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Versions: 00 01 rfc2180                                                 
Network Working Group                                        M. Gahrns
Internet Draft                                               Microsoft
Document: draft-gahrns-imap-practice-00.txt                 March 1997


                   IMAP4 Implementation Practice


Status of this Memo

   This document is an Internet Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups.  Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of six
   months.  Internet Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet
   Drafts as reference material or to cite them other than as a
   "working draft" or "work in progress".

   To learn the current status of any Internet-Draft, please check the
   1id-abstracts.txt listing contained in the Internet-Drafts Shadow
   Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or
   munnari.oz.au.

   A revised version of this draft document will be submitted to the
   RFC editor as a Proposed Standard for the Internet Community.
   Discussion and suggestions for improvement are requested.  This
   document will expire before September 1997. Distribution of this
   draft is unlimited.


1. Abstract

   IMAP4[rfc2060] is rich client/server protocol that allows a client
   to access and manipulate electronic mail messages on a server.
   Within the protocol framework, it is possible to have differing
   results for particular client/server interactions. If a protocol
   does not allow for this, it is often unduly restrictive.

   For example, when multiple clients are accessing a mailbox and one
   attempts to delete the mailbox, an IMAP4 server may choose to
   implement a solution based upon server architectural constraints or
   individual preference.

   With this flexibility comes greater client responsibility.  It is
   not sufficient for a client to be written based upon the behavior of
   a particular IMAP server.  Rather the client must be based upon the
   behavior allowed by the protocol.

   By documenting common IMAP4 server practice for the case of
   simultaneous client access to a mailbox, we hope to ensure the
   widest amount of inter-operation between IMAP4 clients and servers.

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                    IMAP4 Implementation Practice          March 1997


   The behavior described in this document reflects the practice of
   some existing servers or behavior that the consensus of the IMAP
   mailing list has deemed to be reasonable.  The behavior described
   within this document is believed to be [RFC2060] compliant. However,
   this document is not meant to define IMAP4 compliance, nor is it and
   exhaustive list of valid IMAP4 behavior. [RFC2060] must always be
   consulted to determine IMAP4 compliance, especially for server
   behavior not described within this document.

2. Conventions used in this document

   In examples,"C1:", "C2:" and "C3:" indicate lines sent by 3
   different clients (client #1, client #2 and client #3) that are
   connected to a server.  "S1:", "S2:" and "S3:" indicated lines sent
   by the server to client #1, client #2 and client #3 respectively.

   A shared mailbox, is a mailbox that can be used by multiple users.

   A multi-accessed mailbox, is a mailbox that has multiple clients
   simultaneously accessing it.

   A client is said have accessed a mailbox after a successful SELECT
   or EXAMINE command.

   SHOULD and MAY are terms that are defined in accordance with [RFC-
   2060].


3. Deletion/Renaming of a multi-accessed mailbox

   When multiple clients are accessing a mailbox, care must be taken
   when handling the deletion or renaming of the mailbox by one of the
   clients. Following are some strategies an IMAP server may choose to
   use when dealing with this.


3.1. The server MAY fail the DELETE/RENAME command of a multi-accessed
     mailbox

   In some cases, this behavior may not be practical.  For example, if
   a large number of clients are accessing a shared mailbox, the window
   in which no clients have the mailbox accessed may be small or non-
   existent, effectively rendering the mailbox undeletable or
   unrenamable.

   Example:

   <Client #1 and Client #2 have mailbox FOO accessed. Client #1 tries
   to DELETE the mailbox and is refused>

             C1: A001 DELETE FOO
             S1: A001 NO Mailbox FOO is in use by another user.


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                    IMAP4 Implementation Practice          March 1997



3.2. The server MAY allow the DELETE command of a multi-accessed
     mailbox, but keep the information in the mailbox available for
     those clients that currently have access to the mailbox.

   When all clients have finished accessing the mailbox, it is
   permanently removed.  For clients that do not already have access to
   the mailbox, the 'ghosted' mailbox would not be available.  For
   example, it would not be returned to these clients in a subsequent
   LIST or LSUB command and would not be a valid mailbox argument to
   any other IMAP command until the reference count of clients
   accessing the mailbox reached 0.

