caldav-proxy.txt

Calendar Server Extension                                       C. Daboo
                                                              Apple Inc.
                                                       November 13, 2012


              Calendar User Proxy Functionality in CalDAV

Abstract

   This specification defines an extension to CalDAV that makes it easy
   for clients to setup and manage calendar user proxies, using the
   WebDAV Access Control List extension as a basis.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  2
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  Server . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.2.  Client . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  New features in CalDAV . . . . . . . . . . . . . . . . . . . .  4
     5.1.  Feature Discovery  . . . . . . . . . . . . . . . . . . . .  4
     5.2.  Proxy Principal Resource . . . . . . . . . . . . . . . . .  4
     5.3.  New Principal Properties . . . . . . . . . . . . . . . . .  8
       5.3.1.  CS:calendar-proxy-read-for Property  . . . . . . . . .  8
       5.3.2.  CS:calendar-proxy-write-for Property . . . . . . . . .  8
     5.4.  Privilege Provisioning . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  Normative References . . . . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 11
   Appendix B.  Change History  . . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
















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1.  Introduction

   CalDAV [RFC4791] provides a way for calendar users to store calendar
   data and exchange this data via scheduling operations.  Based on the
   WebDAV protocol [RFC2518], it also includes the ability to manage
   access to calendar data via the WebDAV ACL extension [RFC3744].

   It is often common for a calendar user to delegate some form of
   responsibility for their calendar and schedules to another calendar
   user (e.g., a boss allows an assistant to check a calendar or to send
   and accept scheduling invites on his behalf).  The user handling the
   calendar data on behalf of someone else is often referred to as a
   "calendar user proxy".

   Whilst CalDAV does have fine-grained access control features that can
   be used to setup complex sharing and management of calendars, often
   the proxy behavior required is an "all-or-nothing" approach - i.e.
   the proxy has access to all the calendars or to no calendars (in
   which case they are of course not a proxy).  So a simple way to
   manage access to an entire set of calendars and scheduling ability
   would be handy.

   In addition, calendar user agents will often want to display to a
   user who has proxy access to their calendars, or to whom they are
   acting as a proxy.  Again, CalDAV's access control discovery and
   report features can be used to do that, but with fine-grained control
   that exists, it can be hard to tell who is a "real" proxy as opposed
   to someone just granted rights to some subset of calendars.  Again, a
   simple way to discover proxy information would be handy.


2.  Conventions Used in This Document

   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].

   When XML element types in the namespace "DAV:" are referenced in this
   document outside of the context of an XML fragment, the string "DAV:"
   will be prefixed to the element type names.

   When XML element types in the namespaces "DAV:" and
   "urn:ietf:params:xml:ns:caldav" are referenced in this document
   outside of the context of an XML fragment, the string "DAV:" and
   "CALDAV:" will be prefixed to the element type names respectively.

   The namespace "http://calendarserver.org/ns/" is used for XML
   elements defined in this specification.  When XML element types in



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   this namespace are referenced in this document outside of the context
   of an XML fragment, the string "CS:" will be prefixed to the element
   type names respectively.


3.  Overview

3.1.  Server

   For each calendar user principal on the server, the server will
   generate two group principals - "proxy groups".  One is used to hold
   the list of principals who have read-only proxy access to the main
   principal's calendars, the other holds the list of principals who
   have read-write and scheduling proxy access.  NB these new group
   principals would have no equivalent in Open Directory.

   Privileges on each "proxy group" principal will be set so that the
   "owner" has the ability to change property values.

   The "proxy group" principals will be child resources of the user
   principal resource with specific resource types and thus are easy to
   discover.  As a result the user principal resources will also be
   collection resources.

   When provisioning the calendar user home collection, the server will:

   a.  Add an ACE to the calendar home collection giving the read-only
       "proxy group" inheritable read access.

   b.  Add an ACE to the calendar home collection giving the read-write
       "proxy group" inheritable read-write access.

   c.  Add an ACE to each of the calendar Inbox and Outbox collections
       giving the CALDAV:schedule privilege [RFC6638] to the read-write
       "proxy group".

