Network Working Group M. Gahrns
Request for Comments: 3348 R. Cheng
Category: Informational Microsoft
July 2002
The Internet Message Action Protocol (IMAP4)
Child Mailbox Extension
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
The Internet Message Action Protocol (IMAP4) CHILDREN extension
provides a mechanism for a client to efficiently determine if a
particular mailbox has children, without issuing a LIST "" * or a
LIST "" % for each mailbox.
1. 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.
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].
2. Introduction and Overview
Many IMAP4 [RFC-2060] clients present to the user a hierarchical view
of the mailboxes that a user has access to. Rather than initially
presenting to the user the entire mailbox hierarchy, it is often
preferable to show to the user a collapsed outline list of the
mailbox hierarchy (particularly if there is a large number of
mailboxes). The user can then expand the collapsed outline hierarchy
as needed. It is common to include within the collapsed hierarchy a
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RFC 3348 IMAP4 Child Mailbox Extension July 2002
visual clue (such as a "+") to indicate that there are child
mailboxes under a particular mailbox. When the visual clue is
clicked the hierarchy list is expanded to show the child mailboxes.
Several IMAP vendors implemented this proposal, and it is proposed to
document this behavior and functionality as an Informational RFC.
There is interest in addressing the general extensibility of the IMAP
LIST command through an IMAP LIST Extension draft. Similar
functionality to the \HasChildren and \HasNoChildren flags could be
incorporated into this new LIST Extension. It is proposed that the
more general LIST Extension draft proceed on the standards track with
this proposal being relegated to informational status only.
If the functionality of the \HasChildren and \HasNoChildren flags
were incorporated into a more general LIST extension, this would have
the advantage that a client could then have the opportunity to
request whether or not the server should return this information.
This would be an advantage over the current draft for servers where
this information is expensive to compute, since the server would only
need to compute the information when it knew that the client
requesting the information was able to consume it.
3. Requirements
IMAP4 servers that support this extension MUST list the keyword
CHILDREN in their CAPABILITY response.
The CHILDREN extension defines two new attributes that MAY be
returned within a LIST response.
\HasChildren - The presence of this attribute indicates that the
mailbox has child mailboxes.
Servers SHOULD NOT return \HasChildren if child mailboxes exist, but
none will be displayed to the current user in a LIST response (as
should be the case where child mailboxes exist, but a client does not
have permissions to access them.) In this case, \HasNoChildren
SHOULD be used.
In many cases, however, a server may not be able to efficiently
compute whether a user has access to all child mailboxes, or multiple
users may be accessing the same account and simultaneously changing
the mailbox hierarchy. As such a client MUST be prepared to accept
the \HasChildren attribute as a hint. That is, a mailbox MAY be
flagged with the \HasChildren attribute, but no child mailboxes will
appear in a subsequent LIST response.
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Example 3.1:
============
/*** Consider a server that has the following mailbox hierarchy:
INBOX
ITEM_1
ITEM_1A
ITEM_2
TOP_SECRET
Where INBOX, ITEM_1 and ITEM_2 are top level mailboxes. ITEM_1A is a
child mailbox of ITEM_1 and TOP_SECRET is a child mailbox of ITEM_2
that the currently logged on user does NOT have access to.
Note that in this case, the server is not able to efficiently compute
access rights to child mailboxes and responds with a \HasChildren
attribute for mailbox ITEM_2, even though ITEM_2/TOP_SECRET does not
appear in the list response. ***/
C: A001 LIST "" *
S: * LIST (\HasNoChildren) "/" INBOX
S: * LIST (\HasChildren) "/" ITEM_1
S: * LIST (\HasNoChildren) "/" ITEM_1/ITEM_1A
S: * LIST (\HasChildren) "/" ITEM_2
S: A001 OK LIST Completed
\HasNoChildren - The presence of this attribute indicates that the
mailbox has NO child mailboxes that are accessible to the currently
authenticated user. If a mailbox has the \Noinferiors attribute, the
\HasNoChildren attribute is redundant and SHOULD be omitted in the
LIST response.
In some instances a server that supports the CHILDREN extension MAY
NOT be able to determine whether a mailbox has children. For example
it may have difficulty determining whether there are child mailboxes
when LISTing mailboxes while operating in a particular namespace.
In these cases, a server MAY exclude both the \HasChildren and
\HasNoChildren attributes in the LIST response. As such, a client
can not make any assumptions about whether a mailbox has children
based upon the absence of a single attribute.
It is an error for the server to return both a \HasChildren and a
\HasNoChildren attribute in a LIST response.
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RFC 3348 IMAP4 Child Mailbox Extension July 2002
It is an error for the server to return both a \HasChildren and a
\NoInferiors attribute in a LIST response.
Note: the \HasNoChildren attribute should not be confused with the
IMAP4 [RFC-2060] defined attribute \Noinferiors which indicates that
no child mailboxes exist now and none can be created in the future.
The \HasChildren and \HasNoChildren attributes might not be returned
in response to a LSUB response. Many servers maintain a simple
mailbox subscription list that is not updated when the underlying
mailbox structure is changed. A client MUST NOT assume that
hierarchy information will be maintained in the subscription list.
RLIST is a command defined in [RFC-2193] that includes in a LIST
response mailboxes that are accessible only via referral. That is, a
client must explicitly issue an RLIST command to see a list of these
mailboxes. Thus in the case where a mailbox has child mailboxes that
are available only via referral, the mailboxes would appear as
\HasNoChildren in response to the LIST command, and \HasChildren in
response to the RLIST command.
5. Formal Syntax
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in [ABNF].
Two new mailbox attributes are defined as flag_extensions to the
IMAP4 mailbox_list response:
HasChildren = "\HasChildren"
HasNoChildren = "\HasNoChildren"
6. Security Considerations
This extension provides a client a more efficient means of
determining whether a particular mailbox has children. If a mailbox
has children, but the currently authenticated user does not have
access to any of them, the server SHOULD respond with a
\HasNoChildren attribute. In many cases, however, a server may not
be able to efficiently compute whether a user has access to all child
mailboxes. If such a server responds with a \HasChildren attribute,
when in fact the currently authenticated user does not have access to
any child mailboxes, potentially more information is conveyed about
the mailbox than intended. A server designed with such levels of
security in mind SHOULD NOT attach the \HasChildren attribute to a
mailbox unless the server is certain that the user has access to at
least one of the child mailboxes.
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RFC 3348 IMAP4 Child Mailbox Extension July 2002
7. 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.
[RFC-2234] Crocker, D. and P. Overell, Editors, "Augmented BNF for
Syntax Specifications: ABNF", RFC 2234, November 1997.
[RFC-2193] Gahrns, M., "IMAP4 Mailbox Referrals", RFC 2193, September
1997.
8. Acknowledgments
The authors would like to thank the participants of several IMC Mail
Connect events for their input when this idea was originally
presented and refined.
9. Author's Address
Mike Gahrns
Microsoft
One Microsoft Way
Redmond, WA, 98052
Phone: (425) 936-9833
EMail: mikega@microsoft.com
Raymond Cheng
Microsoft
One Microsoft Way
Redmond, WA, 98052
Phone: (425) 703-4913
EMail: raych@microsoft.com
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RFC 3348 IMAP4 Child Mailbox Extension July 2002
10. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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The limited permissions granted above are perpetual and will not be
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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