Network Working Group                                         P. Resnick
Request for Comments: 4469                         QUALCOMM Incorporated
Updates: 3501, 3502                                           April 2006
Category: Standards Track


       Internet Message Access Protocol (IMAP) CATENATE Extension

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   The CATENATE extension to the Internet Message Access Protocol (IMAP)
   extends the APPEND command to allow clients to create messages on the
   IMAP server that may contain a combination of new data along with
   parts of (or entire) messages already on the server.  Using this
   extension, the client can catenate parts of an already existing
   message onto a new message without having to first download the data
   and then upload it back to the server.

1.  Introduction

   The CATENATE extension to the Internet Message Access Protocol (IMAP)
   [1] allows the client to create a message on the server that can
   include the text of messages (or parts of messages) that already
   exist on the server without having to FETCH them and APPEND them back
   to the server.  The CATENATE extension extends the APPEND command so
   that, instead of a single message literal, the command can take as
   arguments any combination of message literals (as described in IMAP
   [1]) and message URLs (as described in the IMAP URL Scheme [2]
   specification).  The server takes all the pieces and catenates them
   into the output message.  The CATENATE extension can also coexist
   with the MULTIAPPEND extension [3] to APPEND multiple messages in a
   single command.

   There are some obvious uses for the CATENATE extension.  The
   motivating use case was to provide a way for a resource-constrained
   client to compose a message for subsequent submission that contains
   data that already exists in that client's IMAP store.  Because the
   client does not have to download and re-upload potentially large
   message parts, bandwidth and processing limitations do not have as
   much impact.  In addition, since the client can create a message in
   its own IMAP store, the command also addresses the desire of the
   client to archive a copy of a sent message without having to upload
   the message twice.  (Mechanisms for sending the message are outside
   the scope of this document.)

   The extended APPEND command can also be used to copy parts of a
   message to another mailbox for archival purposes while getting rid of
   undesired parts.  In environments where server storage is limited, a
   client could get rid of large message parts by copying over only the
   necessary parts and then deleting the original message.  The
   mechanism could also be used to add data to a message (such as
   prepending message header fields) or to include other data by making
   a copy of the original and catenating the new data.

2.  The CATENATE Capability

   A server that supports this extension returns "CATENATE" as one of
   the responses to the CAPABILITY command.

3.  The APPEND Command

   Arguments:  mailbox name
               (The following can be repeated in the presence of the
               MULTIAPPEND extension [3])
               OPTIONAL flag parenthesized list
               OPTIONAL date/time string
               a single message literal or one or more message parts to
               catenate, specified as:
                           message literal
                           or
                           message (or message part) URL

   Responses:  OPTIONAL NO responses: BADURL, TOOBIG

   Result:     OK -  append completed
               NO -  append error: can't append to that mailbox, error
                     in flags or date/time or message text, or can't
                     fetch that data
               BAD - command unknown or arguments invalid

   The APPEND command concatenates all the message parts and appends
   them as a new message to the end of the specified mailbox.  The
   parenthesized flag list and date/time string set the flags and the
   internal date, respectively, as described in IMAP [1].  The
   subsequent command parameters specify the message parts that are
   appended sequentially to the output message.

   If the original form of APPEND is used, a message literal follows the
   optional flag list and date/time string, which is appended as
   described in IMAP [1].  If the extended form is used, "CATENATE" and
   a parenthesized list of message literals and message URLs follows,
   each of which is appended to the new message.  If a message literal
   is specified (indicated by "TEXT"), the octets following the count
   are appended.  If a message URL is specified (indicated by "URL"),
   the octets of the body part pointed to by that URL are appended, as
   if the literal returned in a FETCH BODY response were put in place of
   the message part specifier.  The APPEND command does not cause the
   \Seen flag to be set for any catenated body part.  The APPEND command
   does not change the selected mailbox.

