Internet Engineering Task Force (IETF) A. Gulbrandsen
Request for Comments: 6858 March 2013
Updates: 3501
Category: Standards Track
ISSN: 2070-1721
Internet Engineering Task Force (IETF) A. Gulbrandsen
Request for Comments: 6858 March 2013
Updates: 3501
Category: Standards Track
ISSN: 2070-1721
Simplified POP and IMAP Downgrading for Internationalized Email
针对国际化电子邮件简化POP和IMAP降级
Abstract
摘要
This document specifies a method for IMAP and POP servers to serve internationalized messages to conventional clients. The specification is simple, easy to implement, and provides only rudimentary results.
本文档指定了IMAP和POP服务器向传统客户端提供国际化消息的方法。该规范简单、易于实现，并且只提供基本的结果。
Status of This Memo
关于下段备忘
This is an Internet Standards Track document.
这是一份互联网标准跟踪文件。
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
本文件是互联网工程任务组（IETF）的产品。它代表了IETF社区的共识。它已经接受了公众审查，并已被互联网工程指导小组（IESG）批准出版。有关互联网标准的更多信息，请参见RFC 5741第2节。
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6858.
有关本文件当前状态、任何勘误表以及如何提供反馈的信息，请访问http://www.rfc-editor.org/info/rfc6858.
Copyright Notice
版权公告
Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.
版权所有（c）2013 IETF信托基金和确定为文件作者的人员。版权所有。
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
本文件受BCP 78和IETF信托有关IETF文件的法律规定的约束(http://trustee.ietf.org/license-info)自本文件出版之日起生效。请仔细阅读这些文件，因为它们描述了您对本文件的权利和限制。从本文件中提取的代码组件必须包括信托法律条款第4.e节中所述的简化BSD许可证文本，并提供简化BSD许可证中所述的无担保。
A conventional IMAP or POP client may open a mailbox containing internationalized messages or may even attempt to read internationalized messages, for instance, when a user has both internationalized and conventional Mail User Agents (MUAs).
例如，当用户同时拥有国际化和传统邮件用户代理（MUA）时，传统IMAP或POP客户端可以打开包含国际化邮件的邮箱，甚至可以尝试读取国际化邮件。
Some operations cannot be performed by conventional clients. Most importantly, an internationalized message usually contains at least one internationalized address, so address-based operations are rarely possible. This includes displaying the addresses, replying to messages, and the processing of most types of address-based signature or security.
某些操作无法由传统客户端执行。最重要的是，国际化消息通常至少包含一个国际化地址，因此很少可能进行基于地址的操作。这包括显示地址、回复消息以及处理大多数类型的基于地址的签名或安全性。
However, the sender's name, message subject, body of text, and attachments can easily be displayed, so a helpful IMAP or POP server may prefer to display as much of the message as possible, rather than hide the message entirely.
但是，发件人的姓名、邮件主题、正文和附件可以很容易地显示出来，因此有帮助的IMAP或POP服务器可能更愿意显示尽可能多的邮件，而不是完全隐藏邮件。
This document specifies a way to present such messages to the client. It values simplicity of implementation over fidelity of representation, since implementing a high-fidelity downgrade algorithm such as the one specified in a companion document [RFC6857] is likely more work than implementing proper UTF-8 support for POP [RFC6856] and/or IMAP [RFC6855].
本文档指定了向客户机呈现此类消息的方式。它重视实现的简单性而不是表示的保真度，因为实现高保真降级算法（如配套文档[RFC6857]中指定的算法）可能比实现对POP[RFC6856]和/或IMAP[RFC6855]的适当UTF-8支持要多。
The server is assumed to be internationalized internally and to store messages that are internationalized messages natively. When it needs to present an internationalized message to a conventional client, the server synthesizes a conventional message containing most of the information and presents the "surrogate message".
假定服务器在内部国际化，并存储本机国际化的消息。当服务器需要向传统客户端呈现国际化消息时，服务器合成包含大部分信息的传统消息并呈现“代理消息”。
This specification modifies the base IMAP specification [RFC3501] by relaxing a requirement that sizes be exact and adding a reporting requirement as discussed in Section 3 below.
