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rfc:rfc8508

Internet Engineering Task Force (IETF) S. Brandt Request for Comments: 8508 Verizon Category: Standards Track January 2019 ISSN: 2070-1721

                       IMAP REPLACE Extension

Abstract

 This document defines an IMAP extension that can be used to replace
 an existing message in a message store with a new message.  Message
 replacement is a common operation for clients that automatically save
 drafts or notes as a user composes them.

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 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8508.

Copyright Notice

 Copyright (c) 2019 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
 (https://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.

Brandt Standards Track [Page 1] RFC 8508 IMAP REPLACE Extension January 2019

Table of Contents

 1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
 3.  REPLACE and UID REPLACE . . . . . . . . . . . . . . . . . . .   3
   3.1.  Advertising Support for REPLACE . . . . . . . . . . . . .   3
   3.2.  REPLACE Command . . . . . . . . . . . . . . . . . . . . .   3
   3.3.  UID REPLACE Command . . . . . . . . . . . . . . . . . . .   4
   3.4.  Semantics of REPLACE and UID REPLACE  . . . . . . . . . .   5
   3.5.  IMAP State Diagram Impacts  . . . . . . . . . . . . . . .   6
 4.  Interaction with Other Extensions . . . . . . . . . . . . . .   6
   4.1.  ACL . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.2.  CATENATE  . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.3.  UIDPLUS . . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.4.  IMAP Events in Sieve  . . . . . . . . . . . . . . . . . .   8
   4.5.  CONDSTORE/QRESYNC . . . . . . . . . . . . . . . . . . . .   8
   4.6.  OBJECTID  . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.7.  MULTIAPPEND . . . . . . . . . . . . . . . . . . . . . . .   8
 5.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .   9
 6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
 7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
 8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
   8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
 Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1. Overview

 This document defines an IMAP ([RFC3501]) extension to facilitate the
 replacement of an existing message with a new one.  This is
 accomplished by defining a new REPLACE command and extending the
 Unique Identifier (UID) command to allow UID REPLACE.
 Since there is no replace function in the base IMAP specification,
 clients have instead had to use a combination of three separate
 commands issued in serial fashion; APPEND, STORE, and EXPUNGE.
 Pipelining of these three commands is not recommended since failure
 of any individual command should prevent subsequent commands from
 being executed lest the original message version be lost.
 Because of the non-atomic nature of the existing sequence,
 interruptions can leave messages in intermediate states that can be
 seen and acted upon by other clients.  Such interruptions can also
 strand older revisions of messages, thereby forcing the user to
 manually clean up multiple revisions of the same message in order to
 avoid wasteful quota consumption.  Additionally, the existing
 sequence can fail on APPEND due to an over-quota condition even

Brandt Standards Track [Page 2] RFC 8508 IMAP REPLACE Extension January 2019

 though the subsequent STORE/EXPUNGE would free up enough space for
 the newly revised message.  And finally, server efficiencies may be
 possible with a single logical message replacement operation as
 compared to the existing APPEND/STORE/EXPUNGE sequence.
 In its simplest form, the REPLACE command is a single-command
 encapsulation of APPEND, STORE +flags \DELETED, and UID EXPUNGE for a
 message, except that it avoids any of the quota implications or
 intermediate states associated with the three-command sequence.
 Server developers are encouraged to implement REPLACE as an atomic
 operation to simplify error handling, minimize operational concerns,
 and reduce potential security problems.  For systems where this is
 not possible, communication with the requesting client must ensure no
 confusion of message store state.  A server MUST NOT generate a
 response code for the STORE +flags \DELETED portion of the sequence.
 Additionally, servers supporting the REPLACE command MUST NOT infer
 any inheritance of content, flags, or annotations from the message
 being replaced.

2. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.
 Formal syntax is defined by [RFC5234].
 Example lines prefaced by "C:" are sent by the client, and ones
 prefaced by "S:" are sent by the server.

3. REPLACE and UID REPLACE

3.1. Advertising Support for REPLACE

 Servers that implement the REPLACE extension will return "REPLACE" as
 one of the supported capabilities in the CAPABILITY command response.

