RFC 5258
Internet Message Access Protocol version 4 - LIST Command Extensions
Pages: 31
Proposed Standard
Proposed Standard
Network Working Group B. Leiba Request for Comments: 5258 IBM T.J. Watson Research Center Obsoletes: 3348 A. Melnikov Updates: 2193 Isode Limited Category: Standards Track June 2008 Internet Message Access Protocol version 4 - LIST Command Extensions 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.Abstract
IMAP4 has two commands for listing mailboxes: LIST and LSUB. As we have added extensions, such as Mailbox Referrals, that have required specialized lists we have had to expand the number of list commands, since each extension must add its function to both LIST and LSUB, and these commands are not, as they are defined, extensible. If we've needed the extensions to work together, we've had to add a set of commands to mix the different options, the set increasing in size with each new extension. This document describes an extension to the base LIST command that will allow these additions to be done with mutually compatible options to the LIST command, avoiding the exponential increase in specialized list commands.
Table of Contents
1. Introduction and Overview . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 4 3. Extended LIST Command . . . . . . . . . . . . . . . . . . . . 4 3.1. Initial List of Selection Options . . . . . . . . . . . . 7 3.2. Initial List of Return Options . . . . . . . . . . . . . . 8 3.3. General Principles for Returning LIST Responses . . . . . 9 3.4. Additional Requirements on LIST-EXTENDED Clients . . . . . 9 3.5. CHILDINFO Extended Data Item . . . . . . . . . . . . . . . 10 4. The CHILDREN Return Option . . . . . . . . . . . . . . . . . . 11 5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 19 7. Internationalization Considerations . . . . . . . . . . . . . 22 8. Security Considerations . . . . . . . . . . . . . . . . . . . 23 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23 9.1. Guidelines for IANA . . . . . . . . . . . . . . . . . . . 23 9.2. Registration Procedure and Change Control . . . . . . . . 23 9.3. Registration Template for LIST-EXTENDED Options . . . . . 25 9.4. Initial LIST-EXTENDED Option Registrations . . . . . . . . 25 9.5. Registration Template for LIST-EXTENDED Extended Data Item . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 9.6. Initial LIST-EXTENDED Extended Data Item Registrations . . 28 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29 11.1. Normative References . . . . . . . . . . . . . . . . . . . 29 11.2. Informative References . . . . . . . . . . . . . . . . . . 30
1. Introduction and Overview
The LIST command is extended by amending the syntax to allow options and multiple patterns to be specified. The list of options replaces the several commands that are currently used to mix and match the information requested. The new syntax is backward compatible, with no ambiguity: the new syntax is being used if one of the following conditions is true: 1. if the first word after the command name begins with a parenthesis ("LIST selection options") 2. if the second word after the command name begins with a parenthesis ("multiple mailbox patterns") 3. if the LIST command has more than 2 parameters ("LIST return options") Otherwise the original syntax is used. By adding options to the LIST command, we are announcing the intent to phase out and eventually to deprecate the RLIST and RLSUB commands described in [MBRef]. We are also defining the mechanism to request extended mailbox information, such as is described in the Child Mailbox Extension [CMbox]. The base 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. This document defines an IMAP4 extension that is identified by the capability string "LIST-EXTENDED". The LIST-EXTENDED extension makes the following changes to the IMAP4 protocol, which are described in more detail in Section 3 and Section 4: a. defines new syntax for LIST command options. b. extends LIST to allow for multiple mailbox patterns. c. adds LIST command selection options: SUBSCRIBED, REMOTE, and RECURSIVEMATCH. d. adds LIST command return options: SUBSCRIBED and CHILDREN. e. adds new mailbox attributes: "\NonExistent", "\Subscribed", "\Remote", "\HasChildren", and "\HasNoChildren".
f. adds CHILDINFO extended data item.2. Conventions Used in This Document
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. The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" are used in this document as specified in RFC 2119 [Kwds]. The term "canonical LIST pattern" refers to the canonical pattern constructed internally by the server from the reference and mailbox name arguments (Section 6.3.8 of [IMAP4]). The [IMAP4] LIST command returns only mailboxes that match the canonical LIST pattern. Other terms are introduced where they are referenced for the first time.3. Extended LIST Command
This extension updates the syntax of the LIST command to allow for multiple mailbox patterns to be specified, if they are enclosed in parentheses. A mailbox name matches a list of mailbox patterns if it matches at least one mailbox pattern. If a mailbox name matches multiple mailbox patterns from the list, the server SHOULD return only a single LIST response. Note that the non-extended LIST command is required to treat an empty ("" string) mailbox name argument as a special request to return the hierarchy delimiter and the root name of the name given in the reference parameter (as per [IMAP4]). However, ANY extended LIST command (extended in any of 3 ways specified in Section 1, or any combination thereof) MUST NOT treat the empty mailbox name as such a special request, and any regular processing described in this document applies. In particular, if an extended LIST command has multiple mailbox names and one (or more) of them is the empty string, the empty string MUST be ignored for the purpose of matching. Some servers might restrict which patterns are allowed in a LIST command. If a server doesn't accept a particular pattern, it MUST silently ignore it. The LIST command syntax is also extended in two additional ways: by adding a parenthesized list of command options between the command name and the reference name (LIST selection options) and an optional
