Network Working Group                                A. Gulbrandsen, Ed.Request for Comments: 5161                        Oryx Mail Systems GmbHCategory: Standards Track                               A. Melnikov, Ed.                                                           Isode Limited                                                              March 2008                       The IMAP ENABLE ExtensionStatus 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   Most IMAP extensions are used by the client when it wants to and the   server supports it.  However, a few extensions require the server to   know whether a client supports that extension.  The ENABLE extension   allows an IMAP client to say which extensions it supports.1.  Overview   Several IMAP extensions allow the server to return unsolicited   responses specific to these extensions in certain circumstances.   However, servers cannot send those unsolicited responses until they   know that the clients support such extensions and thus won't choke on   the extension response data.   Up until now, extensions have typically stated that a server cannot   send the unsolicited responses until after the client has used a   command with the extension data (i.e., at that point the server knows   the client is aware of the extension).  CONDSTORE ([RFC4551]),   ANNOTATE ([ANNOTATE]), and some extensions under consideration at the   moment use various commands to enable server extensions.  For   example, CONDSTORE uses a SELECT or FETCH parameter, and ANNOTATE   uses a side effect of FETCH.   The ENABLE extension provides an explicit indication from the client   that it supports particular extensions.  This is done using a new   ENABLE command.   An IMAP server that supports ENABLE advertises this by including the   word ENABLE in its capability list.Gulbrandsen & Melnikov      Standards Track                     [Page 1]RFC 5161               The IMAP ENABLE Extension              March 2008   Most IMAP extensions do not require the client to enable the   extension in any way.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].   Formal syntax is defined by [RFC5234] and [RFC3501].   Example lines prefaced by "C:" are sent by the client and ones   prefaced by "S:" by the server.  The five characters [...] means that   something has been elided.3.  Protocol Changes3.1.  The ENABLE Command   Arguments: capability names   Result:    OK: Relevant capabilities enabled              BAD: No arguments, or syntax error in an argument   The ENABLE command takes a list of capability names, and requests the   server to enable the named extensions.  Once enabled using ENABLE,   each extension remains active until the IMAP connection is closed.   For each argument, the server does the following:   - If the argument is not an extension known to the server, the server     MUST ignore the argument.   - If the argument is an extension known to the server, and it is not     specifically permitted to be enabled using ENABLE, the server MUST     ignore the argument.  (Note that knowing about an extension doesn't     necessarily imply supporting that extension.)   - If the argument is an extension that is supported by the server and     that needs to be enabled, the server MUST enable the extension for     the duration of the connection.  At present, this applies only to     CONDSTORE ([RFC4551]).  Note that once an extension is enabled,     there is no way to disable it.   If the ENABLE command is successful, the server MUST send an untagged   ENABLED response (see Section 3.2).Gulbrandsen & Melnikov      Standards Track                     [Page 2]RFC 5161               The IMAP ENABLE Extension              March 2008   Clients SHOULD only include extensions that need to be enabled by the   server.  At the time of publication, CONDSTORE is the only such   extension (i.e., ENABLE CONDSTORE is an additional "CONDSTORE   enabling command" as defined in [RFC4551]).  Future RFCs may add to   this list.   The ENABLE command is only valid in the authenticated state (see   [RFC3501]), before any mailbox is selected.  Clients MUST NOT issue   ENABLE once they SELECT/EXAMINE a mailbox; however, server   implementations don't have to check that no mailbox is selected or   was previously selected during the duration of a connection.   The ENABLE command can be issued multiple times in a session.  It is   additive; i.e., "ENABLE a b", followed by "ENABLE c" is the same as a   single command "ENABLE a b c".  When multiple ENABLE commands are   issued, each corresponding ENABLED response SHOULD only contain   extensions enabled by the corresponding ENABLE command.   