Network Working Group C. Daboo

Network Working Group C. Daboo
Request for Comments: 5464 Apple, Inc.
Category: Standards Track February 2009

The IMAP METADATA Extension

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

The METADATA extension to the Internet Message Access Protocol
permits clients and servers to maintain "annotations" or "metadata"
on IMAP servers. It is possible to have annotations on a per-mailbox
basis or on the server as a whole. For example, this would allow
comments about the purpose of a particular mailbox to be "attached"
to that mailbox, or a "message of the day" containing server status
information to be made available to anyone logging in to the server.

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Table of Contents

1. Introduction and Overview . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Data Model . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Namespace of Entries . . . . . . . . . . . . . . . . . . . 4
3.2.1. Entry Names . . . . . . . . . . . . . . . . . . . . . 5
3.3. Private versus Shared and Access Control . . . . . . . . . 6
4. IMAP Protocol Changes . . . . . . . . . . . . . . . . . . . . 7
4.1. General Considerations . . . . . . . . . . . . . . . . . . 7
4.2. GETMETADATA Command . . . . . . . . . . . . . . . . . . . 8
4.2.1. MAXSIZE GETMETADATA Command Option . . . . . . . . . . 9
4.2.2. DEPTH GETMETADATA Command Option . . . . . . . . . . . 10
4.3. SETMETADATA Command . . . . . . . . . . . . . . . . . . . 10
4.4. METADATA Response . . . . . . . . . . . . . . . . . . . . 12
4.4.1. METADATA Response with Values . . . . . . . . . . . . 13
4.4.2. Unsolicited METADATA Response without Values . . . . . 13
5. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6.1. Entry and Attribute Registration Template . . . . . . . . 16
6.2. Server Entry Registrations . . . . . . . . . . . . . . . . 16
6.2.1. /shared/comment . . . . . . . . . . . . . . . . . . . 17
6.2.2. /shared/admin . . . . . . . . . . . . . . . . . . . . 17
6.3. Mailbox Entry Registrations . . . . . . . . . . . . . . . 17
6.3.1. /shared/comment . . . . . . . . . . . . . . . . . . . 18
6.3.2. /private/comment . . . . . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8. Normative References . . . . . . . . . . . . . . . . . . . . . 19
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 19

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RFC 5464 The IMAP METADATA Extension February 2009

1. Introduction and Overview

The goal of the METADATA extension is to provide a means for clients
to set and retrieve "annotations" or "metadata" on an IMAP server.
The annotations can be associated with specific mailboxes or the
server as a whole. The server can choose to support only server
annotations or both server and mailbox annotations.

A server that supports both server and mailbox annotations indicates
the presence of this extension by returning "METADATA" as one of the
supported capabilities in the CAPABILITY command response.

A server that supports only server annotations indicates the presence
of this extension by returning "METADATA-SERVER" as one of the
supported capabilities in the CAPABILITY command response.

A server that supports unsolicited annotation change responses MUST
support the "ENABLE" [RFC5161] extension to allow clients to turn
that feature on.

The METADATA extension adds two new commands and one new untagged
response to the IMAP base protocol.

This extension makes the following changes to the IMAP protocol:

o adds a new SETMETADATA command

o adds a new GETMETADATA command

o adds a new METADATA untagged response

o adds a new METADATA response code

The rest of this document describes the data model and protocol
changes more rigorously.

2. Conventions Used in This Document

In examples, "C:" and "S:" indicate lines sent by the client and
server, respectively.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

Whitespace and line breaks have been added to the examples in this
document to promote readability.

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3. Data Model

3.1. Overview

Mailboxes or the server as a whole may have zero or more annotations
associated with them. An annotation contains a uniquely named entry,
which has a value. Annotations can be added to mailboxes when a
mailbox name is provided as the first argument to the SETMETADATA
command, or to the server as a whole when the empty string is
provided as the first argument to the command.

For example, a general comment being added to a mailbox may have an
entry name of "/comment" and a value of "Really useful mailbox".

The protocol changes to IMAP described below allow a client to access
or change the values of any annotation entry, assuming it has
sufficient access rights to do so.

3.2. Namespace of Entries

Each annotation is an entry that has a hierarchical name, with each
component of the name separated by a slash ("/"). An entry name MUST
NOT contain two consecutive "/" characters and MUST NOT end with a
"/" character.

