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


This document clarifies how IMAP (RFC 3501) server implementations should handle CAPABILITY command.

Status of This Memo

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Table of Contents

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. ([CREF1]UNAUTHENTICATE extension would add a third point.) 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 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


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.

Appendix A. Acknowledgements


Author's Address

Alexey Melnikov Isode Ltd 14 Castle Mews Hampton, Middlesex TW12 2NP UK EMail: