Internet DRAFT - draft-ietf-lemonade-reconnect
draft-ietf-lemonade-reconnect
Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Expires: December 7, 2006 C. Wilson
Nokia
June 5, 2006
IMAP4 Extensions for Quick Reconnect
draft-ietf-lemonade-reconnect-07.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines a quick reconnect IMAP4 extension, which gives
a mobile client the ability to resume a previously abandoned session,
without the need to perform a full synchronization sequence. The
goal is to minimize the amount of data transfered at login after a
dropped connection.
Changes since draft-ietf-lemonade-reconnect-06
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o Multiple spelling and editorial corrections from Arnt, Randy and
Dave Cridland.
o Renamed FOLDER response to SELECTED and replaced "()" with nothing
(i.e. the mailbox name is now optional).
o Deleted the requirement to support at least 5 per-user resumable
sessions. This is a deployment choice and the client can't rely
that there is always a number of sessions available to it.
o Define generic syntax for LOGOUT parameters consistent with RFC
4466.
Changes since draft-ietf-lemonade-reconnect-05
o Fixed some formatting issues.
o Clarified what state is to be associated with a session.
Clarified interaction with IMAP ACL extension.
o Clarified that creation of a new session when there is an active
one makes the older one inactive. Added DELETEDSID response in
order to convey this information.
o Clarified that the server can still return NO if per-user limit on
number of sessions is reached and all of them are active.
o Added paragraph clarifying that any session can only be active on
a single TCP connection at a time.
o Clarified that if (on session resumption) UIDVALIDITY provided by
the client doesn't match the server's value, than the mailbox is
selected in the same read-write mode as it was originally
selected, but all other state must be forgotten.
o Specified that the second (and subsequent) request to create a new
session is treated as the first one. (Behavior change from -05)
o Added the DELETESID command that allows to terminate a no longer
needed resumable session without the need to close the connection.
Changes since draft-ietf-lemonade-reconnect-04
o Spelling/wording corrections from Zoltan.
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o Replaced extended Login/Authenticate with a new SID command. This
made the document much simpler.
o Removed the NOSID untagged response as the SID command can return
the tagged NO response.
o Made UIDVALIDITY/modification sequence parameters to the SID
command required if the SID parameter is also specified.
o Made UNSEEN untagged OK response optional.
o Specified that LOGOUT (PRESERVE) should be just treated as LOGOUT
if there is no active session. [Zoltan]
o Specified that the second (and subsequent) request to create a new
session is treated as NOOP.
o Clarified that when the server reaches the global limit on number
of sessions, then it returns NO to the SID command.
o Clarified that when the server reaches the per-user limit on
number of sessions, then it expires the oldest inactive session
belonging to the same user.
o Updated examples and ABNF.
o Changed section 3 arround to be clearer.
o Added new text to security section on New SID flooding.
o Completed IANA Considerations section.
Changes since draft-ietf-lemonade-reconnect-03
o Clarified which response codes/responses are required on
successful AUTHENTICATE/LOGIN.
o FOLDER untagged response code is not needed if NEWSID parameter
was specified.
o One of tagged READ-WRITE/READ-ONLY is required if resuming a
session in selected state. Updated examples as the result.
o Spelling/wording corrections from Eric.
Changes since draft-ietf-lemonade-reconnect-02
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o Removed minimal session expiration timeout.
o Moved EXPUNGED response to a separate draft. Updated the document
as a result of this change.
o Updated references and completed IANA considerations.
o Added description of the updated synchronization sequence.
Changes since draft-ietf-lemonade-reconnect-01
o Added NOSID response.
o Added UIDVALIDITY argument to the SID parameter.
o Added EXPUNGED response.
o Added the following example: an opened mailbox was deleted and
another was renamed on top, while the client was offline.
o Various editorial changes, mostly rewriting text for clarity.
