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
AbstractThis 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.
RFC6857] 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 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". 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. RFC6856] and/or IMAP [RFC6855] support for UTF-8.
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. 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]). 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. 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. Given a field such as Content-Disposition: attachment; filename=FOO the field is presented as Content-Disposition: attachment
RFC 2047. 2.1-2.3, is silently excised. RFC822.SIZE, BODY.SIZE and so on. The IMAP specification [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 size of the surrogate message 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:". RFC1939] refers to the surrogate message. The message size reported by, for example, LIST may refer to either the internationalized or the surrogate message.
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. 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 surrogate 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 surrogate message from its cache rather than retrieve the real message from the server. [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.
[RFC2606] Eastlake, D., 3rd 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. [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, March 2013. [RFC6856] Gellens, R., Newman, C., Yao, J., and K. Fujiwara, "Post Office Protocol Version 3 (POP3) Support for UTF-8", RFC 6856, March 2013. [RFC6857] Fujiwara, K., "Post-Delivery Message Downgrading for Internationalized Email Messages", RFC 6857, March 2013. RFC1925].