draft-ietf-imapext-list-extensions-00.txt   draft-ietf-imapext-list-extensions-01.txt 
IMAP Extensions Working Group B. Leiba IMAP Extensions Working Group B. Leiba
Internet Draft IBM T.J. Watson Research Center Internet Draft IBM T.J. Watson Research Center
Document: draft-ietf-imapext-list-extensions-00.txt October 2000 Document: draft-ietf-imapext-list-extensions-01.txt July 2001
Expires April 2001 Expires January 2002
IMAP4 LIST Command Extensions IMAP4 LIST Command Extensions
Status of this Document Status of this Document
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts. working documents as Internet-Drafts.
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The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
A revised version of this draft document will be submitted to the RFC A revised version of this draft document will be submitted to the RFC
editor as an Proposed Standard for the Internet Community. editor as an Proposed Standard for the Internet Community.
Discussion and suggestions for improvement are requested, and should Discussion and suggestions for improvement are requested, and should
be sent to ietf-imapext@imc.org. This document will expire before 30 be sent to ietf-imapext@imc.org. This document will expire before 31
April 2001. Distribution of this memo is unlimited. January 2002. Distribution of this memo is unlimited.
1. Abstract 1. Abstract
IMAP4 has two commands for listing mailboxes: LIST and LSUB. As we IMAP4 has two commands for listing mailboxes: LIST and LSUB. As we
add extensions that require specialized lists (see [MboxRefer] for an add extensions that require specialized lists (see [MboxRefer] for an
example) we expand the number of list commands, as each extension example) we expand the number of list commands, as each extension
must add its function to both LIST and LSUB. This document describes must add its function to both LIST and LSUB. This document describes
extensions to the LIST command that allow these additions to be done extensions to the LIST command that allow these additions to be done
in mutually compatible options to the LIST command, avoiding the in mutually compatible options to the LIST command, avoiding the
exponential increase in specialized list commands. exponential increase in specialized list commands.
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2. Conventions used in this document 2. Conventions used in this document
In examples, "C:" indicates lines sent by a client that is connected In examples, "C:" indicates lines sent by a client that is connected
to a server. "S:" indicates lines sent by the server to the client. to a server. "S:" indicates lines sent by the server to the client.
The words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" are The words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" are
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The extensions to the LIST command will be accomplished by amending The extensions to the LIST command will be accomplished by amending
the syntax to allow options to be specified. The list of options the syntax to allow options to be specified. The list of options
will replace the several commands that are currently used to mix and will replace the several commands that are currently used to mix and
match the information requested. The new syntax is backward- match the information requested. The new syntax is backward-
compatible, with no ambiguity: if the first word after the command compatible, with no ambiguity: if the first word after the command
name begins with a parenthesis, the new syntax is being used; if it name begins with a parenthesis, the new syntax is being used; if it
does not, it's in the original syntax. does not, it's in the original syntax.
By adding options to the LIST command, we are announcing the intent By adding options to the LIST command, we are announcing the intent
to phase out and eventually to deprecate the base LSUB command as to phase out and eventually to deprecate the RLIST and RLSUB commands
well as the RLIST and RLSUB commands described in [MboxRefer]. We described in [MboxRefer]. We are also defining the mechanism to
are also defining the mechanism to request extended mailbox request extended mailbox information, such as is described in the
information, such as is described in the now-expired "Child Mailbox now-expired "Child Mailbox Extension" draft [ChildMbox]. The base
Extension" draft [ChildMbox]. LSUB command is not deprecated by this extension; rather, this
extension adds a way to obtain subscription information with more
options, with those server implementations that support it. Clients
that simply need a list of subscribed mailboxes, as provided by the
LSUB command, SHOULD continue to use that command.
4. LIST Command Options 4. LIST Command Options
The LIST command syntax is extended by adding a parenthesized list of The LIST command syntax is extended by adding a parenthesized list of
command options between the command name and the reference name (see command options between the command name and the reference name (see
the formal syntax in section 6 for specific details). Command the formal syntax in section 6 for specific details). Command
options will be defined in this document and in approved extension options will be defined in this document and in approved extension
documents; each option will be enabled by a capability string (one documents; each option will be enabled by a capability string (one
capability may enable multiple options), and a client MUST NOT send capability may enable multiple options), and a client MUST NOT send
an option for which the server has not advertised support. A server an option for which the server has not advertised support. A server
MAY ignore options it does not recognize. MAY ignore options it does not recognize.