   On each user principal resource, the server maintains two WebDAV
   properties containing lists of other user principals for which the
   target principal is a read-only or read-write proxy.

3.2.  Client

   A client can see who the proxies are for the current principal by
   examining the principal resource for the two "proxy group" properties
   and then looking at the DAV:group-member-set property of each.

   The client can edit the list of proxies for the current principal by
   editing the DAV:group-member-set property on the relevant "proxy



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   group" principal resource.

   The client can find out who the current principal is a proxy for by
   examining the CS:calendar-proxy-read-for and CS:calendar-proxy-write-
   for properties, possibly using the DAV:expand-property REPORT to get
   other useful properties about the principals being proxied for.


4.  Open Issues

   1.  Do we want to separate read-write access to calendars vs the
       ability to schedule as a proxy?

   2.  We may want to restrict changing properties on the proxy group
       collections to just the DAV:group-member-set property?

   3.  There is no way for a proxy to be able to manage the list of
       proxies.  We could allow the main calendar user DAV:write-acl on
       their "proxy group" principals, in which case they could grant
       others the right to modify the group membership.

   4.  Should the "proxy group" principals also be collections given
       that the regular principal resources will be?


5.  New features in CalDAV

5.1.  Feature Discovery

   A server that supports the features described in this document MUST
   include "calendar-proxy" as a field in the DAV response header from
   an OPTIONS request on any resource that supports these features.

5.2.  Proxy Principal Resource

   Each "regular" principal resource that needs to allow calendar user
   proxy support MUST be a collection resource. i.e. in addition to
   including the DAV:principal XML element in the DAV:resourcetype
   property on the resource, it MUST also include the DAV:collection XML
   element.

   Each "regular" principal resource MUST contain two child resources
   with names "calendar-proxy-read" and "calendar-proxy-write" (note
   that these are only suggested names - the server could choose any
   unique name for these).  These resources are themselves principal
   resources that are groups contain the list of principals for calendar
   users who can act as a read-only or read-write proxy respectively.




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   The server MUST include the CS:calendar-proxy-read or CS:calendar-
   proxy-write XML elements in the DAV:resourcetype property of the
   child resources, respectively.  This allows clients to discover the
   "proxy group" principals by using a PROPFIND, Depth:1 request on the
   current user's principal resource and requesting the DAV:resourcetype
   property be returned.  The element type declarations are:

   <!ELEMENT calendar-proxy-read EMPTY>

   <!ELEMENT calendar-proxy-write EMPTY>

   The server MUST allow the "parent" principal to change the DAV:group-
   member-set property on each of the "child" "proxy group" principal
   resources.  When a principal is listed as a member of the "child"
   resource, the server MUST include the "child" resource URI in the
   DAV:group-membership property on the included principal resource.
   Note that this is just "normal" behavior for a group principal.

   An example principal resource layout might be:

           + /
             + principals/
               + users/
                 + cyrus/
                     calendar-proxy-read
                     calendar-proxy-write
                 + red/
                     calendar-proxy-read
                     calendar-proxy-write
                 + wilfredo/
                     calendar-proxy-read
                     calendar-proxy-write

   If the principal "cyrus" wishes to have the principal "red" act as a
   calendar user proxy on his behalf and have the ability to change
   items on his calendar or schedule meetings on his behalf, then he
   would add the principal resource URI for "red" to the DAV:group-
   member-set property of the principal resource /principals/users/
   cyrus/calendar-proxy-write, giving:

   <DAV:group-member-set>
     <DAV:href>/principals/users/red/</DAV:href>
   </DAV:group-member-set>

   The DAV:group-membership property on the resource /principals/users/
   red/ would be:





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   <DAV:group-membership>
     <DAV:href>/principals/users/cyrus/calendar-proxy-write</DAV:href>
   </DAV:group-membership>

   If the principal "red" was also a read-only proxy for the principal
   "wilfredo", then the DA:group-membership property on the resource
   /principals/users/red/ would be:

   <DAV:group-membership>
     <DAV:href>/principals/users/cyrus/calendar-proxy-write</DAV:href>
     <DAV:href>/principals/users/wilfredo/calendar-proxy-read</DAV:href>
   </DAV:group-membership>

   Thus a client can discover to which principals a particular principal
   is acting as a calendar user proxy for by examining the DAV:group-
   membership property.