Changes since draft-ietf-lemonade-reconnect-00
o Added example protocol traces.
o Added interaction with SASL-IR (SASL initial response extension).
o SIDs are now server generated. Added NEWSID parameter to LOGIN/
AUTHENTICATE and NEWSID response.
o Removed SESSION response. The FOLDER response can now tell if
there is a mailbox selected or not.
o Added LOGOUT (PRESERVE).
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Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 6
2. Introduction and Overview . . . . . . . . . . . . . . . . . . 6
2.1. Typical IMAP Session without Reconnect . . . . . . . . . . 7
2.2. Reconnect IMAP Session . . . . . . . . . . . . . . . . . . 9
3. Session Control . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. SID command . . . . . . . . . . . . . . . . . . . . . . . 10
3.2. Creating a new session . . . . . . . . . . . . . . . . . . 12
3.3. Resuming an existing session . . . . . . . . . . . . . . . 13
3.4. DELETESID command . . . . . . . . . . . . . . . . . . . . 17
4. PRESERVE parameter to the LOGOUT command . . . . . . . . . . . 18
5. New responses . . . . . . . . . . . . . . . . . . . . . . . . 18
5.1. SELECTED Response . . . . . . . . . . . . . . . . . . . . 18
5.2. NEWSID Response . . . . . . . . . . . . . . . . . . . . . 19
5.3. DELETEDSID Response . . . . . . . . . . . . . . . . . . . 19
6. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 19
7. Security Considerations . . . . . . . . . . . . . . . . . . . 20
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1. Normative References . . . . . . . . . . . . . . . . . . . 21
9.2. Informative References . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
Intellectual Property and Copyright Statements . . . . . . . . . . 24
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1. Requirements notation
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].
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
"Resumable session" (or just "session") refers to the entire sequence
of client/server interaction from the initial session creation during
SID command (with no parameters) until the session is explicitly
terminated (or deleted) upon client request or until the session
expires some time after disconnect. A resumable session can be
terminated by the [RFC3501] LOGOUT or DELETESID command. Note that a
resumable session doesn't terminate when the connection is lost or
closed with LOGOUT (PRESERVE) command (see Section 4). Also note
that this term has a different meaning from the term "session" used
by [RFC3501].
A resumable session is said to be "inactive" if it is not associated
with any active TCP connection.
The term "event" is as defined in [C2S].
2. Introduction and Overview
The IMAP4 protocol does not distinguish between a session that has
been disconnected for 15 seconds and one that has been disconnected
for 1 week. The assumption is the same: The client and server may be
completely out of sync and the client must ensure that it is
completely synchronized with the server.
In today's mobile environments, accidental disconnects due to
environmental factors (battery life, signal strength, etc.) can be
quite common. In these situtations, the time between the disconnect
and reconnect can be quite short. It is these situations which we
are interested in.
If the time between connections is sufficiently short, an assumption
can be made that the state of the last connection is still valid and
that the client can avoid having to query much of the state data at
reconnect.
[[anchor3: Note to RFC editor: This document is compliant with
"transitional IMAP capabilities" document [TRANS-CAPA]. Please
change the capability name below to "RECONNECT"]]
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The quick reconnect IMAP extension is present if an IMAP4 server
returns "X-DRAFT-W07-RECONNECT" as one of the supported capabilities
to the CAPABILITY command. Note, that this extension REQUIREs
support for the [CONDSTORE] and the [EXPUNGED] IMAP extensions, so it
MUST be announced in the CAPABILITY response as well.
The text below outlines how the quick reconnect extension is to be
used. The description assumes that the quick reconnect is mainly
used when the client closes the TCP connection (intentionally or
accidentally) and tries to reconnect within a preset amount of time.
1. The client authenticates and asks the server to create a new
session by sending a SID command. The server generates a new
Session ID (SID) and returns it to the client.
2. The client performs operations as normal, typically selecting a
mailbox and accessing messages.
3. At some point the client loses connectivity. The server retains
all information about the client's session, including information
about the currently selected mailbox, for some amount of time.
(case a) The client reconnects before the session timeout.
