IMAP4 Keyword Registry

Abstract

The aim of this document is to establish a new IANA registry for IMAP keywords and to define a procedure for keyword registration, in order to improve interoperability between different IMAP clients.

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 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc5788.

Copyright Notice

Copyright © 2010 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 (http://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.

Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................2
   3. IANA Considerations .............................................3
      3.1. Review Guidelines for the Designated Expert Reviewer .......4
      3.2. Comments on IMAP Keywords' Registrations ...................5
      3.3. Change Control .............................................5
      3.4. Initial Registrations ......................................6
           3.4.1. $MDNSent IMAP Keyword Registration ..................6
           3.4.2. $Forwarded IMAP Keyword Registration ................7
           3.4.3. $SubmitPending IMAP Keyword Registration ............8
           3.4.4. $Submitted IMAP Keyword Registration ................9
   4. Security Considerations ........................................10
   5. Acknowledgements ...............................................10
   6. References .....................................................10
      6.1. Normative References ......................................10
      6.2. Informative References ....................................11

1. Introduction

IMAP keywords [RFC3501] are boolean named flags that can be used by clients to annotate messages in an IMAP mailbox. Although IMAP keywords are an optional feature of IMAP, the majority of IMAP servers can store arbitrary keywords. Many mainstream IMAP clients use a limited set of specific keywords, and some can manage (create, edit, display) arbitrary IMAP keywords.

Over the years, some IMAP keywords have become de-facto standards, with some specific semantics associated with them. In some cases, different client implementors decided to define and use keywords with different names, but the same semantics. Some server implementors decided to map such keywords automatically in order to improve cross- client interoperability.

In other cases, the same keywords have been used with different semantics, thus causing interoperability problems.

This document attempts to prevent further incompatible uses of IMAP keywords by establishing an "IMAP Keywords" registry and allocating a special prefix for standardized keywords.

2. Conventions Used in This Document

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 RFC 2119 [Kwds].

3. IANA Considerations

IANA has established a new registry for IMAP keywords.

Registration of an IMAP keyword is requested by filling in the following template and following the instructions on the IANA pages for the registry to submit it to IANA:

   Subject:    Registration of IMAP keyword X

IMAP keyword name:

Purpose (description):

   Private or Shared on a server:  (One of PRIVATE, SHARED, or BOTH.
                                   PRIVATE means that each different
                                   user has a distinct copy of the
                                   keyword.  SHARED means that all
                                   different users see the same value of
                                   the keyword.  BOTH means that an IMAP
                                   server can have the keyword as either
                                   private or shared.)

Is it an advisory keyword or may it cause an automatic action:

When/by whom the keyword is set/cleared:

   Related keywords:  (for example, "mutually exclusive with keywords Y
   
                      and Z")

Related IMAP capabilities:

Security considerations:

Published specification (recommended):

Person & email address to contact for further information:

   Intended usage:  (One of COMMON, LIMITED USE, or DEPRECATED (i.e.,
   
                    not recommended for use))
   
   Owner/Change controller:  (MUST be "IESG" for any "common use"
                             keyword registration specified in an IETF
                             Review document.  See definition of "common
                             use" below in this section.  When the
                             Owner/Change controller is not a
                             Standardization Organization, the
                             registration request MUST make it clear if

the registration is controlled by a company, or the individual performing the registration.)

   Note:       (Any other information that the author deems interesting
   
               may be added here, for example, if the keyword(s) is
               supported by existing clients.)

Registration of an IMAP keyword requires Expert Review [RFC5226]. Registration of any IMAP keyword is initiated by posting a registration request to the Message Organization WG mailing list <[email protected]> (or its replacement as chosen by the responsible Application Area Director) and CCing IANA (<[email protected]>). After allowing for at least two weeks for community input on the designated mailing list, the expert will determine the appropriateness of the registration request and either approve or disapprove the request with notice to the requestor, the mailing list, and IANA. Any refusal must come with a clear explanation.

The IESG appoints one or more Expert Reviewers for the IMAP keyword registry established by this document.

The Expert Reviewer should strive for timely reviews. The Expert Reviewer should take no longer than six weeks to make and announce the decision, or notify the mailing list that more time is required.

Decisions (or lack of) made by an Expert Reviewer can be first appealed to Application Area Directors and, if the appellant is not satisfied with the response, to the full IESG.

There are two types of IMAP keywords in the "IMAP Keywords" registry: intended for "common use" and vendor-/organization-specific use (also known as "limited use"). An IMAP keyword is said to be for "common use" if it is reasonably expected to be implemented in at least two independent client implementations. The two types of IMAP keywords have different levels of requirements for registration (see below).

