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.
AbstractMost 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. 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. 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. 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
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) RFC3501].
[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. [ANNOTATE] Daboo, C. and R. Gellens, "IMAP ANNOTATE Extension", Work in Progress, August 2006.
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