RFC4469: Internet Message Access Protocol (IMAP) CATENATE Extension
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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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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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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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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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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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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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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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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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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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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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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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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
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