RFC8457: IMAP "$Important" Keyword and "\Important" Special-Use Attribute
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Internet Engineering Task Force (IETF) B. Leiba, Ed.
Request for Comments: 8457 Huawei Technologies
Category: Standards Track September 2018
ISSN: 2070-1721
IMAP "$Important" Keyword and "\Important" Special-Use Attribute
Abstract
RFC 6154 created an IMAP special-use LIST extension and defined an
initial set of attributes. This document defines a new attribute,
"\Important", and establishes a new IANA registry for IMAP folder
attributes, which include the attributes defined in RFCs 5258, 3501,
and 6154. This document also defines a new IMAP keyword,
"$Important", and registers it in the registry defined in RFC 5788.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8457.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions Used in This Document . . . . . . . . . . . . . 3
2. Definition of the "$Important" Message Keyword . . . . . . . 3
3. Definition of the 'Important' Mailbox Attribute . . . . . . . 3
3.1. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2.1. Example of a LIST Response . . . . . . . . . . . . . . . 4
3.2.2. Examples of Creating a New Mailbox Using "\Important" . . 4
4. Implementation Notes . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6.1. Registration of the "$Important" Keyword . . . . . . . . . 6
6.2. Creation of the IMAP Mailbox Name Attributes Registry . . . 7
6.2.1. Instructions to the Designated Expert . . . . . . . . . . 8
6.3. Initial Entries for the IMAP Mailbox Name Attributes
Registry . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . . 10
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The Internet Message Access Protocol (IMAP) specification [RFC3501]
defines the use of message keywords, and an "IMAP Keywords" registry
is created in [RFC5788]. [RFC6154] defines an extension to the IMAP
LIST command for special-use mailboxes. The extension allows servers
to provide extra information (attributes) about the purpose of a
mailbox and defines an initial set of special-use attributes.
This document does the following:
o defines a new message keyword, "$Important", to apply to messages
that are considered important for the user by some externally
defined criteria;
o registers the "$Important" keyword in the "IMAP Keywords"
registry;
o defines a new special-use attribute, "\Important", to designate a
mailbox that will hold messages that are considered important for
the user by some externally defined criteria; and
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o creates a registry for IMAP mailbox attributes and registers the
new attribute and those defined in [RFC5258], [RFC3501], and
[RFC6154].
1.1. Conventions Used in This Document
In the examples used in this document, "C:" indicates lines sent by a
client that is connected to a server, and "S:" indicates lines sent
by the server to the client.
2. Definition of the "$Important" Message Keyword
The "$Important" keyword is a signal that a message is likely
important to the user. The keyword is generally expected to be set
automatically by the system based on available signals (such as who
the message is from, who else the message is addressed to, evaluation
of the subject or content, or other heuristics). While the keyword
also can be set by the user, that is not expected to be the primary
usage.
This is distinct from the "\Flagged" system flag in two ways:
1. "$Important" carries a meaning of general importance, as opposed
to follow-up or urgency. It is meant to be used for a form of
triage, with "\Flagged" remaining as a designation of special
attention, need for follow-up, or time sensitivity. In
particular, the sense of "$Important" is that other messages that
are "like this one" according to some server-applied heuristics
will also be considered "$Important".
2. The setting of "$Important" is expected to be based at least
partly on heuristics (generally set automatically by the server),
whereas "\Flagged" is only intended to be set by the user with
some sort of "flag this message" or "put a star on this message"
interface.
3. Definition of the 'Important' Mailbox Attribute
The "\Important" mailbox attribute is a signal that the mailbox
contains messages that are likely important to the user. In an
implementation that also supports the "$Important" keyword, this
special use is likely to represent a virtual mailbox collecting
messages (from other mailboxes) that are marked with the "$Important"
keyword. In other implementations, the system might automatically
put messages there based on the same sorts of heuristics that are
noted for the "$Important" keyword (see Section 2). The distinctions
between "\Important" and "\Flagged" for mailboxes are similar to
those between "$Important" and "\Flagged" for messages.
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3.1. Formal Syntax
The following syntax specification adds to the one in Section 6 of
[RFC6154] using Augmented Backus-Naur Form (ABNF) as described in
[RFC5234]. Be sure to see the ABNF notes at the beginning of
Section 9 of [RFC3501].
use-attr =/ "\Important"
3.2. Examples
3.2.1. Example of a LIST Response
In the following example, the mailbox called "Important Messages" is
the one designated with the "\Important" attribute.
C: t1 LIST "" "Imp*"
S: * LIST (\HasNoChildren \Important) "/" "Important Messages"
S: * LIST (\HasNoChildren) "/" "Imported Wine"
S: t1 OK Success
3.2.2. Examples of Creating a New Mailbox Using "\Important"
In the following example, the mailbox called "Important Messages" is
created with the "\Important" attribute on a server that advertises
the "CREATE-SPECIAL-USE" capability string.