3.1. Review Guidelines for the Designated Expert Reviewer

Expert Reviewers should focus on the following requirements.

Registration of a vendor-/organization-specific ("limited use") IMAP keyword is easier. The Expert Reviewer only needs to check that the requested name doesn't conflict with an already registered name, and that the name is not too generic, misleading, etc. The Expert Reviewer MAY request the IMAP keyword name change before approving the registration. The Expert Reviewer SHOULD refuse a registration if there is an already registered IMAP keyword that serves the same purpose, but has a different name.

When registering an IMAP keyword for "common use", the Expert Reviewer performs the checks described for vendor-/ organization-specific IMAP keywords, plus additional checks as detailed below.

Keywords intended for "common use" SHOULD start with the "$" prefix. (Note that this is a SHOULD because some of the commonly used IMAP keywords in widespread use don't follow this convention.)

IMAP keywords intended for "common use" SHOULD be standardized in IETF Review [RFC5226] documents. (Note that IETF Review documents still require Expert Review.)

Values in the "IMAP Keywords" IANA registry intended for "common use" must be clearly documented and likely to ensure interoperability. They must be useful, not harmful to the Internet. In cases when an IMAP keyword being registered is already deployed, Expert Reviewers should favor registering it over requiring perfect documentation and/or requesting a change to the name of the IMAP keyword.

The Expert Reviewer MAY automatically "upgrade" registration requests for a "limited use" IMAP keyword to "common use" level. The Expert Reviewer MAY also request that a registration targeted for "common use" be registered as "limited use" instead.

3.2. Comments on IMAP Keywords' Registrations

Comments on registered IMAP keywords should be sent to both the "owner" of the mechanism and to the mailing list designated to IMAP keyword review (see Section 3). This improves the chances of getting a timely response.

Submitters of comments may, after a reasonable attempt to contact the owner and after soliciting comments on the IMAP mailing list, request the designated Expert Reviewer to attach their comment to the IMAP keyword registration itself. The procedure is similar to requesting an Expert Review for the affected keyword.

3.3. Change Control

Once an IMAP keyword registration has been published by IANA, the owner may request a change to its definition. The change request (including a change to the "intended usage" field) follows the same procedure as the initial registration request, with the exception of changes to the "Person & email address to contact for further information" and "Owner/Change controller" fields. The latter can be changed by the owner by informing IANA; this can be done without discussion or review.

The IESG may reassign responsibility for an IMAP keyword. The most common case of this will be to enable clarifications to be made to keywords where the owner of the registration has died, moved out of contact, or is otherwise unable to make changes that are important to the community.

IMAP keyword registrations MUST NOT be deleted; keywords that are no longer believed appropriate for use can be declared DEPRECATED by a change to their "intended usage" field.

The IESG is considered the owner of all "common use" IMAP keywords that are published in an IETF Review document.

3.4. Initial Registrations

IANA has registered the IMAP keywords specified in following subsections in the registry established by this document.

3.4.1. $MDNSent IMAP Keyword Registration

   Subject:    Registration of IMAP keyword $MDNSent
   
   IMAP keyword name:  $MDNSent
   
   Purpose (description):  Specifies that a Message Disposition
                           Notification (MDN) must not be sent for any
                           message annotated with the $MDNSent IMAP
                           keyword.
   
   Private or Shared on a server:  SHARED

Is it an advisory keyword or may it cause an automatic action:

This keyword can cause automatic action by the client. See [RFC3503] for more details.

When/by whom the keyword is set/cleared:

This keyword is set by an IMAP client when it decides to act on an MDN request, or when uploading a sent or draft message. It can also be set by a delivery agent. Once set, the flag SHOULD NOT be cleared.

   Related keywords:  None
   
   Related IMAP capabilities:  None
   
   Security considerations:  See Section 6 of [RFC3503]
   
   Published specification (recommended):  [RFC3503]

Person & email address to contact for further information:

                           Alexey Melnikov <[email protected]>
   
   Intended usage:  COMMON
   
   Owner/Change controller:  IESG

Note:

3.4.2. $Forwarded IMAP Keyword Registration

   Subject:    Registration of the IMAP keyword $Forwarded
   
   IMAP keyword name:  $Forwarded
   
   Purpose (description):  $Forwarded is used by several IMAP clients to
                           specify that the message was resent to
                           another email address, embedded within or
                           attached to a new message.  This keyword is
                           set by the mail client when it successfully
                           forwards the message to another email
                           address.  Typical usage of this keyword is to
                           show a different (or additional) icon for a
                           message that has been forwarded.
   