This extension is identified by the capability string "LISTEXT", and This extension is identified by the capability string "LISTEXT", and
support for it is a prerequisite for any future extensions that support for it is a prerequisite for any future extensions that
require specialized forms of the LIST command. Such extensions MUST require specialized forms of the LIST command. Such extensions MUST
refer to this document and MUST add their function through command refer to this document and MUST add their function through command
options as described herein. options as described herein. This document also defines the "LIST-
SUBSCRIBED" capability string; see the "SUBSCRIBED" option below.
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The options defined in this specification are The options defined in this specification are
SUBSCRIBED - causes the LIST command to list subscribed SUBSCRIBED - causes the LIST command to list subscribed
mailboxes, rather than the actual mailboxes. This will often mailboxes, rather than the actual mailboxes. This will often
be a subset of the actual mailboxes. It's also possible for be a subset of the actual mailboxes. It's also possible for
this list to contain the names of mailboxes that don't exist. this list to contain the names of mailboxes that don't exist.
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In any case, the list MUST include exactly those mailbox names In any case, the list MUST include exactly those mailbox names
that match the selection criteria and are subscribed to. This that match the selection criteria and are subscribed to. This
option is intended to replace the LSUB command, and MUST be option is intended to supplement the LSUB command, and support
supported by all servers. for it is optional -- a server that supports the SUBSCRIBED
option indicates so through the LIST-SUBSCRIBED capability.
Of particular note are the mailbox flags as returned by this
option, compared with what is returned by LSUB. With the
latter, the flags returned may not reflect the actual flag
status on the mailbox, and the \NoSelect flag has a special
meaning (it indicates that this mailbox is not, itself,
subscribed, but that it has child mailboxes that are). With
the SUBSCRIBED option described here, the flags are accurate
and complete, and have no special meanings.
"LSUB" and "LIST (SUBSCRIBED)" are, thus, not the same thing,
and some servers must do significant extra work to respond to
"LIST (SUBSCRIBED)". Because of this, clients SHOULD continue
to use "LSUB" unless they specifically want the additional
information offered by "LIST (SUBSCRIBED)". At the same time,
servers SHOULD support the LIST-SUBSCRIBED capability even if
it entails extra work, because a client that wants the
information will still obtain it by using LSUB followed by a
series of LIST commands, so servers might as well make it
easier.
This option defines a new mailbox flag, "\NonExistent", that
indicates that a mailbox is subscribed to, but does not
actually exist. The "\NonExistent" flag MUST be supported and
MUST be accurately computed.
REMOTE - causes the LIST command to show remote mailboxes as REMOTE - causes the LIST command to show remote mailboxes as
well as local ones, as described in [MboxRefer]. This option well as local ones, as described in [MboxRefer]. This option
is intended to replace the RLIST command and, in conjunction is intended to replace the RLIST command and, in conjunction
with the SUBSCRIBED option, the RLSUB command. This option is with the SUBSCRIBED option, the RLSUB command. This option is
only available on servers that also support RFC-2193. only available on servers that also support RFC-2193.
CHILDREN - Requests mailbox child information as originally CHILDREN - Requests mailbox child information as originally
proposed in [ChildMbox]. See section 5, below, for details. proposed in [ChildMbox]. See section 5, below, for details.
Support for this is optional, but this option MUST be accepted Support for this is optional, but this option MUST be accepted
by all servers (though it MAY be ignored). by all servers (though it MAY be ignored).
The LISTEXT capability also defines a new mailbox flag,
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"\PlaceHolder", that indicates that the designated mailbox does not
meet the selection criteria of the given LIST command, but that it
has one or more child mailboxes that do [EDITORIAL NOTE: "might"?].
The LSUB command indicates this condition by using the "\NoSelect"
flag, but the LIST (SUBSCRIBED) command MUST NOT do that, since
"\NoSelect" retains its original meaning here. Further, the
"\PlaceHolder" flag is more general, in that it can be used with any
extended set of selection criteria.
5. The CHILDREN Option 5. The CHILDREN Option
The CHILDREN option implements the Child Mailbox Extension, The CHILDREN option implements the Child Mailbox Extension,
originally proposed by Mike Gahrns and Raymond Cheng, of Microsoft originally proposed by Mike Gahrns and Raymond Cheng, of Microsoft
Corporation. Most of the information in this section is taken Corporation. Most of the information in this section is taken
directly from their original specification [ChildMbox]. The CHILDREN directly from their original specification [ChildMbox]. The CHILDREN
option is simply an indication that the client wants this option is simply an indication that the client wants this
information; a server MAY provide it even if the option is not information; a server MAY provide it even if the option is not
specified, or MAY ignore the option entirely. specified, or MAY ignore the option entirely.
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The Child Mailbox Extension defines two new attributes that MAY be The Child Mailbox Extension defines two new attributes that MAY be
returned within a LIST response: \HasChildren and \HasNoChildren. returned within a LIST response: \HasChildren and \HasNoChildren.