   In the extended APPEND command, the string following "URL" is an IMAP
   URL [2] and is interpreted according to the rules of [2].  The
   present document only describes the behavior of the command using
   IMAP URLs that refer to specific messages or message parts on the
   current IMAP server from the current authenticated IMAP session.
   Because of that, only relative IMAP message or message part URLs
   (i.e., those having no scheme or <iserver>) are used.  The base URL

   for evaluating the relative URL is considered "imap://user@server/",
   where "user" is the user name of the currently authenticated user and
   "server" is the domain name of the current server.  When in the
   selected state, the base URL is considered
   "imap://user@server/mailbox", where "mailbox" is the encoded name of
   the currently selected mailbox.  Additionally, since the APPEND
   command is valid in the authenticated state of an IMAP session, no
   further LOGIN or AUTHENTICATE command is performed for URLs specified
   in the extended APPEND command.

      Note: Use of an absolute IMAP URL or any URL that refers to
      anything other than a message or message part from the current
      authenticated IMAP session is outside the scope of this document
      and would require an extension to this specification, and a server
      implementing only this specification would return NO to such a
      request.

   The client is responsible for making sure that the catenated message
   is in the format of an Internet Message Format (RFC 2822) [4] or
   Multipurpose Internet Mail Extension (MIME) [5] message.  In
   particular, when a URL is catenated, the server copies octets,
   unchanged, from the indicated message or message part to the
   catenated message.  It does no data conversion (e.g., MIME transfer
   encodings) nor any verification that the data is appropriate for the
   MIME part of the message into which it is inserted.  The client is
   also responsible for inserting appropriate MIME boundaries between
   body parts, and writing MIME Content-Type and Content-Transfer-
   Encoding lines as needed in the appropriate places.

   Responses behave just as the original APPEND command described in
   IMAP [1].  If the server implements the IMAP UIDPLUS extension [6],
   it will also return an APPENDUID response code in the tagged OK
   response.  Two response codes are provided in Section 4 that can be
   used in the tagged NO response if the APPEND command fails.

4.  Response Codes

   When a APPEND command fails, it may return a response code that
   describes a reason for the failure.

4.1.  BADURL Response

   The BADURL response code is returned if the APPEND fails to process
   one of the specified URLs.  Possible reasons for this are bad URL
   syntax, unrecognized URL schema, invalid message UID, or invalid body
   part.  The BADURL response code contains the first URL specified as a
   parameter to the APPEND command that has caused the operation to
   fail.

4.2.  TOOBIG Response

   The TOOBIG response code is returned if the resulting message will
   exceed the 4-GB IMAP message limit.  This might happen, for example,
   if the client specifies 3 URLs for 2-GB messages.  Note that even if
   the server doesn't return TOOBIG, it still has to be defensive
   against misbehaving or malicious clients that try to construct a
   message over the 4-GB limit.  The server may also wish to return the
   TOOBIG response code if the resulting message exceeds a server-
   specific message size limit.

5.  Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) [7] notation.  Elements not defined here can be found in
   the formal syntax of the ABNF [7], IMAP [1], and IMAP ABNF extensions
   [8] specifications.  Note that capability and resp-text-code are
   extended from the IMAP [1] specification and append-data is extended
   from the IMAP ABNF extensions [8] specification.

   append-data =/ "CATENATE" SP "(" cat-part *(SP cat-part) ")"

   cat-part = text-literal / url

   text-literal = "TEXT" SP literal

   url = "URL" SP astring

   resp-text-code =/ toobig-response-code / badurl-response-code

   toobig-response-code = "TOOBIG"

   badurl-response-code = "BADURL" SP url-resp-text

   url-resp-text = 1*(%x01-09 /
                      %x0B-0C /
                      %x0E-5B /
                      %x5D-FE) ; Any TEXT-CHAR except "]"

   capability =/ "CATENATE"

   The astring in the definition of url and the url-resp-text in the
   definition of badurl-response-code each contain an imapurl as defined
   by [2].

6.  Acknowledgements

   Thanks to the members of the LEMONADE working group for their input.
   Special thanks to Alexey Melnikov for the examples.