本规范修改了基本IMAP规范[RFC3501]，放宽了尺寸精确的要求，并增加了报告要求，如下文第3节所述。
The surrogate message is intended to convey the most important information to the user. Where information is lost, the user should consider the message incomplete rather than modified.
代理消息旨在向用户传达最重要的信息。当信息丢失时，用户应该考虑消息不完整而不是修改。
The surrogate message is not intended to convey any information to the client software that would require or enable it to apply special handling to the message. Client authors who wish to handle internationalized messages are encouraged to implement POP [RFC6856] and/or IMAP [RFC6855] support for UTF-8.
代理消息并不打算向客户端软件传递任何需要或使其能够对消息应用特殊处理的信息。鼓励希望处理国际化消息的客户作者为UTF-8实现POP[RFC6856]和/或IMAP[RFC6855]支持。
Uppercase letters in examples represent non-ASCII characters. example.com is a plain domain; EXAMPLE.com represents a non-ASCII domain in the .com top-level domain.
示例中的大写字母表示非ASCII字符。example.com是一个普通域；EXAMPLE.com表示.com顶级域中的非ASCII域。
Each internationalized email address in the header fields listed below is replaced with an invalid email address whose display-name tells the user what happened.
下面列出的标题字段中的每个国际化电子邮件地址都将替换为一个无效电子邮件地址，该地址的显示名称会告诉用户发生了什么。
The format of the display-name is explicitly unspecified. Anything that tells the user what happened is good. Anything that produces an email address that might belong to someone else is bad.
显示名称的格式未明确指定。任何告诉用户发生了什么的事情都是好的。任何产生可能属于其他人的电子邮件地址的东西都是不好的。
Given an internationalized address "Fred Foo <fred@EXAMPLE.com>", an implementation may choose to render it as one of these examples:
给了一个国际化的地址“Fred Foo”<fred@EXAMPLE.com>“，实现可以选择将其呈现为以下示例之一：
"fred@EXAMPLE.com" <invalid@internationalized-address.invalid>
Fred Foo <invalid@internationalized.invalid>
internationalized-address:;
fred:;
"fred@EXAMPLE.com" <invalid@internationalized-address.invalid>
Fred Foo <invalid@internationalized.invalid>
internationalized-address:;
fred:;
The .invalid top-level domain is reserved as a Top Level DNS Name [RFC2606]; therefore, the first two examples are syntactically valid, but they will never belong to anyone. Note that the display-name often needs encoding (see the Message Header Extensions document [RFC2047]).
无效的顶级域保留为顶级DNS名称[RFC2606]；因此，前两个例子在语法上是有效的，但它们永远不会属于任何人。请注意，显示名称通常需要编码（请参阅消息头扩展文档[RFC2047]）。
The affected header fields are "Bcc:", "Cc:", "From:", "Reply-To:", "Resent-Bcc:", "Resent-Cc:", "Resent-From:", "Resent-Sender:", "Resent-To:", "Return-Path:", "Sender:", and "To:". Any addresses present in other header fields, such as "Received:", are not regarded as addresses by this specification.
受影响的标题字段为“密件抄送:”、“抄送:”、“答复:”、“重新发送密件抄送:”、“重新发送发件人:”、“重新发送至:”、“返回路径:”、“发件人：”、“收件人：”。其他标头字段（如“Received:”）中存在的任何地址均不被本规范视为地址。
Any MIME parameter [RFC2045] (whether in the message header or a body part header) that cannot be presented to the client exactly as it appears in the incoming message is silently excised.
任何MIME参数[RFC2045]（无论是在消息头中还是在正文部分头中）如果不能完全按照传入消息中的显示方式呈现给客户端，则会以静默方式删除。
Given a field such as
给定一个字段，如
Content-Disposition: attachment; filename=FOO
Content-Disposition: attachment; filename=FOO
the field is presented as
该字段显示为
Content-Disposition: attachment
内容配置：附件
If the Subject field cannot be presented to the client exactly as it appears in the incoming message, the server presents a representation encoded as specified in RFC 2047.