list of options at the end that control what kind of information should be returned (LIST return options). See the formal syntax in Section 6 for specific details. A LIST selection option tells the server which mailbox names should be selected by the LIST operation. The server should return information about all mailbox names that match any of the "canonical LIST pattern" (as described above) and satisfy additional selection criteria (if any) specified by the LIST selection options. Let's call any such mailbox name a "matched mailbox name". When multiple selection options are specified, the server MUST return information about mailbox names that satisfy every selection option, unless a description of a particular specified option prescribes special rules. An example of an option prescribing special rules is the RECURSIVEMATCH selection option described later in this section. We will use the term "selection criteria" when referring collectively to all selection options specified in a LIST command. A LIST return option controls which information is returned for each matched mailbox name. Note that return options MUST NOT cause the server to report information about additional mailbox names. If the client has not specified any return option, only information about attributes should be returned by the server. (Of course, the server is allowed to include any other information at will.) Both selection and return command options will be defined in this document and in approved extension documents; each option will be enabled by a capability string (one capability may enable multiple options), and a client MUST NOT send an option for which the server has not advertised support. A server MUST respond to options it does not recognize with a BAD response. The client SHOULD NOT specify any option more than once; however, if the client does this, the server MUST act as if it received the option only once. The order in which options are specified by the client is not significant. In general, each selection option except RECURSIVEMATCH will have a corresponding return option. The REMOTE selection option is an anomaly in this regard, and does not have a corresponding return option. That is because it expands, rather than restricts, the set of mailboxes that are returned. Future extensions to this specification should keep parallelism in mind and define a pair of corresponding options. This extension is identified by the capability string "LIST-EXTENDED", and support for it is a prerequisite for any future extensions that require specialized forms of the LIST command. Such extensions MUST refer to this document and MUST add their function through command options as described herein. Note that extensions
that don't require support for an extended LIST command, but use extended LIST responses (see below), don't need to advertise the "LIST-EXTENDED" capability string. This extension also defines extensions to the LIST response, allowing a series of extended fields at the end, a parenthesized list of tagged data (also referred to as "extended data item"). The first element of an extended field is a tag, which identifies the type of data. Tags MUST be registered with IANA, as described in Section 9.5 of this document. An example of such an extended set might be tablecloth (("edge" "lacy") ("color" "red"))) (X-Sample "text")) or tablecloth ("edge" "lacy")) (X-Sample "text" "more text")) See the formal syntax, in Section 6, for the full syntactic details. The server MUST NOT return any extended data item unless the client has expressed its ability to support extended LIST responses, for example, by using an extended LIST command. The server MAY return data in the extended fields that was not directly solicited by the client in the corresponding LIST command. For example, the client can enable extra extended fields by using another IMAP extension that make use of the extended LIST responses. The client MUST ignore all extended fields it doesn't recognize. The LIST-EXTENDED capability also defines several new mailbox attributes. The "\NonExistent" attribute indicates that a mailbox name does not refer to an existing mailbox. Note that this attribute is not meaningful by itself, as mailbox names that match the canonical LIST pattern but don't exist must not be returned unless one of the two conditions listed below is also satisfied: a. The mailbox name also satisfies the selection criteria (for example, it is subscribed and the "SUBSCRIBED" selection option has been specified). b. "RECURSIVEMATCH" has been specified, and the mailbox name has at least one descendant mailbox name that does not match the LIST pattern and does match the selection criteria. In practice, this means that the "\NonExistent" attribute is usually returned with one or more of "\Subscribed", "\Remote", "\HasChildren", or the CHILDINFO extended data item (see their description below).
The "\NonExistent" attribute implies "\NoSelect". The "\NonExistent" attribute MUST be supported and MUST be accurately computed.3.1. Initial List of Selection Options
The selection options defined in this specification are as follows: SUBSCRIBED - causes the LIST command to list subscribed names, rather than the existing mailboxes. This will often be a subset of the actual mailboxes. It's also possible for this list to contain the names of mailboxes that don't exist. In any case, the list MUST include exactly those mailbox names that match the canonical list pattern and are subscribed to. This option is intended to supplement the LSUB command. Of particular note are the mailbox attributes as returned by this option, compared with what is returned by LSUB. With the latter, the attributes returned may not reflect the actual attribute status on the mailbox name, and the \NoSelect attribute has a second special meaning (it indicates that this mailbox is not, itself, subscribed, but that it has descendant mailboxes that are). With the SUBSCRIBED selection option described here, the attributes 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)". This option defines a new mailbox attribute, "\Subscribed", that indicates that a mailbox name is subscribed to. The "\Subscribed" attribute MUST be supported and MUST be accurately computed when the SUBSCRIBED selection option is specified. Note that the SUBSCRIBED selection option implies the SUBSCRIBED return option (see below). REMOTE - causes the LIST command to show remote mailboxes as well as local ones, as described in [MBRef]. This option is intended to replace the RLIST command and, in conjunction with the SUBSCRIBED selection option, the RLSUB command. This option defines a new mailbox attribute, "\Remote", that indicates that a mailbox is a remote mailbox. The "\Remote" attribute MUST be accurately computed when the REMOTE option is specified. The REMOTE selection option has no interaction with other options. Its effect is to tell the server to apply the other options, if
any, to remote mailboxes, in addition to local ones. In
particular, it has no interaction with RECURSIVEMATCH (see below).
A request for (REMOTE RECURSIVEMATCH) is invalid, because a
request for (RECURSIVEMATCH) is. A request for (REMOTE
RECURSIVEMATCH SUBSCRIBED) is asking for all subscribed mailboxes,
both local and remote.
RECURSIVEMATCH - this option forces the server to return information
about parent mailboxes that don't match other selection options,
but have some submailboxes that do. Information about children is
returned in the CHILDINFO extended data item, as described in
Section 3.5.
Note 1: In order for a parent mailbox to be returned, it still has
to match the canonical LIST pattern.
Note 2: When returning the CHILDINFO extended data item, it
doesn't matter whether or not the submailbox matches the canonical
LIST pattern. See also example 9 in Section 5.
The RECURSIVEMATCH option MUST NOT occur as the only selection
option (or only with REMOTE), as it only makes sense when other
selection options are also used. The server MUST return BAD
tagged response in such case.
Note that even if the RECURSIVEMATCH option is specified, the
client MUST still be able to handle a case when a CHILDINFO
extended data item is returned and there are no submailboxes that
meet the selection criteria of the subsequent LIST command, as
they can be deleted/renamed after the LIST response was sent, but
before the client had a chance to access them.
