





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.








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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



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   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



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   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



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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.






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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.























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   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






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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.










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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>.



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   [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>.







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   [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































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