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.  However, this document itself may not be modified in any 
   way, such as by removing the copyright notice or references to the 
   Internet Society or other Internet organizations, except as needed 
   For the purpose of developing Internet standards in which case the 
   procedures for copyrights defined in the Internet Standards 
   process must be followed, or as required to translate it into 
   languages other than English. 
    
   The limited permissions granted above are perpetual and will not 
   be revoked by the Internet Society or its successors or assigns. 
    
   This document and the information contained herein is provided on 
   an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET 
   ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR 
   IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    


  
Gahrns and Cheng          Expires June 2000                         7