C: t1 CREATE "Important Messages" (USE (\Important))
S: t1 OK Mailbox created
The following example is similar to the previous one, but the server
is not able to assign the "\Important" attribute to the new mailbox.
C: t1 CREATE "Important Messages" (USE (\Important))
S: t1 NO [USEATTR] An \Important mailbox already exists
The following example is similar to the previous one, but the server
does not support this extension.
C: t1 CREATE "Important Messages" (USE (\Important))
S: t1 NO [USEATTR] Mailbox not created; unsupported use \Important
In both of the failure-mode examples, the "USEATTR" response code
lets the client know that the problem is in the "USE" parameters.
Note that the same response code is given in both cases, and the
human-readable text is the only way to tell the difference. That
text is not parsable by the client (it can only be logged and/or
reported to the user).
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4. Implementation Notes
This section is non-normative and is intended to describe the
intended (and current as of this publication) usage of "$Important"
in contrast with "\Flagged" on a message.
On the server:
o "\Flagged" is set or cleared in response to an explicit command
from the client.
o "$Important" is set via a heuristic process performed by the
server and usually involves analysis of header fields, what
mailbox the message is filed in, perhaps message content,
attachments, and such. It may then be set or cleared in response
to an explicit command from the client, and the server may use
that to adjust the heuristics in the future. It's also possible
that the server will re-evaluate this and make a message
"$Important" later if the user accesses the message frequently,
for example.
On the client:
o Typically, an icon such as a flag or a star (or an indication,
such as red or bold text) is associated with "\Flagged", and the
UI provides a way for the user to turn that icon or indication on
or off. Manipulation of this results in a command to the server.
o Typically, a lesser indication is used for "$Important". The
client might or might not provide the user with a way to
manipulate it. If it does, manipulation results in a command to
the server.
5. Security Considerations
The security considerations in Section 7 of [RFC6154] apply equally
to this extension, in particular, "Conveying special-use information
to a client exposes a small bit of extra information that could be of
value to an attacker." Moreover, identifying important messages or a
place where important messages are kept could give an attacker a
strategic starting point. If the algorithm by which messages are
determined to be important is well known, still more information is
exposed -- perhaps, for example, there is an implication that the
senders of these messages are particularly significant to the mailbox
owner, and perhaps that is information that should not be made
public.
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As noted in RFC 6154, it is wise to protect the IMAP channel from
passive eavesdropping and to defend against unauthorized discernment
of the identity of a user's "\Important" mailbox or of a user's
"$Important" messages. See Section 11 of [RFC3501] for security
considerations about using the IMAP STARTTLS command to protect the
IMAP channel.
6. IANA Considerations
IANA has completed three actions, which are specified in the sections
below:
1. registration of the "$Important" keyword;
2. creation of a new "IMAP Mailbox Name Attributes" registry; and
3. registration of initial entries in the "IMAP Mailbox Name
Attributes" registry.
6.1. Registration of the "$Important" Keyword
IANA has registered the "$Important" keyword in the "IMAP Keywords"
registry, as follows, using the template in [RFC5788].
IMAP keyword name: $Important
Purpose (description): The "$Important" keyword is a signal that a
message is likely important to the user.
Private or Shared on a server: PRIVATE
Is it an advisory keyword or may it cause an automatic action:
Advisory (but see the reference for details).
When/by whom the keyword is set/cleared: The keyword can be set by
the user, or automatically by the system based on available
signals (such as who the message is from, who else the message
is addressed to, evaluation of the subject or content, or other
heuristics).
Related keywords: None (see the reference for the related mailbox
name attribute).
Related IMAP capabilities: None.
Security considerations: See Section 5 of RFC 8457.
Published specification: RFC 8457
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Person & email address to contact for further information:
IETF Applications and Real-Time Area <art@ietf.org>
Intended usage: COMMON
Owner/Change controller: IESG
Note: None.
6.2. Creation of the IMAP Mailbox Name Attributes Registry
IANA has created a new registry in the group "Internet Message Access
Protocol (IMAP)". It is called "IMAP Mailbox Name Attributes", and
it has two references: "RFC 3501, Section 7.2.2", and "RFC 8457,
Section 6". This registry will be shared with the JSON Meta
Application Protocol (JMAP) for Mail [JMAP-MAIL].
The registry entries contain the following fields:
1. Attribute Name
2. Description
3. Reference
4. Usage Notes
IANA keeps this list in alphabetical order by Attribute Name, which
is registered without the initial backslash ("\"). The names are
generally registered with initial capital letters but are treated as
case-insensitive US-ASCII strings.
The "Usage Notes" field is free-form US-ASCII text that will normally
be empty (and is empty if it's not specified in the registration
request). It is intended to hold things such as "not used by JMAP"
and "JMAP only". The field is for human use, and there is no need
for a registry of strings that may appear here.
