理这些消息。

7. Experimental Targets
7. 实验目标

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


8. IANA Considerations
8. IANA考虑

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


9. Security Considerations
9. 安全考虑

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


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


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

电子邮件地址和邮件头的国际化不能使互联网的安全性低于没有所需扩展的情况。这组规范中记录的需求和机制通常不会引起任何新的安全问题。它们确实需要审查与可混淆字符相关的问题——这是一个在其他地方正在深入探讨的主题(参见,例如,[RFC4690])——以及[RFC3629]中讨论的UTF-8规范化的一些潜在问题和其他转换。规范化和其他与转换和标准表单相关的问题也是[Net Unicode]、[IDNAbis BIDI]和其他地方正在进行的工作主题的一部分。本集中的其他文档将更详细地讨论与国际化地址和标题相关的一些问题。但是,特别要注意的是,任何“降级”机制或使用降级地址都不会不适当地假定国际化地址和ASCII地址之间存在经过身份验证的绑定。

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


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


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


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


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


10. Acknowledgements
10. 致谢

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

本文件及相关文件最初来源于John Klensin和JET集团[Klensin emailaddr],[JET-IMA]的文件。这项工作的灵感来自互联网邮件联盟赞助的“IMAA”邮件列表讨论,特别是保罗·霍夫曼(Paul Hoffman)和亚当·科斯特洛(Adam Costello)[Hoffman IMAA]的早期文件,该文件试图为地址国际化问题定义一个只有MUA的解决方案。

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

最近的文件得益于IETF EAI工作组内的大量讨论,特别是Martin Duerst、Frank Ellermann、Philip Guenther、Kari Hurtta和Alexey Melnikov提供的建议和文本,以及IETF EAI工作组之间的广泛讨论

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

第3节引用的核心文件的编辑和作者:Harald Alvestrand、Kazunori Fujiwara、Chris Newman、Pete Resnick、姚建康、Jeff Yeh和Yoseya Yoshiro。

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

IETF上次通话期间收到的其他评论,包括保罗·霍夫曼(Paul Hoffman)和罗伯特·斯帕克斯(Robert Sparks)的评论,有助于使文件更加清晰和全面。

11. References
11. 工具书类
11.1. Normative References
11.1. 规范性引用文件

[ASCII] American National Standards Institute (formerly United States of America Standards Institute), "USA Code for Information Interchange", ANSI X3.4-1968, 1968.

[ASCII]美国国家标准协会(前美国标准协会),“美国信息交换代码”,ANSI X3.4-1968,1968年。

ANSI X3.4-1968 has been replaced by newer versions with slight modifications, but the 1968 version remains definitive for the Internet.

ANSI X3.4-1968已被稍作修改的较新版本所取代,但1968年版本仍然是互联网的最终版本。

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

[RFC1652]Klensin,J.,Freed,N.,Rose,M.,Stefferud,E.,和D.Crocker,“8bit MIMEtransport的SMTP服务扩展”,RFC 16521994年7月。

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

[RFC2119]Bradner,S.,“RFC中用于表示需求水平的关键词”,RFC 2119,BCP 14,1997年3月。

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

[RFC2821]Klensin,J.,“简单邮件传输协议”,RFC 28212001年4月。

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

[RFC3490]Faltstrom,P.,Hoffman,P.,和A.Costello,“应用程序中的域名国际化(IDNA)”,RFC 34902003年3月。

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

[RFC3629]Yergeau,F.,“UTF-8,ISO 10646的转换格式”,STD 63,RFC 3629,2003年11月。

11.2. Informative References
11.2. 资料性引用

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

[DKIM章程]IETF,“域密钥识别邮件(DKIM)”,2006年10月< html.charters/dkim charter.html>。

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


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

[EAI SMTPext]Yao,J.,Ed.和W.Mao,Ed.,“国际化电子邮件地址的SMTP扩展”,正在进行的工作,2007年6月。

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


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


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

[EAI imap]Resnick,P.和C.Newman,“对UTF-8的imap支持”,正在进行的工作,2007年3月。

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

[EAI pop]Newman,C.,“对UTF-8的POP3支持”,正在进行的工作,2007年1月。

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


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

[Hoffman IMAA]Hoffman,P.和A.Costello,“应用程序中的邮件地址国际化(IMAA)”,正在进行的工作,2003年10月。

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

[IDNAbis BIDI]Alvestrand,H.和C.Karp,“从右到左脚本中的IDNA问题”,正在进行的工作,2006年10月。

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


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

[Klensin emailaddr]Klensin,J.,“电子邮件地址的国际化”,正在进行的工作,2005年7月。

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

[Net Unicode]Klensin,J.和M.Padlipsky,“网络交换的Unicode格式”,正在进行的工作,2007年3月。

[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996.

[RFC2045]Freed,N.和N.Borenstein,“多用途Internet邮件扩展(MIME)第一部分:Internet邮件正文格式”,RFC 20451996年11月。

[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, November 1996.

[RFC2046]Freed,N.和N.Borenstein,“多用途Internet邮件扩展(MIME)第二部分:媒体类型”,RFC 20461996年11月。

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

[RFC2047]Moore,K.,“MIME(多用途互联网邮件扩展)第三部分:非ASCII文本的消息头扩展”,RFC 2047,1996年11月。

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

[RFC2231]Freed,N.和K.Moore,“MIME参数值和编码字扩展:字符集、语言和连续体”,RFC 22311997年11月。

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

[RFC2368]Hoffman,P.,Masinter,L.,和J.Zawinski,“邮件URL方案”,RFC 2368,1998年7月。

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

[RFC2822]Resnick,P.,“互联网信息格式”,RFC 2822,2001年4月。

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


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

[RFC3156]Elkins,M.,Del Torto,D.,Levien,R.,和T.Roessler,“OpenPGP的MIME安全性”,RFC 3156,2001年8月。

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

[RFC3461]Moore,K.,“用于传递状态通知(DSN)的简单邮件传输协议(SMTP)服务扩展”,RFC 3461,2003年1月。

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

[RFC3464]Moore,K.和G.Vaudreuil,“交付状态通知的可扩展消息格式”,RFC 3464,2003年1月。

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

[RFC3851]Ramsdell,B.,“安全/多用途Internet邮件扩展(S/MIME)版本3.1消息规范”,RFC 38512004年7月。

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

[RFC3987]Duerst,M.和M.Suignard,“国际化资源标识符(IRIs)”,RFC 3987,2005年1月。

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

[RFC4155]Hall,E.“应用程序/mbox媒体类型”,RFC 4155,2005年9月。

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

[RFC4409]Gellens,R.和J.Klensin,“邮件邮件提交”,RFC 4409,2006年4月。

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

[RFC4690]Klensin,J.,Faltstrom,P.,Karp,C.,和IAB,“国际化域名(IDN)的审查和建议”,RFC 46902006年9月。

Authors' Addresses


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


   Phone: +1 617 491 5735
   Phone: +1 617 491 5735

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

大韩民国大田市阳湖ICU 119 Munjiro Yuseong gu 305-732


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