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Versions: 00 01 02 03

Network Working Group                                           C. Daboo
Internet-Draft                                                     Apple
Intended status: Standards Track                             A. Quillaud
Expires: August 9, 2014                                           Oracle
                                                        February 5, 2014


                       CalDAV Managed Attachments
                   draft-daboo-caldav-attachments-03

Abstract

   This specification defines an extension to the calendar access
   protocol (CalDAV) to allow attachments associated with iCalendar
   data, to be stored and managed on the server.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on August 9, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  3
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  Discovering Support for Managed Attachments  . . . . . . .  4
     3.3.  POST Request for Managing Attachments  . . . . . . . . . .  5
     3.4.  Adding attachments . . . . . . . . . . . . . . . . . . . .  6
     3.5.  Updating Attachments . . . . . . . . . . . . . . . . . . .  9
     3.6.  Removing Attachments via POST  . . . . . . . . . . . . . . 12
     3.7.  Adding Existing Managed Attachments via PUT  . . . . . . . 13
     3.8.  Updating Attachments via PUT . . . . . . . . . . . . . . . 13
     3.9.  Removing Attachments via PUT . . . . . . . . . . . . . . . 14
     3.10. Retrieving Attachments . . . . . . . . . . . . . . . . . . 14
     3.11. Additional Considerations  . . . . . . . . . . . . . . . . 14
   4.  Modifications to iCalendar Syntax  . . . . . . . . . . . . . . 16
     4.1.  SIZE Property Parameter  . . . . . . . . . . . . . . . . . 16
     4.2.  FILENAME Property Parameter  . . . . . . . . . . . . . . . 17
     4.3.  MANAGED-ID Property Parameter  . . . . . . . . . . . . . . 17
   5.  Additional Message Header Fields . . . . . . . . . . . . . . . 18
     5.1.  Cal-Managed-ID Response Header . . . . . . . . . . . . . . 18
   6.  Additional WebDAV properties . . . . . . . . . . . . . . . . . 18
     6.1.  CALDAV:managed-attachments-server-URL property . . . . . . 18
     6.2.  CALDAV:max-attachment-size property  . . . . . . . . . . . 19
     6.3.  CALDAV:max-attachments-per-resource property . . . . . . . 20
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
     8.1.  Parameter Registrations  . . . . . . . . . . . . . . . . . 21
     8.2.  Message Header Field Registrations . . . . . . . . . . . . 22
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 22
     10.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Change History (To be removed by RFC Editor
                before publication) . . . . . . . . . . . . . . . . . 23
   Appendix B.  Example Involving Recurring Events  . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28













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1.  Introduction

   The iCalendar [RFC5545] data format is used to represent calendar
   data and is used with iTIP [RFC5546] to handle scheduling operations
   between calendar users.

   [RFC4791] defines the CalDAV Calendar Access protocol, based on HTTP
   [RFC2616], for accessing calendar data stored on a server.

   Calendar users often want to include attachments with their calendar
   data events or tasks (for example a copy of a presentation, or the
   meeting agenda). iCalendar provides an "ATTACH" property whose value
   is either the inline Base64 encoded attachment data, or a URL
   specifying the location of the attachment data.

   Use of inline attachment data is not ideal with CalDAV because the
   data would need to be uploaded to the server each time a change to
   the calendar data is done - even minor changes such as a change to
   the summary.  Whilst a client could choose to use a URL value
   instead, the problem then becomes where and how the client discovers
   an appropriate URL to use and how it ensures that only those
   attendees listed in the event or task are able to access it.

   This specification solves this problem by having the client send the
   attachment to the server, separately from the iCalendar data, and the
   server takes care of adding appropriate "ATTACH" properties in the
   iCalendar data as well as managing access privileges .  The server
   can also provide additional information to the client about each
   attachment in the iCalendar data, such as the size and an identifier.


2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   The notation used in this memo is the ABNF notation of [RFC5234] as
   used by iCalendar [RFC5545].  Any syntax elements shown below that
   are not explicitly defined in this specification come from iCalendar
   [RFC5545].


