WDIFF

draft-gahrns-imap-child-mailbox-02.txt --> rfc3348.txt

Network Working Group M. Gahrns, Microsoft Gahrns
Request for Comments: 3348 R. Cheng, Cheng
Category: Informational Microsoft
July 2002

The Internet Draft
Document: draft-gahrns-imap-child-mailbox-02.txt December 1999

IMAP4 Message Action Protocol (IMAP4)
Child Mailbox Extension

Status of this Memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of memo provides information for the Internet Engineering
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The list does
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0. Meta Information on this draft

This information
memo is intended to facilitate discussion. It will be
removed when this document leaves the Internet-Draft stage.

This draft is now being discussed on the IMAPEXT mailing list at
ietf-imapext@imc.org. Subscription requests can be sent to ietf-
imapext-request@imc.org (send an email message with the word
"subscribe" in the body). More information on the mailing list
along with unlimited.

Abstract

The Internet Message Action Protocol (IMAP4) CHILDREN extension
provides a WWW archive of back messages is available at
HTTP://www.imc.org. Earlier discussion took place on the IMAP
Mailing List imap@u.washington.edu

At the 45th IETF in Oslo, and 46th IETF in Washington, there was
further discussion regarding this draft. I am making what I believe
should be the final editorial changes to this draft.

At several of the previous IMC interop events, several IMAP vendors
have done implementations of this proposal. Further discussions are
needed as to whether this draft should be submitted as mechanism for a proposed
standard or as an informational or historical RFC. Submitting as an
information or historical RFC would serve client to document the current

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IMAP4 Child Mailbox Extension December 1999

implementations and elements of it could become efficiently determine if a basis of
particular mailbox has children, without issuing a general LIST extension.

There is some interest in starting "" * or a completely new LIST Extension
draft that addresses general extensibility of the
LIST command.
Similar functionality to the \HasChildren and \HasNoChildren flags
could be incorporated into this new LIST Extension, but the client
would then have an opportunity to request whether or not the server
should return this information. This could be an advantage over the
current draft "" % for servers where each mailbox.

1. Conventions used in this information is expensive to
compute, since the server would only need to compute the information
when it knew that document

In examples, "C:" and "S:" indicate lines sent by the client requesting the information was able to
use it.

Changes since April 99, draft-01

Incorporated comments from the list discussions:

1) Explicitly note that it is an error for the server to return both
a \HasChildren and
server respectively. If such lines are wrapped without a \NoInferiors attribute in a LIST response.

2) Note that new "C:" or
"S:" label, then the \HasChildren wrapping is for editorial clarity and \HasNoChildren attribute may is not
apply to
part of the LSUB command.

3) Note how \HasChildren

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and \HasNoChildren apply in the referral
case.

4) Added back advertising CHILDREN "OPTIONAL" in the capability response as was
the consensus during the Washington IMAP-EXT WG. Rational was that this made it easier for clients.

1. Abstract
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 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 show informational status only.

If the child
mailboxes.

The CHILDREN extension provides functionality of the \HasChildren and \HasNoChildren flags
were incorporated into a mechanism for more general LIST extension, this would have
the advantage that a client could then have the opportunity to
efficiently determine if a particular mailbox has children, without
issuing a LIST "" *
request whether or a LIST "" % for each mailbox name.

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IMAP4 Child Mailbox Extension December 1999

2. Conventions used in this document

In examples, "C:" and "S:" indicate lines sent by not the client and server respectively. If such lines are wrapped without a new "C:"
or "S:" label, then should return this information.
This would be an advantage over the wrapping is current draft for editorial clarity and servers where
this information is not
part of expensive to compute, since the command.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are server would only
need to be interpreted as described in [RFC-2119]. 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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RFC 3348 IMAP4 Child Mailbox Extension July 2002

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.

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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

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IMAP4 Child Mailbox Extension December 1999 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],

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

[RFC-2119],

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

[RFC-2234], 1997.

[RFC-2234] Crocker, D. Crocker and P. Overell, Editors, "Augmented BNF for
Syntax Specifications: ABNF", RFC 2234, Internet Mail Consortium, November 1997

[RFC-2193], 1997.

[RFC-2193] Gahrns, M, M., "IMAP4 Mailbox Referrals", RFC 2193,
Microsoft Corporation, September 1997
1997.

8. Acknowledgments

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IMAP4 Child Mailbox Extension December 1999

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, 98072 98052
Phone: (425) 936-9833
Email:
EMail: mikega@microsoft.com

Raymond Cheng
Microsoft
One Microsoft Way
Redmond, WA, 98072 98052
Phone: (425) 703-4913
Email:
EMail: raych@microsoft.com

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10. Full Copyright Statement

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 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
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This document and the information contained herein is provided on an
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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 and Cheng Expires June 2000 7

Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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