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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on June 13, 2016. Copyright Notice Copyright (c) 2015 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. Melnikov Expires June 13, 2016 [Page 1] Internet-Draft IMAP CAPABILITY clarification December 2015 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 Melnikov Expires June 13, 2016 [Page 2] Internet-Draft IMAP CAPABILITY clarification December 2015 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, . [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, . Melnikov Expires June 13, 2016 [Page 3] Internet-Draft IMAP CAPABILITY clarification December 2015 Appendix A. Acknowledgements TBD. Author's Address Alexey Melnikov Isode Ltd 14 Castle Mews Hampton, Middlesex TW12 2NP UK EMail: Alexey.Melnikov@isode.com Melnikov Expires June 13, 2016 [Page 4]