3.2. Initial List of Return Options
The return options defined in this specification are as follows:
SUBSCRIBED - causes the LIST command to return subscription state
for all matching mailbox names. The "\Subscribed" attribute MUST
be supported and MUST be accurately computed when the SUBSCRIBED
return option is specified. Further, all mailbox flags MUST be
accurately computed (this differs from the behavior of the LSUB
command).
CHILDREN - requests mailbox child information as originally proposed
in [CMbox]. See Section 4, below, for details. This option MUST
be supported by all servers.
3.3. General Principles for Returning LIST Responses
This section outlines several principles that can be used by server implementations of this document to decide whether a LIST response should be returned, as well as how many responses and what kind of information they may contain. 1. At most one LIST response should be returned for each mailbox name that matches the canonical LIST pattern. Server implementors must not assume that clients will be able to assemble mailbox attributes and other information returned in multiple LIST responses. 2. There are only two reasons for including a matching mailbox name in the responses to the LIST command (note that the server is allowed to return unsolicited responses at any time, and such responses are not governed by this rule): A. The mailbox name also satisfies the selection criteria. B. The mailbox name doesn't satisfy the selection criteria, but it has at least one descendant mailbox name that satisfies the selection criteria and that doesn't match the canonical LIST pattern. For more information on this case, see the CHILDINFO extended data item described in Section 3.5. Note that the CHILDINFO extended data item can only be returned when the RECURSIVEMATCH selection option is specified. 3. Attributes returned in the same LIST response must be treated additively. For example, the following response S: * LIST (\Subscribed \NonExistent) "/" "Fruit/Peach" means that the "Fruit/Peach" mailbox doesn't exist, but it is subscribed.3.4. Additional Requirements on LIST-EXTENDED Clients
All clients that support this extension MUST treat an attribute with a stronger meaning as implying any attribute that can be inferred from it. For example, the client must treat the presence of the \NoInferiors attribute as if the \HasNoChildren attribute was also sent by the server.
The following table summarizes inference rules described in Section 3. +--------------------+-------------------+ | returned attribute | implied attribute | +--------------------+-------------------+ | \NoInferiors | \HasNoChildren | | \NonExistent | \NoSelect | +--------------------+-------------------+3.5. CHILDINFO Extended Data Item
The CHILDINFO extended data item MUST NOT be returned unless the client has specified the RECURSIVEMATCH selection option. The CHILDINFO extended data item in a LIST response describes the selection criteria that has caused it to be returned and indicates that the mailbox has at least one descendant mailbox that matches the selection criteria. The LSUB command indicates this condition by using the "\NoSelect" attribute, but the LIST (SUBSCRIBED) command MUST NOT do that, since "\NoSelect" retains its original meaning here. Further, the CHILDINFO extended data item is more general, in that it can be used with any extended set of selection criteria. Note: Some servers allow for mailboxes to exist without requiring their parent to exist. For example, a mailbox "Customers/ABC" can exist while the mailbox "Customers" does not. As CHILDINFO extended data item is not allowed if the RECURSIVEMATCH selection option is not specified, such servers SHOULD use the "\NonExistent \HasChildren" attribute pair to signal to the client that there is a descendant mailbox that matches the selection criteria. See example 11 in Section 5. The returned selection criteria allow the client to distinguish a solicited response from an unsolicited one, as well as to distinguish among solicited responses caused by multiple pipelined LIST commands that specify different criteria. Servers SHOULD ONLY return a non-matching mailbox name along with CHILDINFO if at least one matching child is not also being returned. That is, servers SHOULD suppress redundant CHILDINFO responses. Examples 8 and 10 in Section 5 demonstrate the difference between present CHILDINFO extended data item and the "\HasChildren" attribute.
The following table summarizes interaction between the "\NonExistent" attribute and CHILDINFO (the first column indicates whether the parent mailbox exists): +--------+--------------+--------------------+----------------------+ | exists | meets the | has a child that | returned | | | selection | meets the | LIST-EXTENDED | | | criteria | selection criteria | attributes and | | | | | CHILDINFO | +--------+--------------+--------------------+----------------------+ | no | no | no | no LIST response | | | | | returned | | yes | no | no | no LIST response | | | | | returned | | no | yes | no | (\NonExistent | | | | | <attr>) | | yes | yes | no | (<attr>) | | no | no | yes | (\NonExistent) + | | | | | CHILDINFO | | yes | no | yes | () + CHILDINFO | | no | yes | yes | (\NonExistent | | | | | <attr>) + CHILDINFO | | yes | yes | yes | (<attr>) + CHILDINFO | +--------+--------------+--------------------+----------------------+ where <attr> is one or more attributes that correspond to the selection criteria; for example, for the SUBSCRIBED option the <attr> is \Subscribed.4. The CHILDREN Return Option
The CHILDREN return option implements the Child Mailbox Extension, originally proposed by Mike Gahrns and Raymond Cheng, of Microsoft Corporation. Most of the information in this section is taken directly from their original specification [CMbox]. The CHILDREN return option is simply an indication that the client wants this information; a server MAY provide it even if the option is not specified. Many IMAP4 [IMAP4] 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 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.
The CHILDREN return option provides a mechanism for a client to
efficiently determine whether a particular mailbox has children,
without issuing a LIST "" * or a LIST "" % for each mailbox name.
The CHILDREN return option defines two new attributes that MUST be
returned within a LIST response: \HasChildren and \HasNoChildren.
Although these attributes MAY be returned in response to any LIST
command, the CHILDREN return option is provided to indicate that the
client particularly wants this information. If the CHILDREN return
option is present, the server MUST return these attributes even if
their computation is expensive.
\HasChildren
The presence of this attribute indicates that the mailbox has child
mailboxes. A server SHOULD NOT set this attribute if there are
child mailboxes and the user does not have permission to access
any of 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 any child mailbox. Note
that even though the \HasChildren attribute for a mailbox must
be correct at the time of processing of the mailbox, a client
must be prepared to deal with a situation when a mailbox is
marked with the \HasChildren attribute, but no child mailbox
appears in the response to the LIST command. This might happen,
for example, due to children mailboxes being deleted or made
inaccessible to the user (using access control) by another
client before the server is able to list them.