It authenticates as the same user as before and specifies the
previously used session ID in the SID command. The client and
server begin at the same mailbox as before the connection
loss.
(case b) The client reconnects after the session timeout.
It authenticates as the same user and specifies the previously
used session ID in the SID command. Since the session has
timed out on the server, the server has no record of this
session and creates a new session. This behaviour falls back
to a normal IMAPv4 login sequence.
2.1. Typical IMAP Session without Reconnect
A typical IMAP client session looks as follows: (Modified dump from
network sniffer)
S: * OK [CAPABILITY IMAP4REV1 LOGIN-REFERRALS STARTTLS
AUTH=DIGEST-MD5] imap.mail.com IMAP4rev1 2001.315rh at Thu,
15 Jul 2004 11:47:49-0400 (EDT)
C: 2 AUTHENTICATE DIGEST-MD5
S: + cmVhbG09ImVsd29vZC5pbm5vc29mdC5jb20iLG5vbmNlPSJPQTZNRzl0
RVFHbTJoaCIscW9wPSJhdXRoIixhbGdvcml0aG09bWQ1LXNlc3MsY2hh
cnNldD11dGYtOA==
C: Y2hhcnNldD11dGYtOCx1c2VybmFtZT0iY2hyaXMiLHJlYWxtPSJlbHdvb2
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QuaW5ub3NvZnQuY29tIixub25jZT0iT0E2TUc5dEVRR20yaGgiLG5jPTAw
MDAwMDAxLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLGRpZ2VzdC11cmk9Im
ltYXAvZWx3b29kLmlubm9zb2Z0LmNvbSIscmVzcG9uc2U9ZDM4OGRhZDkw
ZDRiYmQ3NjBhMTUyMzIxZjIxNDNhZjcscW9wPWF1dGg=
S: + cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZA==
C:
S: 2 OK [CAPABILITY IMAP4REV1 NAMESPACE] User chris
authenticated
C: 3 namespace
S: * NAMESPACE (("" "/")("#mhinbox" NIL)("#mh/" "/")) (("~" "/"))
(("#shared/" "/")("#ftp/" "/")("#news." ".")("#public/" "/"))
S: 3 OK completed
C: 4 lsub "" "*"
S: 4 OK LSUB completed
C: 5 lsub "" "#mhinbox*"
S: 5 OK LSUB completed
C: 6 lsub "" "#mh/*"
S: * NO /home/test8/.mh_profile not found, mh format names disabled
S: 6 OK LSUB completed
C: 7 lsub "" "~*"
S: 7 OK LSUB completed
C: 8 lsub "" "#shared/*"
S: 8 OK LSUB completed
C: 9 lsub "" "#ftp/*"
S: 9 OK LSUB completed
C: 10 lsub "" "#news.*"
S: 10 OK LSUB completed
C: 11 lsub "" "#public/*"
S: 11 OK LSUB completed
C: 12 list "" "INBOX"
S: * LIST (\NoInferiors) NIL INBOX
S: 12 OK LIST completed
C: 15 select "INBOX"
S: * 464 EXISTS
S: * 3 RECENT
S: * OK [UNSEEN 12] Message 12 is first unseen
S: * OK [UIDVALIDITY 3857529045] UIDs valid
S: * OK [UIDNEXT 550] Predicted next UID
S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
S: 15 OK [READ-WRITE] SELECT completed
C: 16 UID fetch 1:500 (FLAGS)
S: * 1 FETCH (UID 41 FLAGS (\Seen))
S: * 2 FETCH (UID 43 FLAGS (\Seen \Flagged))
S: ...
S: * 464 FETCH (UID 541 FLAGS (Junk))
S: 16 fetch completed
C: ...
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(Note, that the client has recognized the CAPABILITY response code in
the initial OK response, it has omitted the CAPABILITY command.)
The above dump was generated using Ethereal and it reported that the
above process generated just under 20 KB of data.
The above protocol sequence simply logged the user into the account,
selected the INBOX mailbox and downloaded the updated flags for a
subset of messages.