   An alternative to discovering which principals a user can proxy as is
   to use the WebDAV ACL principal-match report, targeted at the
   principal collections available on the server.

   Example:

   >> Request <<

   REPORT /principals/ HTTP/1.1
   Host: cal.example.com
   Depth: 0
   Content-Type: application/xml; charset="utf-8"
   Content-Length: xxxx
   Authorization: Digest username="red",
    realm="cal.example.com", nonce="...",
    uri="/principals/", response="...", opaque="..."

   <?xml version="1.0" encoding="utf-8" ?>
   <D:principal-match xmlns:D="DAV:">
     <D:self/>
     <D:prop>
       <D:resourcetype/>
     </D:prop>
   </D:principal-match>










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   >> Response <<

   HTTP/1.1 207 Multi-Status
   Date: Fri, 10 Nov 2006 09:32:12 GMT
   Content-Type: application/xml; charset="utf-8"
   Content-Length: xxxx

   <?xml version="1.0" encoding="utf-8" ?>
   <D:multistatus xmlns:D="DAV:"
                  xmlns:A="http://calendarserver.org/ns/">
     <D:response>
       <D:href>/principals/users/red/</D:href>
       <D:propstat>
         <D:prop>
           <D:resourcetype>
             <D:principal/>
             <D:collection/>
           </D:resourcetype>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
     </D:response>
     <D:response>
       <D:href>/principals/users/cyrus/calendar-proxy-write</D:href>
       <D:propstat>
         <D:prop>
           <D:resourcetype>
             <D:principal/>
             <A:calendar-proxy-write/>
           </D:resourcetype>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
     </D:response>
     <D:response>
       <D:href>/principals/users/wilfredo/calendar-proxy-read</D:href>
       <D:propstat>
         <D:prop>
           <D:resourcetype>
             <D:principal/>
             <A:calendar-proxy-read/>
           </D:resourcetype>
         </D:prop>
         <D:status>HTTP/1.1 200 OK</D:status>
       </D:propstat>
     </D:response>
   </D:multistatus>




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5.3.  New Principal Properties

   Each "regular" principal that is a proxy for other principals MUST
   have the CS:calendar-proxy-read-for and CS:calendar-proxy-write-for
   WebDAV properties available on its principal resource, to allow
   clients to quickly find the "proxy for" information.

5.3.1.  CS:calendar-proxy-read-for Property

   Name:  calendar-proxy-read-for

   Namespace:  http://calendarserver.org/ns/

   Purpose:  Lists principals for whom the current principal is a read-
      only proxy for.

   Protected:  This property MUST be protected.

   PROPFIND behavior:  This property SHOULD NOT be returned by a
      PROPFIND allprop request (as defined in Section 14.2 of
      [RFC4918]).

   Description:  This property allows a client to quickly determine the
      principal for whom the current principal is a read-only proxy for.
      The server MUST account for any group memberships of the current
      principal that are either direct or indirect members of a proxy
      group. e.g., if principal "A" assigns a group "G" as a read-only
      proxy, and principal "B" is a member of group "G", then principal
      "B" will see principal "A" listed in the CS:calendar-proxy-read-
      for property on their principal resource.

   Definition:

     <!ELEMENT calendar-proxy-read-for (DAV:href*)>

5.3.2.  CS:calendar-proxy-write-for Property

   Name:  calendar-proxy-write-for

   Namespace:  http://calendarserver.org/ns/

   Purpose:  Lists principals for whom the current principal is a read-
      write proxy for.

   Protected:  This property MUST be protected.






