Internet Engineering Task Force (IETF) M. Bettini
Request for Comments: 9585 Open-Xchange Oy
Category: Standards Track May 2024
ISSN: 2070-1721
IMAP Response Code for Command Progress Notifications
Abstract
This document defines a new IMAP untagged response code,
"INPROGRESS", that provides progress notifications regarding the
status of long-running commands.
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/rfc9585.
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Table of Contents
1. Introduction
2. Conventions Used in This Document
3. CAPABILITY Identification
4. The "INPROGRESS" Response Code
5. Formal Syntax
6. Security Considerations
7. IANA Considerations
8. Normative References
Author's Address
1. Introduction
IMAP commands [RFC9051] can require a considerable amount of time to
be completed by the server. In these cases, the client has no
information about the progress of the commands. It is already
possible to expose updates with a generic untagged response, like "*
OK Still on it, 57% done"; however, this does not provide a standard
way to communicate with the client and does not allow the server to
inform the client of the progress of the long-running actions.
This document extends the Internet Message Access Protocol (IMAP)
[RFC9051] with:
* a new "INPROGRESS" response code [RFC5530]. The new response code
provides a consistent means for a client to receive progress
notifications on command completion status.
* a new "INPROGRESS" capability [RFC9051]. The new capability
informs the client that the server emits progress notifications
via the "INPROGRESS" response code.
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.
The word "can" (not "may") is used to refer to a possible
circumstance or situation, as opposed to an optional facility of the
protocol.
Conventions for notations are as in [RFC9051] and [RFC5530].
In examples, "C:" and "S:" indicate lines sent by the client and
server, respectively. Note that each line includes the terminating
CRLF.
3. CAPABILITY Identification
IMAP servers that support this extension MUST include "INPROGRESS" in
the response list to the CAPABILITY command.
4. The "INPROGRESS" Response Code
The server MAY send the "INPROGRESS" response code to notify the
client about the progress of the commands in execution or simply to
prevent the client from timing out and terminating the connection.
The notifications MAY be sent for any IMAP command. If the server
elects to send notifications, it is RECOMMENDED that these are sent
every 10-15 seconds.
The response code is meant to appear embedded inside an untagged OK
reply. The response code MUST NOT appear in a tagged response (the
command has completed and further progress notifications make no
sense).
The response code MAY embed a list of details, which appear in the
following order:
1. CMD-TAG: the tag [RFC9051] that originated the long-running
command. If the tag is not available or if it contains the "]"
character, it MUST be set to NIL. This still produces a usable
notification, unless multiple commands are in flight
simultaneously. A client can ensure reception of notifications
with tags by simply refraining from the use of the character "]"
in the originating command tags.
2. PROGRESS: a number indicating the number of items processed so
far. The number MUST be non-negative and SHOULD be monotonically
increasing. If the PROGRESS is not available, both PROGRESS and
GOAL MUST be set to NIL.
3. GOAL: a number indicating the total number of items to be
processed. The number MUST be positive, and it SHOULD NOT change
between successive notifications for the same command tag. This
is the number that PROGRESS is expected to reach after the
completion of the command; therefore, it MUST be greater than
PROGRESS. If the GOAL is not known, it MUST be set to NIL.
If the response code does not embed a list of details, all details
are to be interpreted as NIL.
The server can provide the progress details with different degrees of
completeness:
- bare keepalive
* OK [INPROGRESS] Hang in there...
- keepalive with an indication of the command tag
* OK [INPROGRESS ("tag" NIL NIL)] Hang in there...
- progress notification with unknown GOAL
* OK [INPROGRESS ("tag" 175 NIL)] Processed 175 items so far
- progress notification with an indication of the GOAL
* OK [INPROGRESS ("tag" 175 1000)] Processed 17% of the items
Examples:
C: A001 search text "this will be slow"
[13 seconds later]
S: * OK [INPROGRESS ("A001" 454 1000)] Processed 45% of the items
[14 seconds later]
S: * OK [INPROGRESS ("A001" 999 1000)] Processed 99% of the items
[5 seconds later]
S: * SEARCH 447 735
S: A001 OK Search completed (23.387 + 0.004 + 0.017 secs).
C: A003 COPY 2000:4000 Meeting-Minutes
[12 seconds later]
S: * OK [INPROGRESS ("A003" 175 2001)] Still working on this...