The value of an entry is NIL (has no value), or a string or binary
data of zero or more octets. A string MAY contain multiple lines of
text. Clients MUST use the CRLF (0x0D 0x0A) character octet sequence
to represent line ends in a multi-line string value.

Entry names MUST NOT contain asterisk ("*") or percent ("%")
characters and MUST NOT contain non-ASCII characters or characters
with octet values in the range 0x00 to 0x19. Invalid entry names
result in a BAD response in any IMAP command in which they are used.

Entry names are case-insensitive.

Use of control or punctuation characters in entry names is strongly
discouraged.

This specification defines an initial set of entry names available
for use with mailbox and server annotations. In addition, an
extension mechanism is described to allow additional names to be
added for extensibility.

The first component in entry names defines the scope of the
annotation. Currently, only the prefixes "/private" or "/shared" are
defined. These prefixes are used to indicate whether an annotation

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is stored on a per-user basis ("/private") and not visible to other
users, or whether an annotation is shared between authorized users
("/shared") with a single value that can be read and changed by
authorized users with appropriate access. See Section 3.3 for
details.

Entry names can have any number of components starting at 2, unless
they fall under the vendor namespaces (i.e., have a /shared/vendor/
<vendor-token> or /private/vendor/<vendor-token> prefix as described
below), in which case they have at least 4 components.

3.2.1. Entry Names

Entry names MUST be specified in a Standards Track or IESG-approved
Experimental RFC, or fall under the vendor namespace. See
Section 6.1 for the registration template.

3.2.1.1. Server Entries

These entries are set or retrieved when the mailbox name argument to
the new SETMETADATA or GETMETADATA command is the empty string.

/shared/comment

Defines a comment or note that is associated with the server and
that is shared with authorized users of the server.

/shared/admin

Indicates a method for contacting the server administrator. The
value MUST be a URI (e.g., a mailto: or tel: URL). This entry is
always read-only -- clients cannot change it. It is visible to
authorized users of the system.

/shared/vendor/<vendor-token>

Defines the top level of shared entries associated with the
server, as created by a particular product of some vendor. This
entry can be used by vendors to provide server- or client-specific
annotations. The vendor-token MUST be registered with IANA, using
the Application Configuration Access Protocol (ACAP) [RFC2244]
vendor subtree registry.

/private/vendor/<vendor-token>

Defines the top level of private entries associated with the
server, as created by a particular product of some vendor. This
entry can be used by vendors to provide server- or client-specific

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annotations. The vendor-token MUST be registered with IANA, using
the ACAP [RFC2244] vendor subtree registry.

3.2.1.2. Mailbox Entries

These entries are set or retrieved when the mailbox name argument to
the new SETMETADATA or GETMETADATA command is not the empty string.

/shared/comment

Defines a shared comment or note associated with a mailbox.

/private/comment

Defines a private (per-user) comment or note associated with a
mailbox.

/shared/vendor/<vendor-token>

Defines the top level of shared entries associated with a specific
mailbox, as created by a particular product of some vendor. This
entry can be used by vendors to provide client-specific
annotations. The vendor-token MUST be registered with IANA, using
the ACAP [RFC2244] vendor subtree registry.

/private/vendor/<vendor-token>

Defines the top level of private entries associated with a
specific mailbox, as created by a particular product of some
vendor. This entry can be used by vendors to provide client-
specific annotations. The vendor-token MUST be registered with
IANA, using the ACAP [RFC2244] vendor subtree registry.

3.3. Private versus Shared and Access Control

In the absence of the ACL (Access Control List) extension [RFC4314],
users can only set and retrieve private or shared mailbox annotations
on a mailbox that exists and is returned to them via a LIST or LSUB
command, and on which they have either read or write access to the
actual message content of the mailbox (as determined by the READ-ONLY
and READ-WRITE response codes as described in Section 5.2 of
[RFC4314]).

When the ACL extension [RFC4314] is present, users can only set and
retrieve private or shared mailbox annotations on a mailbox on which
they have the "l" right and any one of the "r", "s", "w", "i", or "p"
rights.

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If a client attempts to set or retrieve annotations on mailboxes that
do not satisfy the conditions above, the server MUST respond with a
NO response.

Users can always retrieve private or shared server annotations if
they exist. Servers MAY restrict the creation of private or shared
server annotations as appropriate. When restricted, the server MUST
return a NO response when the SETMETADATA command is used to try to
create a server annotation.