3.  Overview

   There are four main operations a client needs to do with attachments
   for calendar data: add, update, remove, and retrieve.  The first



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   three operations are carried out by the client issuing an HTTP POST
   request on the calendar object resource to which the attachment is
   associated and specifying the appropriate "action" query parameter.
   In the case of the remove operation, the client can alternatively
   directly update the calendar object resource and remove the relevant
   "ATTACH" properties.  The retrieve operation is accomplished by
   simply issuing an HTTP GET request targeting the attachment URI
   specified by the calendar resource's "ATTACH" property.

   iCalendar data stored in a CalDAV calendar object resource can
   contain multiple components when recurrences are involved.  In such a
   situation, the client needs to be able to target a specific
   recurrence instance or multiple instances when adding or deleting
   attachments.  As a result, the POST request needs to provide a way
   for the client to specify which recurrence instances should be
   targeted for the attachment operation.  This is accomplished through
   use of additional query parameters on the POST request-URI.

3.1.  Requirements

   A server that supports the features described in this specification
   is REQUIRED to support the CalDAV "calendar-access" [RFC4791]
   features.

   In addition, such a server MUST support the "return=representation"
   Prefer header value [I-D.snell-http-prefer] on successful HTTP PUT
   and POST requests targeting existing calendar object resources, by
   returning the new representation of that calendar resource (including
   its new Etag header value) in the response.

3.2.  Discovering Support for Managed Attachments

   A server supporting the features described in this specification MUST
   include "calendar-managed-attachments" as a field in the DAV response
   header from an OPTIONS request on a calendar home collection.

   A server might choose to not support storing managed attachments on a
   per-recurrence instance basis (i.e., they can only be added to all
   instances as a whole).  If that is the case, the server MUST include
   "calendar-managed-attachments-no-recurrence" as a field in the DAV
   response header from an OPTIONS request on a calendar home
   collection.  When that field is present, clients MUST NOT attempt any
   managed attachment operations that target specific recurrence
   instances.







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3.3.  POST Request for Managing Attachments

   An HTTP POST request is used to add, update, or remove attachments.
   The request-URI will contain various query parameters to specify the
   behavior.

3.3.1.  action= Query Parameter

   The "action" query parameter is used to identify which attachment
   operation the client is requesting.  This parameter MUST be present
   once on each POST request used to manage attachments.  One of these
   three values MUST be used:

   attachment-add  Indicates an operation that is adding an attachment
      to a calendar object resource.  See Section 3.4 for more details.

   attachment-update  Indicates an operation that is updating an
      existing attachment on a calendar object resource.  See
      Section 3.5 for more details.

   attachment-remove  Indicates an operation that is removing an
      attachment on a calendar object resource.  See Section 3.6 for
      more details.

   Example:
   http://calendar.example.com/events/1.ics?action=attachment-add

3.3.2.  rid= Query Parameter

   The "rid" query parameter is used to identify which recurrence
   instances are being targeted by the client for the attachment
   operation.  This query parameter MUST contain one or more items,
   separated by commas (0x2C).  The item values can be in one of two
   forms:

   Master instance  The value "M" (case-insensitive) refers to the
      "master" recurrence instance, i.e., the component that does not
      include a "RECURRENCE-ID" property.  This item MUST be present
      only once.

   Specific instance  A specific iCalendar instance is targeted by using
      its "RECURRENCE-ID" value as the item value.  That value MUST
      correspond to the RECURRENCE-ID value as stored in the calendar
      object resource (i.e. without any conversion to UTC).  If multiple
      items of this form are used, they MUST be unique values.

   If the "rid" query parameter is not present, all recurrence instances
   in the calendar object resource are targeted.



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   The "rid" query parameter MUST NOT be present in the case of an
   update operation, or if the server chooses not to support per-
   recurrence instance managed attachments (see Section 3.1).

   Example:
   http://calendar.example.com/events/1.ics?
     action=attachment-add&rid=M,20111022T160000

3.3.3.  managed-id= Query Parameter

   The "managed-id" query parameter is used to identify which "ATTACH"
   property is being updated or removed.  The value of this query
   parameter MUST match the "MANAGED-ID" property parameter value on the
   "ATTACH" property in the calendar object resource instance(s)
   targeted by the request.

