The IMAP ENABLE Extension
An IMAP command that lets a client tell the server which extensions it understands.
Gulbrandsen, A., Ed. and A. Melnikov, Ed., "The IMAP ENABLE Extension", RFC 5161, DOI 10.17487/RFC5161, March 2008, <https://www.rfc-editor.org/info/rfc5161>.
@misc{rfc5161,
series = {Request for Comments},
number = 5161,
howpublished = {RFC 5161},
publisher = {RFC Editor},
doi = {10.17487/RFC5161},
url = {https://www.rfc-editor.org/info/rfc5161},
author = {A. Gulbrandsen and A. Melnikov},
title = {{The IMAP ENABLE Extension}},
pagetotal = 7,
year = 2008,
month = mar,
} RFC text
Contents
Network Working Group A. Gulbrandsen, Ed.
Request for Comments: 5161 Oryx Mail Systems GmbH
Category: Standards Track A. Melnikov, Ed.
Isode Limited
March 2008
The IMAP ENABLE Extension
Status 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.
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 Changes
3.1. 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).
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 enabled
3.2. 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.
7. 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.
Editors' 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.com
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ISO Plain Language
On this page
§1Why a Client Needs to Say What It Supports
Several IMAP extensions allow the server to return unsolicited responses specific to those extensions in certain circumstances. However, servers cannot send those unsolicited responses until they know that the clients support such extensions and thus will not choke on the extension response data.
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, at which point the server knows the client is aware of the extension. CONDSTORE, ANNOTATE and some extensions under consideration at the time 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, using a new ENABLE command. An IMAP server that supports ENABLE advertises this by including the word ENABLE in its capability list. Most IMAP extensions do not require the client to enable the extension in any way.
Some extensions let the server volunteer information the client did not ask for. Sending that to a client that does not understand it would break the session. Before this RFC, the only way to find out was for the client to use the extension first and for the server to notice. This RFC makes the client say so up front.
§3.1The ENABLE Command
The ENABLE command takes a list of capability names and requests the server to enable the named extensions. Once enabled, each extension remains active until the IMAP connection is closed. For each argument the server does one of three things. If the argument is not an extension known to the server, the server must ignore the argument. If it is an extension known to the server but not specifically permitted to be enabled using ENABLE, the server must also ignore the argument, because knowing about an extension does not necessarily imply supporting it. If the argument is an extension that is supported by the server and that needs to be enabled, the server must enable it for the duration of the connection. At present this applies only to CONDSTORE, and once an extension is enabled there is no way to disable it.
Clients should only include extensions that need to be enabled by the server. The ENABLE command is only valid in the authenticated state, before any mailbox is selected, and clients must not issue it once they have selected or examined a mailbox, though server implementations do not have to check that no mailbox was selected earlier in the connection.
The command can be issued multiple times in a session and is additive, so ENABLE a b followed by ENABLE c is the same as a single ENABLE a b c. When multiple ENABLE commands are issued, each corresponding ENABLED response should contain only the extensions enabled by that command. There are no limitations on pipelining ENABLE, so 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. A capability command issued right after an ENABLE command must list the same capabilities as one issued before it.
The client sends a list of names and the server switches on the ones it supports and is allowed to switch on. Anything else in the list is quietly ignored. The command has to come after login and before the client opens a mailbox, and it can be repeated; each time, the server lists only what it turned on in that call.
- CAPABILITYThe client asks what the server supports.
- CAPABILITY IMAP4rev1 ID LITERAL+ ENABLE
- ENABLE CONDSTORE
- ENABLED CONDSTOREThe server names what it turned on.
- Not a known extensionThe server ignores the argument.
- Known but not permitted to be enabledThe server ignores the argument, because knowing an extension does not mean supporting it.
- Supported and needs enablingThe server enables it for the rest of the connection. There is no way to turn it off again.
§3.2The ENABLED Response
If the ENABLE command is successful, the server must send an untagged ENABLED response. The ENABLED response occurs as a result of an ENABLE command, and its 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 none of the extensions listed by the client were successfully enabled. The server's reply to the command itself is OK with the text Relevant capabilities enabled, or BAD with the text No arguments, or syntax error in an argument.
After the command the server sends an untagged line naming the extensions it turned on. If it turned none of them on, the line is empty, and the client can tell that from the response.
§4Formal Syntax
The syntax is given in Augmented Backus-Naur Form, with the non-terminals capability and command-any defined in the base IMAP specification. Except as noted otherwise, all alphabetic characters are case-insensitive, and implementations must accept these strings in a case-insensitive fashion.
The capability rule adds the word ENABLE to the list of capabilities. The command-any rule adds ENABLE followed by one or more capability names separated by spaces. The response-data rule adds the untagged form, and enable-data defines the ENABLED response and its optional list of capability names.
The formal grammar adds one command name, one response name, and a list of extension names after each. The names are not case-sensitive.
capability =/ "ENABLE"
command-any =/ "ENABLE" 1*(SP capability)
response-data =/ "*" SP enable-data CRLF
enable-data = "ENABLED" *(SP capability)§3.3A Note to Extension Designers
Designers of IMAP extensions are discouraged from creating extensions that require ENABLE unless there is no good alternative design. 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.
The authors of the RFC ask extension designers not to lean on this command. An extension that changes what the server sends, so that clients must be opted in, costs more to implement than one that does not.
§5Security Considerations
It is believed that this extension does not add any security considerations that are not already present in the base IMAP protocol.
The command adds nothing new in security terms beyond what the base IMAP protocol already has.