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From: draft-ietf-extra-quota-10                        Proposed Standard
Internet Engineering Task Force (IETF)                       A. Melnikov
Request for Comments: 9208                                         Isode
Obsoletes: 2087                                               March 2022
Category: Standards Track
ISSN: 2070-1721


                          IMAP QUOTA Extension

Abstract

   This document defines a QUOTA extension of the Internet Message
   Access Protocol (IMAP) (see RFCs 3501 and 9051) that permits
   administrative limits on resource usage (quotas) to be manipulated
   through the IMAP protocol.

   This document obsoletes RFC 2087 but attempts to remain backwards
   compatible whenever possible.

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/rfc9208.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   than English.

Table of Contents

   1.  Introduction and Overview
   2.  Document Conventions
   3.  Terms
     3.1.  Resource
       3.1.1.  Name
       3.1.2.  Definition
     3.2.  Quota Root
   4.  Definitions
     4.1.  Commands
       4.1.1.  GETQUOTA
       4.1.2.  GETQUOTAROOT
       4.1.3.  SETQUOTA
       4.1.4.  New STATUS attributes
     4.2.  Responses
       4.2.1.  QUOTA
       4.2.2.  QUOTAROOT
     4.3.  Response Codes
       4.3.1.  OVERQUOTA
   5.  Resource Type Definitions
     5.1.  STORAGE
     5.2.  MESSAGE
     5.3.  MAILBOX
     5.4.  ANNOTATION-STORAGE
   6.  Interaction with IMAP ACL Extension (RFC 4314)
   7.  Formal Syntax
   8.  Security Considerations
   9.  IANA Considerations
     9.1.  Changes/Additions to the IMAP Capabilities Registry
     9.2.  IMAP Quota Resource Type Registry
   10. Changes Since RFC 2087
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Acknowledgments
   Contributors
   Author's Address

1.  Introduction and Overview

   This document defines a couple of extensions to the Internet Message
   Access Protocol [RFC3501] [RFC9051] for querying and manipulating
   administrative limits on resource usage (quotas).  This extension is
   compatible with both IMAP4rev1 [RFC3501] and IMAP4rev2 [RFC9051].

   The "QUOTA" capability denotes a server compliant with [RFC2087].
   Some responses and response codes defined in this document are not
   present in such servers (see Section 10 for more details), and
   clients MUST NOT rely on their presence in the absence of any
   capability beginning with "QUOTA=".

   Any server compliant with this document MUST also return at least one
   capability starting with the "QUOTA=RES-" prefix, as described in
   Section 3.1.

   Any server compliant with this document that implements the SETQUOTA
   command (see Section 4.1.3) MUST also return the "QUOTASET"
   capability.

   This document also reserves all other capabilities starting with the
   "QUOTA=" prefix for future IETF Stream Standard Track, Informational,
   or Experimental extensions to this document.

   Quotas can be used to restrict clients for administrative reasons,
   but the QUOTA extension can also be used to indicate system limits
   and current usage levels to clients.

   Although the IMAP4 QUOTA extension specified in [RFC2087] has seen
   deployment in servers, it has seen little deployment in clients.
   Since the meaning of the resources was implementation dependent, it
   was impossible for a client implementation to determine which
   resources were supported, and it was impossible to determine which
   mailboxes were in a given quota root (see Section 3.2) without a
   priori knowledge of the implementation.

2.  Document Conventions

   In protocol examples, this document uses a prefix of "C: " to denote
   lines sent by the client to the server and "S: " for lines sent by
   the server to the client.  Lines prefixed with "//" are comments
   explaining the previous protocol line.  These prefixes and comments
   are not part of the protocol.  Lines without any of these prefixes
   are continuations of the previous line, and no line break is present
   in the protocol before such lines unless specifically mentioned.

   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.

   Other capitalized words are IMAP keywords [RFC3501] [RFC9051] or
   keywords from this document.

3.  Terms

3.1.  Resource

   A resource has a name, a formal definition.

3.1.1.  Name

   The resource name is an atom, as defined in IMAP4rev1 [RFC3501].
   These MUST be registered with IANA.