7.  Security Considerations

   The CATENATE extension does not raise any security considerations
   that are not present for the base protocol or in the use of IMAP
   URLs, and these issues are discussed in the IMAP [1] and IMAP URL [2]
   documents.

8.  IANA Considerations

   IMAP4 capabilities are registered by publishing a standards track or
   IESG approved experimental RFC.  The registry is currently located at
   <http://www.iana.org/assignments/imap4-capabilities>.  This document
   defines the CATENATE IMAP capability.  The IANA has added this
   capability to the registry.

Appendix A.  Examples

   Lines not starting with "C: " or "S: " are continuations of the
   previous lines.

   The original message in examples 1 and 2 below (UID = 20) has the
   following structure:


      multipart/mixed MIME message with two body parts:

      1.  text/plain

      2.  application/x-zip-compressed

   Example 1: The following example demonstrates how a CATENATE client
   can replace an attachment in a draft message, without the need to
   download it to the client and upload it back.

   C: A003 APPEND Drafts (\Seen \Draft $MDNSent) CATENATE
    (URL "/Drafts;UIDVALIDITY=385759045/;UID=20/;section=HEADER"
    TEXT {42}
   S: + Ready for literal data
   C:
   C: --------------030308070208000400050907
   C:  URL "/Drafts;UIDVALIDITY=385759045/;UID=20/;section=1.MIME"
    URL "/Drafts;UIDVALIDITY=385759045/;UID=20/;section=1" TEXT {42}
   S: + Ready for literal data
   C:
   C: --------------030308070208000400050907
   C:  URL "/Drafts;UIDVALIDITY=385759045/;UID=30" TEXT {44}
   S: + Ready for literal data
   C:
   C: --------------030308070208000400050907--
   C: )
   S: A003 OK catenate append completed

   Example 2: The following example demonstrates how the CATENATE
   extension can be used to replace edited text in a draft message, as
   well as header fields for the top level message part (e.g., Subject
   has changed).  The previous version of the draft is marked as
   \Deleted.  Note that the server also supports the UIDPLUS extension,
   so the APPENDUID response code is returned in the successful OK
   response to the APPEND command.

   C: A003 APPEND Drafts (\Seen \Draft $MDNSent) CATENATE (TEXT {738}
   S: + Ready for literal data
   C: Return-Path: <bar@example.org>
   C: Received: from [127.0.0.2]
   C:           by rufus.example.org via TCP (internal) with ESMTPA;
   C:           Thu, 11 Nov 2004 16:57:07 +0000
   C: Message-ID: <419399E1.6000505@example.org>
   C: Date: Thu, 12 Nov 2004 16:57:05 +0000
   C: From: Bob Ar <bar@example.org>
   C: X-Accept-Language: en-us, en
   C: MIME-Version: 1.0
   C: To: foo@example.net
   C: Subject: About our holiday trip
   C: Content-Type: multipart/mixed;
   C:               boundary="------------030308070208000400050907"
   C:
   C: --------------030308070208000400050907
   C: Content-Type: text/plain; charset=us-ascii; format=flowed
   C: Content-Transfer-Encoding: 7bit
   C:
   C: Our travel agent has sent the updated schedule.
   C:
   C: Cheers,
   C: Bob
   C: --------------030308070208000400050907
   C:  URL "/Drafts;UIDVALIDITY=385759045/;UID=20/;Section=2.MIME"
    URL "/Drafts;UIDVALIDITY=385759045/;UID=20/;Section=2" TEXT {44}
   S: + Ready for literal data
   C:
   C: --------------030308070208000400050907--
   C: )
   S: A003 OK [APPENDUID 385759045 45] append Completed
   C: A004 UID STORE 20 +FLAGS.SILENT (\Deleted)
   S: A004 OK STORE completed

   Example 3: The following example demonstrates how the CATENATE
   extension can be used to strip attachments.  Below, a PowerPoint
   attachment was replaced by a small text part explaining that the
   attachment was stripped.