   The "managed-id" query parameter MUST NOT be present in the case of
   an add operation.

   Example:
   http://calendar.example.com/events/1.ics?
     action=attachment-update&managed-id=aUNhbGVuZGFy

3.4.  Adding attachments

   To add an attachment to an existing calendar object resource, the
   following occurs:

   1.  The client issues a POST request targeted at the calendar object
       resource.

       A.  The request-URI will include an "action" query parameter with
           the value "attachment-add" (see Section 3.3.1).

       B.  If all recurrence instances are having an attachment added,
           the "rid" query parameter is not present in the request-URI.
           If one or more specific recurrence instances are targeted,
           then the request-URI will include a "rid" query parameter
           containing the list of instances (see Section 3.3.2).

       C.  The body of the request contains the data for the attachment.

       D.  The client MUST include a valid Content-Type HTTP header
           describing the media type of the attachment (as required by
           HTTP).

       E.  The client SHOULD include a Content-Disposition HTTP header
           [RFC6266] with a "type" parameter set to "attachment", and a



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           "filename" parameter that indicates the name of the
           attachment.

   2.  When the server receives the POST request it does the following:

       A.  Validates that any recurrence instances referred to via the
           "rid" query parameter are valid for the calendar object
           resource being targeted.

       B.  Stores the supplied attachment data into a resource and
           generates an appropriate URI for clients to access the
           resource.

       C.  For each affected recurrence instance in the calendar object
           resource targeted by the request, the server adds an "ATTACH"
           property, whose value is the URI of the stored attachment.
           The "ATTACH" property MUST contain a "MANAGED-ID" parameter
           whose value is a unique identifier (within the context of the
           server as a whole).  The "ATTACH" property SHOULD contain an
           "FMTTYPE" parameter whose value matches the Content-Type
           header value from the request.  The "ATTACH" property SHOULD
           contain an "FILENAME" parameter whose value matches the
           Content-Disposition header "filename" parameter value from
           the request, taking into account the restrictions expressed
           in Section 4.2.  The "ATTACH" property SHOULD include a
           "SIZE" parameter whose value represents the size in octets of
           the attachment.  If a specified recurrence instance does not
           have a matching component in the calendar object resource,
           then the server MUST modify the calendar object resource to
           include the overridden component with the appropriate
           "RECURRENCE-ID" property included.

       D.  Upon successful creation of the attachment resource, and
           modification of the targeted calendar object resource, the
           server MUST return an appropriate HTTP success status
           response and include a "Cal-Managed-ID" HTTP header
           containing the "MANAGED-ID" parameter value of the newly
           created "ATTACH" property.  The client can use the "Cal-
           Managed-ID" header value to correlate the attachment with
           "ATTACH" properties added to the calendar object resource.
           It is expected that the client will immediately reload the
           calendar object resource to refresh any local cache, or use
           the Prefer header "return=representation" option
           [I-D.snell-http-prefer] to have the server return the
           modified calendar object resource data in the HTTP response.

   In the following example, the client adds a new attachment to a non
   recurring event and asks the server (via the Prefer



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   [I-D.snell-http-prefer] HTTP header) to return the updated version of
   that event in the response.

   >> Request <<


   POST /events/64.ics?action=attachment-add HTTP/1.1
   Host: cal.example.com
   Content-Type: text/html; charset="utf-8"
   Content-Disposition:attachment;filename=agenda.html
   Content-Length: xxxx
   Prefer: return=representation

   <html>
     <body>
       <h1>Agenda</h1>
     </body>
   </html>



   >> Response <<


   HTTP/1.1 201 Created
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: yyyy
   Content-Location: http://cal.example.com/events/64.ics
   ETag: "123456789-000-111"
   Cal-Managed-ID: 97S

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Server//EN
   BEGIN:VEVENT
   UID:20010712T182145Z-123401@example.com
   DTSTAMP:20120201T203412Z
   DTSTART:20120714T170000Z
   DTEND:20120715T040000Z
   SUMMARY:One-off meeting
   ATTACH;MANAGED-ID=97S;FMTTYPE=text/html;SIZE=xxxx;
    FILENAME=agenda.html:https://cal.example.com/attach/64/34X22R
   END:VEVENT
   END:VCALENDAR







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3.5.  Updating Attachments

   When an attachment is updated the server MUST change the associated
   "MANAGED-ID" parameter and MAY change the "ATTACH" property value.
   With this approach, clients are able to determine when an attachment
   has been updated by some other client by looking for a change to
   either the "ATTACH" property value, or the "MANAGED-ID" parameter
   value.

