IMAP STATUS=SIZE: IMAP Extension for STATUS=SIZE
Ask an IMAP server for the total size of a mailbox with one STATUS command.
Bosch, S., "IMAP Extension for STATUS=SIZE", RFC 8438, DOI 10.17487/RFC8438, August 2018, <https://www.rfc-editor.org/info/rfc8438>.
@misc{rfc8438,
series = {Request for Comments},
number = 8438,
howpublished = {RFC 8438},
publisher = {RFC Editor},
doi = {10.17487/RFC8438},
url = {https://www.rfc-editor.org/info/rfc8438},
author = {S. Bosch},
title = {{IMAP Extension for STATUS=SIZE}},
pagetotal = 6,
year = 2018,
month = aug,
} RFC text
Contents
Internet Engineering Task Force (IETF) S. Bosch
Request for Comments: 8438 Dovecot Oy
Category: Standards Track August 2018
ISSN: 2070-1721
IMAP Extension for STATUS=SIZE
Abstract
This document adds a new capability called "STATUS=SIZE" to the
Internet Message Access Protocol (IMAP). It allows retrieving the
total storage size of a mailbox with a single STATUS command rather
than retrieving and summing the sizes of all individual messages in
that mailbox.
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/rfc8438.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. STATUS Command and Response Extensions . . . . . . . . . . . 3
4. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Normative References . . . . . . . . . . . . . . . . . . . . 5
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
This document extends the Internet Message Access Protocol (IMAP)
[IMAP4rev1] with a new capability called "STATUS=SIZE". To determine
the total storage size of a mailbox, an IMAP client currently needs
to retrieve all message sizes individually using the FETCH command
with the RFC822.SIZE data item. The STATUS=SIZE capability provides
a more efficient means of achieving this. It extends the STATUS
command with a new "SIZE" data item, which indicates the total size
of all messages in the target mailbox. This way, this information
can be queried with just one STATUS command. When the LIST-STATUS
IMAP capability [LIST-STATUS] is also available, the SIZE data item
can be queried for many mailboxes at once using just one LIST
command.
This capability is particularly useful for IMAP clients that do not
cache the state of the message store, such as most webmail clients.
Without the "STATUS=SIZE" capability, such clients need to fetch all
message sizes from the server when the size of an individual mailbox
needs to be determined. For example, a user may request detailed
quota usage information for each mailbox to find out which specific
mailboxes consume most of the available storage resources. Using
this information, the user can get an overview of which mailboxes
need to be cleaned up to reduce quota usage. The QUOTA capability
[QUOTA] is no help in that scenario, since the provided QUOTAROOT
command can only yield the STORAGE resource usage of a whole quota
root, not each individual mailbox within that root.
2. Conventions Used in This Document
In examples, "C:" indicates lines sent by a client that is connected
to a server. "S:" indicates lines sent by the server to the client.
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 [KEYWORDS] [KEYWORDS2] when, and only when, they appear in all
capitals, as shown here.
3. STATUS Command and Response Extensions
This extension defines one new status data item for the STATUS
command and response:
SIZE
The total size of the mailbox in octets. This is not strictly
required to be an exact value, but it MUST be equal to or greater
than the sum of the values of the RFC822.SIZE FETCH message data
item [IMAP4rev1] of all messages in the mailbox. When the QUOTA
capability [QUOTA] is also supported, this value SHOULD be equal
to the storage usage value used to enforce the STORAGE resource
limit for this mailbox. This way, the client can directly infer
the quota usage.
Since the total storage size of a mailbox can easily exceed 4 GB,
clients MUST be capable of receiving 63-bit SIZE data item values.
The message size is chosen to be at most 63 bits wide rather than 64
bits to make implementations on various platforms (such as Java)
easier.
Example:
C: A01 STATUS frop (MESSAGES SIZE UIDNEXT)
S: * STATUS frop (MESSAGES 8 SIZE 44421 UIDNEXT 242344)
S: A01 OK STATUS completed
The same information can be obtained by using the following commands,
albeit less efficiently:
C: A02 SELECT "frop"
S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
S: * 8 EXISTS
S: * 1 RECENT
S: * OK [UNSEEN 7] First unseen.
S: * OK [UIDVALIDITY 1364851417] UIDs valid
S: * OK [UIDNEXT 242344] Predicted next UID
S: * OK [HIGHESTMODSEQ 3914] Highest
S: A02 OK [READ-WRITE] Select completed.
C: A03 FETCH 1:* (RFC822.SIZE)
S: * 1 FETCH (RFC822.SIZE 3224)
S: * 2 FETCH (RFC822.SIZE 1222)
S: * 3 FETCH (RFC822.SIZE 444)
S: * 4 FETCH (RFC822.SIZE 4516)
S: * 5 FETCH (RFC822.SIZE 544)
S: * 6 FETCH (RFC822.SIZE 922)
S: * 7 FETCH (RFC822.SIZE 31126)
S: * 8 FETCH (RFC822.SIZE 2423)
S: A03 OK Fetch completed.
