IMAP NAMESPACE: IMAP4 Namespace

Network Working Group                                         M. Gahrns
Request for Comments: 2342                                    Microsoft
Category: Standards Track                                     C. Newman
                                                               Innosoft
                                                               May 1998


                            IMAP4 Namespace

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

1. Abstract

   IMAP4 [RFC-2060] does not define a default server namespace. As a
   result, two common namespace models have evolved:

   The "Personal Mailbox" model, in which the default namespace that is
   presented consists of only the user's personal mailboxes. To access
   shared mailboxes, the user must use an escape mechanism to reach
   another namespace.

   The "Complete Hierarchy" model, in which the default namespace that
   is presented includes the user's personal mailboxes along with any
   other mailboxes they have access to.

   These two models, create difficulties for certain client operations.
   This document defines a NAMESPACE command that allows a client to
   discover the prefixes of namespaces used by a server for personal
   mailboxes, other users' mailboxes, and shared mailboxes.  This allows
   a client to avoid much of the manual user configuration that is now
   necessary when mixing and matching IMAP4 clients and servers.

2. Conventions used in this document

   In examples, "C:" and "S:" indicate lines sent by the client and
   server respectively.  If such lines are wrapped without a new "C:" or
   "S:" label, then the wrapping is for editorial clarity and is not
   part of the command.

   Personal Namespace: A namespace that the server considers within the
   personal scope of the authenticated user on a particular connection.
   Typically, only the authenticated user has access to mailboxes in
   their Personal Namespace. It is the part of the namespace that
   belongs to the user that is allocated for mailboxes. If an INBOX
   exists for a user, it MUST appear within the user's personal
   namespace.  In the typical case, there SHOULD be only one Personal
   Namespace on a server.

   Other Users' Namespace: A namespace that consists of mailboxes from
   the Personal Namespaces of other users.  To access mailboxes in the
   Other Users' Namespace, the currently authenticated user MUST be
   explicitly granted access rights.  For example, it is common for a
   manager to grant to their secretary access rights to their mailbox.
   In the typical case, there SHOULD be only one Other Users' Namespace
   on a server.

   Shared Namespace: A namespace that consists of mailboxes that are
   intended to be shared amongst users and do not exist within a user's
   Personal Namespace.

   The namespaces a server uses MAY differ on a per-user basis.

   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].

3. Introduction and Overview

   Clients often attempt to create mailboxes for such purposes as
   maintaining a record of sent messages (e.g. "Sent Mail") or
   temporarily saving messages being composed (e.g. "Drafts").  For
   these clients to inter-operate correctly with the variety of IMAP4
   servers available, the user must enter the prefix of the Personal
   Namespace used by the server.  Using the NAMESPACE command, a client
   is able to automatically discover this prefix without manual user
   configuration.

   In addition, users are often required to manually enter the prefixes
   of various namespaces in order to view the mailboxes located there.
   For example, they might be required to enter the prefix of #shared to
   view the shared mailboxes namespace. The NAMESPACE command allows a
   client to automatically discover the namespaces that are available on
   a server. This allows a client to present the available namespaces to
   the user in what ever manner it deems appropriate.  For example, a
   client could choose to initially display only personal mailboxes, or
   it may choose to display the complete list of mailboxes available,
   and initially position the user at the root of their Personal
   Namespace.

   A server MAY choose to make available to the NAMESPACE command only a
   subset of the complete set of namespaces the server supports. To
   provide the ability to access these namespaces, a client SHOULD allow
   the user the ability to manually enter a namespace prefix.

4. Requirements

   IMAP4 servers that support this extension MUST list the keyword
   NAMESPACE in their CAPABILITY response.

   The NAMESPACE command is valid in the Authenticated and Selected
   state.

5. NAMESPACE Command

   Arguments: none

   Response:  an untagged NAMESPACE response that contains the prefix
                 and hierarchy delimiter to the server's Personal
                 Namespace(s), Other Users' Namespace(s), and Shared
                 Namespace(s) that the server wishes to expose. The
                 response will contain a NIL for any namespace class
                 that is not available. Namespace_Response_Extensions
                 MAY be included in the response.
                 Namespace_Response_Extensions which are not on the IETF
                 standards track, MUST be prefixed with an "X-".