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   PROPFIND behavior:  This property SHOULD NOT be returned by a
      PROPFIND allprop request (as defined in Section 14.2 of
      [RFC4918]).

   Description:  This property allows a client to quickly determine the
      principal for whom the current principal is a read-write proxy
      for.  The server MUST account for any group memberships of the
      current principal that are either direct or indirect members of a
      proxy group. e.g., if principal "A" assigns a group "G" as a read-
      write proxy, and principal "B" is a member of group "G", then
      principal "B" will see principal "A" listed in the CS:calendar-
      proxy-write-for property on their principal resource.

   Definition:

     <!ELEMENT calendar-proxy-write-for (DAV:href*)>

5.4.  Privilege Provisioning

   In order for a calendar user proxy to be able to access the calendars
   of the user they are proxying for the server MUST ensure that the
   privileges on the relevant calendars are setup accordingly:

      The DAV:read privilege MUST be granted for read-only and read-
      write calendar user proxy principals

      The DAV:write privilege MUST be granted for read-write calendar
      user proxy principals.

   Additionally, the CalDAV scheduling Inbox and Outbox calendar
   collections for the user allowing proxy access, MUST have the CALDAV:
   schedule privilege [RFC6638] granted for read-write calendar user
   proxy principals.

   Note that with a suitable repository layout, a server may be able to
   grant the appropriate privileges on a parent collection and ensure
   that all the contained collections and resources inherit that.  For
   example, given the following repository layout:













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           + /
             + calendars/
               + users/
                 + cyrus/
                     inbox
                     outbox
                     home
                     work
                 + red/
                     inbox
                     outbox
                     work
                     soccer
                 + wilfredo/
                     inbox
                     outbox
                     home
                     work
                     flying

   In order for the principal "red" to act as a read-write proxy for the
   principal "cyrus", the following WebDAV ACE will need to be granted
   on the resource /calendars/users/cyrus/ and all children of that
   resource:

   <DAV:ace>
     <DAV:principal>
       <DAV:href>/principals/users/cyrus/calendar-proxy-write</DAV:href>
     </DAV:principal>
     <DAV:privileges>
       <DAV:grant><DAV:read/><DAV:write/></DAV:grant>
     </DAV:privileges>
   </DAV:ace>


6.  Security Considerations

   TBD


7.  IANA Considerations

   This document does not require any actions on the part of IANA.


8.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate



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              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2518]  Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D.
              Jensen, "HTTP Extensions for Distributed Authoring --
              WEBDAV", RFC 2518, February 1999.

   [RFC3744]  Clemm, G., Reschke, J., Sedlar, E., and J. Whitehead, "Web
              Distributed Authoring and Versioning (WebDAV)
              Access Control Protocol", RFC 3744, May 2004.

   [RFC4791]  Daboo, C., Desruisseaux, B., and L. Dusseault,
              "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
              March 2007.

   [RFC4918]  Dusseault, L., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918, June 2007.

   [RFC6638]  Daboo, C. and B. Desruisseaux, "Scheduling Extensions to
              CalDAV", RFC 6638, June 2012.


Appendix A.  Acknowledgments

   This specification is the result of discussions between the Apple
   calendar server and client teams.


Appendix B.  Change History

   Changes in -03:

   1.  Added OPTIONS DAV header token.

   2.  Added CS:calendar-proxy-read-for and CS:calendar-proxy-write-for
       properties for faster discovery of proxy relationships.

   Changes in -02:

   1.  Updated to RFC 4791 reference.

   Changes in -01:

   1.  Added more details on actual CalDAV protocol changes.

   2.  Changed namespace from http://apple.com/ns/calendarserver/ to
       http://calendarserver.org/ns/.





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   3.  Made "proxy group" principals child resources of their "owner"
       principals.

   4.  The "proxy group" principals now have their own resourcetype.


Author's Address

   Cyrus Daboo
   Apple, Inc.
   1 Infinite Loop
   Cupertino, CA  95014
   USA

   Email: cyrus@daboo.name
   URI:   http://www.apple.com/



































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