When the LIST-STATUS IMAP capability [LIST-STATUS] is also available,
the STATUS command can be combined with the LIST command to further
improve efficiency. This way, the sizes of many mailboxes can be
queried with just one LIST command.
Example:
C: A04 LIST "" % RETURN (STATUS (MESSAGES SIZE))
S: * LIST () "." "INBOX"
S: * STATUS "INBOX" (MESSAGES 17 SIZE 16234)
S: * LIST () "." "frop"
S: * STATUS "frop" (MESSAGES 8 SIZE 44421)
S: A04 OK List completed.
4. Formal Syntax
The following syntax specification augments the grammar specified in
[IMAP4rev1] and [IMAP4-ABNF]. It uses the Augmented Backus-Naur Form
(ABNF) notation as specified in [ABNF]. Elements not defined here
are taken from [IMAP4rev1] and [IMAP4-ABNF].
capability =/ "STATUS=SIZE"
status-att =/ "SIZE"
status-att-val =/ "SIZE" SP number64
number64 = 1*DIGIT
; Unsigned 63-bit integer
; (0 <= n <= 9,223,372,036,854,775,807)
5. Security Considerations
There are no known additional security issues with this extension
beyond those described for the base protocol [IMAP4rev1] and the
LIST-STATUS extension [LIST-STATUS].
6. IANA Considerations
IANA has added "STATUS=SIZE" to the "IMAP Capabilities" registry
located at <http://www.iana.org/assignments/imap-capabilities>.
7. Normative References
[ABNF] 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>.
[IMAP4-ABNF]
Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
ABNF", RFC 4466, DOI 10.17487/RFC4466, April 2006,
<https://www.rfc-editor.org/info/rfc4466>.
[IMAP4rev1]
Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>.
[KEYWORDS] 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>.
[KEYWORDS2]
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>.
[LIST-STATUS]
Melnikov, A. and T. Sirainen, "IMAP4 Extension for
Returning STATUS Information in Extended LIST", RFC 5819,
DOI 10.17487/RFC5819, March 2010,
<https://www.rfc-editor.org/info/rfc5819>.
[QUOTA] Myers, J., "IMAP4 QUOTA extension", RFC 2087,
DOI 10.17487/RFC2087, January 1997,
<https://www.rfc-editor.org/info/rfc2087>.
Acknowledgements
Thanks to Bron Gondwana, Alexey Melnikov, Stan Kalisch, and Michael
Slusarz for reviews and suggestions.
Author's Address
Stephan Bosch
Dovecot Oy
Lars Sonckin Kaari 12
Espoo 02600
Finland
Email: stephan.bosch@dovecot.fi
AI ELI5
On this page
§1The Problem of Mailbox Size
Without this extension, a client must retrieve every message size with FETCH and the RFC822.SIZE data item to find the total storage size of a mailbox. That is costly for clients that do not cache the state of the message store, such as most webmail clients.
The RFC's example is a user asking for detailed quota usage per mailbox to find which ones need cleaning up. The QUOTA capability (RFC 2087) does not help, because its QUOTAROOT command only reports STORAGE usage for a whole quota root, not each mailbox inside it.
Adding up every letter's weight takes ages. The post office already knows the total, so you can just ask.
§3The SIZE Data Item
The extension adds one new status data item, SIZE, to the STATUS command and its response. It is the total size of the mailbox in octets. It need not be exact, but it MUST be equal to or greater than the sum of the RFC822.SIZE values of all messages in the mailbox.
When QUOTA is also supported, the value SHOULD equal the storage usage used to enforce the STORAGE limit for that mailbox, so the client can infer quota usage directly.
A server advertises the feature with the capability STATUS=SIZE.
The server gives one number for the whole box. It can round up a bit, but never says the box is lighter than it really is.
§3.1Examples and LIST-STATUS
One STATUS command returns the size along with other items. The same information otherwise needs a SELECT followed by FETCH 1:* (RFC822.SIZE) and manual addition, which the RFC calls less efficient.
When the LIST-STATUS capability (RFC 5819) is available, STATUS can be combined with LIST, so the sizes of many mailboxes come back from one LIST command.
One question gets you the size of one box, and with a bigger question you get the sizes of lots of boxes at once.
- A01 STATUS frop (MESSAGES SIZE UIDNEXT)
- * STATUS frop (MESSAGES 8 SIZE 44421 UIDNEXT 242344)
- A01 OK STATUS completed
§463-bit Values and Formal Syntax
A mailbox can easily exceed 4 GB, so clients MUST be able to receive 63-bit SIZE values. The limit is 63 bits rather than 64 to make implementations easier on platforms such as Java.
The ABNF adds STATUS=SIZE as a capability and SIZE as a status attribute, whose value is SIZE SP number64. A number64 is an unsigned 63-bit integer, from 0 up to 9,223,372,036,854,775,807.
Mailboxes can be huge, so the number field is made big enough for very large totals.
§5Security and IANA Considerations
The RFC says there are no known additional security issues beyond those of the base IMAP protocol (RFC 3501) and the LIST-STATUS extension.
IANA added STATUS=SIZE to the IMAP Capabilities registry.
Telling you a box's size does not open any new doors. A registry keeps a list of the feature names.