   Supported resource names MUST be advertised as a capability by
   prepending the resource name with "QUOTA=RES-".  A server compliant
   with this specification is not required to support all reported
   resource types on all quota roots.

3.1.2.  Definition

   The resource definition or document containing it, while not visible
   through the protocol, SHOULD be registered with IANA.

   The usage of a resource MUST be represented as a 63-bit unsigned
   integer.  0 indicates that the resource is exhausted.  Usage integers
   don't necessarily represent proportional use, so clients MUST NOT
   compare an available resource between two separate quota roots on the
   same or different servers.

   Limits will be specified as, and MUST be represented as, an integer.
   0 indicates that any usage is prohibited.

   Limits may be hard or soft; that is, an implementation MAY choose, or
   be configured, to disallow any command if the limit on a resource is
   or would be exceeded.

   All resources that the server handles MUST be advertised in a
   CAPABILITY response/response code consisting of the resource name
   prefixed by "QUOTA=RES-".

   The resources STORAGE (Section 5.1), MESSAGE (Section 5.2), MAILBOX
   (Section 5.3), and ANNOTATION-STORAGE (Section 5.4) are defined in
   this document.

3.2.  Quota Root

   This document introduces the concept of a "quota root", as resource
   limits can apply across multiple IMAP mailboxes.

   Each mailbox has zero or more implementation-defined named "quota
   roots".  Each quota root has zero or more resource limits (quotas).
   All mailboxes that share the same named quota root share the resource
   limits of the quota root.

   Quota root names need not be mailbox names, nor is there any
   relationship defined by this document between a quota root name and a
   mailbox name.  A quota root name is an astring, as defined in IMAP4
   [RFC3501] [RFC9051].  It SHOULD be treated as an opaque string by any
   clients.

   Quota roots are used since not all implementations may be able to
   calculate usage, or apply quotas, on arbitrary mailboxes or mailbox
   hierarchies.

   Not all resources may be limitable or calculable for all quota roots.
   Furthermore, not all resources may support all limits; some limits
   may be present in the underlying system.  A server implementation of
   this memo SHOULD advise the client of such inherent limits, by
   generating QUOTA (Section 4.2.1) responses, and SHOULD advise the
   client of which resources are limitable for a particular quota root.
   A SETQUOTA (Section 4.1.3) command MAY also round a quota limit in an
   implementation-dependent way, if the granularity of the underlying
   system demands it.  A client MUST be prepared for a SETQUOTA
   (Section 4.1.3) command to fail if a limit cannot be set.

   Implementation Notes: This means that, for example, under UNIX, a
   quota root may have a MESSAGE (Section 5.2) quota always set due to
   the number of inodes available on the filesystem; similarly, STORAGE
   (Section 5.1) may be rounded to the nearest block and limited by free
   filesystem space.

4.  Definitions

4.1.  Commands

   The following commands exist for manipulation and querying quotas.

4.1.1.  GETQUOTA

   Arguments:  quota root

   Responses:  REQUIRED untagged responses: QUOTA

   Result:     OK - getquota completed

               NO - getquota error: no such quota root, permission
               denied

               BAD - command unknown or arguments invalid

   The GETQUOTA command takes the name of a quota root and returns the
   quota root's resource usage and limits in an untagged QUOTA response.
   (Names of quota roots applicable to a particular mailbox can be
   discovered by issuing the GETQUOTAROOT command; see Section 4.1.2.)
   Note that the server is not required to support any specific resource
   type (as advertised in the CAPABILITY response, i.e., all capability
   items with the "QUOTA=RES-" prefix) for any particular quota root.

   Example:

      S: * CAPABILITY [...] QUOTA QUOTA=RES-STORAGE [...]

      [...]

      C: G0001 GETQUOTA "!partition/sda4"

      S: * QUOTA "!partition/sda4" (STORAGE 104 10923847)

      S: G0001 OK Getquota complete

4.1.2.  GETQUOTAROOT

   Arguments:  mailbox name

   Responses:  REQUIRED untagged responses: QUOTAROOT, QUOTA

   Result:     OK - getquotaroot completed

               NO - getquotaroot error: permission denied

               BAD - command unknown or arguments invalid

   The GETQUOTAROOT command takes a mailbox name and returns the list of
   quota roots for the mailbox in an untagged QUOTAROOT response.  For
   each listed quota root, it also returns the quota root's resource
   usage and limits in an untagged QUOTA response.