\HasNoChildren
The presence of this attribute indicates that the mailbox has NO
child mailboxes that are accessible to the currently
authenticated user.
It is an error for the server to return both a \HasChildren and a
\HasNoChildren attribute in the same LIST response.
Note: the \HasNoChildren attribute should not be confused with the
IMAP4 [IMAP4] defined attribute \NoInferiors, which indicates that no
child mailboxes exist now and none can be created in the future.
5. Examples
1: The first example shows the complete local hierarchy that will be used for the other examples. C: A01 LIST "" "*" S: * LIST (\Marked \NoInferiors) "/" "inbox" S: * LIST () "/" "Fruit" S: * LIST () "/" "Fruit/Apple" S: * LIST () "/" "Fruit/Banana" S: * LIST () "/" "Tofu" S: * LIST () "/" "Vegetable" S: * LIST () "/" "Vegetable/Broccoli" S: * LIST () "/" "Vegetable/Corn" S: A01 OK done 2: In the next example, we will see the subscribed mailboxes. This is similar to, but not equivalent with, <LSUB "" "*">. Note that the mailbox called "Fruit/Peach" is subscribed to, but does not actually exist (perhaps it was deleted while still subscribed). The "Fruit" mailbox is not subscribed to, but it has two subscribed children. The "Vegetable" mailbox is subscribed and has two children; one of them is subscribed as well. C: A02 LIST (SUBSCRIBED) "" "*" S: * LIST (\Marked \NoInferiors \Subscribed) "/" "inbox" S: * LIST (\Subscribed) "/" "Fruit/Banana" S: * LIST (\Subscribed \NonExistent) "/" "Fruit/Peach" S: * LIST (\Subscribed) "/" "Vegetable" S: * LIST (\Subscribed) "/" "Vegetable/Broccoli" S: A02 OK done 3: The next example shows the use of the CHILDREN option. The client, without having to list the second level of hierarchy, now knows which of the top-level mailboxes have submailboxes (children) and which do not. Note that it's not necessary for the server to return the \HasNoChildren attribute for the inbox, because the \NoInferiors attribute already implies that, and has a stronger meaning. C: A03 LIST () "" "%" RETURN (CHILDREN) S: * LIST (\Marked \NoInferiors) "/" "inbox" S: * LIST (\HasChildren) "/" "Fruit" S: * LIST (\HasNoChildren) "/" "Tofu" S: * LIST (\HasChildren) "/" "Vegetable" S: A03 OK done
4: In this example, we see more mailboxes that reside on another
server. This is similar to the command <RLIST "" "%">.
C: A04 LIST (REMOTE) "" "%" RETURN (CHILDREN)
S: * LIST (\Marked \NoInferiors) "/" "inbox"
S: * LIST (\HasChildren) "/" "Fruit"
S: * LIST (\HasNoChildren) "/" "Tofu"
S: * LIST (\HasChildren) "/" "Vegetable"
S: * LIST (\Remote) "/" "Bread"
S: * LIST (\HasChildren \Remote) "/" "Meat"
S: A04 OK done
5: The following example also requests the server to include
mailboxes that reside on another server. The server returns
information about all mailboxes that are subscribed. This is
similar to the command <RLSUB "" "*">. We also see the use of
two selection options.
C: A05 LIST (REMOTE SUBSCRIBED) "" "*"
S: * LIST (\Marked \NoInferiors \Subscribed) "/" "inbox"
S: * LIST (\Subscribed) "/" "Fruit/Banana"
S: * LIST (\Subscribed \NonExistent) "/" "Fruit/Peach"
S: * LIST (\Subscribed) "/" "Vegetable"
S: * LIST (\Subscribed) "/" "Vegetable/Broccoli"
S: * LIST (\Remote \Subscribed) "/" "Bread"
S: A05 OK done
6: The following example requests the server to include mailboxes
that reside on another server. The server is asked to return
subscription information for all returned mailboxes. This is
different from the example above.
Note that the output of this command is not a superset of the
output in the previous example, as it doesn't include LIST
response for the non-existent "Fruit/Peach".
C: A06 LIST (REMOTE) "" "*" RETURN (SUBSCRIBED)
S: * LIST (\Marked \NoInferiors \Subscribed) "/" "inbox"
S: * LIST () "/" "Fruit"
S: * LIST () "/" "Fruit/Apple"
S: * LIST (\Subscribed) "/" "Fruit/Banana"
S: * LIST () "/" "Tofu"
S: * LIST (\Subscribed) "/" "Vegetable"
S: * LIST (\Subscribed) "/" "Vegetable/Broccoli"
S: * LIST () "/" "Vegetable/Corn"
S: * LIST (\Remote \Subscribed) "/" "Bread"
S: * LIST (\Remote) "/" "Meat"
S: A06 OK done
7: In the following example, the client has specified multiple
mailbox patterns. Note that this example does not use the
mailbox hierarchy used in the previous examples.
C: BBB LIST "" ("INBOX" "Drafts" "Sent/%")
S: * LIST () "/" "INBOX"
S: * LIST (\NoInferiors) "/" "Drafts"
S: * LIST () "/" "Sent/March2004"
S: * LIST (\Marked) "/" "Sent/December2003"
S: * LIST () "/" "Sent/August2004"
S: BBB OK done
8: The following example demonstrates the difference between the
\HasChildren attribute and the CHILDINFO extended data item.
