    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.

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    RFC 4469                IMAP CATENATE Extension               April 2006

    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.

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    RFC 4469                IMAP CATENATE Extension               April 2006

    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

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    RFC 4469                IMAP CATENATE Extension               April 2006

       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.

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    RFC 4469                IMAP CATENATE Extension               April 2006

    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].

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    RFC 4469                IMAP CATENATE Extension               April 2006

    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.

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    RFC 4469                IMAP CATENATE Extension               April 2006

    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

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    RFC 4469                IMAP CATENATE Extension               April 2006

       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

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    RFC 4469                IMAP CATENATE Extension               April 2006

       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

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    RFC 4469                IMAP CATENATE Extension               April 2006

       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.

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    RFC 4469                IMAP CATENATE Extension               April 2006

    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.

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    RFC 4469                IMAP CATENATE Extension               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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    RFC 4469                IMAP CATENATE Extension               April 2006

    Full Copyright Statement

       Copyright (C) The Internet Society (2006).

       This document is subject to the rights, licenses and restrictions
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    Acknowledgement

       Funding for the RFC Editor function is provided by the IETF
       Administrative Support Activity (IASA).

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