   Note that the mailbox name parameter doesn't have to reference an
   existing mailbox.  This can be handy in order to determine which
   quota root would apply to a mailbox when it gets created.

   Example:

      S: * CAPABILITY [...] QUOTA QUOTA=RES-STORAGE QUOTA=RES-MESSAGE
      [...]

      [...]

      C: G0002 GETQUOTAROOT INBOX

      S: * QUOTAROOT INBOX "#user/alice" "!partition/sda4"

      S: * QUOTA "#user/alice" (MESSAGE 42 1000)

      S: * QUOTA "!partition/sda4" (STORAGE 104 10923847)

      S: G0002 OK Getquotaroot complete

4.1.3.  SETQUOTA

   Arguments:  quota root list of resource limits

   Responses:  untagged responses: QUOTA

   Result:     OK - setquota completed

               NO - setquota error: can't set that data

               BAD - command unknown or arguments invalid

   Note that unlike other command/responses/response codes defined in
   this document, support for the SETQUOTA command requires the server
   to advertise the "QUOTASET" capability.

   The SETQUOTA command takes the name of a mailbox quota root and a
   list of resource limits.  The resource limits for the named quota
   root are changed to the specified limits.  Any previous resource
   limits for the named quota root are discarded, even resource limits
   not explicitly listed in the SETQUOTA command.  (For example, if the
   quota root had both STORAGE and MESSAGE limits assigned to the quota
   root before the SETQUOTA is called and the SETQUOTA only includes the
   STORAGE limit, then the MESSAGE limit is removed from the quota
   root.)

   If the named quota root did not previously exist, an implementation
   may optionally create it and change the quota roots for any number of
   existing mailboxes in an implementation-defined manner.

   If the implementation chooses to change the quota roots for some
   existing mailboxes, such changes SHOULD be announced with untagged
   QUOTA responses.

   Example:

      S: * CAPABILITY [...] QUOTA QUOTASET QUOTA=RES-STORAGE QUOTA=RES-
      MESSAGE [...]

      [...]

      C: S0000 GETQUOTA "#user/alice"

      S: * QUOTA "#user/alice" (STORAGE 54 111 MESSAGE 42 1000)

      S: S0000 OK Getquota completed

      C: S0001 SETQUOTA "#user/alice" (STORAGE 510)

      S: * QUOTA "#user/alice" (STORAGE 58 512)

      // The server has rounded the STORAGE quota limit requested to
      the nearest 512 blocks of 1024 octets; otherwise, another client
      has performed a near-simultaneous SETQUOTA using a limit of 512.

      S: S0001 OK Rounded quota

      C: S0002 SETQUOTA "!partition/sda4" (STORAGE 99999999)

      S: * QUOTA "!partition/sda4" (STORAGE 104 10923847)

      // The server has not changed the quota, since this is a
      filesystem limit, and it cannot be changed. The QUOTA
      response here is entirely optional.

      S: S0002 NO Cannot change system limit

4.1.4.  New STATUS attributes

   The DELETED and DELETED-STORAGE status data items allow for
   estimation of the amount of resources that could be freed by an
   EXPUNGE on a mailbox.

   The DELETED status data item requests the server to return the number
   of messages with the \Deleted flag set.  The DELETED status data item
   is only required to be implemented when the server advertises the
   "QUOTA=RES-MESSAGE" capability.

   The DELETED-STORAGE status data item requests the server to return
   the amount of storage space that can be reclaimed by performing
   EXPUNGE on the mailbox.  The server SHOULD return the exact value;
   however, it is recognized that the server may have to do a non-
   trivial amount of work to calculate it.  If the calculation of the
   exact value would take a long time, the server MAY instead return the
   sum of the RFC822.SIZE of the messages with the \Deleted flag set.
   The DELETED-STORAGE status data item is only required to be
   implemented when the server advertises the "QUOTA=RES-STORAGE"
   capability.

