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Versions: 00 01 02 03 04 05 06 07 08 09 10                              
IMAPEXT Working Group                                        A. Melnikov
Internet Draft                                                    Editor
Document: draft-ietf-imapext-acl-04.txt                        June 2002


                          IMAP4 ACL extension

Status of this Memo

   This document is an Internet Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet Drafts are working documents of the Internet Engineering
   Task Force (IETF), its Areas, and its Working Groups.  Note that
   other groups may also distribute working documents as Internet
   Drafts. Internet Drafts are draft documents valid for a maximum of
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   Internet Drafts as reference material or to cite them other than as
   ``work in progress''.

     The list of current Internet-Drafts can be accessed at
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     Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or
     munnari.oz.au.

   A revised version of this draft document will be submitted to the RFC
   editor as a Proposed Standard for the Internet Community.  Discussion
   and suggestions for improvement are requested.  Distribution of this
   draft is unlimited.


0.   Open issues

   This section will be removed when the draft will be published as RFC.
   It is intended to simplify discussion.

    1). Should the document define exact syntax for identifiers starting
        with '$'? For example, the following was proposed by Mark Crispin:

      Definable identifiers.  Server implementations are *NOT* required to
      use any of these, but if they do, these are their semantics.  Note
      that definable identifiers may expand to other definable identifiers.

      $G$xxx    global identifier xxx.  Valid everywhere
      $U$xxx    per-user identifier xxx.  Valid only for the logged-in user
      $M$xxx    per-mailbox identifer xxx.  Valid only for this mailbox
      $xxxxx    reserved for future definition

      Non-definable identifiers.  Server implementations SHOULD support $WORLD,
      and MUST support anyone and user names.  However, a server can respond
      with a NO.

      $WORLD            all authorized users but not anonymous
      $ANONYMOUS                anonymous users
      $DISABLE          disabled rights

    2). Do we need a mechanism to encode login names if they start with
        $, -?

    3). Should the annotations (ANNOTATE) be controlled by "w" right?

    4). Other editorial comments/questions are marked in the text with << and >>.


1.   Abstract

   The ACL extension of the Internet Message Access Protocol [IMAP4]
   permits access control lists to be manipulated through the IMAP
   protocol.


2.   Conventions Used in this Document

   In examples, "C:" and "S:" indicate lines sent by the client and
   server respectively.

   In all examples "/" character is used as hierarchy separator.

   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 RFC 2119 [KEYWORDS].


3.   Introduction and Overview

   The ACL extension is present in any IMAP4 implementation which
   returns a capability starting with "ACL2=" as one of the supported
   capabilities to the CAPABILITY command.

   An access control list is a set of <identifier,rights> pairs.

   Identifier is a UTF-8 string. All user name strings accepted by the LOGIN or
   AUTHENTICATE commands to authenticate to the IMAP server are reserved
   as identifiers for the corresponding user.  Identifiers starting with
   a dash ("-") are reserved for "negative rights", described below.
   Several special identifiers are defined by this document:
    a). The identifier "$anyone" is reserved to refer to the universal identity
        (all authentications, including anonymous).
    b). The identifier "$authuser" is reserved to refer to all authenticated users,
        but not anonymous.
    c). The identifier "$owner" refers to the owner of a mailbox (if any).
    d). The identifier "$administrators" refers to all users with administrative rights.

   Note that a server is not required to implement any special identifier mentioned
   above. However if it allows a user to perform ACL operations on any one of them,
   server MUST use the semantic as described above.

   All other identifiers starting with a dollar sign ("$") are reserved for
   groups and implementation defined aliases. (As a consequence, user name strings
   accepted by LOGIN or AUTHENTICATE MUST NOT start with "$".) All other identifier
   strings are interpreted in an implementation-defined manner.

