IMAP QUOTA Extension

IMAP QUOTA Extension Isode Limited

alexey.melnikov@isode.com https://www.isode.com

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.

Introduction and Overview This document defines a couple of extensions to the Internet Message Access Protocol for querying and manipulating administrative limits on resource usage (quotas). This extension is compatible with both IMAP4rev1 and IMAP4rev2 . The "QUOTA" capability denotes a server compliant with . Some responses and response codes defined in this document are not present in such servers (see 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 . Any server compliant with this document that implements the SETQUOTA command (see ) 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 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 ) without a priori knowledge of the implementation.

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 when, and only when, they appear in all capitals, as shown here. Other capitalized words are IMAP keywords or keywords from this document.

Terms

Resource A resource has a name, a formal definition.

Name The resource name is an atom, as defined in IMAP4rev1. 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.

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, MESSAGE, MAILBOX, and ANNOTATION-STORAGE are defined in this document.

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 . 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 responses, and SHOULD advise the client of which resources are limitable for a particular quota root. A SETQUOTA 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 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 quota always set due to the number of inodes available on the filesystem; similarly, STORAGE may be rounded to the nearest block and limited by free filesystem space.

Definitions

Commands The following commands exist for manipulation and querying quotas.

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 .) 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:

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:

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:

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:

Responses The following responses may be sent by the server.

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:

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:

Response Codes

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: Subject: afternoon meeting C: To: mooch@owatagu.siam.edu.example C: Message-Id: 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) 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: Subject: afternoon meeting C: To: mooch@owatagu.siam.edu.example C: Message-Id: 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:

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.

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 . This document provides a more precise definition.

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 . This document provides a more precise definition.

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.

ANNOTATION-STORAGE "ANNOTATION-STORAGE" is the maximum size of all annotations , 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.

Interaction with IMAP ACL Extension (RFC 4314) This section lists rights required to execute quota-related commands when both RFC 4314 and this document are implemented.

Operations\Rights	r	a	Non
GETQUOTA			+
GETQUOTAROOT	*		*
SETQUOTA		+

See 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.

Formal Syntax The following syntax specification uses the Augmented Backus-Naur Form (ABNF) notation as specified in . Non-terminals referenced but not defined below are as defined by IMAP4 . 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. ]]>

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.

IANA Considerations

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 . 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, . See 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.

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" . 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 . 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: STORAGE

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

MESSAGE

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

MAILBOX

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/responses:	N/A
Extra optional IMAP commands/responses:	N/A
Author:	Alexey Melnikov <alexey.melnikov@isode.com>
Change Controller:	IESG <iesg@ietf.org>
Reference:	Section 5.3 of RFC 9208

ANNOTATION-STORAGE

field name	field value
Name of the quota resource type:	ANNOTATION-STORAGE
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 IMAP commands/responses:	N/A
Extra optional IMAP commands/responses:	N/A
Author:	Alexey Melnikov <alexey.melnikov@isode.com>
Change Controller:	IESG <iesg@ietf.org>
Reference:	Section 5.4 of RFC 9208

Changes Since RFC 2087 This document is a revision of , 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 , 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.

References Normative References Informative References

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 : , , , , , and . This document is a revision of , and it borrows a lot of text from that RFC. Thus, the work of , the author of , is appreciated.

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