   Example:

      S: * CAPABILITY [...] QUOTA QUOTA=RES-STORAGE QUOTA=RES-
      MESSAGE [...]

      [...]

      C: S0003 STATUS INBOX (MESSAGES DELETED DELETED-STORAGE)

      S: * STATUS INBOX (MESSAGES 12 DELETED 4 DELETED-STORAGE 8)

      // 12 messages, 4 of which would be deleted when an EXPUNGE
      happens.

      S: S0003 OK Status complete.

4.2.  Responses

   The following responses may be sent by the server.

4.2.1.  QUOTA

      Data: quota root name

      list of resource names, usages, and limits

   This response occurs as a result of a GETQUOTA, GETQUOTAROOT, or
   SETQUOTA command.  The first string is the name of the quota root for
   which this quota applies.

   The name is followed by an S-expression format list of the resource
   usage and limits of the quota root.  The list contains zero or more
   triplets.  Each triplet contains a resource name, the current usage
   of the resource, and the resource limit.

   Resources not named in the list are not limited in the quota root.
   Thus, an empty list means there are no administrative resource limits
   in the quota root.

   Example:

      S: * QUOTA "" (STORAGE 10 512)

4.2.2.  QUOTAROOT

      Data: mailbox name

      zero or more quota root names

   This response occurs as a result of a GETQUOTAROOT command.  The
   first string is the mailbox and the remaining strings are the names
   of the quota roots for the mailbox.

   Examples:

      S: * QUOTAROOT INBOX ""

      // The INBOX mailbox is covered by a single quota root with
      name "".

      S: * QUOTAROOT comp.mail.mime

      // The comp.mail.mime mailbox has no quota root associated
      with it, but one can be created.

4.3.  Response Codes

4.3.1.  OVERQUOTA

   The OVERQUOTA response code SHOULD be returned in the tagged NO
   response to an APPEND/COPY/MOVE when the addition of the message(s)
   puts the target mailbox over any one of its quota limits.

   Example 1:

      C: A003 APPEND saved-messages (\Seen) {326}
      S: + Ready for literal data
      C: Date: Mon, 7 Feb 1994 21:52:25 -0800 (PST)
      C: From: Fred Foobar <foobar@Blurdybloop.example>
      C: Subject: afternoon meeting
      C: To: mooch@owatagu.siam.edu.example
      C: Message-Id: <B27397-0100000@Blurdybloop.example>
      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:
      S: A003 NO [OVERQUOTA] APPEND Failed

   The OVERQUOTA response code MAY also be returned in an untagged NO
   response in the authenticated or the selected state when a mailbox
   exceeds soft quota.  For example, such OVERQUOTA response codes might
   be sent as a result of an external event (e.g., Local Mail Transfer
   Protocol (LMTP) [RFC2033] delivery or COPY/MOVE/APPEND in another
   IMAP connection) that causes the currently selected mailbox to exceed
   soft quota.  Note that such an OVERQUOTA response code might be
   ambiguous because it might relate to the target mailbox (as specified
   in COPY/MOVE/APPEND) or to the currently selected mailbox.  (The
   EXTRA WG chose not to address this deficiency due to syntactic
   limitations of IMAP response codes and because such events are likely
   to be rare.)  This form of the OVERQUOTA response codes MUST NOT be
   returned if there is no mailbox selected and no command in progress
   that adds a message to a mailbox (e.g., APPEND).

   Example 2:

      C: A003 APPEND saved-messages (\Seen) {326}
      S: + Ready for literal data
      C: Date: Mon, 7 Feb 1994 21:52:25 -0800 (PST)
      C: From: Fred Foobar <foobar@Blurdybloop.example>
      C: Subject: afternoon meeting
      C: To: mooch@owatagu.siam.edu.example
      C: Message-Id: <B27397-0100000@Blurdybloop.example>
      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:
      S: * NO [OVERQUOTA] Soft quota has been exceeded
      S: A003 OK [APPENDUID 38505 3955] APPEND completed

   Example 3:

      C: A004 COPY 2:4 MEETING
      S: * NO [OVERQUOTA] Soft quota has been exceeded
      S: A004 OK [COPYUID 38505 304,319:320 3956:3958] COPY
          command completed

5.  Resource Type Definitions

   The following resource types are defined in this memo.  A server
   supporting a resource type MUST advertise this as a CAPABILITY with a
   name consisting of the resource name prefixed by "QUOTA=RES-".  A
   server MAY support multiple resource types and MUST advertise all
   resource types it supports.