2.2. Reconnect IMAP Session
Now let's look at the same mailbox, but see how the protocol would
react if the client had dropped the connection and was reconnecting
within a few minutes.
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As in the previous section, the client has recognized the CAPABILITY
response code in the initial OK response, so it has omitted the
CAPABILITY command.
S: * OK [CAPABILITY IMAP4REV1 LOGIN-REFERRALS STARTTLS
AUTH=DIGEST-MD5] imap.example.com IMAP4rev1 2001.315rh at Thu,
15 Jul 2004 11:47:49-0400 (EDT)
C: 2 AUTHENTICATE DIGEST-MD5
S: + cmVhbG09ImVsd29vZC5pbm5vc29mdC5jb20iLG5vbmNlPSJPQTZNRzl0
RVFHbTJoaCIscW9wPSJhdXRoIixhbGdvcml0aG09bWQ1LXNlc3MsY2hh
cnNldD11dGYtOA==
C: Y2hhcnNldD11dGYtOCx1c2VybmFtZT0iY2hyaXMiLHJlYWxtPSJlbHdvb2
QuaW5ub3NvZnQuY29tIixub25jZT0iT0E2TUc5dEVRR20yaGgiLG5jPTAw
MDAwMDAxLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLGRpZ2VzdC11cmk9Im
ltYXAvZWx3b29kLmlubm9zb2Z0LmNvbSIscmVzcG9uc2U9ZDM4OGRhZDkw
ZDRiYmQ3NjBhMTUyMzIxZjIxNDNhZjcscW9wPWF1dGg=
S: + cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZA==
C:
S: 2 OK [CAPABILITY IMAP4REV1 NAMESPACE MAILBOX-REFERRALS
CONDSTORE X-DRAFT-I01-EXPUNGED X-DRAFT-W07-RECONNECT] User chris
authenticated
C: 3 SID P1234567890 56789 20010715194045000 41,43:211,214:541
S: * SELECTED INBOX
S: * 464 EXISTS
S: * 3 RECENT
S: * OK [UIDVALIDITY 3857529045] UIDVALIDITY
S: * OK [UIDNEXT 550] Predicted next UID
S: * OK [HIGHESTMODSEQ 20010715194045007]
S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
S: * 1 FETCH (UID 1 FLAGS (\Seen))
S: * EXPUNGED 205,207,209,215:321
S: 3 OK [READ-WRITE] session continued
C: ...
That's it! And since no new messages arrived, no headers were
downloaded. The total byte count is about 500 bytes, significantly
less than the amount of data transferred during a normal connect
request.
3. Session Control
3.1. SID command
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Arguments: optional SID parameter
optional mailbox UIDVALIDITY
optional last known modification sequence
optional list of known UIDs
Responses: OPTIONAL untagged responses: NEWSID (REQUIRED if no SID
parameter was specified; also required if SID parameter
was specified but the given session ID is invalid)
OPTIONAL untagged responses: SELECTED (REQUIRED if SID
parameter was specified)
The following responses are REQUIRED if the client remains
in the selected state upon reconnect (See Section 3.3).
Untagged responses: FLAGS, EXISTS, RECENT
OK untagged responses: PERMANENTFLAGS, UIDNEXT,
UIDVALIDITY, HIGHESTMODSEQ
OPTIONAL OK untagged responses: UNSEEN
OK tagged responses: READ-WRITE or READ-ONLY (as described
in section 6.3.1 [RFC3501])
Result: OK - session resumed or created; see Section 3.3
NO - session creation failure, e.g. session number limit
exceeded
BAD - arguments invalid
This section defines the new SID command. This command is only
available in authenticated and selected IMAP states.
If the optional session identifier (SID) parameter is not provided,
the server will generate a new session as described in Section 3.2.
If the optional SID parameter is defined, the server MUST atempt to
locate and resume the session with the given SID as described in
Section 3.3.
Whether a session was created or resumed, the server is now required
to remember the session state associated with the (username, SID)
pair.