   In some cases, this behavior may not be desirable. For example if
   someone created a mailbox with offensive or sensitive information,
   one might prefer to have the mailbox deleted and all access to the
   information contained within removed immediately, rather than
   continuing to allow access until the client closes the mailbox.

   Furthermore, this behavior, prevents 'recycling' of the same mailbox
   name until all clients have finished accessing the original mailbox.

   Example:

   <Client #1 and Client #2 have mailbox FOO accessed. Client #1
   DELETEs mailbox FOO>

             C1: A001 DELETE FOO
             S1: A001 OK Mailbox FOO is deleted.

   <Client #2 is still able to operate on the deleted mailbox>

             C2: B001 STATUS FOO (MESSAGES)
             S2: * STATUS FOO (MESSAGES 6)
             S2: B001 OK STATUS completed

   <Client #3 which did not have access to the mailbox prior to the
   deletion by client #1 does not have access to the mailbox>

             C3: C001 STATUS FOO (MESSAGES)
             S3: C001 NO Mailbox does not exist

   <Nor is client #3 able to create a mailbox with the name FOO, while
   the reference count is non zero>

             C3: C002 CREATE FOO
             S3: C002 NO Mailbox FOO is still in use. Try again later.

   <Client #2 closes its access to the mailbox, no other clients have
   access to the mailbox FOO and reference count becomes 0>

             C2: B002 CLOSE
             S2: B002 OK CLOSE Completed

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                    IMAP4 Implementation Practice          March 1997



   <Now that the reference count on FOO has reached 0, the mailbox name
   can be recycled>

             C3: C003 CREATE FOO
             S3: C003 OK CREATE Completed


3.3. The server MAY allow the DELETE/RENAME of a multi-accessed
     mailbox, but disconnect all other clients who have the mailbox
     accessed by sending a untagged BYE response.

   A server may often choose to disconnect clients in the DELETE case,
   but may choose to implement a "friendlier" method for the RENAME
   case.

   Example:

   <Client #1 and Client #2 have mailbox FOO accessed. Client #1
   DELETEs the mailbox FOO>

             C1: A002 DELETE FOO
             S1: A002 OK DELETE completed.

   <Server disconnects all other users of the mailbox>
             S2: * BYE Mailbox FOO has been deleted.


3.4. The server MAY allow the RENAME of a multi-accessed mailbox by
     simply changing the name attribute on the mailbox.

   Other clients that have access to the mailbox can continue issuing
   commands such as FETCH that do not reference the mailbox name.
   Clients would discover the renaming the next time they referred to
   the old mailbox name.  Some servers MAY choose to include the
   [NEWNAME] response code in their tagged NO response to a command
   that contained the old mailbox name, as a hint to the client that
   the operation can succeed if the command is issued with the new
   mailbox name.


   Example:

   <Client #1 and Client #2 have mailbox FOO accessed. Client #1
   RENAMEs the mailbox.>

             C1: A001 RENAME FOO BAR
             S1: A001 OK RENAME completed.

   <Client #2 is still able to do operations that do not reference the
   mailbox name>

             C2: B001 FETCH 2:4 (FLAGS)

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                    IMAP4 Implementation Practice          March 1997


             S2: * 2 FETCH . . .
             S2: * 3 FETCH . . .
             S2: * 4 FETCH . . .
             S2: B001 OK FETCH completed

   <Client #2 is not able to do operations that reference the mailbox
   name>

             C2: B002 STATUS FOO (MESSAGES)
             S2: B002 NO [NEWNAME FOO BAR] Mailbox has been renamed


4. Expunging of messages on a multi-accessed mailbox

   When multiple clients are accessing a mailbox, care must be taken
   when handling the EXPUNGE of messages.  Other clients accessing the
   mailbox may be in the midst of issuing a command that depends upon
   message sequence numbers.  Because an EXPUNGE response can not be
   sent while responding to a FETCH, STORE or SEARCH command, it is not
   possible to immediately notify the client of the EXPUNGE.  This can
   result in ambiguity if the client issues a FETCH, STORE or SEARCH
   operation on a message that has been EXPUNGED.


4.1. Fetching of EXPUNGED messages

   Following are some strategies an IMAP server may choose to use when
   dealing with a FETCH command on expunged messages.