   To change the data of an existing managed attachment in a calendar
   object resource, the following occurs:

   1.  The client issues a POST request targeted at the calendar object
       resource.

       A.  The request-URI will include an "action" query parameter with
           the value "attachment-update" (see Section 3.3.1).

       B.  The request-URI will include a "managed-id" query parameter
           with the value matching that of the "MANAGED-ID" parameter
           for the "ATTACH" property being updated (see Section 3.3.3).

       C.  The body of the request contains the updated data for the
           attachment.

       D.  The client MUST include a valid Content-Type header
           describing the media type of the attachment (as required by
           HTTP).

       E.  The client SHOULD include a Content-Disposition header
           [RFC6266] with a "type" parameter set to "attachment", and a
           "filename" parameter that indicates the name of the
           attachment.

   2.  When the server receives the POST request it does the following:

       A.  Validates that the "managed-id" query parameter is valid for
           the calendar object resource.

       B.  Updates the content of the attachment resource corresponding
           to that managed-id with the supplied attachment data.

       C.  For each affected recurrence instance in the calendar object
           resource targeted by the request, the server updates the
           "ATTACH" property whose "MANAGED-ID" property parameter value
           matches the "managed-id" query parameter.  The "MANAGED-ID"
           parameter value is changed to allow other clients to detect
           the update, and the property value (attachment URI) might



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           also be changed.  The "ATTACH" property SHOULD contain a
           "FMTTYPE" parameter whose value matches the Content-Type
           header value from the request - this could differ from the
           original value if the media type of the updated attachment is
           different.  The "ATTACH" property SHOULD contain a "FILENAME"
           parameter whose value matches the Content-Disposition header
           "filename" parameter value from the request, taking into
           account the restrictions expressed in Section 4.2.  The
           "ATTACH" property SHOULD include a "SIZE" parameter whose
           value represents the size in octets of the updated
           attachment.

       D.  Upon successful update of the attachment resource, and
           modification of the targeted calendar object resource, the
           server MUST return an appropriate HTTP success status
           response, and include a "Cal-Managed-ID" HTTP header
           containing the new value of the "MANAGED-ID" parameter.  It
           is expected that the client will immediately reload the
           calendar object resource to refresh any local cache, or use
           the Prefer header "return=representation" option
           [I-D.snell-http-prefer] to have the server return the
           modified calendar object resource data in the HTTP response.

   The update operation does not take a "rid" parameter and does not
   add, or remove, any "ATTACH" property in the targetted calendar
   object resource.  To link an existing attachment to a new instance,
   the client simply does a PUT on the calendar object resource, adding
   an "ATTACH" property which duplicates the existing one (see
   Section 3.7).

   In the following example, the client updates an existing attachment
   and asks the server (via the Prefer [I-D.snell-http-prefer] HTTP
   header) to return the updated version of that event in the response.


















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   >> Request <<


   POST /events/64.ics?action=attachment-update&managed-id=97S HTTP/1.1
   Host: cal.example.com
   Content-Type: text/html; charset="utf-8"
   Content-Disposition:attachment;filename=agenda.html
   Content-Length: xxxx
   Prefer: return=representation

   <html>
     <body>
       <h1>Agenda</h1>
       <p>Discuss attachment draft</p>
     </body>
   </html>



   >> Response <<


   HTTP/1.1 200 OK
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: yyyz
   Content-Location: http://cal.example.com/events/64.ics
   Cal-Managed-ID: 98S
   ETag: "123456789-000-222"

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Server//EN
   BEGIN:VEVENT
   UID:20010712T182145Z-123401@example.com
   DTSTAMP:20120201T203412Z
   DTSTART:20120714T170000Z
   DTEND:20120715T040000Z
   SUMMARY:One-off meeting
   ATTACH;MANAGED-ID=98S;FMTTYPE=text/html;SIZE=xxxy;
    FILENAME=agenda.html:https://cal.example.com/attach/64/34X22R
   END:VEVENT
   END:VCALENDAR









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3.6.  Removing Attachments via POST

   To remove an existing attachment from a calendar object, the
   following occurs:

   1.  The client issues a POST request targeted at the calendar object
       resource.

       A.  The request-URI will include an "action" query parameter with
           the value "attachment-remove" (see Section 3.3.1).