Let's assume there is the following hierarchy:
C: C01 LIST "" "*"
S: * LIST (\Marked \NoInferiors) "/" "inbox"
S: * LIST () "/" "Foo"
S: * LIST () "/" "Foo/Bar"
S: * LIST () "/" "Foo/Baz"
S: * LIST () "/" "Moo"
S: C01 OK done
If the client asks RETURN (CHILDREN), it will get this:
C: CA3 LIST "" "%" RETURN (CHILDREN)
S: * LIST (\Marked \NoInferiors) "/" "inbox"
S: * LIST (\HasChildren) "/" "Foo"
S: * LIST (\HasNoChildren) "/" "Moo"
S: CA3 OK done
A) Let's also assume that the mailbox "Foo/Baz" is the only
subscribed mailbox. Then we get this result:
C: C02 LIST (SUBSCRIBED) "" "*"
S: * LIST (\Subscribed) "/" "Foo/Baz"
S: C02 OK done
Now, if the client issues <LIST (SUBSCRIBED) "" "%">, the server will
return no mailboxes (as the mailboxes "Moo", "Foo", and "Inbox" are
NOT subscribed). However, if the client issues this:
C: C04 LIST (SUBSCRIBED RECURSIVEMATCH) "" "%"
S: * LIST () "/" "Foo" ("CHILDINFO" ("SUBSCRIBED"))
S: C04 OK done
(i.e., the mailbox "Foo" is not subscribed, but it has a child that
is.)
A1) If the mailbox "Foo" had also been subscribed, the last command
would return this:
C: C04 LIST (SUBSCRIBED RECURSIVEMATCH) "" "%"
S: * LIST (\Subscribed) "/" "Foo" ("CHILDINFO" ("SUBSCRIBED"))
S: C04 OK done
or even this:
C: C04 LIST (SUBSCRIBED RECURSIVEMATCH) "" "%"
S: * LIST (\Subscribed \HasChildren) "/" "Foo" ("CHILDINFO"
("SUBSCRIBED"))
S: C04 OK done
A2) If we assume instead that the mailbox "Foo" is not part of the
original hierarchy and is not subscribed, the last command will give
this result:
C: C04 LIST (SUBSCRIBED RECURSIVEMATCH) "" "%"
S: * LIST (\NonExistent) "/" "Foo" ("CHILDINFO" ("SUBSCRIBED"))
S: C04 OK done
B) Now, let's assume that no mailbox is subscribed. In this case,
the command <LIST (SUBSCRIBED RECURSIVEMATCH) "" "%"> will return no
responses, as there are no subscribed children (even though "Foo" has
children).
C) And finally, suppose that only the mailboxes "Foo" and "Moo" are
subscribed. In that case, we see this result:
C: C04 LIST (SUBSCRIBED RECURSIVEMATCH) "" "%" RETURN (CHILDREN)
S: * LIST (\HasChildren \Subscribed) "/" "Foo"
S: * LIST (\HasNoChildren \Subscribed) "/" "Moo"
S: C04 OK done
(which means that the mailbox "Foo" has children, but none of them is
subscribed).
9: The following example demonstrates that the CHILDINFO extended
data item is returned whether or not children mailboxes match
the canonical LIST pattern.
Let's assume there is the following hierarchy:
C: D01 LIST "" "*"
S: * LIST (\Marked \NoInferiors) "/" "inbox"
S: * LIST () "/" "foo2"
S: * LIST () "/" "foo2/bar1"
S: * LIST () "/" "foo2/bar2"
S: * LIST () "/" "baz2"
S: * LIST () "/" "baz2/bar2"
S: * LIST () "/" "baz2/bar22"
S: * LIST () "/" "baz2/bar222"
S: * LIST () "/" "eps2"
S: * LIST () "/" "eps2/mamba"
S: * LIST () "/" "qux2/bar2"
S: D01 OK done
And that the following mailboxes are subscribed:
C: D02 LIST (SUBSCRIBED) "" "*"
S: * LIST (\Subscribed) "/" "foo2/bar1"
S: * LIST (\Subscribed) "/" "foo2/bar2"
S: * LIST (\Subscribed) "/" "baz2/bar2"
S: * LIST (\Subscribed) "/" "baz2/bar22"
S: * LIST (\Subscribed) "/" "baz2/bar222"
S: * LIST (\Subscribed) "/" "eps2"
S: * LIST (\Subscribed) "/" "eps2/mamba"
S: * LIST (\Subscribed) "/" "qux2/bar2"
S: D02 OK done
The client issues the following command first:
C: D03 LIST (RECURSIVEMATCH SUBSCRIBED) "" "*2"
S: * LIST () "/" "foo2" ("CHILDINFO" ("SUBSCRIBED"))
S: * LIST (\Subscribed) "/" "foo2/bar2"
S: * LIST (\Subscribed) "/" "baz2/bar2"
S: * LIST (\Subscribed) "/" "baz2/bar22"
S: * LIST (\Subscribed) "/" "baz2/bar222"
S: * LIST (\Subscribed) "/" "eps2" ("CHILDINFO" ("SUBSCRIBED"))
S: * LIST (\Subscribed) "/" "qux2/bar2"
S: D03 OK done
and the server may also include (but this would violate a SHOULD NOT
in Section 3.5, because CHILDINFO is redundant)
S: * LIST () "/" "baz2" ("CHILDINFO" ("SUBSCRIBED"))
S: * LIST (\NonExistent) "/" "qux2" ("CHILDINFO" ("SUBSCRIBED"))
The CHILDINFO extended data item is returned for mailboxes "foo2",
"baz2", and "eps2", because all of them have subscribed children,
even though for the mailbox "foo2" only one of the two subscribed
children matches the pattern, for the mailbox "baz2" all the
subscribed children match the pattern, and for the mailbox "eps2"
none of the subscribed children matches the pattern.