如果主题字段不能完全按照传入消息中的显示方式呈现给客户端，则服务器将呈现RFC 2047中指定的编码表示。
Any header field that cannot be presented to the client, even with the modifications listed in Sections 2.1-2.3, is silently excised.
即使在第2.1-2.3节中列出了修改，也无法显示给客户机的任何标题字段都会被自动删除。
IMAP allows clients to retrieve the message size without downloading the message, using RFC822.SIZE, BODY.SIZE[] and so on. The IMAP specification [RFC3501] requires that the returned size be exact.
IMAP允许客户端使用RFC822.size、BODY.size[]等在不下载消息的情况下检索消息大小。IMAP规范[RFC3501]要求返回的大小精确。
This specification relaxes that requirement. When a conventional client requests size information for a message, the IMAP server is permitted to return size information for the internationalized message, even though the size of the surrogate message differs.
本规范放宽了这一要求。当传统客户端请求消息的大小信息时，允许IMAP服务器返回国际化消息的大小信息，即使代理消息的大小不同。
When an IMAP server performs downgrading as part of generating FETCH responses, it reports which messages were synthesized using a response code and attendant UID (Unique Identifier) set. This can be helpful to humans debugging the server and/or client.
当IMAP服务器在生成获取响应的过程中执行降级时，它会报告使用响应代码和助理UID（唯一标识符）集合成的消息。这有助于调试服务器和/或客户机。
C: a UID FETCH 1:* BODY.PEEK[HEADER.FIELDS(To From Cc)]
S: 1 FETCH (UID 65 [...]
S: 2 FETCH (UID 70 [...]
S: a OK [DOWNGRADED 70,105,108,109] Done
C: a UID FETCH 1:* BODY.PEEK[HEADER.FIELDS(To From Cc)]
S: 1 FETCH (UID 65 [...]
S: 2 FETCH (UID 70 [...]
S: a OK [DOWNGRADED 70,105,108,109] Done
The message-set argument to DOWNGRADED contains UIDs.
要降级的消息集参数包含UID。
Note that DOWNGRADED does not necessarily mention all the internationalized messages in the mailbox. In the example above, we know that UID 65 does not contain internationalized addresses in the "From:", "To:", and "Cc:" fields. It may, for example, contain an internationalized "Subject:".
请注意，降级并不一定会提及邮箱中的所有国际化邮件。在上面的示例中，我们知道UID65在“From:”、“To:”和“Cc:”字段中不包含国际化地址。例如，它可能包含一个国际化的“主题：”。
The number of lines specified in the TOP command [RFC1939] refers to the surrogate message. The message size reported by, for example, LIST may refer to either the internationalized or the surrogate message.
TOP命令[RFC1939]中指定的行数表示代理消息。例如，列表报告的消息大小可能引用国际化消息或代理消息。
If the internationalized message uses any sort of signature that covers header fields, the signature of the surrogate message almost certainly is invalid and may be invalid in other cases. This is a necessary limitation of displaying internationalized messages in legacy clients, since those clients do not support internationalized header fields. These cases are discussed in more detail in the POP or IMAP Downgrade document [RFC6857]. Even though invalid, these signatures should not be removed from the surrogate message, to preserve as much of the information as possible from the original message.
如果国际化消息使用任何类型的签名（包括头字段），那么代理消息的签名几乎肯定是无效的，并且在其他情况下可能是无效的。这是在旧客户端中显示国际化消息的必要限制，因为这些客户端不支持国际化头字段。POP或IMAP降级文档[RFC6857]中详细讨论了这些情况。即使这些签名无效，也不应从代理消息中删除，以尽可能保留原始消息中的信息。
If any excised information is significant, then that information does not arrive at the recipient. Notably, the "Message-Id:", "In-Reference-To:", and "References:" fields may be excised, which might cause a lack of context when the recipient reads the message.