   Consider the following scenario:

   - Client #1 and Client #2 have mailbox FOO selected.
   - There are 7 messages in the mailbox.
   - Messages 4:7 are marked for deletion.
   - Client #1 issues an EXPUNGE, to expunge messages 4:7


4.1.1. The server MAY allow the EXPUNGE of a multi-accessed mailbox but
       keep the messages available to satisfy subsequent FETCH commands
       until it is able to send an EXPUNGE response to each client.

   In some cases, the behavior of keeping "ghosted" messages may not be
   desirable.  For example if a message contained offensive or
   sensitive information, one might prefer to instantaneously remove
   all access to the information, regardless of whether another client
   is in the midst of accessing it.

   Example:  (Building upon the scenario outlined in 4.1.)


   <Client #2 is still able to access the expunged messages because the
   server has kept a 'ghosted' copy of the messages until it is able to
   notify client #2 of the EXPUNGE>

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                    IMAP4 Implementation Practice          March 1997



             C2: B001 FETCH 4:7 RFC822
             S2: * 4 FETCH RFC822 . . . (RFC822 info returned)
             S2: * 5 FETCH RFC822 . . . (RFC822 info returned)
             S2: * 6 FETCH RFC822 . . . (RFC822 info returned)
             S2: * 7 FETCH RFC822 . . . (RFC822 info returned)
             S2: B001 OK FETCH Completed

   <Client #2 issues a command where it can get notified of the
   EXPUNGE>

             C2: B002 NOOP
             S2: * 4 EXPUNGE
             S2: * 4 EXPUNGE
             S2: * 4 EXPUNGE
             S2: * 4 EXPUNGE
             S2: * 3 EXISTS
             S2: B002 OK NOOP Complete

   <Client #2 no longer has access to the expunged messages>

             C2: B003 FETCH 4:7 RFC822
             S2: B003 NO Messages 4:7 are no longer available.


4.1.2 The server MAY allow the EXPUNGE of a multi-access mailbox,
      and on subsequent FETCH commands return a tagged NO, and FETCH
      responses only for the non-expunged messages.

   If all of the messages in the subsequent FETCH command have been
   expunged, the server SHOULD return only a tagged NO.

   After receiving a tagged NO FETCH response, the client SHOULD issue
   a NOOP command so that it will be informed of any pending EXPUNGE
   responses.  The client may then either reissue the failed FETCH
   command, or by examining the EXPUNGE response from the NOOP and the
   FETCH response from the FETCH, determine that the FETCH failed
   because of pending expunges.

   Example:  (Building upon the scenario outlined in 4.1.)

   <Client #2 attempts to FETCH a mix of expunged and non-expunged
   messages.  A FETCH response is returned only for then non-expunged
   messages along with a tagged NO>

             C2: B001 FETCH 3:5 ENVELOPE
             S2: * 3 FETCH ENVELOPE . . . (ENVELOPE info returned)
             S2: B001 NO Some of the requested messages no longer exist

   <Upon receiving a tagged NO FETCH response, Client #2 issues a NOOP
   to be informed of any pending EXPUNGE responses>

             C2: B002 NOOP

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                    IMAP4 Implementation Practice          March 1997


             S2: * EXPUNGE 4
             S2: * EXPUNGE 4
             S2: * EXPUNGE 4
             S2: * EXPUNGE 4
             S2: * 3 EXISTS
             S2: B002 OK NOOP Completed.

   <By receiving a FETCH response for message 3, and an EXPUNGE
   response that indicates messages 4:7 have been expunged, the client
   does not need to re-issue the FETCH>



4.1.3 The server MAY allow the EXPUNGE of a multi-access mailbox, and
      on subsequent FETCH commands return a tagged OK, "NIL FETCH
      Responses" for expunged messages, and FETCH responses for non
      -expunged messages.

   If all of the messages in the subsequent FETCH command have been
   expunged, the server SHOULD return only a tagged NO.  In this case,
   the client SHOULD issue a NOOP command so that it will be informed
   of any pending EXPUNGE responses.  The client may then either
   reissue the failed FETCH command, or by examining the EXPUNGE
   response from the NOOP, determine that the FETCH failed because of
   pending expunges.