[14 seconds later]
S: * OK [INPROGRESS ("A003" 440 2001)] Still working on this...
[13 seconds later]
S: * OK [INPROGRESS ("A003" 987 2001)] Still working on this...
[14 seconds later]
S: * OK [INPROGRESS ("A003" 1388 2001)] Still working on this...
[14 seconds later]
S: * OK [INPROGRESS ("A003" 1876 2001)] Still working on this...
[9 seconds later]
S: A003 OK Copy completed
PROGRESS and GOAL SHOULD be counts of the kind of item being
processed -- in most cases, messages counts. If that is not
possible, the counts SHOULD be percentages, with GOAL set to 100 and
PROGRESS varying between 0 and 99.
The server SHOULD NOT send a progress notification where PROGRESS
equals GOAL, as that would mean the command is completed. In that
case, the proper tagged response should be emitted instead.
If the command completes before the first server notification
deadline, there will be no notifications at all. The client MUST
assume PROGRESS to be 0 and GOAL to be unknown until the server
issues a notification for the command.
While the server SHOULD keep GOAL constant and PROGRESS monotonically
increasing, there are circumstances where this might not be possible.
The client MUST be prepared to handle cases where the server cannot
keep GOAL constant and/or PROGRESS monotonically increasing. When
the GOAL changes or the PROGRESS goes backward, the RECOMMENDED
interpretation is that the previous GOAL has been reached, but the
server discovered that further (long-running) work is required (with
a new known or unknown GOAL).
The client MAY disregard progress notifications entirely or process
them only in relation to specific commands. If a user interface is
involved, it is the client's duty to decide which of these
notifications should emerge to the user interface and/or modify the
user's ability to interact in their presence, since this may differ
based on implementation details.
Also, the client MUST NOT consider the values to be authoritative for
any other use than evaluating the progress of the commands. For
example, the client must not use the GOAL field in place of the
proper output of a SEARCH command to know the number of messages in a
folder.
5. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) [RFC5234] notation. Elements not defined here can be
found in the formal syntax of the ABNF [RFC5234] and IMAP [RFC9051]
specifications.
Except as noted otherwise, all alphabetic characters are case-
insensitive. The use of uppercase or lowercase characters to define
token strings are for editorial clarity only. Implementations MUST
accept these strings in a case-insensitive fashion.
inprogress-tag = quoted / nil
inprogress-state-unknown = nil SP nil
inprogress-state-counting = number SP nil
inprogress-state-known-goal = number SP nz-number
inprogress-state = inprogress-state-unknown
/ inprogress-state-counting
/ inprogress-state-known-goal
resp-text-code =/ "INPROGRESS" [ SP "(" inprogress-tag SP
inprogress-state ")" ]
6. Security Considerations
The details of the response code are not expected to disclose any
information that isn't currently available from the command output.
The progress details could be obtained anyway by sending a series of
commands with different workloads -- by either constructing data sets
or searching in the appropriate way.
The client must protect itself against data sent by a malicious
server. Specifically, the client should guard against values that
can cause arithmetic exceptions, like GOAL = 0, GOAL/VALUE < 0, GOAL/
VALUE ≥ 2^32 (these are not possible within a correct implementation
of the ABNF syntax above), and VALUE > GOAL. In these cases, the
notification MUST be disregarded.
7. IANA Considerations
IANA has added "INPROGRESS" to the "IMAP Response Codes" registry
located at <https://www.iana.org/assignments/imap-response-codes>,
with a reference to this document.
IANA had added "INPROGRESS" to the "IMAP Capabilities" registry
located at <https://www.iana.org/assignments/imap-capabilities>, with
a reference to this document.
8. 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>.
[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>.
[RFC5530] Gulbrandsen, A., "IMAP Response Codes", RFC 5530,
DOI 10.17487/RFC5530, May 2009,
<https://www.rfc-editor.org/info/rfc5530>.
[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>.
[RFC9051] Melnikov, A., Ed. and B. Leiba, Ed., "Internet Message
Access Protocol (IMAP) - Version 4rev2", RFC 9051,
DOI 10.17487/RFC9051, August 2021,
<https://www.rfc-editor.org/info/rfc9051>.
Author's Address