There are no limitations on pipelining ENABLE.  For example, it is   possible to send ENABLE and then immediately SELECT, or a LOGIN   immediately followed by ENABLE.   The server MUST NOT change the CAPABILITY list as a result of   executing ENABLE; i.e., a CAPABILITY command issued right after an   ENABLE command MUST list the same capabilities as a CAPABILITY   command issued before the ENABLE command.  This is demonstrated in   the following example:      C: t1 CAPABILITY      S: * CAPABILITY IMAP4rev1 ID LITERAL+ ENABLE X-GOOD-IDEA      S: t1 OK foo      C: t2 ENABLE CONDSTORE X-GOOD-IDEA      S: * ENABLED X-GOOD-IDEA      S: t2 OK foo      C: t3 CAPABILITY      S: * CAPABILITY IMAP4rev1 ID LITERAL+ ENABLE X-GOOD-IDEA      S: t3 OK foo again   In the following example, the client enables CONDSTORE:      C: a1 ENABLE CONDSTORE      S: * ENABLED CONDSTORE      S: a1 OK Conditional Store enabledGulbrandsen & Melnikov      Standards Track                     [Page 3]RFC 5161               The IMAP ENABLE Extension              March 20083.2.  The ENABLED Response   Contents:   capability listing   The ENABLED response occurs as a result of an ENABLE command.  The   capability listing contains a space-separated listing of capability   names that the server supports and that were successfully enabled.   The ENABLED response may contain no capabilities, which means that no   extensions listed by the client were successfully enabled.3.3.  Note to Designers of Extensions That May Use the ENABLE Command   Designers of IMAP extensions are discouraged from creating extensions   that require ENABLE unless there is no good alternative design.   Specifically, extensions that cause potentially incompatible behavior   changes to deployed server responses (and thus benefit from ENABLE)   have a higher complexity cost than extensions that do not.4.  Formal Syntax   The following syntax specification uses the Augmented Backus-Naur   Form (ABNF) notation as specified in [RFC5234] including the core   rules in Appendix B.1.  [RFC3501] defines the non-terminals   "capability" and "command-any".   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    =/ "ENABLE"      command-any   =/ "ENABLE" 1*(SP capability)      response-data =/ "*" SP enable-data CRLF      enable-data   = "ENABLED" *(SP capability)5.  Security Considerations   It is believed that this extension doesn't add any security   considerations that are not already present in the base IMAP protocol   [RFC3501].6.  IANA Considerations   The IANA has added ENABLE to the IMAP4 Capabilities Registry.Gulbrandsen & Melnikov      Standards Track                     [Page 4]RFC 5161               The IMAP ENABLE Extension              March 20087.  Acknowledgments   The editors would like to thank Randy Gellens, Chris Newman, Peter   Coates, Dave Cridland, Mark Crispin, Ned Freed, Dan Karp, Cyrus   Daboo, Ken Murchison, and Eric Burger for comments and corrections.   However, this doesn't necessarily mean that they endorse this   extension, agree with all details, or are responsible for errors   introduced by the editors.8.  Normative References   [RFC2119]  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.   [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for              Syntax Specifications: ABNF", STD 68, RFC 5234, January              2008.   [RFC4551]  Melnikov, A. and S. Hole, "IMAP Extension for Conditional              STORE Operation or Quick Flag Changes Resynchronization",              RFC 4551, June 2006.9.  Informative References   [ANNOTATE] Daboo, C. and R. Gellens, "IMAP ANNOTATE Extension", Work              in Progress, August 2006.Gulbrandsen & Melnikov      Standards Track                     [Page 5]RFC 5161               The IMAP ENABLE Extension              March 2008Editors' Addresses   Arnt Gulbrandsen   Oryx Mail Systems GmbH   Schweppermannstr. 8   D-81671 Muenchen   Germany   Fax: +49 89 4502 9758   EMail: arnt@oryx.com   Alexey Melnikov   Isode Ltd   5 Castle Business Village   36 Station Road   Hampton, Middlesex  TW12 2BX   UK   EMail: Alexey.Melnikov@isode.comGulbrandsen & Melnikov      Standards Track                     [Page 6]RFC 5161               The IMAP ENABLE Extension              March 2008Full Copyright Statement   Copyright (C) The IETF Trust (2008).   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.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found in BCP 78 and BCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository at   http://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Gulbrandsen & Melnikov      Standards Track                     [Page 7]