Note that if the client issues
C: D03 LIST (RECURSIVEMATCH SUBSCRIBED) "" "*"
S: * LIST () "/" "foo2" ("CHILDINFO" ("SUBSCRIBED"))
S: * LIST (\Subscribed) "/" "foo2/bar1"
S: * LIST (\Subscribed) "/" "foo2/bar2"
S: * LIST () "/" "baz2" ("CHILDINFO" ("SUBSCRIBED"))
S: * LIST (\Subscribed) "/" "baz2/bar2"
S: * LIST (\Subscribed) "/" "baz2/bar22"
S: * LIST (\Subscribed) "/" "baz2/bar222"
S: * LIST (\Subscribed) "/" "eps2" ("CHILDINFO" ("SUBSCRIBED"))
S: * LIST (\Subscribed) "/" "eps2/mamba"
S: * LIST (\Subscribed) "/" "qux2/bar2"
S: D03 OK done
The LIST responses for mailboxes "foo2", "baz2", and "eps2" still
have the CHILDINFO extended data item, even though this information
is redundant and the client can determine it by itself.
10: The following example shows usage of multiple mailbox patterns.
It also demonstrates that the presence of the CHILDINFO extended
data item doesn't necessarily imply \HasChildren.
C: a1 LIST "" ("foo" "foo/*")
S: * LIST () "/" foo
S: a1 OK done
C: a2 LIST (SUBSCRIBED) "" "foo/*"
S: * LIST (\Subscribed \NonExistent) "/" foo/bar
S: a2 OK done
C: a3 LIST (SUBSCRIBED RECURSIVEMATCH) "" foo RETURN (CHILDREN)
S: * LIST (\HasNoChildren) "/" foo ("CHILDINFO" ("SUBSCRIBED"))
S: a3 OK done
11: The following example shows how a server that supports missing
mailbox hierarchy elements can signal to a client that didn't
specify the RECURSIVEMATCH selection option that there is a
child mailbox that matches the selection criteria.
C: a1 LIST (REMOTE) "" *
S: * LIST () "/" music/rock
S: * LIST (\Remote) "/" also/jazz
S: a1 OK done
C: a2 LIST () "" %
S: * LIST (\NonExistent \HasChildren) "/" music
S: a2 OK done
C: a3 LIST (REMOTE) "" %
S: * LIST (\NonExistent \HasChildren) "/" music
S: * LIST (\NonExistent \HasChildren) "/" also
S: a3 OK done
C: a3.1 LIST "" (% music/rock)
S: * LIST () "/" music/rock
S: a3.1 OK done
Because "music/rock" is the only mailbox under "music", there's no
need for the server to also return "music". However clients must
handle both cases.
6. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) as described in [ABNF]. Terms not defined here are taken
from [IMAP4]. In particular, note that the version of "mailbox-list"
below, which defines the payload of the LIST response, updates the
version defined in the IMAP specification. It is pointed to by
"mailbox-data", which is defined in [IMAP4].
"vendor-token" is defined in [ACAP]. Note that this normative
reference to ACAP will be an issue in moving this spec forward, since
it introduces a dependency on ACAP. The definitions of
"vendor-token" and of the IANA registry must eventually go somewhere
else, in a document that can be moved forward on the standards track
independently of ACAP.
childinfo-extended-item = "CHILDINFO" SP "("
list-select-base-opt-quoted
*(SP list-select-base-opt-quoted) ")"
; Extended data item (mbox-list-extended-item)
; returned when the RECURSIVEMATCH
; selection option is specified.
; Note 1: the CHILDINFO tag can be returned
; with and without surrounding quotes, as per
; mbox-list-extended-item-tag production.
; Note 2: The selection options are always returned
; quoted, unlike their specification in
; the extended LIST command.
child-mbox-flag = "\HasChildren" / "\HasNoChildren"
; attributes for CHILDREN return option, at most one
; possible per LIST response
eitem-standard-tag = atom
; a tag for extended list data defined in a Standard
; Track or Experimental RFC.
eitem-vendor-tag = vendor-token "-" atom
; a vendor-specific tag for extended list data
list = "LIST" [SP list-select-opts] SP mailbox SP mbox-or-pat
[SP list-return-opts]
list-return-opts = "RETURN" SP
"(" [return-option *(SP return-option)] ")"
; list return options, e.g., CHILDREN
list-select-base-opt = "SUBSCRIBED" / option-extension
; options that can be used by themselves
list-select-base-opt-quoted = DQUOTE list-select-base-opt DQUOTE
list-select-independent-opt = "REMOTE" / option-extension
; options that do not syntactically interact with
; other options
list-select-mod-opt = "RECURSIVEMATCH" / option-extension
; options that require a list-select-base-opt
; to also be present
list-select-opt = list-select-base-opt / list-select-independent-opt
/ list-select-mod-opt
; An option registration template is described in
; Section 9.3 of this document.
list-select-opts = "(" [
(*(list-select-opt SP) list-select-base-opt
*(SP list-select-opt))
/ (list-select-independent-opt
*(SP list-select-independent-opt))
] ")"
; Any number of options may be in any order.
; If a list-select-mod-opt appears, then a
; list-select-base-opt must also appear.
; This allows these:
; ()
; (REMOTE)
; (SUBSCRIBED)
; (SUBSCRIBED REMOTE)
; (SUBSCRIBED RECURSIVEMATCH)
; (SUBSCRIBED REMOTE RECURSIVEMATCH)
; But does NOT allow these:
; (RECURSIVEMATCH)
; (REMOTE RECURSIVEMATCH)
mailbox-list = "(" [mbx-list-flags] ")" SP
(DQUOTE QUOTED-CHAR DQUOTE / nil) SP mailbox
[SP mbox-list-extended]
; This is the list information pointed to by the ABNF
; item "mailbox-data", which is defined in [IMAP4]
mbox-list-extended = "(" [mbox-list-extended-item
*(SP mbox-list-extended-item)] ")"
mbox-list-extended-item = mbox-list-extended-item-tag SP
tagged-ext-val
mbox-list-extended-item-tag = astring
; The content MUST conform to either "eitem-vendor-tag"
; or "eitem-standard-tag" ABNF productions.