At minimum this state includes:
o The currently selected mailbox
o If mailbox is selected read-only or read-write
o The number of recent messages (RECENT)
o Non persistent flags created while mailbox was opened, if the
server supports not persistent IMAP flags
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o Any state associated with the current IMAP connection, but is not
the state of the selected mailbox
Note that if the server also supports [ACL] extension it can either
cache the MYRIGHTS value or can refresh it on reconnect. In the
latter case the server SHOULD send the untagged MYRIGHTS response on
reconnect.
Note that other IMAP extensions can define additional state that has
to be preserved.
The following state doesn't have to be preserved by the server,
because it is either supplied by the client on reconnect, or the
server is required to send it to the client on reconnect:
o The mailbox UIDVALIDITY value
o The mailbox HIGHESTMODSEQ value [CONDSTORE]
o The highest message UID used in the mailbox
o The total number of messages (EXISTS)
An implementation may choose to keep all session state in memory and
expire sessions some time after client disconnects due to dropped TCP
connection or explicit execution of LOGOUT (PRESERVE) command. Upon
session expiration the session ID becomes invalid and its associated
state is deleted. While there is no required minimum expiration
timeout, server implementations are encouraged to support as long a
time as possible, subject to denial-of-service and related concerns.
Note that SIDs are chosen by the server and are opaque to the client.
The same SID used with two different usernames refers to two
different sessions, unless the two usernames refer to the same user
account, e.g. one username is an alias for another.
3.2. Creating a new session
This section describes the server behavior if the client has not
specified a session identifier parameter to the SID command. This
instructs the server to create a new session regardless if a
previous, active resumable session is available. If an active
session is already available and a new one can be created, the
existing session becomes inactive and can be resumed at a later time.
This operation can be performed at any point during the IMAP session.
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Example: The client issues a SID command without a SID parameter, the
server creates a new session and responds with a new NEWSID
response:
C: A01 SID
S: * NEWSID ABC412423rs
S: A01 OK session created
In order to limit Denial of Service attacks a server implementation
SHOULD have a per-user limit on maximum number of resumable sessions.
When this limit is reached the server SHOULD expire the oldest
inactive (not associated with an active TCP connection) session
associated with the same user. If no inactive session associated
with the same user is available, the server MUST return the tagged NO
response to the SID command.
A server implementation might also have a per-server limit on maximum
number of resumable sessions. This document doesn't define this
limit. If the server has reached the limit, it MUST return the
tagged NO response to the SID command.
3.3. Resuming an existing session
This section describes server behaviour if the client has specified
the SID parameter to a SID command.
The SID command takes four arguments: the session identifier, the
mailbox UIDVALIDITY, the last known modification sequence and the
optional list of known UIDs.
The server verifies all arguments for syntactic validity. If any
parameter is not syntactically valid, the server return the tagged
BAD response.
After that the server checks the session identifier (SID). If the
SID is not recognized (for example, the SID is invalid because it is
either not known to the server or has expired), the server creates a
new session as described in Section 3.2.
If the SID is valid, the server MUST inform the client of the state
of the session by sending an untagged SELECTED response. The
SELECTED response contains the name of the currently selected mailbox
name or nothing if there is no mailbox selected. Note that the
mailbox name MUST be reported verbatim as originally specified by the
client in SELECT/EXAMINE. I.e. before server applies any mailbox
name canonicalization (like converting "Inbox" or "users.andrew" to
"INBOX"). If, before the disconnect, the session had a mailbox
selected and the mailbox has been deleted since then (or the user no
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longer has permissions to access it), the server MUST return a
special SELECTED untagged response that contains no mailbox name.
This tells the client that the session has returned to the
unauthenticated state.
Note that a session can only be active on a single TCP connection.
An attempt to resume an active session on a different TCP connection
forces the server to close the session on the other connection and to
resume it on the current connection. The other connection is
notified about this with the untagged DELETEDSID response (See
Section 5.3).
If at this point there is no mailbox selected, the remaining
arguments MUST be ignored by the server.