3.2. REPLACE Command

 Arguments:  message sequence number
             mailbox name
             OPTIONAL flag parenthesized list
             OPTIONAL date/time string
             message literal
 Responses: no specific responses for this command

Brandt Standards Track [Page 3] RFC 8508 IMAP REPLACE Extension January 2019

 Result:     OK - replace completed
             NO - replace error; can't remove specified message
                  or can't add new message content
             BAD - command unknown or arguments invalid
 Example:
   C: A003 REPLACE 4 Drafts (\Seen \Draft) {312}
   S: + Ready for literal data
   C: Date: Thu, 1 Jan 2015 00:05:00 -0500 (EST)
   C: From: Fritz Schmidt <fritz.ze@example.org>
   C: Subject: happy new year !!
   C: To: miss.mitzy@example.org
   C: Message-Id: <B238822388-0100000@example.org>
   C: MIME-Version: 1.0
   C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
   C:
   C: Just saw the best fireworks show. Wish you were here.
   C:
   S: * OK [APPENDUID 1 2000] Replacement Message ready
   S: * 5 EXISTS
   S: * 4 EXPUNGE
   S: A003 OK Replace completed

3.3. UID REPLACE Command

 This extends the first form of the UID command (see Section 6.4.8 of
 [RFC3501]) to add the REPLACE command defined above as a valid
 argument.  This form of REPLACE uses a UID rather than a sequence
 number as its first parameter.
 Example:
   C: A004 UID REPLACE 2000 Drafts (\Seen \Draft) {350}
   S: + Ready for literal data
   C: Date: Thu, 1 Jan 2015 00:06:00 -0500 (EST)
   C: From: Fritz Schmidt <fritz.ze@example.org>
   C: Subject: happy new year !!
   C: To: miss.mitzy@example.org
   C: Message-Id: <B238822389-0100000@example.org>
   C: MIME-Version: 1.0
   C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
   C:
   C: Just saw the best fireworks show. Wish you were here.
   C: Hopefully next year you can join us.
   C:
   S: * OK [APPENDUID 1 2001] Replacement Message ready
   S: * 5 EXISTS
   S: * 4 EXPUNGE
   S: A004 OK Replace completed

Brandt Standards Track [Page 4] RFC 8508 IMAP REPLACE Extension January 2019

3.4. Semantics of REPLACE and UID REPLACE

 The REPLACE and UID REPLACE commands take five arguments: a message
 identifier, a named mailbox, an optional parenthesized flag list, an
 optional message date/time string, and a message literal.  The
 message literal will be appended to the named mailbox, and the
 message specified by the message identifier will be removed from the
 selected mailbox.  These operations will appear to the client as a
 single action.  This has the same effect as the following sequence:
    1. APPEND
    2. [UID] STORE +FLAGS.SILENT \DELETED
    3. UID EXPUNGE
 In the cited sequence, the quota implications of APPEND are evaluated
 within the context of the pending EXPUNGE so that only the net quota
 consumption is considered.  Additionally, the EXPUNGE portion of the
 sequence only applies to the specified message, not all messages
 flagged as "\Deleted".
 Although the effect of REPLACE is identical to the steps above, the
 semantics are not identical; similar to MOVE [RFC6851], the
 intermediate states do not occur and the response codes are
 different.  In particular, the response codes for APPEND and EXPUNGE
 will be returned while those for the STORE operation MUST NOT be
 generated.
 When an error occurs while processing REPLACE or UID REPLACE, the
 server MUST NOT leave the selected mailbox in an inconsistent state;
 any untagged EXPUNGE response MUST NOT be sent until all actions are
 successfully completed.
 While it may be common for the named mailbox argument to match the
 selected mailbox for the common use case of replacing a draft, the
 REPLACE extension intentionally does not require the two to be the
 same.  As an example, it's possible to use the REPLACE command to
 replace a message in the \Drafts special-use mailbox (see Section 2
 of [RFC6154]) with a message in the \Sent special-use mailbox
 following message submission.
 Because of the similarity of REPLACE to APPEND, extensions that
 affect APPEND affect REPLACE in the same way.  Response codes such as
 TRYCREATE (see Section 6.3.11 of [RFC3501]), along with those defined
 by extensions, are sent as appropriate.  See Section 4 for more
 information about how REPLACE interacts with other IMAP extensions.

Brandt Standards Track [Page 5] RFC 8508 IMAP REPLACE Extension January 2019

3.5. IMAP State Diagram Impacts

 Unlike the APPEND command, which is valid in the authenticated state,
 the REPLACE and UID REPLACE commands MUST only be valid in the
 selected state.  This difference from APPEND is necessary since
 REPLACE operates on message sequence numbers.  Additionally, the
 REPLACE extension intentionally follows the convention for UID
 commands found in Section 6.4.8 of [RFC3501] in that the UID variant
 of the command does not support use from the authenticated state.

4. Interaction with Other Extensions

 This section describes how REPLACE interacts with some other IMAP
 extensions.