While these attributes MAY be returned in response to any LIST While these attributes MAY be returned in response to any LIST
command, the CHILDREN option is provided to indicate that the client command, the CHILDREN option is provided to indicate that the client
particularly wants this information. If the CHILDREN option is particularly wants this information. If the CHILDREN option is
present, the server SHOULD return these attributes even if their present, the server SHOULD return these attributes even if their
computation is expensive. computation is expensive.
\HasChildren - The presence of this attribute indicates that the mailbox \HasChildren - The presence of this attribute indicates that the mailbox
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has child mailboxes. has child mailboxes.
A server SHOULD NOT set this attribute if there are child A server SHOULD NOT set this attribute if there are child
mailboxes, and the user does not have permissions to access any mailboxes, and the user does not have permissions to access any
of them. In this case, \HasNoChildren SHOULD be used. of them. In this case, \HasNoChildren SHOULD be used.
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In many cases, however, a server may not be able to efficiently In many cases, however, a server may not be able to efficiently
compute whether a user has access to all child mailboxes. As compute whether a user has access to all child mailboxes. As
such a client MUST be prepared to accept the \HasChildren such a client MUST be prepared to accept the \HasChildren
attribute as a hint. That is, a mailbox MAY be flagged with the attribute as a hint. That is, a mailbox MAY be flagged with the
\HasChildren attribute, but no child mailboxes will appear in \HasChildren attribute, but no child mailboxes will appear in
the LIST response. the LIST response.
\HasNoChildren - The presence of this attribute indicates that the \HasNoChildren - The presence of this attribute indicates that the
mailbox has NO child mailboxes that are accessible to the mailbox has NO child mailboxes that are accessible to the
currently authenticated user. currently authenticated user.
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6. Examples 6. Examples
The first example shows the complete local hierarchy that will be The first example shows the complete local hierarchy that will be
used for the other examples. used for the other examples.
C: A01 LIST "" "*" C: A01 LIST "" "*"
S: * LIST (\Marked \NoInferiors) "/" "inbox" S: * LIST (\Marked \NoInferiors) "/" "inbox"
S: * LIST () "/" "Fruit" S: * LIST () "/" "Fruit"
S: * LIST () "/" "Fruit/Apple" S: * LIST () "/" "Fruit/Apple"
S: * LIST () "/" "Fruit/Banana" S: * LIST () "/" "Fruit/Banana"
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S: * LIST () "/" "Tofu" S: * LIST () "/" "Tofu"
S: * LIST () "/" "Vegetable" S: * LIST () "/" "Vegetable"
S: * LIST () "/" "Vegetable/Broccoli" S: * LIST () "/" "Vegetable/Broccoli"
S: A01 OK done S: A01 OK done
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In the next example, we'll see the subscribed mailboxes. This is In the next example, we'll see the subscribed mailboxes. This is
equivalent to <LSUB "" "*">. Note that the mailbox called similar, but not equivalent, to <LSUB "" "*">. Note that the mailbox
"Fruit/Peach" is subscribed to, but does not actually exist (perhaps called "Fruit/Peach" is subscribed to, but does not actually exist
it was deleted while still subscribed). (perhaps it was deleted while still subscribed). And the "Fruit"
mailbox is not subscribed to, but it has two subscribed children.
C: A02 LIST (SUBSCRIBE) "" "*" C: A02 LIST (SUBSCRIBE) "" "*"
S: * LIST (\Marked \NoInferiors) "/" "inbox" S: * LIST (\Marked \NoInferiors) "/" "inbox"
S: * LIST () "/" "Fruit" S: * LIST (\PlaceHolder) "/" "Fruit"
S: * LIST () "/" "Fruit/Banana" S: * LIST () "/" "Fruit/Banana"
S: * LIST () "/" "Fruit/Peach" S: * LIST (\NonExistent) "/" "Fruit/Peach"
S: A02 OK done S: A02 OK done
The next example shows the use of the CHILDREN option. The client, The next example shows the use of the CHILDREN option. The client,
without having to list the second level of hierarchy, now knows which without having to list the second level of hierarchy, now knows which
of the top-level mailboxes have sub-mailboxes (children) and which do of the top-level mailboxes have sub-mailboxes (children) and which do
not. Note that it's not necessary for the server to return the not. Note that it's not necessary for the server to return the
\HasNoChildren flag for the inbox, because the \NoInferiors flag \HasNoChildren flag for the inbox, because the \NoInferiors flag
already implies that, and has a stronger meaning. already implies that, and has a stronger meaning.