   Rights is a string listing a (possibly empty) set of alphanumeric
   characters, each character listing a set of operations which is being
   controlled.  Letters are reserved for ''standard'' rights, listed
   below.  The set of standard rights may only be extended by a
   standards-track document.  Digits are reserved for implementation or
   site defined rights.  The currently defined standard rights are:

   l - lookup (mailbox is visible to LIST/LSUB commands, SUBSCRIBE mailbox)
   r - read (SELECT the mailbox, perform STATUS, CHECK, FETCH, SEARCH,
       COPY from mailbox)
   s - keep seen/unseen information across sessions (set or clear \SEEN flag
       via STORE, APPEND or COPY)
   w - write (set or clear flags other than \SEEN and \DELETED via STORE,
       APPEND or COPY)
   i - insert (perform APPEND, COPY into mailbox)
   p - post (send mail to submission address for mailbox,
       not enforced by IMAP4 itself)
   c - create mailboxes (CREATE new sub-mailboxes in any
       implementation-defined hierarchy, parent mailbox for the new
       mailbox name in RENAME).
       When a new mailbox is created it SHOULD inherit rights from
       the parent mailbox (if one exists) in the defined hierarchy.
   x - delete mailbox (DELETE mailbox, old mailbox name in RENAME)
   t - delete messages (set or clear \DELETED flag via STORE, set \DELETED flag
       during APPEND/COPY)
   e - perform EXPUNGE and expunge as a part of CLOSE
   d - This right is defined for backward compatibility with ACL
       extension (RFC 2086). If a client sets "d" right, the server MUST
       set "x", "e" and "t" rights. When the client clears the "d" right,
       the server MUST clear "x", "e" and "t" rights. When all three of "x",
       "e" and "t" are set, the server MUST return "d" right in response to
       a GETACL command. If "x", "e" and "t" rights are not tied together,
       "d" right MUST NOT be returned in a LISTRIGHTS response.
   a - administer (perform SETACL and DELETEACL)

   An implementation may tie rights together or may force rights to
   always or never be granted to particular identifiers.  For example,
   in an implementation that uses unix mode bits, the rights "wisd" are
   tied, the "a" right is always granted to the owner of a mailbox and
   is never granted to another user.  If rights are tied in an
   implementation, the implementation must be conservative in granting
   rights in response to SETACL commands--unless all rights in a tied
   set are specified, none of that set should be included in the ACL
   entry for that identifier.  A client may discover the set of rights
   which may be granted to a given identifier in the ACL for a given
   mailbox by using the LISTRIGHTS command.

   When an identifier in an ACL starts with a dash ("-"), that indicates
   that associated rights are to be removed from the identifier that is
   prefixed by the dash. This is referred to as a "negative right". For
   example, if the identifier "-fred" is granted the "w" right, that
   indicates that the "w" right is to be removed from users matching the
   identifier "fred".  Server implementations are not required to
   support "negative right" identifiers.

   It is possible for multiple identifiers in an access control list to
   apply to a given user (or other authentication identity).  For
   example, an ACL may include rights to be granted to the identifier
   matching the user, one or more implementation-defined identifiers
   matching groups which include the user, and/or the identifier
   "anyone". How these rights are combined to determine the user's
   access is implementation-defined. The set of rules that describes
   how access is calculated is defined by a rule identifier (rule-ID).

   A client may determine the set of rights granted to the logged-in user
   for a given mailbox by using the MYRIGHTS command.

   If a server implementing ACL2 uses the union of the rights granted to
   the applicable identifiers minus the union of the negative rights
   in order to calculate access, it MUST report "ACL2=UNION" in the server's
   capability list.

   An implementation may instead choose to only use those rights granted
   to the most specific identifier present in the ACL. In this case the
   server MUST report "ACL2=MOST-SPECIFIC" in the server's capability
   list.

   If the server implements any other policy for rights calculation,
   it MUST be either registered with IANA using the template provided in 7.1
   or start with "X-". The server MUST report "ACL2=<rule-ID>" capability.


3.1. Rights required to perform different IMAP4rev1 commands

   Before executing a command an ACL2 compliant server must check which rights
   are required to perform it. This section groups command by functions
   they perform and list the rights required. It also gives the detailed description
   of any special processing required.

   Listing and subscribing/unsubscribing mailboxes:
    LIST - "l" right is required.

    Note, that if the user has "l" right to a mailbox "A/B", but not to its parent
    mailbox "A", the LIST command should behave as if the mailbox "A" doesn't exist,
    for example:
               C: A777 LIST "" *
               S: * LIST (\NoInferiors) "/" "A/B"
               S: * LIST () "/" "C"
               S: * LIST (\NoInferiors) "/" "C/D"
               S: A777 OK LIST completed

    SUBSCRIBE - "l" right is required only if the server checks for mailbox existence
           when performing SUBSCRIBE.