If the METADATA extension is present, support for shared annotations
is REQUIRED, whilst support for private annotations is OPTIONAL.
This recognizes the fact that support for private annotations may
introduce significantly more complexity to a server in terms of
tracking ownership of the annotations, how quota is determined for
users based on their own annotations, etc.

4. IMAP Protocol Changes

4.1. General Considerations

The new SETMETADATA command and the METADATA response each have a
mailbox name argument. An empty string is used for the mailbox name
to signify server annotations. A non-empty string is used to signify
mailbox annotations attached to the corresponding mailbox.

Servers SHOULD ensure that mailbox annotations are automatically
moved when the mailbox they refer to is renamed, i.e., the
annotations follow the mailbox. This applies to a rename of the
INBOX, with the additional behavior that the annotations are copied
from the original INBOX to the renamed mailbox, i.e., mailbox
annotations are preserved on the INBOX when it is renamed.

Servers SHOULD delete annotations for a mailbox when the mailbox is
deleted, so that a mailbox created with the same name as a previously
existing mailbox does not inherit the old mailbox annotations.

Servers SHOULD allow annotations on all 'types' of mailboxes,
including ones reporting \Noselect for their LIST response. Servers
can implicitly remove \Noselect mailboxes when all child mailboxes
are removed, and, at that time any annotations associated with the
\Noselect mailbox SHOULD be removed.

The server is allowed to impose limitations on the size of any one
annotation or the total number of annotations for a single mailbox or
for the server as a whole. However, the server MUST accept an
annotation data size of at least 1024 bytes, and an annotation count
per server or mailbox of at least 10.

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Some annotations may be "read-only" -- i.e., they are set by the
server and cannot be changed by the client. Also, such annotations
may be "computed" -- i.e., the value changes based on underlying
properties of the mailbox or server. For example, an annotation
reporting the total size of all messages in the mailbox would change
as messages are added or removed. Or, an annotation containing an
IMAP URL for the mailbox would change if the mailbox was renamed.

Servers MAY support sending unsolicited responses for use when
annotations are changed by some "third-party" (see Section 4.4). In
order to do so, servers MUST support the ENABLE command [RFC5161] and
MUST only send unsolicited responses if the client used the ENABLE
command [RFC5161] extension with the capability string "METADATA" or
"METADATA-SERVER" earlier in the session, depending on which of those
capabilities is supported by the server.

4.2. GETMETADATA Command

This extension adds the GETMETADATA command. This allows clients to
retrieve server or mailbox annotations.

This command is only available in authenticated or selected state
[RFC3501].

Arguments: mailbox-name
options
entry-specifier

Responses: required METADATA response

Result: OK - command completed
NO - command failure: can't access annotations on
the server
BAD - command unknown or arguments invalid

When the mailbox name is the empty string, this command retrieves
server annotations. When the mailbox name is not empty, this command
retrieves annotations on the specified mailbox.

Options MAY be included with this command and are defined below.

Example:

C: a GETMETADATA "" /shared/comment
S: * METADATA "" (/shared/comment "Shared comment")
S: a OK GETMETADATA complete

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In the above example, the contents of the value of the "/shared/
comment" server entry is requested by the client and returned by
the server.

Example:

C: a GETMETADATA "INBOX" /private/comment
S: * METADATA "INBOX" (/private/comment "My own comment")
S: a OK GETMETADATA complete

In the above example, the contents of the value of the "/private/
comment" mailbox entry for the mailbox "INBOX" is requested by the
client and returned by the server.

Entry specifiers can be lists of atomic specifiers, so that multiple
annotations may be returned in a single GETMETADATA command.

Example:

C: a GETMETADATA "INBOX" (/shared/comment /private/comment)
S: * METADATA "INBOX" (/shared/comment "Shared comment"
/private/comment "My own comment")
S: a OK GETMETADATA complete

In the above example, the values of the two server entries
"/shared/comment" and "/private/comment" on the mailbox "INBOX"
are requested by the client and returned by the server.

4.2.1. MAXSIZE GETMETADATA Command Option

When the MAXSIZE option is specified with the GETMETADATA command, it
restricts which entry values are returned by the server. Only entry
values that are less than or equal in octet size to the specified
MAXSIZE limit are returned. If there are any entries with values
larger than the MAXSIZE limit, the server MUST include the METADATA
LONGENTRIES response code in the tagged OK response for the
GETMETADATA command. The METADATA LONGENTRIES response code returns
the size of the biggest entry value requested by the client that
exceeded the MAXSIZE limit.

