RFC9208: IMAP QUOTA Extension
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Obsoletes:
RFC2087
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.
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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 Éric 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