       B.  If all recurrence instances are having an attachment removed,
           the "rid" query parameter is not present in the request-URI.
           If one or more specific recurrence instances are targeted,
           then the request-URI will include a "rid" query parameter
           containing the list of instances (see Section 3.3.2).

       C.  The request-URI will include a "managed-id" query parameter
           with the value matching that of the "MANAGED-ID" property
           parameter for the "ATTACH" property being removed (see
           Section 3.3.3).

       D.  The body of the request will be empty.

   2.  When the server receives the POST request it does the following:

       A.  Validates that any recurrence instances referred to via the
           "rid" query parameter are valid for the calendar object
           resource being targeted.

       B.  Validates that the "managed-id" query parameter is valid for
           the calendar object resource and specific instances being
           targeted.

       C.  For each affected recurrence instance in the calendar object
           resource targeted by the request, the server removes the
           matching "ATTACH" property.  Note that if a specified
           recurrence instance does not have a matching component in the
           calendar object resource, then the server MUST modify the
           calendar object resource to include the overridden component
           with the appropriate "RECURRENCE-ID" property included, and
           the matching "ATTACH" property removed.  This later case is
           actually valid only if the master component does include the
           referenced "ATTACH" property.

       D.  If the attachment resource is no longer referenced by any
           instance of the calendar object resource, the server can
           delete the attachment resource to free up storage space.



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       E.  Upon successful removal of the attachment resource and
           modification of the targeted calendar object resource, the
           server MUST return an appropriate HTTP success status
           response.  It is expected that the client will immediately
           reload the calendar object resource to refresh any local
           cache, or use the Prefer header "return=representation"
           option [I-D.snell-http-prefer] to have the server return the
           modified calendar object resource data in the HTTP response.

   In the following example, the client deletes an existing attachment
   by passing its managed-id in the request.  The Prefer
   [I-D.snell-http-prefer] HTTP header is not set in the request so the
   calendar object resource data is not returned in the response.

   >> Request <<


   POST /events/64.ics?action=attachment-remove&managed-id=98S HTTP/1.1
   Host: cal.example.com
   Content-Length: 0



   >> Response <<


   HTTP/1.1 204 No Content
   Content-Length: 0



3.7.  Adding Existing Managed Attachments via PUT

   Clients can make use of existing managed attachments by adding the
   corresponding "ATTACH" property to calendar object resources (subject
   to the restrictions described in Section 3.11.3).  When this occurs,
   servers SHOULD NOT change either the "MANAGED-ID" parameter value or
   the "ATTACH" property value for any managed attachments - this
   ensures that clients do not have to download the attachment data
   again if they already have it cached, because it is used in another
   calendar resource.

3.8.  Updating Attachments via PUT

   Servers MUST NOT allow clients to update attachment data directly via
   a PUT on the attachment URI (or via any other HTTP method that
   modifies content).  Instead, attachments can only be manipulated via
   use of POST requests on the calendar data.



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3.9.  Removing Attachments via PUT

   Clients can remove attachments by simply re-writing the calendar
   object resource data to remove the appropriate "ATTACH" properties.
   Servers MUST NOT allow clients to delete attachments directly via a
   DELETE request on the attachment URI.

3.10.  Retrieving Attachments

   Clients retrieve attachments by issuing an HTTP GET request using the
   value of the corresponding "ATTACH" property as the request-URI,
   taking into account the substitution mechanism associated with the
   "CALDAV:managed-attachments-server-URL" property (see Section 6.1).

3.11.  Additional Considerations

3.11.1.  Error Handling

   This specification creates additional preconditions for the POST
   method.