<<Allow if the user has "r" right?>>

    UNSUBSCRIBE - no rights required to perform this operation.

    LSUB - "l" right is required only if the server checks for mailbox existence when
           performing SUBSCRIBE.

   Mailbox management:
    CREATE - "c" right on a nearest existing parent mailbox. When a new mailbox
             is created it SHOULD inherit rights from the parent mailbox (if one exists)
             in the defined hierarchy.

    DELETE - "x" right on the mailbox.

    RENAME - Moving a mailbox from one parent to another
             requires "x" right on the mailbox itself and "c" right for the new parent.
             For example, if the user wants to rename mailbox named "A/B/C"
             to "D/E", the user must have "x" right
             for the mailbox "A/B/C" and "c" right for the mailbox "D".

   Copying or appending messages:

    Before performing a COPY/APPEND command the server MUST check if the user has "i" right
    for the target mailbox. If the user doesn't have "i" right, the operation fails.
    Otherwise for each copied/appended message the server MUST check if the user has
     "t" right - when the message has \Deleted flag set
     "s" right - when the message has \Seen flag set
     "w" right for all other message flags.
    Only when the user has a particular right the corresponding flags are stored for the
    newly created message. The server MUST NOT fail a COPY/APPEND if the user has no rights
    to set a particular flag.

<<Add an example here?>>

   Expunging the selected mailbox:
    EXPUNGE - "e" right on the selected mailbox.

    CLOSE - "e" right on the selected mailbox. Server MUST NOT fail CLOSE operation if it is
            unable to expunge the mailbox.

   Fetch information about a mailbox and its messages:
    SELECT/EXAMINE/STATUS - "r" right on the mailbox.

    CHECK - "r" right on the selected mailbox *.

    SEARCH - "r" right on the selected mailbox *.

    FETCH - "r" right on the selected mailbox *, "s" right is required when FETCH implicitly
            sets \Seen flag. FETCH operation MUST NOT fail if the server is required, but
            unable to set \Seen flag.

* - no need to check explicitly, as "r" is required to open a mailbox in the first place.

   Changing flags:
    STORE - the server MUST check if the user has
     "t" right - when the user modifies \Deleted flag
     "s" right - when the user modifies \Seen flag
     "w" right for all other message flags.
     STORE operation SHOULD NOT fail if the user has rights to modify at least one flag
     specified in the STORE.

   Changing ACLs:
    SETACL/DELETEACL/REPLACEACL - "a" right on the mailbox.

   Reading ACLs:
    GETACL - "a" right on the mailbox.
<<Several implementations allow GETACL with "r" right. If this is allowed, GETACL can
  disclose some identifiers existing on a mail system>>

    LISTRIGHTS -
<<Same rights as for GETACL?>>

    MYRIGHTS - any of the following rights is required to perform the operation:
               "l", "r", "i", "c", "x", "e", "a".


4.   Commands


4.1. SETACL

   Arguments:  mailbox name
               authentication identifier
               access right modification

   Data:       no specific data for this command

   Result:     OK - setacl completed
               NO - setacl failure: can't set acl
               BAD - command unknown or arguments invalid

      The SETACL command changes the access control list on the
      specified mailbox so that the specified identifier is granted
      permissions as specified in the third argument.

      The third argument is a string containing an optional plus ("+")
      or minus ("-") prefix, followed by zero or more rights characters.
      If the string starts with a plus, the following rights are added
      to any existing rights for the identifier.  If the string starts
      with a minus, the following rights are removed from any existing
      rights for the identifier.  If the string does not start with a
      plus or minus, the rights replace any existing rights for the
      identifier.


4.2. DELETEACL

   Arguments:  mailbox name
               authentication identifier

   Data:       no specific data for this command

   Result:     OK - deleteacl completed
               NO - deleteacl failure: can't delete acl
               BAD - command unknown or arguments invalid

      The DELETEACL command removes any <identifier,rights> pair for the
      specified identifier from the access control list for the
      specified mailbox.


