Network Working Group M. Gahrns, Microsoft
C. Newman, Innosoft
Internet Draft
Document: draft-gahrns-imap-namespace-01.txt June 1997
IMAP4 Namespace
Status of this Memo
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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 user's 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.
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2. Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
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. The specially defined IMAP4 mailbox INBOX
resides in a user's personal namespace.
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.
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 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 which 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.
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4. Requirements
IMAP4 servers that support this extension MUST list the keyword
NAMESPACE in their CAPABILITY response.
5. NAMESPACE Command
Arguments: none
Response: an untagged NAMESPACE response that contains the prefix
to the server's default Personal Namespace, the Other
Users' Namespace, and the Shared Namespace that the
server wishes to expose. The Personal Namespace and
Other User's Namespace prefix are each to a single
namespace, and as such, MUST end with the hierarchy
character used in that namespace. The Shared Namespace
prefix MAY be to multiple namespaces. If the Shared
Namespace prefix is to multiple namespaces, the
hierarchy character is not included in the prefix.
Result: OK - Command completed
NO - Error: Can't complete command
BAD - argument invalid
If a particular namespace is not available, the prefix to that
namespace is NIL.
Example:
< A server that supports only the personal namespace. No leading
prefix is used on personal mailboxes. >
C: A001 NAMESPACE
S: * NAMESPACE "" NIL NIL
S: A001 OK NAMESPACE command completed
Example:
< A user logged on anonymously to a server. No personal
mailboxes are associated with the anonymous user. No prefix is
required to access shared mailboxes. >
C: A001 NAMESPACE
S: * NAMESPACE NIL NIL ""
S: A001 OK NAMESPACE command completed
The Personal Namespace prefix returned MUST be to a single Personal
Namespace and MUST end with the hierarchy character used in that
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namespace. This allows a client to use the Personal Namespace
prefix to automatically create personal mailboxes.
Example:
< A server that supports only the Personal Namespace, with a
leading prefix of INBOX to personal mailboxes. >
C: A001 NAMESPACE
S: * NAMESPACE "INBOX." NIL NIL
S: A001 OK NAMESPACE command completed
C: A002 CREATE "INBOX.Sent Mail"
S: A002 OK CREATE command completed
The Other Users' Namespace prefix MUST be to a single Other Users'
Namespace and MUST end with the hierarchy character used in that
namespace. 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 authenticate 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:
< 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: A002 OK LIST command completed
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Example:
< 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.
The shared mailboxes prefix MAY be to multiple Shared Namespaces. A
client can construct a LIST command by appending a "%" to the Shared
Namespace prefix to discover available Shared Namespaces.
Example:
< A server that contains a single Shared Namespace. >
C: A001 NAMESPACE
S: * NAMESPACE "" NIL "Public Folders/"
S: A001 OK NAMESPACE command completed
C: A002 LIST "" "Public Folders/%"
S: * LIST () "/" "Public Folders/Foo"
S: * LIST () "/" "Public Folders/Bar"
S: A002 OK LIST command completed.
Example:
< A server that contains multiple Shared Namespaces. Note that
the hierarchy delimiter used within each namespace can be
different. >
C: A001 NAMESPACE
S: * NAMESPACE "~/mail/" NIL "#"
S: A001 OK NAMESPACE command completed
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C: A002 LIST "" "#%"
S: * LIST () "." "#News"
S: * LIST () "/" "#Shared"
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].
Namespace_Command = "NAMESPACE"
Namespace_Response = "*" SPACE "NAMESPACE" SPACE Prefix SPACE Prefix
SPACE Prefix
; The first prefix is a prefix to the Personal Namespace
; The second prefix is a prefix to the Other Users' Namespace
; The third prefix is a prefix to the Shared Namespace
mailbox = <mailbox>
; <mailbox as defined in [RFC-2060]
Prefix = NIL | mailbox
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 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, University of Washington, December 1996.
[RFC-2119], Bradner, S, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, Harvard University, March 1997
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[ABNF], DRUMS working group, Dave Crocker Editor, "Augmented BNF for
Syntax Specifications: ABNF", draft-drums-abnf-02.txt (work in
progress), Internet Mail Consortium, April 1997
[IMAP-URL], Newman, C., "IMAP URL Scheme", draft-newman-url-imap-
09.txt (work in progress), Innosoft, May 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. Author's Addresses
Mike Gahrns
Microsoft
One Microsoft Way
Redmond, WA, 98072, USA
Phone: (206) 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
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