5.1.  STORAGE

   "STORAGE" is the physical space estimate, in units of 1024 octets, of
   the mailboxes governed by the quota root.  This MAY not be the same
   as the sum of the RFC822.SIZE of the messages.  Some implementations
   MAY include metadata sizes for the messages and mailboxes, and other
   implementations MAY store messages in such a way that the physical
   space used is smaller, for example, due to use of compression.
   Additional messages might not increase the usage.  Clients MUST NOT
   use the usage figure for anything other than informational purposes;
   for example, they MUST NOT refuse to APPEND a message if the limit
   less the usage is smaller than the RFC822.SIZE divided by 1024 octets
   of the message, but it MAY warn about such condition.

   The usage figure may change as a result of performing actions not
   associated with adding new messages to the mailbox, such as SEARCH,
   since this may increase the amount of metadata included in the
   calculations.

   When the server supports this resource type, it MUST also support the
   DELETED-STORAGE status data item.

   Support for this resource MUST be indicated by the server by
   advertising the "QUOTA=RES-STORAGE" capability.

   A resource named the same was also given as an example in [RFC2087].
   This document provides a more precise definition.

5.2.  MESSAGE

   "MESSAGE" is the number of messages stored within the mailboxes
   governed by the quota root.  This MUST be an exact number; however,
   clients MUST NOT assume that a change in the usage indicates a change
   in the number of messages available, since the quota root may include
   mailboxes the client has no access to.

   When the server supports this resource type, it MUST also support the
   DELETED status data item.

   Support for this resource MUST be indicated by the server by
   advertising the "QUOTA=RES-MESSAGE" capability.

   A resource named the same was also given as an example in [RFC2087].
   This document provides a more precise definition.

5.3.  MAILBOX

   "MAILBOX" is the number of mailboxes governed by the quota root.
   This MUST be an exact number; however, clients MUST NOT assume that a
   change in the usage indicates a change in the number of mailboxes,
   since the quota root may include mailboxes the client has no access
   to.

   Support for this resource MUST be indicated by the server by
   advertising the "QUOTA=RES-MAILBOX" capability.

5.4.  ANNOTATION-STORAGE

   "ANNOTATION-STORAGE" is the maximum size of all annotations
   [RFC5257], in units of 1024 octets, associated with all messages in
   the mailboxes governed by the quota root.

   Support for this resource MUST be indicated by the server by
   advertising the "QUOTA=RES-ANNOTATION-STORAGE" capability.

6.  Interaction with IMAP ACL Extension (RFC 4314)

   This section lists [RFC4314] rights required to execute quota-related
   commands when both RFC 4314 and this document are implemented.

   +===================+=+=+===+===+===+===+===+===+===+===+=====+=====+
   | Operations\Rights |l|r| s | w | i | c | x | t | e | a | Any | Non |
   +===================+=+=+===+===+===+===+===+===+===+===+=====+=====+
   | GETQUOTA          | | |   |   |   |   |   |   |   |   |     |  +  |
   +-------------------+-+-+---+---+---+---+---+---+---+---+-----+-----+
   | GETQUOTAROOT      | |*|   |   |   |   |   |   |   |   |     |  *  |
   +-------------------+-+-+---+---+---+---+---+---+---+---+-----+-----+
   | SETQUOTA          | | |   |   |   |   |   |   |   | + |     |     |
   +-------------------+-+-+---+---+---+---+---+---+---+---+-----+-----+

                                  Table 1

   See Section 4 of [RFC4314] for conventions used in this table.

   Legend:

   "+":    The right is required

   "*":    Only one of the rights marked with * is required

   "Any":  At least one of the "l", "r", "i", "k", "x", or "a" rights is
           required

   "Non":  No rights required to perform the command

   Note that which permissions are needed in order to perform a
   GETQUOTAROOT command depends on the quota resource type being
   requested.  For example, a quota on the number of messages (MESSAGE
   resource type) or total size of messages (STORAGE resource type)
   requires "r" right on the mailbox in question, since the quota
   involved would reveal information about the number (or total size) of
   messages in the mailbox.  By comparison, the MAILBOX resource type
   doesn't require any right.

