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rfc:rfc2342

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.

Gahrns & Newman Standards Track [Page 1] RFC 2342 IMAP4 Namespace May 1998

 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

Gahrns & Newman Standards Track [Page 2] RFC 2342 IMAP4 Namespace May 1998

 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

Gahrns & Newman Standards Track [Page 3] RFC 2342 IMAP4 Namespace May 1998

 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

Gahrns & Newman Standards Track [Page 4] RFC 2342 IMAP4 Namespace May 1998

    < 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

Gahrns & Newman Standards Track [Page 5] RFC 2342 IMAP4 Namespace May 1998

    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. >

Gahrns & Newman Standards Track [Page 6] RFC 2342 IMAP4 Namespace May 1998

    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.

Gahrns & Newman Standards Track [Page 7] RFC 2342 IMAP4 Namespace May 1998

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.

Gahrns & Newman Standards Track [Page 8] RFC 2342 IMAP4 Namespace May 1998

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

Gahrns & Newman Standards Track [Page 9] RFC 2342 IMAP4 Namespace May 1998

12. Full Copyright Statement

 Copyright (C) The Internet Society (1998).  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
 English.
 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.

Gahrns & Newman Standards Track [Page 10]

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