   Result:    OK - Command completed
                 NO - Error: Can't complete command
                 BAD - argument invalid

   Example 5.1:
   ===========

      < A server that supports a single personal namespace.  No leading
      prefix is used on personal mailboxes and "/" is the hierarchy
      delimiter.>

      C: A001 NAMESPACE
      S: * NAMESPACE (("" "/")) NIL NIL
      S: A001 OK NAMESPACE command completed
   Example 5.2:
   ===========

      < A user logged on anonymously to a server.  No personal mailboxes
      are associated with the anonymous user and the user does not have
      access to the Other Users' Namespace.  No prefix is required to
      access shared mailboxes and the hierarchy delimiter is "." >

      C: A001 NAMESPACE
      S: * NAMESPACE NIL NIL (("" "."))
      S: A001 OK NAMESPACE command completed

   Example 5.3:
   ===========

      < A server that contains a Personal Namespace and a single Shared
      Namespace. >

      C: A001 NAMESPACE
      S: * NAMESPACE (("" "/")) NIL (("Public Folders/" "/"))
      S: A001 OK NAMESPACE command completed

   Example 5.4:
   ===========

      < A server that contains a Personal Namespace, Other Users'
      Namespace and multiple Shared Namespaces.  Note that the hierarchy
      delimiter used within each namespace can be different. >

      C: A001 NAMESPACE
      S: * NAMESPACE (("" "/")) (("~" "/")) (("#shared/" "/")
         ("#public/" "/")("#ftp/" "/")("#news." "."))
      S: A001 OK NAMESPACE command completed

   The prefix string allows a client to do things such as automatically
   creating personal mailboxes or LISTing all available mailboxes within
   a namespace.

   Example 5.5:
   ===========

      < A server that supports only the Personal Namespace, with a
      leading prefix of INBOX to personal mailboxes and a hierarchy
      delimiter of ".">

      C: A001 NAMESPACE
      S: * NAMESPACE (("INBOX." ".")) NIL  NIL
      S: A001 OK NAMESPACE command completed
      < Automatically create a mailbox to store sent items.>

      C: A002 CREATE "INBOX.Sent Mail"
      S: A002 OK CREATE command completed

   Although typically a server will support only a single Personal
   Namespace, and a single Other User's Namespace, circumstances exist
   where there MAY be multiples of these, and a client MUST be prepared
   for them.   If a client is configured such that it is required to
   create a certain mailbox, there can be circumstances where it is
   unclear which Personal Namespaces it should create the mailbox in.
   In these situations a client SHOULD let the user select which
   namespaces to create the mailbox in.

   Example 5.6:
   ===========

      < In this example, a server supports 2 Personal Namespaces.  In
      addition to the regular Personal Namespace, the user has an
      additional personal namespace to allow access to mailboxes in an
      MH format mailstore. >

      < The client is configured to save a copy of all mail sent by the
      user into a mailbox called 'Sent Mail'.  Furthermore, after a
      message is deleted from a mailbox, the client is configured to
      move that message to a mailbox called 'Deleted Items'.>

      < Note that this example demonstrates how some extension flags can
      be passed to further describe the #mh namespace. >

      C: A001 NAMESPACE
      S: * NAMESPACE (("" "/")("#mh/" "/" "X-PARAM" ("FLAG1" "FLAG2")))
         NIL NIL
      S: A001 OK NAMESPACE command completed

      < It is desired to keep only one copy of sent mail. It is unclear
      which Personal Namespace the client should use to create the 'Sent
      Mail' mailbox.  The user is prompted to select a namespace and
      only one 'Sent Mail' mailbox is created. >

      C: A002 CREATE "Sent Mail"
      S: A002 OK CREATE command completed

      < The client is designed so that it keeps two 'Deleted Items'
      mailboxes, one for each namespace. >

      C: A003 CREATE "Delete Items"
      S: A003 OK CREATE command completed
      C: A004 CREATE "#mh/Deleted Items"
      S: A004 OK CREATE command completed

   The next level of hierarchy following the Other Users' Namespace
   prefix SHOULD consist of <username>, where <username> is a user name
   as per the IMAP4 LOGIN or AUTHENTICATE command.

   A client can construct a LIST command by appending a "%" to the Other
   Users' Namespace prefix to discover the Personal Namespaces of other
   users that are available to the currently authenticated user.

   In response to such a LIST command, a server SHOULD NOT return user
   names that have not granted access to their personal mailboxes to the
   user in question.

   A server MAY return a LIST response containing only the names of
   users that have explicitly granted access to the user in question.

   Alternatively, a server MAY return NO to such a LIST command,
   requiring that a user name be included with the Other Users'
   Namespace prefix before listing any other user's mailboxes.

