| Internet-Draft | IMAP QUOTA | October 2021 |
| Melnikov | Expires 25 April 2022 | [Page] |
This document defines a QUOTA extension of the Internet Message Access Protocol (RFC 3501/RFC 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.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
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This Internet-Draft will expire on 25 April 2022.¶
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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 capitalised words are IMAP keywords [RFC3501][RFC9051] or keywords from this document.¶
This document defines a couple of extensions to the Internet Message Access Protocol [RFC3501] for querying and manipulating administrative limits on resource usage (quotas). This extension is compatible with both IMAP4rev1 [RFC3501] and IMAP4rev2 [RFC9051].¶
The capability "QUOTA", denotes a RFC2087 [RFC2087] compliant server. Some responses and response codes defined in this document are not present in such servers (see Section 12 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 "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 "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 RFC2087 [RFC2087] specified an IMAP4 QUOTA extension, and this has seen deployment in servers, it has seen little deployment in clients. Since the meaning of the resources was left implementation-dependent, it was impossible for a client implementation to determine which resources were supported, and impossible to determine which mailboxes were in a given quota root (see Section 3.2), without a priori knowledge of the implementation.¶
A resource has a name, a formal definition.¶
The resource name is an atom, as defined in IMAP4rev1 [RFC3501]. These MUST be registered with IANA. Implementation specific resources begin with "V-" .¶
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.¶
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 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 which 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.¶
This document introduces a 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]. 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. Further, 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, and similarly STORAGE (Section 5.1) may be rounded to the nearest block and limited by free filesystem space.¶
The following commands exist for manipulation and querying quotas.¶
REQUIRED untagged responses: QUOTA¶
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.¶
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 quotaroot would apply to a mailbox when it gets created.¶
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¶
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 SETQUOTA command requires the server to advertise "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 be 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.¶
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)¶
S: S0001 OK Rounded quota¶
C: S0002 SETQUOTA "!partition/sda4" (STORAGE 99999999)¶
S: * QUOTA "!partition/sda4" (STORAGE 104 10923847)¶
DELETED and DELETED-STORAGE status data items allow to estimate the amount of resource freed by an EXPUNGE on a mailbox.¶
The DELETED status data item requests the server to return the number of messages with \Deleted flag set. The DELETED status data item is only required to be implemented when the server advertises 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 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 RFC822.SIZEs of messages with the \Deleted flag set. The DELETED-STORAGE status data item is only required to be implemented when the server advertises QUOTA=RES-STORAGE capability.¶
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)¶
The following responses may be sent by the server.¶
This response occurs as a result of a GETQUOTA, a GETQUOTAROOT or a SETQUOTA command. The first string is the name of the quota root for which this quota applies.¶
The name is followed by a 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)¶
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.¶
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 code might be sent as a result of an external event (e.g. LMTP delivery or COPY/MOVE/APPEND in another IMAP connection) that causes the currently selected mailbox to exceed soft quota. Note that such 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 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
¶
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.¶
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, 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. Client 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 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 CAPABILITY "QUOTA=RES-STORAGE".¶
A resource named the same was also given as an example in RFC2087 [RFC2087]. This document provides a more precise definition.¶
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 CAPABILITY "QUOTA=RES-MESSAGE".¶
A resource named the same was also given as an example in RFC2087 [RFC2087]. This document provides a more precise definition.¶
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 CAPABILITY "QUOTA=RES-MAILBOX".¶
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 CAPABILITY "QUOTA=RES-ANNOTATION-STORAGE".¶
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 | + |
See Section 4 of RFC 4314 for conventions used in this table.¶
Legend:¶
Note that which permissions are needed in order to perform GETQUOTAROOT command depends on the quota resource type being requested. For example, a quota on 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.¶
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].¶
Except as noted otherwise, all alphabetic characters are case-insensitive. The use of upper or lower case characters to define token strings is for editorial clarity only. Implementations MUST accept these strings in a case-insensitive fashion.¶
"STORAGE" / "MESSAGE" / "MAILBOX" /¶
"ANNOTATION-STORAGE" / resource-name-vnd /¶
resource-name-ext¶
"V-" atom¶
;; Vendor specific, must be registered with IANA.¶
;; The "V-" prefix should be followed by a domain name¶
;; under vendor's control.¶
atom¶
;; Not starting with V- and defined¶
;; in an IETF Stream RFC¶
number64¶
;; must be less than corresponding resource-limit¶
capa-quota-res / "QUOTASET"¶
;; One or more capa-quota-res must be returned.¶
;; Also "QUOTASET" can optionally be returned.¶
"DELETED" / "DELETED-STORAGE"¶
;; DELETED status data item MUST be supported¶
;; when "QUOTA=RES-MESSAGE" capability is¶
;; advertised.¶
;; DELETED-STORAGE status data item MUST be¶
;; supported when "QUOTA=RES-STORAGE" capability¶
;; is advertised.¶
status-att-deleted /¶
status-att-deleted-storage¶
"DELETED" SP number¶
;; DELETED status data item MUST be supported¶
;; when "QUOTA=RES-MESSAGE" capability is¶
;; advertised.¶
"DELETED-STORAGE" SP number64¶
;; DELETED-STORAGE status data item MUST be¶
;; supported when "QUOTA=RES-STORAGE" capability¶
;; is advertised.¶
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 SETQUOTA setting 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 load on servers, such as caching of resource usage information or usage of less precise computations when under heavy load.¶
IMAP4 capabilities are registered by publishing a standards track or IESG approved Informational or Experimental RFC. The registry is currently located at:¶
https://www.iana.org/assignments/imap4-capabilities¶
IANA is requested to update definition of the QUOTA extension to point to this document. IANA is also requested to add the "QUOTASET" capability to the IMAP4 capabilities registry, with this document as the reference.¶
IANA is requested to reserve the prefix "QUOTA=RES-" in the IMAP4 capabilities registry and add a pointer to this document and to the IMAP quota resource type registry (see Section 9.2).¶
IANA is requested to reserve all other capabilities starting with "QUOTA=" prefix for future IETF Stream extensions to this document.¶
IANA is requested to create a new registry for IMAP quota resource types. Registration policy for this registry is "Specification Required". When registering a new quota resource type, the registrant need to provide the following: Name of the quota resource type, Author/Change Controller name and email address, short description, extra (if any) required and optional IMAP commands/responses, and a reference to a specification that describes the quota resource type in more details.¶
Designated Experts should check that provided references are correct, that they describe the quota resource type being registered in sufficient details to be implementable, that syntax of any optional commands/responses is correct (e.g. ABNF validates), and their 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.¶
This document includes initial registrations for the following IMAP quota resource type: STORAGE (Section 5.1), MESSAGE (Section 5.2), MAILBOX (Section 5.3) and "ANNOTATION-STORAGE" (Section 5.4). See Section 9.3 for the registration templates.¶
Dave Cridland wrote lots of text in an earlier draft that became the basis for this document.¶
Editor of this document would like to thank the following people who provided useful comments or participated in discussions that lead to this update to RFC 2087: John Myers, Cyrus Daboo, Lyndon Nerenberg, Benjamin Kaduk, Roman Danyliw, Eric Vyncke.¶
This document is a revision of RFC 2087. It borrows a lot of text from RFC 2087. Thus work of the RFC 2087 author John Myers is appreciated.¶
This document is a revision of RFC 2087. It tries to clarify the meaning of different terms used by RFC 2087. It also provides more examples, gives guidance on allowed server behaviour, defines IANA registry for quota resource types and provides initial registrations for 4 of them.¶
When compared with RFC 2087, this document defines two more commonly used resource type, adds 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.¶