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RFC 8438

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IMAP Extension for STATUS=SIZE

 


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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.

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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.

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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

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   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.

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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>.

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   [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