   Example 5.7:
   ===========

      < A server that supports providing a list of other user's
      mailboxes that are accessible to the currently logged on user. >

      C: A001 NAMESPACE
      S: * NAMESPACE (("" "/")) (("Other Users/" "/")) NIL
      S: A001 OK NAMESPACE command completed

      C: A002 LIST "" "Other Users/%"
      S: * LIST () "/" "Other Users/Mike"
      S: * LIST () "/" "Other Users/Karen"
      S: * LIST () "/" "Other Users/Matthew"
      S: * LIST () "/" "Other Users/Tesa"
      S: A002 OK LIST command completed

   Example 5.8:
   ===========

      < A server that does not support providing a list of other user's
      mailboxes that are accessible to the currently logged on user.
      The mailboxes are listable if the client includes the name of the
      other user with the Other Users' Namespace prefix. >
      C: A001 NAMESPACE
      S: * NAMESPACE (("" "/")) (("#Users/" "/")) NIL
      S: A001 OK NAMESPACE command completed

      < In this example, the currently logged on user has access to the
      Personal Namespace of user Mike, but the server chose to suppress
      this information in the LIST response.  However, by appending the
      user name Mike (received through user input) to the Other Users'
      Namespace prefix, the client is able to get a listing of the
      personal mailboxes of user Mike. >

      C: A002 LIST "" "#Users/%"
      S: A002 NO The requested item could not be found.

      C: A003 LIST "" "#Users/Mike/%"
      S: * LIST () "/" "#Users/Mike/INBOX"
      S: * LIST () "/" "#Users/Mike/Foo"
      S: A003 OK LIST command completed.

      A prefix string might not contain a hierarchy delimiter, because
      in some cases it is not needed as part of the prefix.

      Example 5.9:
      ===========

      < A server that allows access to the Other Users' Namespace by
      prefixing the others' mailboxes with a '~' followed by <username>,
      where <username> is a user name as per the IMAP4 LOGIN or
      AUTHENTICATE command.>

      C: A001 NAMESPACE
      S: * NAMESPACE (("" "/")) (("~" "/")) NIL
      S: A001 OK NAMESPACE command completed

      < List the mailboxes for user mark >

      C: A002 LIST "" "~mark/%"
      S: * LIST () "/" "~mark/INBOX"
      S: * LIST () "/" "~mark/foo"
      S: A002 OK LIST command completed

   Historical convention has been to start all namespaces with the "#"
   character.  Namespaces that include the "#" character are not IMAP
   URL [IMAP-URL] friendly requiring the "#" character to be represented
   as %23 when within URLs.  As such, server implementers MAY instead
   consider using namespace prefixes that do not contain the "#"
   character.

6. Formal Syntax

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in [ABNF].

   atom = <atom>
      ; <atom> as defined in [RFC-2060]

   Namespace = nil / "(" 1*( "(" string SP  (<"> QUOTED_CHAR <"> /
      nil) *(Namespace_Response_Extension) ")" ) ")"

   Namespace_Command = "NAMESPACE"

   Namespace_Response_Extension = SP string SP "(" string *(SP string)
      ")"

   Namespace_Response = "*" SP "NAMESPACE" SP Namespace SP Namespace SP
      Namespace

      ; The first Namespace is the Personal Namespace(s)
      ; The second Namespace is the Other Users' Namespace(s)
      ; The third Namespace is the Shared Namespace(s)

      nil = <nil>
         ; <nil> as defined in [RFC-2060]

      QUOTED_CHAR = <QUOTED_CHAR>
         ; <QUOTED_CHAR> as defined in [RFC-2060]

      string = <string>
         ; <string> as defined in [RFC-2060]
         ; Note that  the namespace prefix is to a mailbox and following
         ; IMAP4 convention, any international string in the NAMESPACE
         ; response MUST be of modified UTF-7 format as described in
         ;  [RFC-2060].

7. Security Considerations

   In response to a LIST command containing an argument of the Other
   Users' Namespace prefix, a server SHOULD NOT list users that have not
   granted list access to their personal mailboxes to the currently
   authenticated user.  Providing such a list, could compromise security
   by potentially disclosing confidential information of who is located
   on the server, or providing a starting point of a list of user
   accounts to attack.

8. References

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

   [RFC-2119], Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", BCP 14, RFC 2119, March 1997.

   [ABNF] Crocker, D., Editor, and P. Overell, "Augmented BNF for Syntax
   Specifications: ABNF", RFC 2234, November 1997.

   [IMAP-URL], Newman, C., "IMAP URL Scheme", RFC 2192, September 1997.

9.  Acknowledgments

   Many people have participated in the discussion of IMAP namespaces on
   the IMAP mailing list.  In particular, the authors would like to
   thank Mark Crispin for many of the concepts relating to the Personal
   Namespace and accessing the Personal Namespace of other users, Steve
   Hole for summarizing the two namespace models, John Myers and Jack De
   Winter for their work in a preceding effort trying to define a
   standardized personal namespace, and Larry Osterman for his review
   and collaboration on this document.

11. Authors' Addresses

   Mike Gahrns
   Microsoft
   One Microsoft Way
   Redmond, WA, 98072, USA

   Phone: (425) 936-9833
   EMail: mikega@microsoft.com


   Chris Newman
   Innosoft International, Inc.
   1050 East Garvey Ave. South
   West Covina, CA, 91790, USA

   EMail: chris.newman@innosoft.com

12.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

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