7.  Formal Syntax

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

   Non-terminals referenced but not defined below are as defined by
   IMAP4 [RFC3501] [RFC9051].

   Except as noted otherwise, all alphabetic characters are case
   insensitive.  The use of uppercase or lowercase characters to define
   token strings is for editorial clarity only.  Implementations MUST
   accept these strings in a case-insensitive fashion.

   getquota =         "GETQUOTA" SP quota-root-name

   getquotaroot =     "GETQUOTAROOT" SP mailbox

   quota-list =       "(" quota-resource *(SP quota-resource) ")"

   quota-resource =   resource-name SP resource-usage SP resource-limit

   quota-response =   "QUOTA" SP quota-root-name SP quota-list

   quotaroot-response =  "QUOTAROOT" SP mailbox *(SP quota-root-name)

   setquota =         "SETQUOTA" SP quota-root-name SP setquota-list

   setquota-list =    "(" [setquota-resource *(SP setquota-resource)]
                      ")"

   setquota-resource =  resource-name SP resource-limit

   quota-root-name =  astring

   resource-limit =   number64

   resource-name =    "STORAGE" / "MESSAGE" / "MAILBOX" /
                      "ANNOTATION-STORAGE" / resource-name-ext

   resource-name-ext =  atom
                      ;; Future resource registrations

   resource-usage =  number64
                      ;; must be less than corresponding resource-limit

   capability-quota = capa-quota-res / "QUOTASET"
                      ;; One or more capa-quota-res must be returned.
                      ;; Also "QUOTASET" can optionally be returned.

   capa-quota-res =   "QUOTA=RES-" resource-name

   status-att =/      "DELETED" / "DELETED-STORAGE"
                      ;; DELETED status data item MUST be supported
                      ;; when the "QUOTA=RES-MESSAGE" capability is
                      ;; advertised.
                      ;; DELETED-STORAGE status data item MUST be
                      ;; supported when the "QUOTA=RES-STORAGE"
                      ;; capability is advertised.

   status-att-val =/  status-att-deleted /
                      status-att-deleted-storage

   status-att-deleted =  "DELETED" SP number
                      ;; DELETED status data item MUST be supported
                      ;; when the "QUOTA=RES-MESSAGE" capability is
                      ;; advertised.

   status-att-deleted-storage =  "DELETED-STORAGE" SP number64
                      ;; DELETED-STORAGE status data item MUST be
                      ;; supported when the "QUOTA=RES-STORAGE"
                      ;; capability is advertised.

   resp-text-code =/  "OVERQUOTA"

   number64 =         <Defined in RFC 9051>

8.  Security Considerations

   Implementors should be careful to make sure the implementation of
   these commands does not violate the site's security policy.  The
   resource usage of other users is likely to be considered confidential
   information and should not be divulged to unauthorized persons.  In
   particular, no quota information should be disclosed to anonymous
   users.

   As for any resource shared across users (for example, a quota root
   attached to a set of shared mailboxes), a user that can consume or
   render unusable the resource can affect the resources available to
   the other users; this might occur, for example, by a user with
   permission to execute the SETQUOTA setting, which sets an
   artificially small value.

   Note that computing resource usage might incur a heavy load on the
   server.  Server implementers should consider implementation
   techniques that lower the load on servers such as caching of resource
   usage information or usage of less precise computations when under
   heavy load.

9.  IANA Considerations

9.1.  Changes/Additions to the IMAP Capabilities Registry

   IMAP4 capabilities are registered by publishing a Standards Track or
   an IESG-approved Informational or Experimental RFC.  The "IMAP
   Capabilities" registry is currently located at
   <https://www.iana.org/assignments/imap4-capabilities>.

   IANA has updated the reference for the QUOTA extension to point to
   this document.  IANA has also added the "QUOTA=" prefix and the
   "QUOTASET" capability to the "IMAP Capabilities" registry with this
   document as the reference.