Example: Resuming an old session.
C: A02 SID ABC412423rs 67890007 20050715194045000
S: * SELECTED
S: ...
S: A02 OK session resumed, but no selected mailbox
UIDVALIDITY Parameter:
Once the session has successfully resumed and there is a mailbox
selected, the server checks the UIDVALIDITY value provided by the
client. If the provided UIDVALIDITY doesn't match the UIDVALIDITY
for the currently selected mailbox, the server MUST send the NEWSID
response containing the SID of the resumed session. This is done in
order to force the client to perform full resynchronization. In this
case the server SHOULD [[anchor7: (MUST? What about non permanent
flags?)]] clear all state associated with the mailbox, except for
whether the mailbox was selected read-only or read-write and the
remaining arguments to the SID command are ignored. After that the
server behaves as if SELECT/EXAMINE command for the mailbox was
issued (depending on whether the current mailbox was originally
selected read-write or read-only), i.e. it returns other untagged
responses like EXISTS, UIDVALIDITY untagged OK response, etc.
Example: Attempting to resume an old session with an invalid
UIDVALIDITY parameter.
C: A02 SID ABC412423rs 67890007 20050715194045000 41,43:
211,214:541
S: * SELECTED INBOX
S: * NEWSID ABC412423rs
S: * 464 EXISTS
S: * 3 RECENT
S: * OK [UIDVALIDITY 3857529045] UIDVALIDITY
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S: * OK [UIDNEXT 550] Predicted next UID
S: * OK [HIGHESTMODSEQ 20060128194045007]
S: A02 OK [READ-WRITE] resync required
Note that even though the UIDVALIDITY is invalid, the mailbox
remained selected and the untagged SELECTED response is returned as
the result.
Modification Sequence and UID Parameters:
If the provided UIDVALIDITY matches that of the selected mailbox, the
server then checks the last known modification sequence.
The server sends the client any pending flag changes (using FETCH
responses that MUST contain UIDs) and expunges that have occurred in
this mailbox since the provided modification sequence.
If the list of known UIDs was also provided, the server should only
report flag changes and expunges for the provided messages. If the
client doesn't provide the list of UIDs, the server acts as if the
client has specified "1:*". Thus, the client client can process just
these pending events and need not perform a full resynchronization.
The result of this step is semantically equivalent to the client
issuing:
tag1 UID FETCH "known-uids" (FLAGS) (CHANGEDSINCE
"mod-sequence-value" REPORTEXPUNGES)
If the server was unable to cache all events since the MODSEQ and
hence is unable to send them to the client, the server MUST return
the NEWSID response to signal that the client should perform a state-
comparison based synchronization. Note, that the NEWSID response MAY
contain the same SID as specified in the SID parameter. Section 3.2
describes server behavior in this case, i.e. the server behaves as if
the session can't be resumed.
Example: Resuming an old session in selected state with the INBOX
selected.
C: A02 SID ABC412423rs 67890007 20060115194045000 41,43:211,214:
541)
S: * SELECTED INBOX
S: * 314 EXISTS
S: 15 RECENT
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S: * OK [UIDVALIDITY 67890007] UIDVALIDITY
S: * OK [UIDNEXT 567] Predicted next UID
S: * OK [HIGHESTMODSEQ 20060115205545359]
S: * 49 FETCH (UID 117 FLAGS (\Seen \Answered))
S: * 50 FETCH (UID 119 FLAGS (\Draft $MDNSent))
S: ...
S: * 100 FETCH (UID 541 FLAGS (\Seen $Forwarded))
S: * EXPUNGED 41,43:116,118,120:211,214:540
S: A02 OK session resumed
If, while the client was offline, the selected mailbox was deleted
and another mailbox was renamed to the same name, the server MAY do
either:
1. return the NEWSID response, as well as the new UIDVALIDITY and
all other responses as described in Section 3.1.
2. return "SELECTED" response with no mailbox in order to return the
client to the authenticated state.