   "NIL FETCH responses" are a representation of empty data as
   appropriate for the FETCH argument specified.

   Example:

   * 1 FETCH (ENVELOPE (NIL NIL NIL NIL NIL NIL NIL NIL NIL NIL))
   * 1 FETCH (FLAGS ())
   * 1 FETCH (INTERNALDATE "00-Jan-0000 00:00:00 +0000")
   * 1 FETCH (RFC822 "")
   * 1 FETCH (RFC822.HEADER "")
   * 1 FETCH (RFC822.TEXT "")
   * 1 FETCH (RFC822.SIZE 0)
   * 1 FETCH (BODY ("TEXT" "PLAIN" NIL NIL NIL "7BIT" 0 0)
   * 1 FETCH (BODYSTRUCTURE ("TEXT" "PLAIN" NIL NIL NIL "7BIT" 0 0)
   * 1 FETCH (BODY[<section>] "")
   * 1 FETCH (BODY[<section>]<partial> "")


   In some cases, a client may not be able to distinguish between "NIL
   FETCH responses" received because a message was expunged and those
   received because the data actually was NIL.  For example, a  * 5
   FETCH (FLAGS ()) response could be received if no flags were set on
   message 5, or because message 5 was expunged. In a case of potential
   ambiguity, the client SHOULD issue a command such as NOOP to force
   the sending of the EXPUNGE responses to resolve any ambiguity.



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                    IMAP4 Implementation Practice          March 1997


   Example:  (Building upon the scenario outlined in 4.1.)

   <Client #2 attempts to access a mix of expunged and non-expunged
   messages.  Normal data is returned for non-expunged message, "NIL
   FETCH responses" are returned for expunged messages>

             C2: B002 FETCH 3:5 ENVELOPE
             S2: * 3 FETCH ENVELOPE . . . (ENVELOPE info returned)
             S2: * 4 FETCH ENVELOPE (NIL NIL NIL NIL NIL NIL NIL NIL
                   NIL NIL)
             S2: * 5 FETCH ENVELOPE (NIL NIL NIL NIL NIL NIL NIL NIL
                   NIL NIL)
             S2: B002 OK FETCH Completed

   <Client #2 attempts to FETCH only expunged messages and receives a
   tagged NO response>

             C2: B002 FETCH 4:7 ENVELOPE
             S2: B002 NO Messages 4:7 have been expunged.


4.1.4 To avoid the situation altogether, the server MAY fail the
      EXPUNGE of a multi-accessed mailbox

   In some cases, this behavior may not be practical.  For example, if
   a large number of clients are accessing a shared mailbox, the window
   in which no clients have the mailbox accessed may be small or non-
   existent, effectively rendering the message unexpungeable.


4.2. Storing of expunged messages

   Following are some strategies an IMAP server may choose to use when
   dealing with a STORE command on expunged messages.

4.2.1 If the ".SILENT" suffix is used, and the STORE completed
      successfully for all the non-expunged messages, the server SHOULD
      return a tagged OK.

   Example:  (Building upon the scenario outlined in 4.1.)

   <Client #2 tries to SILENETLY STORE flags on expunged and non-
   expunged messages.  The server sets the flags on the non-expunged
   messages and returns OK>

             C2: B001 STORE 1:7 +FLAGS.SILENT (\SEEN)
             S2: B001 OK


4.2.2. If the ".SILENT" suffix is not used, and only expunged messages
       are referenced, the server SHOULD return only a tagged NO.

   Example:  (Building upon the scenario outlined in 4.1.)

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                    IMAP4 Implementation Practice          March 1997



   <Client #2 tries to STORE flags only on expunged messages>

             C2: B001 STORE 5:7 +FLAGS (\SEEN)
             S2: B001 NO  Messages have been expunged


4.2.3. If the ".SILENT" suffix is not used, and a mixture of expunged
       and non-expunged messages are referenced, the server MAY set the
       flags and return a FETCH response for the non-expunged messages
       along with a tagged NO.

   After receiving a tagged NO STORE response, the client SHOULD issue
   a NOOP command so that it will be informed of any pending EXPUNGE
   responses.  The client may then either reissue the failed STORE
   command, or by examining the EXPUNGE responses from the NOOP and
   FETCH responses from the STORE, determine that the STORE failed
   because of pending expunges.