   IANA has added the following notes to the "IMAP Capabilities"
   registry:

   The prefix "QUOTA=RES-" is reserved per RFC 9208, Section 9.1.  See
   Section 9.2 of that document for values that follow this prefix.

   All other capabilities starting with the "QUOTA=" prefix are reserved
   for future IETF Stream extensions to RFC 9208.

9.2.  IMAP Quota Resource Type Registry

   IANA has created a new registry for IMAP quota resource types.  The
   registration policy for the "IMAP Quota Resource Types" registry is
   "Specification Required" [RFC8126].

   When registering a new quota resource type, the registrant needs to
   provide the following:

   *  the name of the quota resource type

   *  a short description

   *  extra required IMAP commands/responses (if any)

   *  extra optional IMAP commands/responses (if any)

   *  name and email address of author

   *  name and email address of change controller

   *  a reference to a specification that describes the quota resource
      type in more detail

   Designated experts should check that the provided references are
   correct, the references describe the quota resource type being
   registered in sufficient detail to be implementable, the syntax of
   any optional commands/responses is correct (e.g., ABNF validates),
   and the syntax/description complies with rules and limitations
   imposed by IMAP [RFC3501] [RFC9051].  Designated experts should avoid
   registering multiple identical quota resource types under different
   names and should provide advice to requestors about other possible
   quota resource types to use.

   The initial contents of the "IMAP Quota Resource Types" registry are
   as follows:

   +===================+=======================================+
   | field name        | field value                           |
   +===================+=======================================+
   | Name of the quota | STORAGE                               |
   | resource type:    |                                       |
   +-------------------+---------------------------------------+
   | Description:      | The physical space estimate, in units |
   |                   | of 1024 octets, of the mailboxes      |
   |                   | governed by the quota root.           |
   +-------------------+---------------------------------------+
   | Extra required    | DELETED-STORAGE STATUS request data   |
   | IMAP commands/    | item and response data item           |
   | responses:        |                                       |
   +-------------------+---------------------------------------+
   | Extra optional    | N/A                                   |
   | IMAP commands/    |                                       |
   | responses:        |                                       |
   +-------------------+---------------------------------------+
   | Author:           | Alexey Melnikov                       |
   |                   | <alexey.melnikov@isode.com>           |
   +-------------------+---------------------------------------+
   | Change            | IESG <iesg@ietf.org>                  |
   | Controller:       |                                       |
   +-------------------+---------------------------------------+
   | Reference:        | Section 5.1 of RFC 9208               |
   +-------------------+---------------------------------------+

                          Table 2: STORAGE

   +=====================+==========================================+
   | field name          | field value                              |
   +=====================+==========================================+
   | Name of the quota   | MESSAGE                                  |
   | resource type:      |                                          |
   +---------------------+------------------------------------------+
   | Description:        | The number of messages stored within the |
   |                     | mailboxes governed by the quota root.    |
   +---------------------+------------------------------------------+
   | Extra required IMAP | DELETED STATUS request data item and     |
   | commands/responses: | response data item                       |
   +---------------------+------------------------------------------+
   | Extra optional IMAP | N/A                                      |
   | commands/responses: |                                          |
   +---------------------+------------------------------------------+
   | Author:             | Alexey Melnikov                          |
   |                     | <alexey.melnikov@isode.com>              |
   +---------------------+------------------------------------------+
   | Change Controller:  | IESG <iesg@ietf.org>                     |
   +---------------------+------------------------------------------+
   | Reference:          | Section 5.2 of RFC 9208                  |
   +---------------------+------------------------------------------+

                            Table 3: MESSAGE

   +==================================+=============================+
   | field name                       | field value                 |
   +==================================+=============================+
   | Name of the quota resource type: | MAILBOX                     |
   +----------------------------------+-----------------------------+
   | Description:                     | The number of mailboxes     |
   |                                  | governed by the quota root. |
   +----------------------------------+-----------------------------+
   | Extra required IMAP commands/    | N/A                         |
   | responses:                       |                             |
   +----------------------------------+-----------------------------+
   | Extra optional IMAP commands/    | N/A                         |
   | responses:                       |                             |
   +----------------------------------+-----------------------------+
   | Author:                          | Alexey Melnikov             |
   |                                  | <alexey.melnikov@isode.com> |
   +----------------------------------+-----------------------------+
   | Change Controller:               | IESG <iesg@ietf.org>        |
   +----------------------------------+-----------------------------+
   | Reference:                       | Section 5.3 of RFC 9208     |
   +----------------------------------+-----------------------------+