4.3. REPLACEACL

   Arguments:  mailbox name
               list of (authentication identifier, access rights) pairs

   Data:       no specific data for this command

   Result:     OK - replaceacl completed
               NO - replaceacl failure: can't replace acl
               BAD - command unknown or arguments invalid

      The REPLACEACL command replaces the access control list of the
      specified mailbox with the one provided as the second parameter to
      REPLACEACL. This command is semantically equivalent to the following
      sequence of commands:

      1). GETACL <mailbox name>
      2). For each (authentication identifier AID, access rights RD) pair returned
          in the untagged ACL response that was caused by GETACL, perform
           DELETEACL <mailbox name> AID
      3). For each (authentication identifier AIA, access rights RA) pair from
          the second parameter of REPLACEACL perform
           SETACL <mailbox name> AIA RA


4.4. GETACL

   Arguments:  mailbox name

   Data:       untagged responses: ACL

   Result:     OK - getacl completed
               NO - getacl failure: can't get acl
               BAD - command unknown or arguments invalid

      The GETACL command returns the access control list for mailbox in
      an untagged ACL reply.

   Example:    C: A002 GETACL INBOX
               S: * ACL INBOX Fred rwipslextda
               S: A002 OK Getacl complete


4.5. LISTRIGHTS

   Arguments:  mailbox name
               authentication identifier

   Data:       untagged responses: LISTRIGHTS

   Result:     OK - listrights completed
               NO - listrights failure: can't get rights list
               BAD - command unknown or arguments invalid

      The LISTRIGHTS command takes a mailbox name and an identifier and
      returns information about what rights may be granted to the
      identifier in the ACL for the mailbox.

   Example:    C: a001 LISTRIGHTS ~/Mail/saved smith
               S: * LISTRIGHTS ~/Mail/saved smith la r swi cdext
               S: a001 OK Listrights completed


               C: a005 LISTRIGHTS archive.imap anyone
               S: * LISTRIGHTS archive.imap anyone "" l r s w i p c dtex a
               0 1 2 3 4 5 6 7 8 9
               S: a005 OK Listrights completed


4.6. MYRIGHTS

   Arguments:  mailbox name

   Data:       untagged responses: MYRIGHTS

   Result:     OK - myrights completed
               NO - myrights failure: can't get rights
               BAD - command unknown or arguments invalid

      The MYRIGHTS command returns the set of rights that the user has
      to mailbox in an untagged MYRIGHTS reply.

      The user must have any of the following rights to perform this operation:
        "l", "r", "i", "c", "x", "e", "a".
      If the user doesn't have any right from the above list, the server
      MUST behave as if the mailbox doesn't exist.


   Example:    C: A003 MYRIGHTS INBOX
               S: * MYRIGHTS INBOX rwipsldexta
               S: A003 OK Myrights complete


5.   Responses


5.1. ACL

   Data:       mailbox name
               zero or more identifier rights pairs

      The ACL response occurs as a result of a GETACL command.  The
      first string is the mailbox name for which this ACL applies.  This
      is followed by zero or more pairs of strings, each pair contains
      the identifier for which the entry applies followed by the set of
      rights that the identifier has.


5.2. LISTRIGHTS

   Data:       mailbox name
               identifier
               required rights
               list of optional rights

      The LISTRIGHTS response occurs as a result of a LISTRIGHTS
      command.  The first two strings are the mailbox name and
      identifier for which this rights list applies.  Following the
      identifier is a string containing the (possibly empty) set of
      rights the identifier will always be granted in the mailbox.
      Following this are zero or more strings each containing a set of
      rights the identifier may be granted in the mailbox.  Rights
      mentioned in the same string are tied together--either all must be
      granted to the identifier in the mailbox or none may be granted.

      The same right may not be listed more than once in the LISTRIGHTS
      command.


5.3. MYRIGHTS untagged response

   Data:       mailbox name
               rights

      The MYRIGHTS response occurs as a result of a MYRIGHTS command.
      The first string is the mailbox name for which these rights apply.
      The second string is the set of rights that the client has.


5.4. MYRIGHTS response code

   Data:       rights

      The MYRIGHTS response code is sent in an untagged OK response
      that results from SELECT/EXAMINE. Also this response code can be
      sent at any time after opening a mailbox when the server detects
      that the set of rights allowed for the logged in user has changed.

      The MYRIGHT response code is equivalent to MYRIGHTS untagged response
      for the selected mailbox.