; A tag registration template is described in this
; document in Section 9.5.
mbx-list-oflag =/ child-mbox-flag / "\Subscribed" / "\Remote"
mbx-list-sflag =/ "\NonExistent"
mbox-or-pat = list-mailbox / patterns
option-extension = (option-standard-tag / option-vendor-tag)
[SP option-value]
option-standard-tag = atom
; an option defined in a Standards Track or
; Experimental RFC
option-val-comp = astring /
option-val-comp *(SP option-val-comp) /
"(" option-val-comp ")"
option-value = "(" option-val-comp ")"
option-vendor-tag = vendor-token "-" atom
; a vendor-specific option, non-standard
patterns = "(" list-mailbox *(SP list-mailbox) ")"
return-option = "SUBSCRIBED" / "CHILDREN" / option-extension
tagged-ext-comp = astring /
tagged-ext-comp *(SP tagged-ext-comp) /
"(" tagged-ext-comp ")"
; Extensions that follow this general
; syntax should use nstring instead of
; astring when appropriate in the context
; of the extension.
; Note that a message set or a "number"
; can always be represented as an "atom".
; A URL should be represented as
; a "quoted" string.
tagged-ext-simple = sequence-set / number
tagged-ext-val = tagged-ext-simple /
"(" [tagged-ext-comp] ")"
7. Internationalization Considerations
The LIST command selection option types defined in this specification
involve simple tests of mailbox properties. However, future
extensions to LIST-EXTENDED may define selection options that do more
sophisticated tests. In the case of a test that requires matching
text, in the presence of the COMPARATOR [I18N] extension, the active
comparator must be used to do comparisons. Such LIST-EXTENDED
extensions MUST indicate in their specification the interaction with
the COMPARATOR [I18N] extension.
8. Security Considerations
This document describes syntactic changes to the specification of the IMAP4 commands LIST, LSUB, RLIST, and RLSUB, and the modified LIST command has the same security considerations as those commands. They are described in [IMAP4] and [MBRef]. The Child Mailbox 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 any child mailbox. 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. In most situations, this will not be a security concern, because if information regarding whether a mailbox has children is considered sensitive, a user would not be granted access to that mailbox in the first place. The CHILDINFO extended data item has the same security considerations as the \HasChildren attribute described above.9. IANA Considerations
9.1. Guidelines for IANA
IANA has created two new registries for LIST-EXTENDED options and LIST-EXTENDED response data. The templates and the initial registrations are detailed below.9.2. Registration Procedure and Change Control
Registration of a LIST-EXTENDED option is done by filling in the template in Section 9.3 and sending it via electronic mail to iana@iana.org. Registration of a LIST-EXTENDED extended data item is done by filling in the template in Section 9.5 and sending it via electronic mail to iana@iana.org. IANA has the right to reject obviously bogus registrations, but will perform no review of claims made in the registration form. A LIST-EXTENDED option/extended data item name that starts with "V-" is reserved for vendor-specific options/extended data items. All options, whether they are vendor specific or global, should be registered with IANA. If a LIST-EXTENDED extended data item is returned as a result of requesting a particular LIST-EXTENDED option,
the name of the option SHOULD be used as the name of the
LIST-EXTENDED extended data item.
Each vendor-specific option/extended data item MUST start with its
vendor-token ("vendor prefix"). The vendor-token MUST be registered
with IANA, using the [ACAP] vendor subtree registry.
Standard LIST-EXTENDED option/extended data item names are case
insensitive. If the vendor prefix is omitted from a vendor-specific
LIST-EXTENDED option/extended data item name, the rest is case
insensitive. The vendor prefix itself is not case sensitive, as it
might contain non-ASCII characters. While the registration
procedures do not require it, authors of
LIST-EXTENDED options/extended data items are encouraged to seek
community review and comment whenever that is feasible. Authors may
seek community review by posting a specification of their proposed
mechanism as an
Internet-Draft. LIST-EXTENDED option/extended data items intended
for widespread use should be standardized through the normal IETF
process, when appropriate.
Comments on registered LIST-EXTENDED options/extended response data
should first be sent to the "owner" of the mechanism and/or to the
IMAPEXT WG mailing list. Submitters of comments may, after a
reasonable attempt to contact the owner, request IANA to attach their
comment to the registration itself. If IANA approves of this, the
comment will be made accessible in conjunction with the registration
LIST-EXTENDED options/extended response data itself.
Once a LIST-EXTENDED registration has been published by IANA, the
author may request a change to its definition. The change request
follows the same procedure as the registration request.
The owner of a LIST-EXTENDED registration may pass responsibility for
the registered option/extended data item to another person or agency
by informing IANA; this can be done without discussion or review.
The IESG may reassign responsibility for a LIST-EXTENDED
option/extended data item. The most common case of this will be to
enable changes to be made to mechanisms where the author of the
registration has died, has moved out of contact, or is otherwise
unable to make changes that are important to the community.
LIST-EXTENDED registrations may not be deleted; mechanisms that are
no longer believed appropriate for use can be declared OBSOLETE by a
change to their "intended use" field. Such LIST-EXTENDED
options/extended data items will be clearly marked in the lists
published by IANA.
The IESG is considered to be the owner of all LIST-EXTENDED options/extended data items that are on the IETF standards track.9.3. Registration Template for LIST-EXTENDED Options
To: iana@iana.org Subject: Registration of LIST-EXTENDED option X LIST-EXTENDED option name: LIST-EXTENDED option type: (One of SELECTION or RETURN) Implied return options(s), if the option type is SELECTION: (zero or more) LIST-EXTENDED option description: Published specification (optional, recommended): Security considerations: Intended usage: (One of COMMON, LIMITED USE, or OBSOLETE) Person and email address to contact for further information: Owner/Change controller: (Any other information that the author deems interesting may be added below this line.)9.4. Initial LIST-EXTENDED Option Registrations
The LIST-EXTENDED option registry has been populated with the following entries: 1. To: iana@iana.org Subject: Registration of LIST-EXTENDED option SUBSCRIBED LIST-EXTENDED option name: SUBSCRIBED LIST-EXTENDED option type: SELECTION Implied return options(s): SUBSCRIBED LIST-EXTENDED option description: Causes the LIST command to list subscribed mailboxes, rather than the actual mailboxes.