   Example:  (Building upon the scenario outlined in 4.1.)


   <Client #2 tries to STORE flags on a mixture of expunged and non-
   expunged messages>

             C2: B001 STORE 1:7 +FLAGS (\SEEN)
             S2: * FETCH 1 FLAGS (\SEEN)
             S2: * FETCH 2 FLAGS (\SEEN)
             S2: * FETCH 3 FLAGS (\SEEN)
             S2: B001 NO Some of the messages no longer exist.

             C2: B002 NOOP
             S2: * EXPUNGE 4
             S2: * EXPUNGE 4
             S2: * EXPUNGE 4
             S2: * EXPUNGE 4
             S2: * 3 EXISTS
             S2: B002 OK NOOP Completed.

   <By receiving FETCH responses for messages 1:3, and an EXPUNGE
   response that indicates messages 4:7 have been expunged, the client
   does not need to re-issue the STORE>


4.2.4. If the ".SILENT" suffix is not used, and a mixture of expunged
       and non-expunged messages are referenced, the server MAY return
       only an untagged NO and not set any flags, nor return any FETCH
       responses

   After receiving a tagged NO STORE response, the client SHOULD issue
   a NOOP command so that it will be informed of any pending EXPUNGE
   responses.  The client would then re-issue the STORE command after
   updating its message list per any EXPUNGE response.

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                    IMAP4 Implementation Practice          March 1997



   If a large number of clients are accessing a shared mailbox, the
   window in which there are no pending expunges may be small or non-
   existent, effectively disallowing a client from setting the flags on
   all messages at once.

   Example:  (Building upon the scenario outlined in 4.1.)


   <Client #2 tries to STORE flags on a mixture of expunged and non-
   expunged messages>

             C2: B001 STORE 1:7 +FLAGS (\SEEN)
             S2: B001 NO  Some of the messages no longer exist.

   <Client #2 issues a NOOP to be informed of the EXPUNGED messages>

             C2: B002 NOOP
             S2: * EXPUNGE 4
             S2: * EXPUNGE 4
             S2: * EXPUNGE 4
             S2: * EXPUNGE 4
             S2: * 3 EXISTS
             S2: B002 OK NOOP Completed.

   <Client #2 updates its message list and re-issues the STORE on only
   those messages that have not been expunged>

             C2: B003 STORE 1:3 +FLAGS (\SEEN)
             S2: * FETCH 1 FLAGS (\SEEN)
             S2: * FETCH 2 FLAGS (\SEEN)
             S2: * FETCH 3 FLAGS (\SEEN)
             S2: B003 OK  STORE Completed


4.2. Searching of EXPUNGED messages

   A server MAY simply not return a search response for messages that
   have been expunged and it has not been able to inform the client
   about.  If a client was expecting a particular message to be
   returned in a search result, and it was not, the client SHOULD issue
   a NOOP command to see if the message was expunged by another client.


5. Security Considerations

   This document describes behavior of servers that use the IMAP4
   protocol, and as such, has the same security considerations as
   described in [RFC-2060].

   In particular, some described server behavior does not allow for the
   immediate deletion of information when a mailbox is accessed by


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                    IMAP4 Implementation Practice          March 1997


   multiple clients.  This may be a consideration when dealing with
   sensitive information where immediate deletion would be preferred.


6. References

   [RFC-2060], Crispin, M., "Internet Message Access Protocol – Version
   4rev1", RFC 2060, University of Washington, December 1996.


7.  Acknowledgments

   This document is the result of discussions on the IMAP4 mailing list
   and is meant to reflect consensus of this group.  In particular,
   Raymond Cheng, Mark Crispin, Jack De Winter, Jim Evans, Steve Hole,
   Mark Keasling, Barry Leiba, Pat Moran, Larry Osterman, Chris Newman,
   and Vladimir Vulovic were significant participants in this
   discussion or made suggestions to this document.


8. Author's Address

   Mike Gahrns
   Microsoft
   One Microsoft Way
   Redmond, WA, 98072

   Phone: (206) 936-9833
   Email:            mikega@microsoft.co

                             m

























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