Example:

C: a GETMETADATA "INBOX" (MAXSIZE 1024)
(/shared/comment /private/comment)
S: * METADATA "INBOX" (/private/comment "My own comment")
S: a OK [METADATA LONGENTRIES 2199] GETMETADATA complete

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In the above example, the values of the two server entries
"/shared/comment" and "/private/comment" on the mailbox "INBOX"
are requested by the client, which wants to restrict the size of
returned values to 1024 octets. In this case, the "/shared/
comment" entry value is 2199 octets and is not returned.

4.2.2. DEPTH GETMETADATA Command Option

When the DEPTH option is specified with the GETMETADATA command, it
extends the list of entry values returned by the server. For each
entry name specified in the GETMETADATA command, the server returns
the value of the specified entry name (if it exists), plus all
entries below the entry name up to the specified DEPTH. Three values
are allowed for DEPTH:

"0" - no entries below the specified entry are returned
"1" - only entries immediately below the specified entry are returned
"infinity" - all entries below the specified entry are returned

Thus, "depth 1" for an entry "/a" will match "/a" as well as its
children entries (e.g., "/a/b"), but will not match grandchildren
entries (e.g., "/a/b/c").

If the DEPTH option is not specified, this is the same as specifying
"DEPTH 0".

Example:

C: a GETMETADATA "INBOX" (DEPTH 1)
(/private/filters/values)
S: * METADATA "INBOX" (/private/filters/values/small
"SMALLER 5000" /private/filters/values/boss
"FROM \"[email protected]\"")
S: a OK GETMETADATA complete

In the above example, 2 entries below the /private/filters/values
entry exist on the mailbox "INBOX": "/private/filters/values/
small" and "/private/filters/values/boss".

4.3. SETMETADATA Command

This extension adds the SETMETADATA command. This allows clients to
set annotations.

This command is only available in authenticated or selected state
[RFC3501].

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Arguments: mailbox-name
entry
value
list of entry, values

Responses: no specific responses for this command

Result: OK - command completed
NO - command failure: can't set annotations,
or annotation too big or too many
BAD - command unknown or arguments invalid

This command sets the specified list of entries by adding or
replacing the specified values provided, on the specified existing
mailboxes or on the server (if the mailbox argument is the empty
string). Clients can use NIL for the value of entries it wants to
remove. The server SHOULD NOT return a METADATA response containing
the updated annotation data. Clients MUST NOT assume that a METADATA
response will be sent, and MUST assume that if the command succeeds,
then the annotation has been changed.

If the server is unable to set an annotation because the size of its
value is too large, the server MUST return a tagged NO response with
a "[METADATA MAXSIZE NNN]" response code when NNN is the maximum
octet count that it is willing to accept.

If the server is unable to set a new annotation because the maximum
number of allowed annotations has already been reached, the server
MUST return a tagged NO response with a "[METADATA TOOMANY]" response
code.

If the server is unable to set a new annotation because it does not
support private annotations on one of the specified mailboxes, the
server MUST return a tagged NO response with a "[METADATA NOPRIVATE]"
response code.

When any one annotation fails to be set, resulting in a tagged NO
response from the server, then the server MUST NOT change the values
for other annotations specified in the SETMETADATA command.

Example:

C: a SETMETADATA INBOX (/private/comment {33}
S: + ready for data
My new comment across
two lines.
)
S: a OK SETMETADATA complete

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In the above example, the entry "/private/comment" for the mailbox
"INBOX" is created (if not already present) and the value set to a
multi-line string.

Example:

C: a SETMETADATA INBOX (/private/comment NIL)
S: a OK SETMETADATA complete

In the above example, the entry "/private/comment" is removed from
the mailbox "INBOX".

Multiple entries can be set in a single SETMETADATA command by
listing entry-value pairs in the list.

Example:

C: a SETMETADATA INBOX (/private/comment "My new comment"
/shared/comment "This one is for you!")
S: a OK SETMETADATA complete

In the above example, the entries "/private/comment" and "/shared/
comment" for the mailbox "INBOX" are created (if not already
present) and the values set as specified.

Example:

C: a SETMETADATA INBOX (/private/comment "My new comment")
S: a NO [METADATA TOOMANY] SETMETADATA failed

In the above example, the server is unable to set the requested
(new) annotation as it has reached the limit on the number of
annotations it can support on the specified mailbox.