Example: Resuming an old session in selected (read-only) state with
the INBOX selected.
C: A02 SID ABC412423rs 67890007 20010715194045000 41,43:211,214:541
S: * SELECTED INBOX
S: * OK [UIDVALIDITY 67890007] Current UIDValidity
S: * OK [UIDNEXT 782] Next available UID
S: * 114 EXISTS
S: * 5 RECENT
S: * OK [HIGHESTMODSEQ 20010715235047007] S: * FLAGS (\Answered
\Flagged \Draft \Deleted \Seen)
S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft
\Deleted \Seen \*)] Permanent flags
S: * 49 FETCH (UID 117 FLAGS (\Seen \Answered))
S: ...
S: A02 OK [READ-ONLY] session resumed
Example: Resuming an old session in selected state with the INBOX
selected, but where the server could not cache all the events since
the specified modification sequence. The server sends the NEWSID
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response, which means that the client should perform full
resynchronization.
C: A02 SID ABC412423rs 67890007 20010715194045000 41,43:211,214:541
S: * NEWSID jwejei9qe3r
S: * SELECTED INBOX
S: * OK [UIDVALIDITY 67890007] Current UIDValidity
S: * OK [UIDNEXT 1598] Next available UID
S: * 117 EXISTS
S: * 20 RECENT
S: * OK [HIGHESTMODSEQ 20010715235047019] S: * FLAGS (\Answered
\Flagged \Draft \Deleted \Seen)
S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft
\Deleted \Seen \*)] Permanent flags
S: A02 OK [READ-WRITE] session created
Example: Resuming an old session in selected state with the mailbox
Drafts selected, but the mailbox was deleted and another one was
renamed to become Drafts, while the client was offline. The server
chooses to keep the Drafts mailbox open, however it sends the NEWSID
response, which means that the client should perform full
resynchronization.
C: A02 SID Frt-egf-779 6896834 11 20:35
S: * NEWSID Frt-egf-779
S: * SELECTED Drafts
S: * OK [UIDVALIDITY 1098183472] Current UIDValidity
S: * OK [UIDNEXT 12] Next available UID
S: * OK [HIGHESTMODSEQ 54] S: * 7 EXISTS
S: * 0 RECENT
S: * FLAGS (\Answered \Flagged \Draft \Deleted \Seen)
S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft
\Deleted \Seen \*)] Permanent flags
S: A02 OK [READ-WRITE] session created
Note that as one of READ-WRITE/READ-ONLY tagged response codes is
required when a session is resumed in selected state, the server is
unable to send the CAPABILITY response code in the tagged OK
response.
3.4. DELETESID command
Arguments: none
Responses: no specific responses for this command
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Result: OK - session successfully terminated
NO - session deletion failure
BAD - arguments invalid or no active session
The DELETESID command tells the server to terminate the current
session. If there is no current session, the server MUST respond
with the tagged BAD response.
4. PRESERVE parameter to the LOGOUT command
[[anchor8: This section clearly assumes voluntarily disconnect]]
This section updates the description of the LOGOUT command found in
Section 6.1.3 of [RFC3501]. A LOGOUT command with no parameters
tells the server to terminate the current session (if any).
This document also adds a new parameter "PRESERVE" to the LOGOUT
command. This tells the server that it should not terminate the
current resumable session. If the client doesn't want to terminate
the current session on logout it SHOULD use "LOGOUT (PRESERVE)"
instead of just dropping the TCP connection. This helps to avoid
unnecessary resource consumption on the server caused by a TCP
disconnect.
Note that if "LOGOUT (PRESERVE)" is issued when no resumable session
was created, the server should treat this as if the LOGOUT command
was issued with no parameters.
Example: C: A023 LOGOUT (PRESERVE)
S: * BYE IMAP4rev1 Server logging out, state preserved
S: A023 OK LOGOUT completed
(Server and client then close the connection)
5. New responses
5.1. SELECTED Response
Contents: name of the selected mailbox or nothing if there is no
selected mailbox
The SELECTED response tells the client that a session was
successfully resumed and returns the name of the selected mailbox.