   Private or Shared on a server:  BOTH

Is it an advisory keyword or may it cause an automatic action:

                           advisory

When/by whom the keyword is set/cleared:

This keyword can be set by either a delivery agent or a mail client. Once set, the flag SHOULD NOT be cleared. Notes: There is no way to tell if a message with $Forwarded keyword set was forwarded more than once.

   Related keywords:  None
   
   Related IMAP capabilities:  None
   Security considerations:  A server implementing this keyword as a
                             shared keyword may disclose that a
                             confidential message was forwarded.
   
   Published specification (recommended):  [RFC5550]

Person & email address to contact for further information:

                           Alexey Melnikov <[email protected]>
   
   Intended usage:  COMMON
   
   Owner/Change controller:  IESG

Note:

3.4.3. $SubmitPending IMAP Keyword Registration

   Subject:    Registration of IMAP keyword $SubmitPending
   
   IMAP keyword name:  $SubmitPending
   
   Purpose (description):  The $SubmitPending IMAP keyword designates
                           the message as awaiting to be submitted.
                           This keyword allows storing messages waiting
                           to be submitted in the same mailbox where
                           messages that were already submitted and/or
                           are being edited are stored.

A message that has both $Submitted and $SubmitPending IMAP keywords set is a message being actively submitted.

   Private or Shared on a server:  SHARED

Is it an advisory keyword or may it cause an automatic action:

This keyword can cause automatic action by the client. See Section 5.10 of [RFC5550] for more details.

When/by whom the keyword is set/cleared:

This keyword is set by a mail client when it decides that the message needs to be sent out.

   Related keywords:  $Submitted
   
   Related IMAP capabilities:  None
   Security considerations:  A server implementing this keyword as a
                             shared keyword may disclose that a
                             confidential message is scheduled to be
                             sent out or is being actively sent out.
   
   Published specification (recommended):  [RFC5550]

Person & email address to contact for further information:

                           Alexey Melnikov <[email protected]>
   
   Intended usage:  COMMON
   
   Owner/Change controller:  IESG

Note:

3.4.4. $Submitted IMAP Keyword Registration

   Subject:    Registration of IMAP keyword $Submitted
   
   IMAP keyword name:  $Submitted
   
   Purpose (description):  The $Submitted IMAP keyword designates the
                           message as being sent out.

A message that has both $Submitted and $SubmitPending IMAP keywords set is a message being actively submitted.

   Private or Shared on a server:  SHARED

Is it an advisory keyword or may it cause an automatic action:

This keyword can cause automatic action by the client. See Section 5.10 of [RFC5550] for more details.

When/by whom the keyword is set/cleared:

This keyword is set by a mail client when it decides to start sending it.

   Related keywords:  $SubmitPending
   
   Related IMAP capabilities:  None
   
   Security considerations:  A server implementing this keyword as a
                             shared keyword may disclose that a
                             confidential message was sent or is being
                             actively sent out.
   
   Published specification (recommended):  [RFC5550]

Person & email address to contact for further information:

                           Alexey Melnikov <[email protected]>
   
   Intended usage:  COMMON
   
   Owner/Change controller:  IESG

Note:

4. Security Considerations

IMAP keywords are one of the base IMAP features [RFC3501]. This document doesn't change their behavior, so it does not add new security issues.

A particular IMAP keyword might have specific security considerations, which are documented in the IMAP keyword registration template standardized by this document.

5. Acknowledgements

The creation of this document was prompted by one of many discussions on the IMAP mailing list.

John Neystadt co-authored the first version of this document.

Special thanks to Chris Newman, David Harris, Lyndon Nerenberg, Mark Crispin, Samuel Weiler, Alfred Hoenes, Lars Eggert, and Cullen Jennings for reviewing different versions of this document. However, all errors or omissions must be attributed to the authors of this document.

The authors would also like to thank the developers of Mozilla mail clients for providing food for thought.

6. References

6.1. Normative References

   [Kwds]     Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.
   
   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

6.2. Informative References

   [RFC3503]  Melnikov, A., "Message Disposition Notification (MDN)
              profile for Internet Message Access Protocol (IMAP)",
              RFC 3503, March 2003.
   
   [RFC5550]  Cridland, D., Melnikov, A., and S. Maes, "The Internet
              Email to Support Diverse Service Environments (Lemonade)
              Profile", RFC 5550, August 2009.

Authors' Addresses

   Alexey Melnikov
   Isode Limited
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK
   
   EMail: [email protected]
   URI:   http://www.melnikov.ca/
   
   Dave Cridland
   Isode Limited
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK
   
   EMail: [email protected]