4.4. METADATA Response

The METADATA response displays results of a GETMETADATA command, or
can be returned as an unsolicited response at any time by the server
in response to a change in a server or mailbox annotation.

When unsolicited responses are activated by the ENABLE [RFC5161]
command for this extension, servers MUST send unsolicited METADATA
responses if server or mailbox annotations are changed by a third-
party, allowing servers to keep clients updated with changes.

Unsolicited METADATA responses MUST only contain entry names, not the
values. If the client wants to update any cached values, it must
explicitly retrieve those using a GETMETADATA command.

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The METADATA response can contain multiple entries in a single
response, but the server is free to return multiple responses for
each entry or group of entries, if it desires.

This response is only available in authenticated or selected state
[RFC3501].

4.4.1. METADATA Response with Values

The response consists of a list of entry-value pairs.

Example:

C: a GETMETADATA "" /shared/comment
S: * METADATA "" (/shared/comment "My comment")
S: a OK GETMETADATA complete

In the above example, a single entry with its value is returned by
the server.

Example:

C: a GETMETADATA "INBOX" /private/comment /shared/comment
S: * METADATA "INBOX" (/private/comment "My comment"
/shared/comment "Its sunny outside!")
S: a OK GETMETADATA complete

In the above example, two entries and their values are returned by
the server.

Example:

C: a GETMETADATA "INBOX" /private/comment /shared/comment
S: * METADATA "INBOX" (/private/comment "My comment")
S: * METADATA "INBOX" (/shared/comment "Its sunny outside!")
S: a OK GETMETADATA complete

In the above example, the server returns two separate responses
for each of the two entries requested.

4.4.2. Unsolicited METADATA Response without Values

The response consists of a list of entries, each of which have
changed on the server or mailbox.

Example:

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C: a NOOP
S: * METADATA "" /shared/comment
S: a OK NOOP complete

In the above example, the server indicates that the "/shared/
comment" server entry has been changed.

Example:

C: a NOOP
S: * METADATA "INBOX" /shared/comment /private/comment
S: a OK NOOP complete

In the above example, the server indicates a change to two mailbox
entries.

5. Formal Syntax

The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [RFC5234].

Non-terminals referenced but not defined below are as defined by
[RFC3501], with the new definitions in [RFC4466] superseding those in
[RFC3501].

Except as noted otherwise, all alphabetic characters are case-
insensitive. The use of upper or lower case characters to define
token strings is for editorial clarity only. Implementations MUST
accept these strings in a case-insensitive fashion.

capability =/ "METADATA" / "METADATA-SERVER"
; defines the capabilities for this extension.

command-auth =/ setmetadata / getmetadata
; adds to original IMAP command

entries = entry /
"(" entry *(SP entry) ")"
; entry specifiers

entry = astring
; slash-separated path to entry
; MUST NOT contain "*" or "%"

entry-value = entry SP value

entry-values = "(" entry-value *(SP entry-value) ")"

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entry-list = entry *(SP entry)
; list of entries used in unsolicited
; METADATA response

getmetadata = "GETMETADATA" [SP getmetadata-options]
SP mailbox SP entries
; empty string for mailbox implies
; server annotation.

getmetadata-options = "(" getmetadata-option
*(SP getmetadata-option) ")"

getmetadata-option = tagged-ext-label [SP tagged-ext-val]
; tagged-ext-label and tagged-ext-val
; are defined in [RFC4466].

maxsize-opt = "MAXSIZE" SP number
; Used as a getmetadata-option

metadata-resp = "METADATA" SP mailbox SP
(entry-values / entry-list)
; empty string for mailbox implies
; server annotation.

response-payload =/ metadata-resp
; adds to original IMAP data responses

resp-text-code =/ "METADATA" SP "LONGENTRIES" SP number
; new response codes for GETMETADATA

resp-text-code =/ "METADATA" SP ("MAXSIZE" SP number /
"TOOMANY" / "NOPRIVATE")
; new response codes for SETMETADATA
; failures

scope-opt = "DEPTH" SP ("0" / "1" / "infinity")
; Used as a getmetadata-option

setmetadata = "SETMETADATA" SP mailbox
SP entry-values
; empty string for mailbox implies
; server annotation.

value = nstring / literal8

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6. IANA Considerations

All entries MUST have either "/shared" or "/private" as a prefix.
Entry names MUST be specified in a Standards Track or IESG-approved
Experimental RFC, or fall under the vendor namespace (i.e., use
/shared/vendor/<vendor-token> or /private/vendor/<vendor-token> as
the prefix).