If the response doesn't contain a mailbox name, this means that there
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is no mailbox selected.
5.2. NEWSID Response
Contents: generated session ID
The NEWSID response tells the client the unique identifier that can
be used to resume the created session. It also tells the client to
perform a full state resynchronization.
5.3. DELETEDSID Response
Contents: session ID
The DELETEDSID response tells the client that the current active
session is closed because it was resumed (transferred) on another TCP
connection.
6. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [ABNF].
Non-terminals referenced but not defined below are as defined by
[RFC3501], [CONDSTORE], [EXPUNGED] or [IMAPABNF].
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.
sid = "SID" [SP session-id SP uidvalidity SP
mod-sequence-value [SP known-uids]]
uidvalidity = nz-number
known-uids = sequence-set
;; sequence of UIDs, "*" is not allowed
deletesid = "DELETESID"
uidvalidity = nz-number
known-uids = sequence-set
;; sequence of UIDs, "*" is not allowed
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command-auth =/ sid / deletesid
logout = "LOGOUT" [logout-params]
logout-params = SP "(" logout-param-data ")"
;; modifies the original IMAP4 LOGOUT
;; command to accept optional parameters
logout-param-data= logout-param *(SP logout-param)
logout-param = logout-param-name [SP logout-param-value]
;; parameters to LOGOUT may contain one or
;; more atoms atoms and/or strings and/or lists.
logout-param-name= tagged-ext-label
logout-param-value= tagged-ext-val
;; This non-terminal shows recommended syntax
;; for future extensions.
logout-preserve = "PRESERVE"
;; conforms to logout-param syntax
mailbox-data =/ selected-resp / newsid-resp / deletedsid-resp
selected-resp = "SELECTED" [SP folder]
;; lack of the mailbox parameter denotes
;; that there is no mailbox selected
newsid-resp = "NEWSID" SP session-id
deletedsid-resp = "DELETEDSID" SP session-id
session-id = string ;; a unique session identifier,
;; implementation specific.
7. Security Considerations
In order to mitigate DoS attacks caused by a client that tries to
generate numerous resumable sessions (for example by issuing multiple
SID commands with no parameters on the same TCP connection), a server
implementation should consider imposing a limit on the maximum number
of resumable sessions associated with a particular user and/or coming
from a particular IP address/network. Once this limit has been
reached, the oldest inactive session must be destroyed and the new
one can be created. Inactive sessions SHOULD have an expiration
timeout.
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The Quick Reconnect extension does not make any modifications to the
authentication sequence of the current IMAP4 protocol [RFC3501].
8. IANA Considerations
IMAP4 capabilities are registered by publishing a standards track or
IESG approved experimental RFC. The registry is currently located
at:
http://www.iana.org/assignments/imap4-capabilities
This document defines the X-DRAFT-W07-RECONNECT [[anchor15: Fix
capability name upon publication]] IMAP capability. IANA is
requested to add this capability to the registry.
9. References
9.1. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, Ed., "Augmented BNF for
Syntax Specifications: ABNF", RFC 4234, October 2005.
[ACL] Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
RFC 4314, December 2005.
[CONDSTORE]
Melnikov, A. and S. Hole, "IMAP Extension for Conditional
STORE", November 2003.
[EXPUNGED]
Melnikov, A., "IMAP4 extension to CONDSTORE for reporting
messages expunged since last synchronization",
January 2005.
[IMAPABNF]
Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
ABNF", RFC 4466, April 2006.
[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.
[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
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9.2. Informative References
[C2S] Maes, S. and C. Wilson, "Lemonade Requirements for Server to
Client Notifications", February 2005.
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Authors' Addresses
Alexey Melnikov
Isode Ltd
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
Email: Alexey.Melnikov@isode.com
Corby Wilson
Nokia
5 Wayside Rd.
Burlington, MA 01803
USA
Email: corby@computer.org
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