RFC2177 - IMAP4 IDLE command

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　　Network Working Group B. Leiba
Request for Comments: 2177 IBM T.J. Watson Research Center
Category: Standards Track June 1997

IMAP4 IDLE command

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.

1. Abstract

The Internet Message Access Protocol [IMAP4] requires a client to
poll the server for changes to the selected mailbox (new mail,
deletions). It's often more desirable to have the server transmit
updates to the client in real time. This allows a user to see new
mail immediately. It also helps some real-time applications based on
IMAP, which might otherwise need to poll extremely often (such as
every few seconds). (While the spec actually does allow a server to
push EXISTS responses aysynchronously, a client can't expect this
behaviour and must poll.)

This document specifies the syntax of an IDLE command, which will
allow a client to tell the server that it's ready to accept such
real-time updates.

2. Conventions Used in this Document

In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.

The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as described in RFC2060
[IMAP4].

3. Specification

IDLE Command

Arguments: none

Responses: continuation data will be requested; the client sends
the continuation data "DONE" to end the command

Result: OK - IDLE completed after client sent "DONE"
NO - failure: the server will not allow the IDLE
command at this time
BAD - command unknown or arguments invalid

The IDLE command may be used with any IMAP4 server implementation
that returns "IDLE" as one of the supported capabilities to the
CAPABILITY command. If the server does not advertise the IDLE
capability, the client MUST NOT use the IDLE command and must poll
for mailbox updates. In particular, the client MUST continue to be
able to accept unsolicited untagged responses to ANY command, as
specified in the base IMAP specification.

The IDLE command is sent from the client to the server when the
client is ready to accept unsolicited mailbox update messages. The
server requests a response to the IDLE command using the continuation
("+") response. The IDLE command remains active until the client
responds to the continuation, and as long as an IDLE command is
active, the server is now free to send untagged EXISTS, EXPUNGE, and
other messages at any time.

The IDLE command is terminated by the receipt of a "DONE"
continuation from the client; such response satisfies the server's
continuation request. At that point, the server MAY send any
remaining queued untagged responses and then MUST immediately send
the tagged response to the IDLE command and prepare to process other
commands. As in the base specification, the processing of any new
command may cause the sending of unsolicited untagged responses,
subject to the ambiguity limitations. The client MUST NOT send a
command while the server is waiting for the DONE, since the server
will not be able to distinguish a command from a continuation.

The server MAY consider a client inactive if it has an IDLE command
running, and if such a server has an inactivity timeout it MAY log
the client off implicitly at the end of its timeout period. Because
of that, clients using IDLE are advised to terminate the IDLE and
re-issue it at least every 29 minutes to avoid being logged off.
This still allows a client to receive immediate mailbox updates even
though it need only "poll" at half hour intervals.

Example: C: A001 SELECT INBOX
S: * FLAGS (Deleted Seen)
S: * 3 EXISTS
S: * 0 RECENT
S: * OK [UIDVALIDITY 1]
S: A001 OK SELECT completed
C: A002 IDLE
S: + idling
...time passes; new mail arrives...
S: * 4 EXISTS
C: DONE
S: A002 OK IDLE terminated
...another client expunges message 2 now...
C: A003 FETCH 4 ALL
S: * 4 FETCH (...)
S: A003 OK FETCH completed
C: A004 IDLE
S: * 2 EXPUNGE
S: * 3 EXISTS
S: + idling
...time passes; another client expunges message 3...
S: * 3 EXPUNGE
S: * 2 EXISTS
...time passes; new mail arrives...
S: * 3 EXISTS
C: DONE
S: A004 OK IDLE terminated
C: A005 FETCH 3 ALL
S: * 3 FETCH (...)
S: A005 OK FETCH completed
C: A006 IDLE

4. Formal Syntax

The following syntax specification uses the augmented Backus-Naur
Form (BNF) notation as specified in [RFC-822] as modified by [IMAP4].
Non-terminals referenced but not defined below are as defined by
[IMAP4].

command_auth ::= append / create / delete / examine / list / lsub /
rename / select / status / subscribe / unsubscribe
/ idle
;; Valid only in Authenticated or Selected state

idle ::= "IDLE" CRLF "DONE"

5. References

[IMAP4] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC2060, December 1996.

6. Security Considerations

There are no known security issues with this extension.

7. Author's Address

Barry Leiba
IBM T.J. Watson Research Center
30 Saw Mill River Road
Hawthorne, NY 10532

Email: leiba@watson.ibm.com

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