RFC3502 - Internet Message Access Protocol (IMAP) - MULTIAPPEND Extension

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　　Network Working Group M. Crispin
Request for Comments: 3502 University of Washington
Category: Standards Track March 2003

Internet Message Access Protocol (IMAP) - MULTIAPPEND 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 (2003). All Rights Reserved.

Abstract

This document describes the multiappending extension to the Internet
Message Access Protocol (IMAP) (RFC3501). This extension provides
substantial performance improvements for IMAP clients which upload
multiple messages at a time to a mailbox on the server.

A server which supports this extension indicates this with a
capability name of "MULTIAPPEND".

Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "MAY", and "OPTIONAL" in this document are to
be interpreted as described in [KEYWORDS].

Introduction

The MULTIAPPEND extension permits uploading of multiple messages with
a single command. When used in conjunction with the [LITERAL+]
extension, the entire upload is accomplished in a single
command/response round trip.

A MULTIAPPEND APPEND operation is atomic; either all messages are
successfully appended, or no messages are appended.

In the base IMAP specification, each message must be appended in a
separate command, and there is no mechanism to "unappend" messages if
an error occurs while appending. Also, some mail stores may require

an expensive "open/lock + sync/unlock/close" operation as part of
appending; this can be quite expensive if it must be done on a
per-message basis.

If the server supports both LITERAL+ and pipelining but not
MULTIAPPEND, it may be possible to get some of the performance
advantages of MULTIAPPEND by doing a pipelined "batch" append.
However, it will not work as well as MULTIAPPEND for the following
reasons:

1) Multiple APPEND commands, even as part of a pipelined batch,
are non-atomic by definition. There is no way to revert the
mailbox to the state before the batch append in the event of an
error.

2) It may not be feasible for the server to coalesce pipelined
APPEND operations so as to avoid the "open/lock +
sync/unlock/close" overhead described above. In any case, such
coalescing would be timing dependent and thus potentially
unreliable. In particular, with traditional UNIX mailbox files,
it is assumed that a lock is held only for a single atomic
operation, and many applications disregard any lock that is
older than 5 minutes.

3) If an error occurs, depending upon the nature of the error,
it is possible for additional messages to be appended after the
error. For example, the user wants to append 5 messages, but a
disk quota error occurs with the third message because of its
size. However, the fourth and fifth messages have already been
sent in the pipeline, so the mailbox ends up with the first,
second, fourth, and fifth messages of the batch appended.

6.3.11. APPEND Command

Arguments: mailbox name
one or more messages to upload, specified as:
OPTIONAL flag parenthesized list
OPTIONAL date/time string
message literal

Data: no specific responses for this command

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

The APPEND command appends the literal arguments as new messages
to the end of the specified destination mailbox. This argument
SHOULD be in the format of an [RFC-2822] message. 8-bit
characters are permitted in the message. A server implementation
that is unable to preserve 8-bit data properly MUST be able to
reversibly convert 8-bit APPEND data to 7-bit using a [MIME-IMB]
content transfer encoding.

Note: There MAY be exceptions, e.g., draft messages, in
which required [RFC-2822] header lines are omitted in the
message literal argument to APPEND. The full implications
of doing so MUST be understood and carefully weighed.

If a flag parenthesized list is specified, the flags SHOULD be set
in the resulting message; otherwise, the flag list of the
resulting message is set empty by default.

If a date-time is specified, the internal date SHOULD be set in
the resulting message; otherwise, the internal date of the
resulting message is set to the current date and time by default.

A zero-length message literal argument is an error, and MUST
return a NO. This can be used to cancel the append.

If the append is unsuccessful for any reason (including being
cancelled), the mailbox MUST be restored to its state before the
APPEND attempt; no partial appending is permitted. The server MAY
return an error before processing all the message arguments.

If the destination mailbox does not exist, a server MUST return an
error, and MUST NOT automatically create the mailbox. Unless it
is certain that the destination mailbox can not be created, the
server MUST send the response code "[TRYCREATE]" as the prefix of
the text of the tagged NO response. This gives a hint to the
client that it can attempt a CREATE command and retry the APPEND
if the CREATE is successful.