如果任何删减的信息是重要的，则该信息不会到达收件人。值得注意的是，“Message Id:”、“In Reference:”和“References:”字段可能会被删除，这可能会导致收件人在阅读邮件时缺少上下文。
Some POP or IMAP clients, such as Fetchmail, download messages and delete the versions on the server. This may lead to permanent loss of information when the only remaining version of a message is the surrogate message.
一些POP或IMAP客户端（如Fetchmail）下载邮件并删除服务器上的版本。当消息的唯一剩余版本是代理消息时，这可能会导致信息永久丢失。
Other clients cache messages for a very long time, even across client upgrades, such as the stock Android client. When such a client is internationalized, care must be taken so that it does not use an old surrogate message from its cache rather than retrieve the real message from the server.
其他客户端会将消息缓存很长时间，甚至可以跨客户端升级，例如stock Android客户端。当这样的客户机被国际化时，必须小心，使其不会从缓存中使用旧的代理消息，而不是从服务器检索真正的消息。
IANA has added DOWNGRADED to the "IMAP Response Codes" registry.
IANA已将降级添加到“IMAP响应代码”注册表中。
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, May 1996.
[RFC1939]迈尔斯，J.和M.罗斯，“邮局协议-第3版”，STD 53，RFC 1939，1996年5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。
[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。
[RFC2606] Eastlake, D., 3rd and A. Panitz, "Reserved Top Level DNS Names", BCP 32, RFC 2606, June 1999.
[RFC2606]Eastlake，D.，3rd and A.Panitz，“保留顶级域名”，BCP 32，RFC 26061999年6月。
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003.
[RFC3501]Crispin，M.，“互联网消息访问协议-版本4rev1”，RFC 35012003年3月。
[RFC1925] Callon, R., "The Twelve Networking Truths", RFC 1925, April 1 1996.
[RFC1925]卡隆，R.，“十二个网络真理”，RFC19251996年4月1日。
[RFC6855] Resnick, P., Ed., Newman, C., Ed., and S. Shen, Ed., "IMAP Support for UTF-8", RFC 6855, March 2013.
[RFC6855]Resnick，P.，Ed.，Newman，C.，Ed.，和S.Shen，Ed.，“UTF-8的IMAP支持”，RFC 68552013年3月。
[RFC6856] Gellens, R., Newman, C., Yao, J., and K. Fujiwara, "Post Office Protocol Version 3 (POP3) Support for UTF-8", RFC 6856, March 2013.
[RFC6856]Gellens，R.，Newman，C.，Yao，J.，和K.Fujiwara，“邮局协议版本3（POP3）对UTF-8的支持”，RFC 68562013年3月。
[RFC6857] Fujiwara, K., "Post-Delivery Message Downgrading for Internationalized Email Messages", RFC 6857, March 2013.
[RFC6857]Fujiwara，K.，“国际化电子邮件的投递后邮件降级”，RFC 6857，2013年3月。
Claudio Allocchio, Ned Freed, Kazunori Fujiwara, Ted Hardie, John Klensin, Barry Leiba, John Levine, Alexey Melnikov, Chris Newman, and Joseph Yee. This specification was inspired by the principle stated in Rule 12 of "The Twelve Networking Truths" [RFC1925].
克劳迪奥·阿洛乔、内德·弗里德、卡祖诺里·藤原一郎、特德·哈迪、约翰·克莱辛、巴里·莱巴、约翰·莱文、阿列克谢·梅尔尼科夫、克里斯·纽曼和约瑟夫·耶伊。本规范受“十二个网络真理”[RFC1925]第12条规定的原则启发。
Author's Address
作者地址
Arnt Gulbrandsen Schweppermannstr. 8 D-81671 Muenchen Germany
阿恩特·古尔布兰森·施韦珀·曼斯特。德国慕尼黑8D-81671
Fax: +49 89 4502 9758
EMail: arnt@gulbrandsen.priv.no
Fax: +49 89 4502 9758
EMail: arnt@gulbrandsen.priv.no