                            Table 4: MAILBOX

   +================+=======================================+
   | field name     | field value                           |
   +================+=======================================+
   | Name of the    | ANNOTATION-STORAGE                    |
   | quota resource |                                       |
   | type:          |                                       |
   +----------------+---------------------------------------+
   | Description:   | The maximum size of all annotations   |
   |                | [RFC5257], in units of 1024 octets,   |
   |                | associated with all messages in the   |
   |                | mailboxes governed by the quota root. |
   +----------------+---------------------------------------+
   | Extra required | N/A                                   |
   | IMAP commands/ |                                       |
   | responses:     |                                       |
   +----------------+---------------------------------------+
   | Extra optional | N/A                                   |
   | IMAP commands/ |                                       |
   | responses:     |                                       |
   +----------------+---------------------------------------+
   | Author:        | Alexey Melnikov                       |
   |                | <alexey.melnikov@isode.com>           |
   +----------------+---------------------------------------+
   | Change         | IESG <iesg@ietf.org>                  |
   | Controller:    |                                       |
   +----------------+---------------------------------------+
   | Reference:     | Section 5.4 of RFC 9208               |
   +----------------+---------------------------------------+

                  Table 5: ANNOTATION-STORAGE

10.  Changes Since RFC 2087

   This document is a revision of [RFC2087], and it aims to clarify the
   meaning of different terms that were used in that RFC.  It also
   provides more examples, gives guidance on allowed server behavior,
   defines an IANA registry for quota resource types, and provides
   initial registrations for 4 of them.

   When compared with [RFC2087], this document defines two more commonly
   used resource types, adds an optional OVERQUOTA response code, and
   defines two extra STATUS data items ("DELETED" and "DELETED-
   STORAGE").  The DELETED STATUS data item must be implemented if the
   "QUOTA=RES-MESSAGE" capability is advertised.  The DELETED-STORAGE
   STATUS data item must be implemented if the "QUOTA=RES-STORAGE"
   capability is advertised.  For extensibility, quota usage and quota
   limits are now 63-bit unsigned integers.

11.  References

11.1.  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>.

   [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>.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <https://www.rfc-editor.org/info/rfc3501>.

   [RFC4314]  Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
              RFC 4314, DOI 10.17487/RFC4314, December 2005,
              <https://www.rfc-editor.org/info/rfc4314>.

   [RFC5257]  Daboo, C. and R. Gellens, "Internet Message Access
              Protocol - ANNOTATE Extension", RFC 5257,
              DOI 10.17487/RFC5257, June 2008,
              <https://www.rfc-editor.org/info/rfc5257>.

   [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>.

11.2.  Informative References

   [RFC2033]  Myers, J., "Local Mail Transfer Protocol", RFC 2033,
              DOI 10.17487/RFC2033, October 1996,
              <https://www.rfc-editor.org/info/rfc2033>.

   [RFC2087]  Myers, J., "IMAP4 QUOTA extension", RFC 2087,
              DOI 10.17487/RFC2087, January 1997,
              <https://www.rfc-editor.org/info/rfc2087>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

Acknowledgments

   The editor of this document would like to thank the following people
   who provided useful comments or participated in discussions that lead
   to this update of [RFC2087]: John Myers, Cyrus Daboo, Lyndon
   Nerenberg, Benjamin Kaduk, Roman Danyliw, and √Čric Vyncke.

   This document is a revision of [RFC2087], and it borrows a lot of
   text from that RFC.  Thus, the work of John Myers, the author of
   [RFC2087], is appreciated.

Contributors

   Dave Cridland wrote a lot of text in an earlier draft version that
   became the basis for this document.

Author's Address

   Alexey Melnikov
   Isode Limited
   Email: alexey.melnikov@isode.com
   URI:   https://www.isode.com