If the mailbox is currently selected, the normal new message
actions SHOULD occur. Specifically, the server SHOULD notify the
client immediately via an untagged EXISTS response. If the server
does not do so, the client MAY issue a NOOP command (or failing
that, a CHECK command) after one or more APPEND commands.

Example: C: A003 APPEND saved-messages (\Seen) {329}
S: + Ready for literal data
C: Date: Mon, 7 Feb 1994 21:52:25 -0800 (PST)
C: From: Fred Foobar <foobar@Blurdybloop.example.COM>
C: Subject: afternoon meeting
C: To: mooch@owatagu.example.net
C: Message-Id: <B27397-0100000@Blurdybloop.example.COM>
C: MIME-Version: 1.0
C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
C:
C: Hello Joe, do you think we can meet at 3:30 tomorrow?
C: (\Seen) " 7-Feb-1994 22:43:04 -0800" {295}
S: + Ready for literal data
C: Date: Mon, 7 Feb 1994 22:43:04 -0800 (PST)
C: From: Joe Mooch <mooch@OWaTaGu.example.net>
C: Subject: Re: afternoon meeting
C: To: foobar@blurdybloop.example.com
C: Message-Id: <a0434793874930@OWaTaGu.example.net>
C: MIME-Version: 1.0
C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
C:
C: 3:30 is fine with me.
C:
S: A003 OK APPEND completed
C: A004 APPEND bogusname (\Flagged) {1023}
S: A004 NO [TRYCREATE] No such mailbox as bogusname
C: A005 APPEND test (\Flagged) {99}
S: + Ready for literal data
C: Date: Mon, 7 Feb 2000 22:43:04 -0800 (PST)
C: From: Fred Foobar <fred@example.com>
C: Subject: hmm...
C: {35403}
S: A005 NO APPEND failed: Disk quota exceeded

Note: The APPEND command is not used for message delivery,
because it does not provide a mechanism to transfer [SMTP]
envelope information.

Modification to IMAP4rev1 Base Protocol Formal Syntax

The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [ABNF].

append = "APPEND" SP mailbox 1*append-message

append-message = [SP flag-list] [SP date-time] SP literal

MULTIAPPEND Interaction with UIDPLUS Extension

Servers which support both MULTIAPPEND and [UIDPLUS] will have the
"resp-code-apnd" rule modified as follows:

resp-code-apnd = "APPENDUID" SP nz-number SP set

That is, the APPENDUID response code returns as many UIDs as there
were messages appended in the multiple append. The UIDs returned
should be in the order the articles where appended. The message set
may not contain extraneous UIDs or the symbol "*".

Security Considerations

The MULTIAPPEND extension does not raise any security considerations
that are not present in the base [IMAP] protocol, and these issues
are discussed in [IMAP]. Nevertheless, it is important to remember
that IMAP4rev1 protocol transactions, including electronic mail data,
are sent in the clear over the network unless protection from
snooping is negotiated, either by the use of STARTTLS, privacy
protection is negotiated in the AUTHENTICATE command, or some other
protection mechanism is in effect.

Normative References

[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC2234, November 1997.

[IMAP] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC3501, March 2003.

[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.

[MIME-IMB] Freed, N. and N. Borenstein, "MIME (Multipurpose Internet
Mail Extensions) Part One: Format of Internet Message
Bodies", RFC2045, November 1996.

[RFC-2822] Resnick, P., "Internet Message Format", RFC2822, April
2001.

Informative References

[LITERAL+] Myers, J., "IMAP4 non-synchronizing literals", RFC2088,
January 1997.

[UIDPLUS] Myers, J., "IMAP4 UIDPLUS extension", RFC2359, June 1988.

[SMTP] Klensin, J., Editor, "Simple Mail Transfer Protocol", RFC
2821, April 2001.

Author's Address

Mark R. Crispin
Networks and Distributed Computing
University of Washington
4545 15th Avenue NE
Seattle, WA 98105-4527

Phone: (206) 543-5762
EMail: MRC@CAC.Washington.EDU

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Acknowledgement

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