4.1. ACL

 The Access Control List (ACL) rights [RFC4314] required for UID
 REPLACE are the union of the ACL rights required for UID STORE and
 UID EXPUNGE in the current mailbox, and APPEND in the target mailbox.

4.2. CATENATE

 Servers supporting both REPLACE and CATENATE [RFC4469] MUST support
 the additional append-data and resp-text-code elements defined in
 Section 5 ("Formal Syntax") of [RFC4469] in conjunction with the
 REPLACE command.  When combined with CATENATE, REPLACE can become
 quite an efficient way of message manipulation.

Brandt Standards Track [Page 6] RFC 8508 IMAP REPLACE Extension January 2019

 Example:
   User composes message and attaches photo
   ----------------------------------------
   C: A010 APPEND Drafts (\Seen \Draft) {1201534}
   S: + Ready for literal data
   C: Date: Thu, 1 Jan 2015 00:10:00 -0500 (EST)
   C: From: Fritz Schmidt <fritz.ze@example.org>
   C: Message-ID: <B238822388-0100003@example.org>
   C: MIME-Version: 1.0
   C: Content-Type: multipart/mixed;
   C:         boundary="------------030305060306060609050804"
   C:
   C: --------------030305060306060609050804
   C: Content-Type: text/plain; charset=utf-8; format=flowed
   C: Content-Transfer-Encoding: 7bit
   C:
   C: Here is picture from the fireworks
   C:
   C: Yours...
   C: Fritz
   C:
   C: --------------030305060306060609050804
   C: Content-Type: image/jpeg;
   C:         name="Fireworks.jpg"
   C: Content-Transfer-Encoding: base64
   C: Content-Disposition: attachment;
   C:         filename="Fireworks.jpg"
   C:
     <large base64 encoded part goes here>
   C:
   C: --------------030305060306060609050804--
   S: A010 OK [APPENDUID 1 3002] APPEND complete
   User completes message with To: and Subject: fields
   ---------------------------------------------------
   C: A011 UID REPLACE 3002 Drafts CATENATE (TEXT {71}
   S: + Ready for literal data
   C: To: Mitzy <miss.mitzy@example.org>
   C: Subject: My view of the fireworks
   C:  URL "/Drafts/;UID=3002")
   S: * OK [APPENDUID 1 3003] Replacement Message ready
   S: * 5 EXISTS
   S: * 4 EXPUNGE
   S: A011 OK REPLACE completed

Brandt Standards Track [Page 7] RFC 8508 IMAP REPLACE Extension January 2019

4.3. UIDPLUS

 Servers supporting both REPLACE and UIDPLUS [RFC4315] SHOULD send
 APPENDUID in response to a UID REPLACE command.  For additional
 information, see Section 3 of [RFC4315].  Servers implementing
 REPLACE and UIDPLUS are also advised to send the APPENDUID response
 code in an untagged OK before sending the EXPUNGE or replaced
 responses.  (Sending APPENDUID in the tagged OK as described in the
 UIDPLUS specification means that the client first receives EXPUNGE
 for a message and afterwards APPENDUID for the new message.  It can
 be unnecessarily difficult to process that sequence usefully.)

4.4. IMAP Events in Sieve

 REPLACE applies to IMAP events in Sieve [RFC6785] in the same way
 that APPEND does.  Therefore, REPLACE can cause a Sieve script to be
 invoked with the imap.cause set to "APPEND".  Because the
 intermediate state of STORE +FLAGS.SILENT \DELETED is not exposed by
 REPLACE, no action will be taken that results in an imap.cause of
 FLAG.

4.5. CONDSTORE/QRESYNC

 Servers implementing both REPLACE and CONDSTORE/QRESYNC [RFC7162]
 MUST treat the message being replaced as if it were being removed
 with a UID EXPUNGE command.  Sections 3.2.9 and 3.2.10 of [RFC7162]
 are particularly relevant for this condition.

4.6. OBJECTID

 Servers implementing both REPLACE and OBJECTID [RFC8474] MUST return
 different EMAILIDs for both the replaced and replacing messages.  The
 only exception to this is the case outlined in Section 5.1 ("EMAILID
 Identifier for Identical Messages") of [RFC8474] when the server
 detects that both messages' immutable content is identical.

4.7. MULTIAPPEND

 The REPLACE extension has no interaction with MULTIAPPEND [RFC3502].
 This document explicitly does not outline a method for replacing
 multiple messages concurrently.