C: A03 LIST (CHILDREN) "" "%" C: A03 LIST (CHILDREN) "" "%"
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C: A04 LIST (REMOTE CHILDREN) "" "%" C: A04 LIST (REMOTE CHILDREN) "" "%"
S: * LIST (\Marked \NoInferiors) "/" "inbox" S: * LIST (\Marked \NoInferiors) "/" "inbox"
S: * LIST (\HasChildren) "/" "Fruit" S: * LIST (\HasChildren) "/" "Fruit"
S: * LIST (\HasNoChildren) "/" "Tofu" S: * LIST (\HasNoChildren) "/" "Tofu"
S: * LIST (\HasChildren) "/" "Vegetable" S: * LIST (\HasChildren) "/" "Vegetable"
S: * LIST () "/" "Bread" S: * LIST () "/" "Bread"
S: * LIST (\HasChildren) "/" "Meat" S: * LIST (\HasChildren) "/" "Meat"
S: A04 OK done S: A04 OK done
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7. Formal Syntax 7. Formal Syntax
The following syntax specification uses the augmented Backus-Naur The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in [ABNF]. Terms not defined here are taken Form (BNF) as described in [ABNF]. Terms not defined here are taken
from [IMAP4]. from [IMAP4].
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child-mbox-flag = "\HasChildren" / "\HasNoChildren" child-mbox-flag = "\HasChildren" / "\HasNoChildren"
; flags for Child Mailbox Extension, at most one ; flags for Child Mailbox Extension, at most one
; possible per LIST response ; possible per LIST response
list = "LIST" [SP list-options] SP mailbox SP list-mailbox list = "LIST" [SP list-options] SP mailbox SP list-mailbox
list-options = "(" [option *(SP option)] ")" list-options = "(" [option *(SP option)] ")"
mbox-list-oflag := child-mbox-flag mbox-list-oflag := child-mbox-flag / "\NonExistent" / "\PlaceHolder"
option = "SUBSCRIPTIONS" / "CHILDREN" / "REMOTE" / option = "SUBSCRIPTIONS" / "CHILDREN" / "REMOTE" /
option-extension option-extension
option-extension = atom option-extension = atom
8. Security Considerations 8. Security Considerations
This document describes syntactic changes to the specification of the This document describes syntactic changes to the specification of the
IMAP4 commands LIST, LSUB, RLIST, and RLSUB, and the modified LIST IMAP4 commands LIST, LSUB, RLIST, and RLSUB, and the modified LIST
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the mailbox than intended. In most situations this will not be a the mailbox than intended. In most situations this will not be a
security concern, because if information regarding whether a mailbox security concern, because if information regarding whether a mailbox
has children is considered sensitive, a user would not be granted has children is considered sensitive, a user would not be granted
access to that mailbox in the first place. access to that mailbox in the first place.
9. References 9. References
[IMAP4]; Crispin, M.; "Internet Message Access Protocol - Version [IMAP4]; Crispin, M.; "Internet Message Access Protocol - Version
4rev1"; RFC 2060; University of Washington; December 1996. 4rev1"; RFC 2060; University of Washington; December 1996.
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[MboxRefer]; Gahrns, M.; "IMAP4 Mailbox Referrals"; RFC 2193; [MboxRefer]; Gahrns, M.; "IMAP4 Mailbox Referrals"; RFC 2193;
Microsoft Corporation; September 1997. Microsoft Corporation; September 1997.
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[ChildMbox]; Gahrns, M. & Cheng, R.; "IMAP4 Child Mailbox Extension"; [ChildMbox]; Gahrns, M. & Cheng, R.; "IMAP4 Child Mailbox Extension";
draft-gahrns-imap-child-mailbox-03.txt; Microsoft Corporation; draft-gahrns-imap-child-mailbox-03.txt; Microsoft Corporation;
November 1997 -- expired, listed for historical reference. November 1997 -- expired, listed for historical reference.
[Keywords]; Bradner, S.; "Key words for use in RFCs to Indicate [Keywords]; Bradner, S.; "Key words for use in RFCs to Indicate
Requirement Levels"; RFC 2119; Harvard University; March 1997. Requirement Levels"; RFC 2119; Harvard University; March 1997.
[ABNF]; Crocker, D., and Overell, P. "Augmented BNF for Syntax [ABNF]; Crocker, D., and Overell, P. "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997. Specifications: ABNF", RFC 2234, November 1997.
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11. Author's Address 11. Author's Address
Barry Leiba Barry Leiba
IBM T.J. Watson Research Center IBM T.J. Watson Research Center
30 Saw Mill River Road 30 Saw Mill River Road
Hawthorne, NY 10532 Hawthorne, NY 10532
Phone: 1-914-784-7941 Phone: 1-914-784-7941
Email: leiba@watson.ibm.com Email: leiba@watson.ibm.com
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