6.   Formal Syntax

   Formal syntax is defined using ABNF [ABNF] as modified by [IMAP4].
   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.


   acl_data        = "ACL" SPACE mailbox *(SPACE identifier SPACE rights)

   deleteacl       = "DELETEACL" SPACE mailbox SPACE identifier

   getacl          = "GETACL" SPACE mailbox

   identifier      = astring
                           ;; UTF-8

   listrights      = "LISTRIGHTS" SPACE mailbox SPACE identifier

   listrights_data = "LISTRIGHTS" SPACE mailbox SPACE identifier
                           SPACE rights *(SPACE rights)

   mod_rights      = astring
                           ;; +rights to add, -rights to remove

   myrights        = "MYRIGHTS" SPACE mailbox

   myrights_data   = "MYRIGHTS" SPACE mailbox SPACE rights

   myrights_rspcod = "MYRIGHTS" SPACE rights

   replaceacl      = "REPLACEACL" SPACE mailbox *(SPACE identifier SPACE rights)

   resp-text-code  =/ myrights_data

   rights          = astring

   setacl          = "SETACL" SPACE mailbox SPACE identifier SPACE mod_rights


7.   IANA considerations

7.1.  ACL access calculation rule Registration Template

   When an access calculation rule for ACL2 extension is registered, the
   following information is supplied:

   Rule Identification: specify a string that identifies this
      rule.  Unless the rule is registered with the IANA, the
      rule's identification must start with "X-".
      The server supporting a particular rule <rule-ID> MUST report
      "ACL2=<rule-ID>" in the capability list.

   Rule Semantics: specify how access rights for a mailbox are calculated.

   Negative rights allowed: specify whether "negative right" identifiers are
      allowed.

   Groups allowed: specify whether group identifiers are allowed.

   Special Identifiers: describe whether any implementation defined
      aliases are allowed and define their meaning.

   Contact Information: specify the postal and electronic contact
      information for the author of the feature.


8.   Initial Registrations

8.1.  Registration: UNION access calculation rule

   Rule Identification: UNION

   Rule Semantics: the union of the rights granted to
   the applicable identifiers minus the union of the negative rights.

   Negative rights allowed: Yes.

   Groups allowed: Yes, but not required.

   Special Identifiers: No.

   Contact Information: c.f., the "Editor's Address" section of this
      memo


8.2.  Registration: MOST-SPECIFIC access calculation rule

   Rule Identification: MOST-SPECIFIC

   Rule Semantics: the rights granted to the most specific identifier
      present in the ACL are used, i.e. if the user identifier is present,
      its ACL is used. If no user identifier is present, but a group that
      includes this user as a member is present, the group ACL is used.
      If neither user, nor group identifier is present, but an ACL for
      a special group "anyone" is present, the ACL for "anyone" is used.

   Negative rights allowed: No.

   Groups allowed: Yes, but not required.

   Special Identifiers: No.

   Contact Information: c.f., the "Editor's Address" section of this
      memo


9.   Security Considerations

   An implementation must make sure the ACL commands themselves do not
   give information about mailboxes with appropriately restricted ACL's.
   For example, a GETACL command on a mailbox for which the user has
   insufficient rights should not admit the mailbox exists, much less
   return the mailbox's ACL.


10.  Other considerations


10.1. Compatibility with RFC 2086

   Any server that prohibits a user name in LOGIN/AUTHENTICATE that starts
   with "$" MUST report "ACL" capability in addition to a "ACL=..." capability.


10.2. ACL2 interaction with QUOTA extension

   Server that implement both ACL2 and QUOTA extensions MUST use the following
   table to determine if a quote operation should be allowed for the user:

    GETQUOTAROOT - "r" right

    GETQUOTA - no rights required

    SETQUOTA - "a" right

<< Some implementations allows to set quota when the user has "r" right as well.
   Should we allow that? >>


10.3. Additional requirements and Implementation notes

   Any server implementing an ACL2 extension MUST accurately reflect the current
   user's rights in FLAGS and PERMANENTFLAGS responses. The server SHOULD issue
   a MYRIGHTS response code in an untagged OK response as a result of a SELECT
   or EXAMINE command.