Published specification: RFC 5258, Section 3.
Security considerations: RFC 5258, Section 8.
Intended usage: COMMON
Person and email address to contact for further information:
Alexey Melnikov <Alexey.Melnikov@isode.com>
Owner/Change controller: iesg@ietf.org
2. To: iana@iana.org
Subject: Registration of LIST-EXTENDED option REMOTE
LIST-EXTENDED option name: REMOTE
LIST-EXTENDED option type: SELECTION
Implied return options(s): (none)
LIST-EXTENDED option description: Causes the LIST command to
return remote mailboxes as well as local ones, as described in
RFC 2193.
Published specification: RFC 5258, Section 3.
Security considerations: RFC 5258, Section 8.
Intended usage: COMMON
Person and email address to contact for further information:
Alexey Melnikov <Alexey.Melnikov@isode.com>
Owner/Change controller: iesg@ietf.org
3. To: iana@iana.org
Subject: Registration of LIST-EXTENDED option SUBSCRIBED
LIST-EXTENDED option name: SUBSCRIBED
LIST-EXTENDED option type: RETURN
LIST-EXTENDED option description: Causes the LIST command to
return subscription state.
Published specification: RFC 5258, Section 3.
Security considerations: RFC 5258, Section 8.
Intended usage: COMMON
Person and email address to contact for further information:
Alexey Melnikov <Alexey.Melnikov@isode.com>
Owner/Change controller: iesg@ietf.org
4. To: iana@iana.org
Subject: Registration of LIST-EXTENDED option RECURSIVEMATCH
LIST-EXTENDED option name: RECURSIVEMATCH
LIST-EXTENDED option type: SELECTION
Implied return options(s): (none)
LIST-EXTENDED option description: Requests that CHILDINFO
extended data item (childinfo-extended-item) is to be returned.
Published specification: RFC 5258, Section 3.
Security considerations: RFC 5258, Section 8.
Intended usage: COMMON
Person and email address to contact for further information:
Alexey Melnikov <Alexey.Melnikov@isode.com>
Owner/Change controller: iesg@ietf.org
5. To: iana@iana.org
Subject: Registration of LIST-EXTENDED option CHILDREN
LIST-EXTENDED option name: CHILDREN
LIST-EXTENDED option type: RETURN
LIST-EXTENDED option description: Requests mailbox child
information.
Published specification: RFC 5258, Section 3 and Section 4.
Security considerations: RFC 5258, Section 8.
Intended usage: COMMON
Person and email address to contact for further information:
Alexey Melnikov <Alexey.Melnikov@isode.com>
Owner/Change controller: iesg@ietf.org
9.5. Registration Template for LIST-EXTENDED Extended Data Item
To: iana@iana.org
Subject: Registration of X LIST-EXTENDED extended data item
LIST-EXTENDED extended data item tag:
LIST-EXTENDED extended data item description:
Which LIST-EXTENDED option(s) (and their types) causes this extended
data item to be returned (if any):
Published specification (optional, recommended):
Security considerations:
Intended usage:
(One of COMMON, LIMITED USE, or OBSOLETE)
Person and email address to contact for further information:
Owner/Change controller:
(Any other information that the author deems interesting may be added
below this line.)
9.6. Initial LIST-EXTENDED Extended Data Item Registrations
The LIST-EXTENDED extended data item registry has been populated with
the following entries:
1. To: iana@iana.org
Subject: Registration of CHILDINFO LIST-EXTENDED extended data
item
LIST-EXTENDED extended data item tag: CHILDINFO
LIST-EXTENDED extended data item description: The CHILDINFO
extended data item describes the selection criteria that has
caused it to be returned and indicates that the mailbox has one
or more child mailboxes that match the selection criteria.
Which LIST-EXTENDED option(s) (and their types) causes this
extended data item to be returned (if any): RECURSIVEMATCH
selection option
Published specification: RFC 5258, Section 3.5.
Security considerations: RFC 5258, Section 8.
Intended usage: COMMON
Person and email address to contact for further information:
Alexey Melnikov <Alexey.Melnikov@isode.com>
Owner/Change controller: iesg@ietf.org
10. Acknowledgements
Mike Gahrns and Raymond Cheng of Microsoft Corporation originally
devised the Child Mailbox Extension and proposed it in 1997; the
idea, as well as most of the text in Section 4, is theirs.
This document is the result of discussions on the IMAP4 and IMAPEXT
mailing lists and is meant to reflect consensus of those groups. In
particular, Mark Crispin, Philip Guenther, Cyrus Daboo, Timo
Sirainen, Ken Murchison, Rob Siemborski, Steve Hole, Arnt
Gulbrandsen, Larry Greenfield, Dave Cridland, and Pete Maclean were
active participants in those discussions or made suggestions to this
document.
11. References
11.1. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[ACAP] Newman, C. and J. Myers, "ACAP -- Application Configuration
Access Protocol", RFC 2244, November 1997.
[I18N] Newman, C., Gulbrandsen, A., and A. Melnikov, "Internet
Message Access Protocol Internationalization", RFC 5255,
June 2008.
[IMAP4] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 3501, March 2003.
[Kwds] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[MBRef] Gahrns, M., "IMAP4 Mailbox Referrals", RFC 2193,
September 1997.
11.2. Informative References
[CMbox] Gahrns, M. and R. Cheng, "The Internet Message Action Protocol (IMAP4) Child Mailbox Extension", RFC 3348, July 2002.Authors' Addresses
Barry Leiba IBM T.J. Watson Research Center 19 Skyline Drive Hawthorne, NY 10532 US Phone: +1 914 784 7941 EMail: leiba@watson.ibm.com Alexey Melnikov Isode Limited 5 Castle Business Village 36 Station Road Hampton, Middlesex TW12 2BX UK EMail: Alexey.Melnikov@isode.com URI: http://www.melnikov.ca/
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