Brandt Standards Track [Page 8] RFC 8508 IMAP REPLACE Extension January 2019

5. Formal Syntax

 The following syntax specification uses the Augmented Backus-Naur
 Form (ABNF) notation as specified in [RFC5234].  [RFC3501] defines
 the non-terminals "capability","command-select", "mailbox",
 "seq-number", and "uid".  [RFC4466] defines the non-terminal
 "append-message".
 Except as noted otherwise, all alphabetic characters are case
 insensitive.  The use of uppercase or lowercase characters to define
 token strings is for editorial clarity only.  Implementations MUST
 accept these strings in a case-insensitive fashion.
 capability     =/ "REPLACE"
 command-select =/ replace
 replace        = "REPLACE" SP seq-number SP mailbox append-message
 uid            =/ "UID" SP replace

6. Security Considerations

 This document is believed to add no security problems beyond those
 that may already exist with the base IMAP specification.  The REPLACE
 command may actually prevent some potential security problems because
 it avoids intermediate message states that could possibly be
 exploited by an attacker.

7. IANA Considerations

 The IANA has added REPLACE to the "IMAP Capabilities" registry at
 <https://www.iana.org/assignments/imap-capabilities>.

8. References

8.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <https://www.rfc-editor.org/info/rfc2119>.
 [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
            4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
            <https://www.rfc-editor.org/info/rfc3501>.
 [RFC4314]  Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
            RFC 4314, DOI 10.17487/RFC4314, December 2005,
            <https://www.rfc-editor.org/info/rfc4314>.

Brandt Standards Track [Page 9] RFC 8508 IMAP REPLACE Extension January 2019

 [RFC4315]  Crispin, M., "Internet Message Access Protocol (IMAP) -
            UIDPLUS extension", RFC 4315, DOI 10.17487/RFC4315,
            December 2005, <https://www.rfc-editor.org/info/rfc4315>.
 [RFC4466]  Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
            ABNF", RFC 4466, DOI 10.17487/RFC4466, April 2006,
            <https://www.rfc-editor.org/info/rfc4466>.
 [RFC4469]  Resnick, P., "Internet Message Access Protocol (IMAP)
            CATENATE Extension", RFC 4469, DOI 10.17487/RFC4469, April
            2006, <https://www.rfc-editor.org/info/rfc4469>.
 [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234,
            DOI 10.17487/RFC5234, January 2008,
            <https://www.rfc-editor.org/info/rfc5234>.
 [RFC6785]  Leiba, B., "Support for Internet Message Access Protocol
            (IMAP) Events in Sieve", RFC 6785, DOI 10.17487/RFC6785,
            November 2012, <https://www.rfc-editor.org/info/rfc6785>.
 [RFC7162]  Melnikov, A. and D. Cridland, "IMAP Extensions: Quick Flag
            Changes Resynchronization (CONDSTORE) and Quick Mailbox
            Resynchronization (QRESYNC)", RFC 7162,
            DOI 10.17487/RFC7162, May 2014,
            <https://www.rfc-editor.org/info/rfc7162>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
            2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
            May 2017, <https://www.rfc-editor.org/info/rfc8174>.
 [RFC8474]  Gondwana, B., Ed., "IMAP Extension for Object
            Identifiers", RFC 8474, DOI 10.17487/RFC8474, September
            2018, <https://www.rfc-editor.org/info/rfc8474>.

8.2. Informative References

 [RFC3502]  Crispin, M., "Internet Message Access Protocol (IMAP) -
            MULTIAPPEND Extension", RFC 3502, DOI 10.17487/RFC3502,
            March 2003, <https://www.rfc-editor.org/info/rfc3502>.
 [RFC6154]  Leiba, B. and J. Nicolson, "IMAP LIST Extension for
            Special-Use Mailboxes", RFC 6154, DOI 10.17487/RFC6154,
            March 2011, <https://www.rfc-editor.org/info/rfc6154>.

Brandt Standards Track [Page 10] RFC 8508 IMAP REPLACE Extension January 2019

 [RFC6851]  Gulbrandsen, A. and N. Freed, Ed., "Internet Message
            Access Protocol (IMAP) - MOVE Extension", RFC 6851,
            DOI 10.17487/RFC6851, January 2013,
            <https://www.rfc-editor.org/info/rfc6851>.

Acknowledgements

 The author would like to thank the participants of IMAPEXT with
 particular thanks to Arnt Gulbrandsen, Alexey Melnikov, Chris Newman,
 and Bron Gondwana for their specific contributions.

Author's Address

 Stuart Brandt
 Verizon
 22001 Loudoun County Parkway
 Ashburn, VA  20147
 United States of America
 Email: stujenerin@aol.com

Brandt Standards Track [Page 11]

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