Each entry registration MUST include a content-type that is used to
indicate the nature of the annotation value. Where applicable, a
charset parameter MUST be included with the content-type.

6.1. Entry and Attribute Registration Template

To: [email protected]
Subject: IMAP METADATA Entry Registration

Type: [Either "Mailbox" or "Server"]

Name: [the name of the entry]

Description: [a description of what the entry is for]

Content-type: [MIME Content-Type and charset for the entry value]

RFC Number: [for entries published as RFCs]

Contact: [email and/or physical address to contact for
additional information]

6.2. Server Entry Registrations

The following templates specify the IANA registrations of annotation
entries specified in this document.

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6.2.1. /shared/comment

To: [email protected]
Subject: IMAP METADATA Entry Registration

Type: Server

Name: /shared/comment

Description: Defines a comment or note that is associated
with the server and that is shared with
authorized users of the server.

Content-type: text/plain; charset=utf-8

RFC Number: RFC 5464

Contact: IMAP Extensions mailto:[email protected]

6.2.2. /shared/admin

To: [email protected]
Subject: IMAP METADATA Entry Registration

Type: Server

Name: /shared/admin

Description: Indicates a method for contacting the server
administrator. The value MUST be a URI (e.g., a
mailto: or tel: URL). This entry is always
read-only -- clients cannot change it. It is visible
to authorized users of the system.

Content-type: text/plain; charset=utf-8

RFC Number: RFC 5464

Contact: IMAP Extensions mailto:[email protected]

6.3. Mailbox Entry Registrations

The following templates specify the IANA registrations of annotation
entries specified in this document.

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6.3.1. /shared/comment

To: [email protected]
Subject: IMAP METADATA Entry Registration

Type: Mailbox

Name: /shared/comment

Description: Defines a shared comment or note associated with a
mailbox.

Content-type: text/plain; charset=utf-8

RFC Number: RFC 5464

Contact: IMAP Extensions mailto:[email protected]

6.3.2. /private/comment

To: [email protected]
Subject: IMAP METADATA Entry Registration

Type: Mailbox

Name: /private/comment

Description: Defines a private comment or note associated with a
mailbox.

Content-type: text/plain; charset=utf-8

RFC Number: RFC 5464

Contact: IMAP Extensions mailto:[email protected]

7. Security Considerations

The security considerations in Section 11 of [RFC3501] apply here
with respect to protecting annotations from snooping. Servers MAY
choose to only support the METADATA and/or METADATA-SERVER extensions
after a privacy layer has been negotiated by the client.

Annotations can contain arbitrary data of varying size. As such,
servers MUST ensure that size limits are enforced to prevent a user
from using up all available space on a server and preventing use by
others. Clients MUST treat annotation data values as an "untrusted"
source of data as it is possible for it to contain malicious content.

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RFC 5464 The IMAP METADATA Extension February 2009

Annotations whose values are intended to remain private MUST be
stored only in entries that have the "/private" prefix on the entry
name.

Excluding the above issues, the METADATA extension does not raise any
security considerations that are not present in the base IMAP
protocol, and these issues are discussed in [RFC3501].

8. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2244] Newman, C. and J. Myers, "ACAP -- Application
Configuration Access Protocol", RFC 2244, November 1997.

[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.

[RFC4314] Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
RFC 4314, December 2005.

[RFC4466] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
ABNF", RFC 4466, April 2006.

[RFC5161] Gulbrandsen, A. and A. Melnikov, "The IMAP ENABLE
Extension", RFC 5161, March 2008.

[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.

Appendix A. Acknowledgments

The ideas expressed in this document are based on the message
annotation document that was co-authored by Randall Gellens. The
author would like to thank the following individuals for contributing
their ideas and support for writing this specification: Dave
Cridland, Arnt Gulbrandsen, Dan Karp, Alexey Melnikov, Ken Murchison,
Chris Newman, and Michael Wener.

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RFC 5464 The IMAP METADATA Extension February 2009

Author's Address

Cyrus Daboo
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
USA

EMail: [email protected]
URI: http://www.apple.com/

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RFC 5464 The IMAP METADATA Extension February 2009

Full Copyright Statement

Copyright (C) The IETF Trust (2009).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.

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