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diff --git a/imap/docs/rfc/rfc2180.txt b/imap/docs/rfc/rfc2180.txt new file mode 100644 index 00000000..57607002 --- /dev/null +++ b/imap/docs/rfc/rfc2180.txt @@ -0,0 +1,787 @@ + + + + + + +Network Working Group M. Gahrns +Request for Comments: 2180 Microsoft +Category: Informational July 1997 + + + IMAP4 Multi-Accessed Mailbox Practice + +Status of this Memo + + This memo provides information for the Internet community. This memo + does not specify an Internet standard of any kind. Distribution of + this memo is unlimited. + +1. Abstract + + IMAP4[RFC-2060] 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. + + 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 [RFC-2060] compliant. However, this + document is not meant to define IMAP4 compliance, nor is it an + exhaustive list of valid IMAP4 behavior. [RFC-2060] must always be + consulted to determine IMAP4 compliance, especially for server + behavior not described within this document. + + + + + + + + +Gahrns Informational [Page 1] + +RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997 + + +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 to have accessed a mailbox after a successful SELECT + or EXAMINE command. + + 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 [RFC-2119]. + + +3. Deletion/Renaming of a multi-accessed mailbox + + If an external agent or multiple clients are accessing a mailbox, + care must be taken when handling the deletion or renaming of the + mailbox. Following are some strategies an IMAP server may choose to + use when dealing with this situation. + + +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. + + + + + + + +Gahrns Informational [Page 2] + +RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 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, may prevent 'recycling' of the same + mailbox name until all clients have finished accessing the original + mailbox. + + Example: + + <Client #1 and Client #2 have mailbox FOO selected. 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 STORE 1 +FLAGS (\Seen) + S2: * 1 FETCH FLAGS (\Seen) + S2: B001 OK STORE 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. + + + + +Gahrns Informational [Page 3] + +RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997 + + + <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 + + <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. + + + + + + + +Gahrns Informational [Page 4] + +RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997 + + + 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) + 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 APPEND FOO {300} C2: Date: Mon, 7 Feb 1994 + 21:52:25 0800 (PST) C2: . . . S2: B002 NO [NEWNAME FOO + BAR] Mailbox has been renamed + + +4. Expunging of messages on a multi-accessed mailbox + + If an external agent or 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. + + + + + + + + + +Gahrns Informational [Page 5] + +RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997 + + + 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> + + 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. + + + + + + +Gahrns Informational [Page 6] + +RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997 + + +4.1.2 The server MAY allow the EXPUNGE of a multi-accessed mailbox, + and on subsequent FETCH commands return FETCH responses only for + non-expunged messages and 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 + S2: * 4 EXPUNGE + S2: * 4 EXPUNGE + S2: * 4 EXPUNGE + S2: * 4 EXPUNGE + 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> + + + + + + + + + + + + + + + + +Gahrns Informational [Page 7] + +RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997 + + +4.1.3 The server MAY allow the EXPUNGE of a multi-accessed mailbox, and + on subsequent FETCH commands return the usual FETCH responses for + non-expunged messages, "NIL FETCH Responses" for expunged + messages, and a tagged OK response. + + 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. + + 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> + + + + + + + + +Gahrns Informational [Page 8] + +RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997 + + + 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 silently 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 + + + + + + + + + +Gahrns Informational [Page 9] + +RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997 + + +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.) + + <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: * 4 EXPUNGE + S2: * 4 EXPUNGE + S2: * 4 EXPUNGE + S2: * 4 EXPUNGE + 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> + + + + + + +Gahrns Informational [Page 10] + +RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997 + + +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 + an untagged NO and not set any flags. + + 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. + + 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: * 4 EXPUNGE + S2: * 4 EXPUNGE + S2: * 4 EXPUNGE + S2: * 4 EXPUNGE + 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 + + +4.3. 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. + + + + +Gahrns Informational [Page 11] + +RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997 + + +4.4 Copying of expunged messages + + COPY is the only IMAP4 sequence number command that is safe to allow + an EXPUNGE response on. This is because a client is not permitted to + cascade several COPY commands together. A client is required to wait + and confirm that the copy worked before issuing another one. + +4.4.1 The server MAY disallow the COPY of messages in a multi-access + mailbox that contains expunged messages. + + Pending EXPUNGE response(s) MUST be returned to the COPY command. + + Example: + + C: A001 COPY 2,4,6,8 FRED + S: * 4 EXPUNGE + S: A001 NO COPY rejected, because some of the requested + messages were expunged + + Note: Non of the above messages are copied because if a COPY command + is unsuccessful, the server MUST restore the destination mailbox to + its state before the COPY attempt. + + +4.4.2 The server MAY allow the COPY of messages in a multi-access + mailbox that contains expunged messages. + + Pending EXPUNGE response(s) MUST be returned to the COPY command. + Messages that are copied are messages corresponding to sequence + numbers before any EXPUNGE response. + + Example: + + C: A001 COPY 2,4,6,8 FRED + S: * 3 EXPUNGE + S: A001 OK COPY completed + + In the above example, the messages that are copied to FRED are + messages 2,4,6,8 at the start of the COPY command. These are + equivalent to messages 2,3,5,7 at the end of the COPY command. The + EXPUNGE response can't take place until after the messages from the + COPY command are identified (because of the "no expunge while no + commands in progress" rule). + + + + + + + + +Gahrns Informational [Page 12] + +RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997 + + + Example: + + C: A001 COPY 2,4,6,8 FRED + S: * 4 EXPUNGE + S: A001 OK COPY completed + + In the above example, message 4 was copied before it was expunged, + and MUST appear in the destination mailbox FRED. + + +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 + 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. + + [RFC-2119], Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", RFC 2119, Harvard University, March 1997. + + +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, Jim Evans, Erik Forsberg, Steve Hole, + Mark Keasling, Barry Leiba, Syd Logan, John Mani, Pat Moran, Larry + Osterman, Chris Newman, Bart Schaefer, Vladimir Vulovic, and Jack De + Winter were active participants in this discussion or made + suggestions to this document. + + + + + + + + + + + +Gahrns Informational [Page 13] + +RFC 2180 IMAP4 Multi-Accessed Mailbox Practice July 1997 + + +8. Author's Address + + Mike Gahrns + Microsoft + One Microsoft Way + Redmond, WA, 98072 + + Phone: (206) 936-9833 + EMail: mikega@microsoft.com + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Gahrns Informational [Page 14] + |