Internet Engineering Task Force (IETF) A. Gulbrandsen Request for Comments: 6858 January 2013 Updates: 3501 Category: Standards Track ISSN: 2070-1721 Simplified POP/IMAP Downgrading for Internationalized Email Abstract 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. Status of This Memo This is an Internet Standards Track document. 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. 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. Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. 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. Gulbrandsen Standards Track [Page 1] RFC 6858 Simple POP/IMAP for Internationalized Email January 2013 1. Overview 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). 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, and most types of address-based signature or security processing. However, the sender's name, message subject, body of text, and attachments can easily be displayed, so a helpful IMAP/POP server may prefer to display as much of the message as possible, rather than hide the message entirely. 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 is likely more work than implementing proper UTF-8 support for POP [RFC6856] and/or IMAP [RFC6855]. The server is assumed to be internationalized internally and to store messages 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 "synthetic message". The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. Information Preserved and Lost The synthetic 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. The synthetic 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. Gulbrandsen Standards Track [Page 2] RFC 6858 Simple POP/IMAP for Internationalized Email January 2013 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. 2.1. Email Addresses 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. 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. Given an internationalized address "Fred Foo ", an implementation may choose to render it as one of these examples: "fred@EXAMPLE.com" Fred Foo internationalized-address:; fred:; The .invalid top-level domain is reserved by [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 [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. 2.2. MIME Parameters Any MIME parameter [RFC2045] (whether in the message header or a body part header) that cannot be presented as is to the client is silently excised. Given a field such as Content-Disposition: attachment; filename=FOO the field is presented as Content-Disposition: attachment Gulbrandsen Standards Track [Page 3] RFC 6858 Simple POP/IMAP for Internationalized Email January 2013 2.3. Subject Field If the Subject field cannot be presented as is, the server presents a representation encoded as specified in [RFC2047]. 2.4. Remaining Header Fields Any header field that cannot be presented to the client, even with the modifications listed in Sections 2.1-2.3, is silently excised. 3. IMAP-Specific Details IMAP allows clients to retrieve the message size without downloading the message, using RFC822.SIZE, BODY.SIZE[] and so on. [RFC3501] requires that the returned size be exact. 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 synthetic message's size differs. 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. 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. 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. 4. POP-Specific Details The number of lines specified in the TOP command (see [RFC1939]) refers to the synthetic message. The message size reported by, for example, LIST may refer to either the internationalized or the synthetic message. Gulbrandsen Standards Track [Page 4] RFC 6858 Simple POP/IMAP for Internationalized Email January 2013 5. Security Considerations If the internationalized message uses any sort of signature that covers header fields, the synthetic message's signature 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 [RFC6857]. Even though invalid, these signatures SHOULD NOT be removed from the synthetic message, to preserve as much of the information as possible from the original message. 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. 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 synthetic message. 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 synthetic message from its cache rather than retrieve the real message from the server. 6. IANA Considerations IANA has added DOWNGRADED to the "IMAP Response Codes" registry. 7. References 7.1. Normative References [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, May 1996. [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047, November 1996. Gulbrandsen Standards Track [Page 5] RFC 6858 Simple POP/IMAP for Internationalized Email January 2013 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2606] Eastlake 3rd, D. and A. Panitz, "Reserved Top Level DNS Names", BCP 32, RFC 2606, June 1999. [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003. 7.2. Informative References [RFC1925] Callon, R., "The Twelve Networking Truths", RFC 1925, April 1 1996. [RFC6855] Resnick, P., Ed., Newman, C., Ed., and S. Shen, Ed., "IMAP Support for UTF-8", RFC 6855, January 2013. [RFC6856] Gellens, R., Newman, C., Yao, J., and K. Fujiwara, "POP3 Support for UTF-8", RFC 6856, January 2013. [RFC6857] Fujiwara, K., "Post-Delivery Message Downgrading for Internationalized Email Messages", RFC 6857, January 2013. 8. Acknowledgements Claudio Allocchio, Ned Freed, Kazunori Fujiwara, Ted Hardie, John Klensin, Barry Leiba, John Levine, Alexey Melnikov, Chris Newman, Joseph Yee, and the originator of rule 12 in [RFC1925] helped with this document. Author's Address Arnt Gulbrandsen Schweppermannstr. 8 D-81671 Muenchen Germany Fax: +49 89 4502 9758 EMail: arnt@gulbrandsen.priv.no Gulbrandsen Standards Track [Page 6]