Network Working Group M. Gahrns, Microsoft R. Cheng, Microsoft Internet Draft Document: draft-gahrns-imap-child-mailbox-02.txt December 1999 IMAP4 Child Mailbox Extension Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 0. Meta Information on this draft This information is intended to facilitate discussion. It will be removed when this document leaves the Internet-Draft stage. This draft is now being discussed on the IMAPEXT mailing list at ietf-imapext@imc.org. Subscription requests can be sent to ietf- imapext-request@imc.org (send an email message with the word "subscribe" in the body). More information on the mailing list along with a WWW archive of back messages is available at HTTP://www.imc.org. Earlier discussion took place on the IMAP Mailing List imap@u.washington.edu At the 45th IETF in Oslo, and 46th IETF in Washington, there was further discussion regarding this draft. I am making what I believe should be the final editorial changes to this draft. At several of the previous IMC interop events, several IMAP vendors have done implementations of this proposal. Further discussions are needed as to whether this draft should be submitted as a proposed standard or as an informational or historical RFC. Submitting as an information or historical RFC would serve to document the current Gahrns and Cheng 1 IMAP4 Child Mailbox Extension December 1999 implementations and elements of it could become a basis of a general LIST extension. There is some interest in starting a completely new LIST Extension draft that addresses general extensibility of the LIST command. Similar functionality to the \HasChildren and \HasNoChildren flags could be incorporated into this new LIST Extension, but the client would then have an opportunity to request whether or not the server should return this information. This could be an advantage over the current draft for servers where this information is expensive to compute, since the server would only need to compute the information when it knew that the client requesting the information was able to use it. Changes since April 99, draft-01 Incorporated comments from the list discussions: 1) Explicitly note that it is an error for the server to return both a \HasChildren and a \NoInferiors attribute in a LIST response. 2) Note that the \HasChildren and \HasNoChildren attribute may not apply to the LSUB command. 3) Note how \HasChildren and \HasNoChildren apply in the referral case. 4) Added back advertising CHILDREN in the capability response as was the consensus during the Washington IMAP-EXT WG. Rational was that this made it easier for clients. 1. Abstract Many IMAP4 [RFC-2060] clients present to the user a hierarchical view of the mailboxes that a user has access to. Rather than initially presenting to the user the entire mailbox hierarchy, it is often preferable to show to the user a collapsed outline list of the mailbox hierarchy (particularly if there is a large number of mailboxes). The user can then expand the collapsed outline hierarchy as needed. It is common to include within the collapsed hierarchy a visual clue (such as a "+") to indicate that there are child mailboxes under a particular mailbox. When the visual clue is clicked the hierarchy list is expanded to show the child mailboxes. The CHILDREN extension provides a mechanism for a client to efficiently determine if a particular mailbox has children, without issuing a LIST "" * or a LIST "" % for each mailbox name. Gahrns and Cheng Expires June 2000 2 IMAP4 Child Mailbox Extension December 1999 2. Conventions used in this document In examples, "C:" and "S:" indicate lines sent by the client and server respectively. If such lines are wrapped without a new "C:" or "S:" label, then the wrapping is for editorial clarity and is not part of the 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. Requirements IMAP4 servers that support this extension MUST list the keyword CHILDREN in their CAPABILITY response. The CHILDREN extension defines two new attributes that MAY be returned within a LIST response. \HasChildren - The presence of this attribute indicates that the mailbox has child mailboxes. Servers SHOULD NOT return \HasChildren if child mailboxes exist, but none will be displayed to the current user in a LIST response (as should be the case where child mailboxes exist, but a client does not have permissions to access them.) In this case, \HasNoChildren SHOULD be used. In many cases, however, a server may not be able to efficiently compute whether a user has access to all child mailboxes, or multiple users may be accessing the same account and simultaneously changing the mailbox hierarchy. As such a client MUST be prepared to accept the \HasChildren attribute as a hint. That is, a mailbox MAY be flagged with the \HasChildren attribute, but no child mailboxes will appear in a subsequent LIST response. Example 3.1: ============ /*** Consider a server that has the following mailbox hierarchy: INBOX ITEM_1 ITEM_1A ITEM_2 TOP_SECRET Where INBOX, ITEM_1 and ITEM_2 are top level mailboxes. ITEM_1A is a child mailbox of ITEM_1 and TOP_SECRET is a child mailbox of ITEM_2 that the currently logged on user does NOT have access to. Gahrns and Cheng Expires June 2000 3 IMAP4 Child Mailbox Extension December 1999 Note that in this case, the server is not able to efficiently compute access rights to child mailboxes and responds with a \HasChildren attribute for mailbox ITEM_2, even though ITEM_2/TOP_SECRET does not appear in the list response. ***/ C: A001 LIST "" * S: * LIST (\HasNoChildren) "/" INBOX S: * LIST (\HasChildren) "/" ITEM_1 S: * LIST (\HasNoChildren) "/" ITEM_1/ITEM_1A S: * LIST (\HasChildren) "/" ITEM_2 S: A001 OK LIST Completed \HasNoChildren - The presence of this attribute indicates that the mailbox has NO child mailboxes that are accessible to the currently authenticated user. If a mailbox has the \Noinferiors attribute, the \HasNoChildren attribute is redundant and SHOULD be omitted in the LIST response. In some instances a server that supports the CHILDREN extension MAY NOT be able to determine whether a mailbox has children. For example it may have difficulty determining whether there are child mailboxes when LISTing mailboxes while operating in a particular namespace. In these cases, a server MAY exclude both the \HasChildren and \HasNoChildren attributes in the LIST response. As such, a client can not make any assumptions about whether a mailbox has children based upon the absence of a single attribute. It is an error for the server to return both a \HasChildren and a \HasNoChildren attribute in a LIST response. It is an error for the server to return both a \HasChildren and a \NoInferiors attribute in a LIST response. Note: the \HasNoChildren attribute should not be confused with the IMAP4 [RFC-2060] defined attribute \Noinferiors which indicates that no child mailboxes exist now and none can be created in the future. The \HasChildren and \HasNoChildren attributes might not be returned in response to a LSUB response. Many servers maintain a simple mailbox subscription list that is not updated when the underlying mailbox structure is changed. A client MUST NOT assume that hierarchy information will be maintained in the subscription list. RLIST is a command defined in [RFC-2193] that includes in a LIST response mailboxes that are accessible only via referral. That is, a client must explicitly issue an RLIST command to see a list of these mailboxes. Thus in the case where a mailbox has child mailboxes that are available only via referral, the mailboxes would Gahrns and Cheng Expires June 2000 4 IMAP4 Child Mailbox Extension December 1999 appear as \HasNoChildren in response to the LIST command, and \HasChildren in response to the RLIST command. 5. Formal Syntax The following syntax specification uses the augmented Backus-Naur Form (BNF) as described in [ABNF]. Two new mailbox attributes are defined as flag_extensions to the IMAP4 mailbox_list response: HasChildren = "\HasChildren" HasNoChildren = "\HasNoChildren" 6. Security Considerations This extension provides a client a more efficient means of determining whether a particular mailbox has children. If a mailbox has children, but the currently authenticated user does not have access to any of them, the server SHOULD respond with a \HasNoChildren attribute. In many cases, however, a server may not be able to efficiently compute whether a user has access to all child mailboxes. If such a server responds with a \HasChildren attribute, when in fact the currently authenticated user does not have access to any child mailboxes, potentially more information is conveyed about the mailbox than intended. A server designed with such levels of security in mind SHOULD NOT attach the \HasChildren attribute to a mailbox unless the server is certain that the user has access to at least one of the child mailboxes. 7. 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 [RFC-2234], D. Crocker and P. Overell, Editors, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, Internet Mail Consortium, November 1997 [RFC-2193], Gahrns, M, "IMAP4 Mailbox Referrals", RFC 2193, Microsoft Corporation, September 1997 8. Acknowledgments Gahrns and Cheng Expires June 2000 5 IMAP4 Child Mailbox Extension December 1999 The authors would like to thank the participants of several IMC Mail Connect events for their input when this idea was originally presented and refined. 9. Author's Address Mike Gahrns Microsoft One Microsoft Way Redmond, WA, 98072 Phone: (425) 936-9833 Email: mikega@microsoft.com Raymond Cheng Microsoft One Microsoft Way Redmond, WA, 98072 Phone: (425) 703-4913 Email: raych@microsoft.com Gahrns and Cheng Expires June 2000 6 IMAP4 Child Mailbox Extension December 1999 Full Copyright Statement Copyright (C) The Internet Society (1999). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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