Internet DRAFT - draft-melnikov-imap-capabilities
draft-melnikov-imap-capabilities
Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Updates: 3501 (if approved) December 11, 2015
Intended status: Standards Track
Expires: June 13, 2016
Clarification on IMAP CAPABILITY command behaviour
draft-melnikov-imap-capabilities-00
Abstract
This document clarifies how IMAP (RFC 3501) server implementations
should handle CAPABILITY command.
Status of This Memo
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This Internet-Draft will expire on June 13, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. Clarification on CAPABILITY response/response code . . . . . 2
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3
6. Security Considerations . . . . . . . . . . . . . . . . . . . 3
7. Normative References . . . . . . . . . . . . . . . . . . . . 3
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4
1. Introduction
This document clarifies how IMAP [RFC3501] server implementations
should respond to CAPABILITY command or what they should return in
CAPABILITY response code at different points in IMAP connection.
This document updates RFC 3501.
A CAPABILITY response or CAPABILITY response codes return a listing
of capabilities that the server supports. RFC 3501 didn't specify
whether advertised capabilities can change over time and, if they
can, at which points in IMAP connection. This document clarifies
that.
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 [RFC2119].
The term "IMAP connection" or just "connection" is as specified in
[RFC3501].
3. Clarification on CAPABILITY response/response code
Two successive CAPABILITY commands (with no commands in between them)
MUST return the same list of capabilities.
The list of capabilities is generally static, but it can change at 2
points in IMAP connection ("security state change points"): after a
successful STARTTLS command and after a successful LOGIN or
AUTHENTICATE command. () The list of capabilities MUST NOT change at
any other points.
With a small number of exceptions, capabilities can't be removed,
they can only be added or their parameters might change. Once a
capability is announced, it can't be taken away in a subsequent
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CAPABILITY response, except for a few very limited cases. For
example, after STARTTLS command is successful, the STARTTLS
capability doesn't need to be advertised (but it is not an error if
it is).
Capabilities that include parameter(s) can change their parameters at
security state change points. The later parameter(s) replace any
previously announced parameters.
A CAPABILITY response code can contain the same information as a
CAPABILITY response. Some implementations only return capabilities
that apply in non-authenticated state before authentication and only
capabilities that apply in authenticated/selected state after
authentication.
4. Examples
TBD. One example: after STARTTLS, AUTH=PLAIN and/or AUTH=EXTERNAL
can be advertised.
Second example: Show changing APPENDLIMIT parameter after
authentication.
5. IANA Considerations
This document doesn't require any action from IANA.
6. Security Considerations
TBD
7. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<http://www.rfc-editor.org/info/rfc3501>.
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Appendix A. Acknowledgements
TBD.
Author's Address
Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
UK
EMail: Alexey.Melnikov@isode.com
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