IMAP QUOTA Extension
draft-melnikov-extra-quota-00

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Network Working Group                                        D. Cridland
Internet-Draft
Intended status: Standards Track                             A. Melnikov
Expires: September 12, 2019                                        Isode
                                                          March 11, 2019

                          IMAP QUOTA Extension
                     draft-melnikov-extra-quota-00

Abstract

   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.

Status of This Memo

   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 September 12, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   than English.

Table of Contents

   1.  Document Conventions  . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction and Overview . . . . . . . . . . . . . . . . . .   3
   3.  Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Resource  . . . . . . . . . . . . . . . . . . . . . . . .   4
       3.1.1.  Name  . . . . . . . . . . . . . . . . . . . . . . . .   4
       3.1.2.  Definition  . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Quota Root  . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Commands  . . . . . . . . . . . . . . . . . . . . . . . .   5
       4.1.1.  GETQUOTA  . . . . . . . . . . . . . . . . . . . . . .   5
       4.1.2.  GETQUOTAROOT  . . . . . . . . . . . . . . . . . . . .   6
       4.1.3.  SETQUOTA  . . . . . . . . . . . . . . . . . . . . . .   7
       4.1.4.  New STATUS attributes . . . . . . . . . . . . . . . .   8
     4.2.  Responses . . . . . . . . . . . . . . . . . . . . . . . .   8
       4.2.1.  QUOTA . . . . . . . . . . . . . . . . . . . . . . . .   8
       4.2.2.  QUOTAROOT . . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Response Codes  . . . . . . . . . . . . . . . . . . . . .   9
       4.3.1.  OVERQUOTA . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Resource Type Definitions . . . . . . . . . . . . . . . . . .  10
     5.1.  STORAGE . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  MESSAGE . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.3.  MAILBOX . . . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Formal syntax . . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  13
   10. Changes since RFC 2087  . . . . . . . . . . . . . . . . . . .  14
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     11.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

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1.  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 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", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC2119 [RFC2119].

   Other capitalised words are IMAP4 [RFC3501] keywords or keywords from
   this document.

2.  Introduction and Overview

   The capability "QUOTA", denotes a RFC2087 [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 [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.

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3.  Terms

3.1.  Resource

   A resource has a name, a formal definition.

3.1.1.  Name

   [[CREF1: Fix IANA considerations section.]]

   The resource name is an atom, as defined in IMAP4 [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.

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 32 bit unsigned
   integer.  0 indicates that the resource is exhausted.  Usage integers
   MUST NOT represent proportional use, such that a client can compare
   available resource between two separate quota roots or servers with
   reasonable accuracy.

   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 [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 (Section 5.1), MESSAGE (Section 5.2) and
   MAILBOX (Section 5.3) are defined in this document.

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3.2.  Quota Root

   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
   [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 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 (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 dependant 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.

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

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   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.

      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: no such mailbox, permission denied
      BAD - command unknown or arguments invalid

   The GETQUOTAROOT command takes the name of a mailbox 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.

   [[CREF2: Need to clarify that the mailbox name 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.]]

      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

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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

   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.

   [[CREF3: Should the server be sending untagged QUOTA responses for
   all side effect changes?]]

      Example:

      S: * CAPABILITY [...] QUOTA 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 octects, or else 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 cannot be changed.  The QUOTA response here
      is entirely optional.

      S: S0002 NO Cannot change system limit

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4.1.4.  New STATUS attributes

   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.

      Example:

      S: * CAPABILITY [...] QUOTA QUOTA=RES-STORAGE QUOTA-RES-MESSAGE
      [...]
      [...]
      C: S0003 STATUS INBOX (MESSAGES DELETED-MESSAGES DELETED-STORAGE)
      S: * STATUS INBOX (MESSAGES 12 DELETED-MESSAGES 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 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.

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   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.

   Example:

   S: * QUOTAROOT INBOX ""

   S: * QUOTAROOT comp.mail.mime

4.3.  Response Codes

4.3.1.  OVERQUOTA

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

   Example:

      S: C: A003 APPEND Drafts (\Seen $MDNSent) {310}
      S: + Ready for literal data
      C: Date: Mon, 7 Feb 1994 21:52:25 -0800 (PST)
      C: From: Fred Foobar <foobar@Blurdybloop.COM>
      C: Subject: afternoon meeting
      C: To: mooch@owatagu.siam.edu
      C: Message-Id: <B27397-0100000@Blurdybloop.COM>
      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 when the currently selected mailbox exceeds soft quota.
   [[CREF4: What about per-user quotas when no mailbox is selected or

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   when the destination mailbox (different from the currently selected
   one) is affected?]] The response code MUST be followed by the tag of
   the command that caused this (such as APPEND or COPY).  The tag MUST
   be omitted if an external event (e.g.  LMTP delivery or APPEND/COPY
   in another IMAP connection) caused this event.

   Example:

      S: C: A003 APPEND Drafts (\Seen $MDNSent) {310}
      S: + Ready for literal data
      C: Date: Mon, 7 Feb 1994 21:52:25 -0800 (PST)
      C: From: Fred Foobar <foobar@Blurdybloop.COM>
      C: Subject: afternoon meeting
      C: To: mooch@owatagu.siam.edu
      C: Message-Id: <B27397-0100000@Blurdybloop.COM>
      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 A003] Soft quota has been exceeded
      S: A003 OK [APPENDUID 38505 3955] APPEND 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 mupltiple resource types, and MUST advertise all
   resource types it supports.

5.1.  STORAGE

   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,

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   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
   [RFC2087], 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.
   [[CREF5: ?]]

5.2.  MESSAGE

   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
   [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.

5.3.  MAILBOX

   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".

6.  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].

   Except as noted otherwise, all alphabetic characters are case-
   insensitive.  The use of upper or lower case characters to define

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   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     =  number

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

   resource-name-vnd  =  "V-" atom
                       ;; Vendor specific, must be registered with IANA.
                       ;; The "V-" prefix should be followed by a domain
                       name
                       ;; under vendor's control.

   resource-name-ext  =  atom
                       ;; Not starting with V- and defined
                       ;; in a Standard Track or Experimental RFC

   resource-names =    "(" [resource-name *(SP resource-name)] ")"

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

   capability-quota   =  capa-quota-res

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

   status-att         =/  "DELETED-MESSAGES" / "DELETED-STORAGE"

                       [[CREF6: Should this be optional unless the
                       server implements MESSAGE/STORAGE?]]

   resp-text-code     =/  "OVERQUOTA" [SP tag]

7.  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.

8.  IANA Considerations

   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.

   [[CREF7: Define registry for "QUOTA=RES-" and add initial
   registrations.]]

9.  Acknowledgments

   Authors 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.

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10.  Changes since RFC 2087

   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")

11.  References

11.1.  Normative References

   [ABNF]     Crocker, D., Ed. and P. Overell, Ed., "Augmented BNF for
              Syntax Specifications: ABNF", RFC 4234, October 2005.

   [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>.

11.2.  Informative References

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

Authors' Addresses

   Dave A. Cridland

   Email: dave@cridland.net

   Alexey Melnikov
   Isode Limited

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

Cridland & Melnikov    Expires September 12, 2019              [Page 14]