| Network Working Group | A. Melnikov |
| Internet-Draft | Isode |
| Intended status: Standards Track | June 15, 2020 |
| Expires: December 17, 2020 |
IMAP QUOTA Extension
draft-ietf-extra-quota-01
The QUOTA extension of the Internet Message Access Protocol (RFC 3501) permits administrative limits on resource usage (quotas) to be manipulated through the IMAP protocol.
This memo obsoletes RFC 2087, but attempts to remain backwards compatible whenever possible.
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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 unless specifically mentioned.
Again, for examples, the hierarchy separator on the server is presumed to be "/" throughout. None of these assumptions is required nor recommended by this document.
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 8174 when, and only when, they appear in all capitals, as shown here.
Other capitalised words are IMAP4 keywords or keywords from this document.
The capability "QUOTA", denotes a RFC2087 compliant server. Some commands and responses defined in this document are not present in such servers, 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.
This document also reserves all other capabilities starting with "QUOTA=" prefix to future standard track 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 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-dependant, 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, 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. 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 32 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 constisting of the resource name prefixed by "QUOTA=RES-". For compatability with RFC2087, a client which discovers resources available on the server which are not advertised through this mechanism MUST treat them as if they were completely opaque, and without any meaning.
The resources STORAGE, MESSAGE and MAILBOX are defined in this document.
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 memo 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 arbitary mailboxes or mailbox hierarchies.
Not all resources may be limitable or calculatable 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 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 dependant 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, and similarly STORAGE may be rounded to the nearest block and limited by free filesystem space.
The following commands exist for manipulation and querying quotas.
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. The client can try using any of the resource types returned in CAPABILITY response (i.e. all capability items with "QUOTA=RES-" prefix), however the server is not required to support any specific resource type for any particular quota root.
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.
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.
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.
DELETED-MESSAGES and DELETED-STORAGE status data items allow to estimate the amount of resource freed by an EXPUNGE on a mailbox.
DELETED-MESSAGES status data item requests the server to return the number of messages with \Deleted flag set.
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 following responses may be sent by the server.
This response occurs as a result of a GETQUOTA or GETQUOTAROOT 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.
Example:
S: * QUOTAROOT INBOX ""
S: * QUOTAROOT comp.mail.mime
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:
The OVERQUOTA response code MAY also be returned in an untagged NO response when a mailbox exceeds soft quota. Such responses have 2 forms. If it is followed by a tag, the tag refers to the command that caused this (such as APPEND or COPY) and the OVERQUOTA response code applies to the target mailbox specified by such command. If the OVERQUOTA response code is not followed by the tag, this means that an external event (e.g. LMTP delivery or APPEND/COPY in another IMAP connection) caused this event and the event applies to the currently selected mailbox. In particular, this means that such OVERQUOTA response codes MUST NOT be returned if there is no mailbox selected or if a mailbox other than the currently selected one exceeds soft quota.
Example:
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 mupltiple 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.
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. In absence of information to the contrary, clients conformant to this specification connecting to servers which do not advertise "QUOTA=RES-STORAGE", yet allow a resource named STORAGE, MUST NOT assume that it is the same resource. [CREF1]Is the above restriction useful and will it be obeyed? If the answer is "no", delete it.
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.
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, clients conformant to this specification connecting to servers which do not advertise "QUOTA=RES-MESSAGE", yet allow a resource named MESSAGE, MUST NOT assume that it is the same resource. [CREF2]As above.
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".
[CREF3]Bron to check whether this is a sensible description and whether it is needed at all: 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".
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.
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.
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.
IMAP4 capabilities are registered by publishing a standards track or IESG approved experimental RFC. The registry is currently located at:
http://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 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 and a reference to a specification that describes the quota resource type in more details.
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 details below.
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 quote resource type registry established above.
IANA is requested to reserve all other capabilities starting with "QUOTA=" prefix for future standard track or experimental extensions to this document.
Dave Cridland wrote lots of text in an earlier draft that became the basis for this document.
Editors 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
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 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 3 of them.
When compared with RFC 2087, this document defines one more commonly used resource type, adds optional OVERQUOTA response code and defines two extra STATUS data items ("DELETED-MESSAGES" and "DELETED-STORAGE")
| [ABNF] | Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 5234, January 2008. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC3501] | Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003. |
| [RFC4314] | Melnikov, A., "IMAP4 Access Control List (ACL) Extension", RFC 4314, DOI 10.17487/RFC4314, December 2005. |
| [RFC5257] | Daboo, C. and R. Gellens, "Internet Message Access Protocol - ANNOTATE Extension", RFC 5257, DOI 10.17487/RFC5257, June 2008. |
| [RFC8174] | Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017. |
| [RFC2087] | Myers, J., "IMAP4 QUOTA extension", RFC 2087, DOI 10.17487/RFC2087, January 1997. |