The registration policy for the new registry is listed as "IETF
Review" or "Expert Review" [RFC8126], and new registrations will be
accepted in one of two ways:
1. For registrations requested in an IETF consensus document, the
registration policy will be IETF Review, and the request will be
made in the IANA Considerations section of the document, which
gives the requested values for each of the fields.
2. For other registrations, the policy will be Expert Review policy
(see Section 6.2.1), and the request will be made by sending
email to IANA asking for a new IMAP Mailbox Name Attribute and
giving the requested values for each of the fields. While a
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formal specification is not required, the reference document
should provide a description of the proposed attribute sufficient
for building interoperable implementations. An Informational RFC
(submitted, for example, through the IETF or Independent stream)
is a fine way to publish a reference document (see also
Section 6.2.1).
6.2.1. Instructions to the Designated Expert
The expert reviewer, who will be designated by the IESG, is expected
to provide only a general review of the requested registration,
checking that the reference and description are adequate for
understanding the intent of the registered attribute. Efforts should
also be made to generalize the intent of an attribute so that
multiple implementations with the same requirements may reuse
existing attributes. Except for this check, this is intended to be
very close to a first come first served policy, and the expert should
not block serious registration requests with a reasonable reference.
The reference may be to any form of documentation, including a web
page, but consideration should be given to providing one that is
expected to be long-lived and stable.
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6.3. Initial Entries for the IMAP Mailbox Name Attributes Registry
The registry initially contains these entries:
+===============+===================================+===========+
| Attribute | Description | Reference |
| Name | | |
+===============+===================================+===========+
| All | All messages | [RFC6154] |
+---------------+-----------------------------------+-----------+
| Archive | Archived messages | [RFC6154] |
+---------------+-----------------------------------+-----------+
| Drafts | Messages that are working drafts | [RFC6154] |
+---------------+-----------------------------------+-----------+
| Flagged | Messages with the \Flagged flag | [RFC6154] |
+---------------+-----------------------------------+-----------+
| HasChildren | Has accessible child mailboxes | [RFC5258] | *
+---------------+-----------------------------------+-----------+
| HasNoChildren | Has no accessible child mailboxes | [RFC5258] | *
+---------------+-----------------------------------+-----------+
| Important | Messages deemed important to user | RFC 8457 |
+---------------+-----------------------------------+-----------+
| Junk | Messages identified as Spam/Junk | [RFC6154] |
+---------------+-----------------------------------+-----------+
| Marked | Server has marked the mailbox as | [RFC3501] | *
| | "interesting" | |
+---------------+-----------------------------------+-----------+
| NoInferiors | No hierarchy under this name | [RFC3501] | *
+---------------+-----------------------------------+-----------+
| NonExistent | The mailbox name doesn't actually | [RFC5258] | *
| | exist | |
+---------------+-----------------------------------+-----------+
| Noselect | The mailbox is not selectable | [RFC3501] | *
+---------------+-----------------------------------+-----------+
| Remote | The mailbox exists on a remote | [RFC5258] | *
| | server | |
+---------------+-----------------------------------+-----------+
| Sent | Sent mail | [RFC6154] |
+---------------+-----------------------------------+-----------+
| Subscribed | The mailbox is subscribed to | [RFC5258] |
+---------------+-----------------------------------+-----------+
| Trash | Messages the user has discarded | [RFC6154] |
+---------------+-----------------------------------+-----------+
| Unmarked | No new messages since last select | [RFC3501] | *
+===============+===================================+===========+
The rows marked with "*" at the end have their Usage Notes field set
to "not used by JMAP".
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7. References
7.1. Normative References
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC6154] Leiba, B. and J. Nicolson, "IMAP LIST Extension for
Special-Use Mailboxes", RFC 6154, DOI 10.17487/RFC6154,
March 2011, <https://www.rfc-editor.org/info/rfc6154>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
7.2. Informative References
[JMAP-MAIL]
Jenkins, N. and C. Newman, "JMAP for Mail", Work in
Progress, draft-ietf-jmap-mail-07, August 2018.
[RFC5258] Leiba, B. and A. Melnikov, "Internet Message Access
Protocol version 4 - LIST Command Extensions", RFC 5258,
DOI 10.17487/RFC5258, June 2008,
<https://www.rfc-editor.org/info/rfc5258>.
[RFC5788] Melnikov, A. and D. Cridland, "IMAP4 Keyword Registry",
RFC 5788, DOI 10.17487/RFC5788, March 2010,
<https://www.rfc-editor.org/info/rfc5788>.
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Contributors
The following author was an original contributor to this document in
addition to the editor.
Eric "Iceman"
Google
Email: iceman@google.com
Author's Address
Barry Leiba (editor)
Huawei Technologies
Phone: +1 646 827 0648
Email: barryleiba@computer.org
URI: http://internetmessagingtechnology.org/
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