   Example:    C: A142 SELECT INBOX
               S: * 172 EXISTS
               S: * 1 RECENT
               S: * OK [UNSEEN 12] Message 12 is first unseen
               S: * OK [UIDVALIDITY 3857529045] UIDs valid
               S: * OK [UIDNEXT 4392] Predicted next UID
               S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
               S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
               S: * OK [MYRIGHTS rwiste] Allowed rights
               S: A142 OK [READ-WRITE] SELECT completed

   An ACL2 server is allowed to add/remove some rights
   automatically when executing any of SETACL/DELETEACL/REPLACEACL. If the
   server does that it MUST send untagged ACL response to notify the client
   about the changes made.


11.  References

   [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate
   Requirement Levels", RFC 2119, Harvard University, March 1997.

   [ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications:
   ABNF", RFC 2234, Internet Mail Consortium, Demon Internet Ltd,
   November 1997.

   [IMAP4] Crispin, M., "Internet Message Access Protocol - Version
   4rev1", RFC 2060, University of Washington, December 1996.

   [UTF-8] Yergeau, F., "UTF-8, a transformation format of IS0 10646",
   RFC 2279, Alis Technologies, January 1998.

   [QUOTA] Myers, J., "IMAP4 QUOTA extension", RFC 2087, Carnegie Mellon
   University, January 1997.


12.  Aknowledgement

   This document is a revision of the RFC 2086 written by John G. Myers.

   Editor also appreciate comments received from Mark Crispin, Chris Newman,
   Cyrus Daboo, Curtis King, Lyndon Nerenberg and other members of IMAPEXT
   working group.


13.  Editor's Address

   Alexey Melnikov
   mailto: mel@messagingdirect.com

   ACI WorldWide/MessagingDirect
   22 The Quadrant, Richmond,
   Surrey, United Kingdom, TW9 1BP


14.  Full Copyright Statement

    Copyright (C) The Internet Society 2002.  All Rights Reserved.

    This document and translations of it may be copied and furnished to
    others, and derivative works that comment on or otherwise explain it
    or assist in its implementation may be prepared, copied, published
    and distributed, in whole or in part, without restriction of any
    kind, provided that the above copyright notice and this paragraph
    are included on all such copies and derivative works.  However, this
    document itself may not be modified in any way, such as by removing
    the copyright notice or references to the Internet Society or other
    Internet organizations, except as needed for the purpose of
    developing Internet standards in which case the procedures for
    copyrights defined in the Internet Standards process must be
    followed, or as required to translate it into languages other than
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    The limited permissions granted above are perpetual and will not be
    revoked by the Internet Society or its successors or assigns.

    This document and the information contained herein is provided on an
    "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
    TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
    BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
    HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
    MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Appendix A. Changes since RFC 2086

   1. Changed the charset of "identifier" from US-ASCII to UTF-8.

   2. Specified that identifiers starting with a dollar sign ("$") are
      reserved for groups and implementation defined aliases.

   3. Specified that mailbox deletion is controled by the "x" right and
      EXPUNGE is controlled by "e" right.

   4. Clarified that RENAME requires "c" right for the new parent and "x"
      right for the old name.

   5. Changed capability name from "ACL" to "ACL2" because changes 2 and 3
      are not backward compatible with ACL RFC.

   6. Added "t" right that controls STORE \Deleted. Redefined "d" to be a
      macro for "e", "x" and "t".

   7. Added "ACL2=UNION" and "ACL2=MOST-SPECIFIC" capabilities and IANA
      registration template.

   8. Specified that "a" right also controls DELETEACL

   9. Specified that "r" right also controls STATUS

  10. Specified that "w" controls setting flags other than \Seen and \Deleted
      on APPEND. Same for "s" and "t" flags.

  11. Specified that "l" controls SUBSCRIBE/UNSUBSCRIBE.

  12. Added note about compatibility with RFC 2086

  13. Added "Implementation Notes" section.

  14. Updated "References" section.

  15. Deleted "PARTIAL", this is a depricated feature of RFC1730.

  16. Added MYRIGHT response code as per Cyrus suggestion.

  17. Added REPLACEACL as per Mark and Cyrus request.

  18. Added new section that describes which rights are required and/or
      checked when performing various IMAP commands.

  19. Added section about interaction of ACL2 with the QUOTA extension.