   C: A003 APPEND Drafts (\Seen \Draft $MDNSent) CATENATE
    (URL "/Drafts;UIDVALIDITY=385759045/;UID=21/;section=HEADER"
    TEXT {42}
   S: + Ready for literal data
   C:
   C: --------------030308070208000400050903
   C:  URL "/Drafts;UIDVALIDITY=385759045/;UID=21/;section=1.MIME"
    URL "/Drafts;UIDVALIDITY=385759045/;UID=21/;section=1" TEXT {255}
   S: + Ready for literal data
   C:
   C: --------------030308070208000400050903
   C: Content-type: text/plain; charset="us-ascii"
   C: Content-transfer-encoding: 7bit
   C:
   C: This body part contained a Power Point presentation that was
   C: deleted upon your request.
   C: --------------030308070208000400050903--
   C: )
   S: A003 OK append Completed

   Example 4: The following example demonstrates a failed APPEND
   command.  The server returns the BADURL response code to indicate
   that one of the provided URLs is invalid.  This example also
   demonstrates how the CATENATE extension can be used to construct a
   digest of several messages.

   C: A003 APPEND Sent (\Seen $MDNSent) CATENATE (TEXT {541}
   S: + Ready for literal data
   C: Return-Path: <foo@example.org>
   C: Received: from [127.0.0.2]
   C:           by rufus.example.org via TCP (internal) with ESMTPA;
   C:           Thu, 11 Nov 2004 16:57:07 +0000
   C: Message-ID: <419399E1.6000505@example.org>
   C: Date: Thu, 21 Nov 2004 16:57:05 +0000
   C: From: Farren Oo <foo@example.org>
   C: X-Accept-Language: en-us, en
   C: MIME-Version: 1.0
   C: To: bar@example.org
   C: Subject: Digest of the mailing list for today
   C: Content-Type: multipart/digest;
   C:               boundary="------------030308070208000400050904"
   C:
   C: --------------030308070208000400050904
   C:  URL "/INBOX;UIDVALIDITY=785799047/;UID=11467" TEXT {42}
   S: + Ready for literal data
   C:
   C: --------------030308070208000400050904
   C:  URL "/INBOX;UIDVALIDITY=785799047/;UID=113330/;section=1.5.9"
    TEXT {42}
   S: + Ready for literal data
   C:
   C: --------------030308070208000400050904
   C:  URL "/INBOX;UIDVALIDITY=785799047/;UID=11916" TEXT {44}
   S: + Ready for literal data
   C:
   C: --------------030308070208000400050904--
   C: )
   S: A003 NO [BADURL "/INBOX;UIDVALIDITY=785799047/;UID=113330;
   section=1.5.9"] CATENATE append has failed, one message expunged

   Note that the server could have validated the URLs as they were
   received and therefore could have returned the tagged NO response
   with BADURL response-code in place of any continuation request after
   the URL was received.

9.  Normative References

   [1]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1",
        RFC 3501, March 2003.

   [2]  Newman, C., "IMAP URL Scheme", RFC 2192, September 1997.

   [3]  Crispin, M., "Internet Message Access Protocol (IMAP) -
        MULTIAPPEND Extension", RFC 3502, March 2003.

   [4]  Resnick, P., "Internet Message Format", RFC 2822, April 2001.

   [5]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part One: Format of Internet Message Bodies",
        RFC 2045, November 1996.

   [6]  Crispin, M., "Internet Message Access Protocol (IMAP) - UIDPLUS
        extension", RFC 4315, December 2005.

   [7]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 4234, October 2005.

   [8]  Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4 ABNF",
        RFC 4466, April 2006.

Author's Address

   Peter W. Resnick
   QUALCOMM Incorporated
   5775 Morehouse Drive
   San Diego, CA  92121-1714
   US

   Phone: +1 858 651 4478
   EMail: presnick@qualcomm.com
   URI:   http://www.qualcomm.com/~presnick/

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