   The new preconditions are:

      (CALDAV:max-attachment-size): The attachment submitted in the POST
      request MUST have an octet size less than or equal to the value of
      the CALDAV:max-attachment-size property value (Section 6.2) on the
      calendar collection of the target calendar resource;

      (CALDAV:max-attachments-per-resource): The addition of the
      attachment submitted in the POST request MUST result in the target
      calendar resource having a number of managed attachments less than
      or equal to the value of the CALDAV:max-attachments-per-resource
      property value (Section 6.3) on the calendar collection of the
      target calendar resource;

      (CALDAV:valid-managed-id): The managed-id POST request query
      parameter MUST contain a value corresponding to a "MANAGED-ID"
      property parameter value in the iCalendar data targeted by the
      request.

   A POST request to add, modify, or delete a managed attachment results
   in an implicit modification of the targeted calendar resource
   (equivalent of a PUT).  As a consequence, clients should also be
   prepared to handle preconditions associated with this implicit PUT.
   This includes (but is not limited to):

      (CALDAV:max-resource-size) (from Section 5.3.2.1 of [RFC4791])




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      (DAV:quota-not-exceeded) (from Section 6 of [RFC4331])

      (DAV:sufficient-disk-space) (from Section 6 of [RFC4331])

   A PUT request to add or modify and existing calendar object resource
   can make reference to a managed attachment.  The following new
   preconditions is defined:

      (CALDAV:valid-managed-id-parameter): a "MANAGED-ID" property
      parameter value in the iCalendar data in the PUT request is not
      valid (e.g., does not match any existing managed attachment).

3.11.2.  Quotas

   The WebDAV Quotas [RFC4331] specification defines two live WebDAV
   properties (DAV:quota-available-bytes and DAV:quota-used-bytes) to
   communicate storage quota information to clients.  Server
   implementations MAY choose to include managed attachments sizes when
   calculating the amount of storage used by a particular resource.

3.11.3.  Access Control

   Access to the managed attachments store in a calendar object resource
   SHOULD be restricted to only those calendar users who have access to
   that calendar object either directly, or indirectly (via being an
   attendee who would receive a scheduling message).

   When accessing a managed attachment, clients SHOULD be prepared to
   authenticate with the server storing the attachment resource.  The
   credentials required to access the managed attachment store could be
   different from the ones used to access the CalDAV server.

   This specification only allows organizers of scheduled events to add
   managed attachments.  Servers MUST prevent attendees of scheduled
   events from adding, updating or removing managed attachments.  In
   addition, the server MUST prevent a calendar user from re-using a
   managed attachment (based on its managed-id value), unless that user
   is the one who originally created the managed attachment.

3.11.4.  Redirects

   For POST requests that add or update attachment data, the server MAY
   issue an HTTP redirect to require the client to re-issue the POST
   request using a different request-URI.  As a result, it is always
   best for clients to use the "100 Continue" behavior defined in
   Section 8.2.3 of [RFC2616].  Using this mechanism ensures that, if a
   redirect does occur, the client does not needlessly send the
   attachment data.



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3.11.5.  Automatic Clean-up by servers

   Servers MAY automatically remove attachment data, for example to
   regain the storage taken by unused attachments, or as the result of a
   virus scanning.  When doing so they SHOULD NOT modify calendar data
   referencing those attachments.  Instead they SHOULD return a 410 HTTP
   status response to any request on the removed attachment URI.

3.11.6.  Sending Scheduling Messages with Attachments

   When a managed attachment is added, updated or removed from a
   calendar object resource, the server MUST ensure that a scheduling
   message is sent to update any attendees with the changes, as per
   [RFC6638].

3.11.7.  Other Client Considerations

   Clients can expect servers to take a while to respond to POST
   requests that include large attachment bodies.  Servers SHOULD use
   the "100 Continue" behavior defined in Section 8.2.3 of [RFC2616] to
   keep the client connection alive if the response will take some time.

   When exporting calendar data from a CalDAV server supporting managed
   attachments, clients SHOULD remove all "MANAGED-ID" property
   parameters from "ATTACH" properties in the calendar data.  Similarly
   when importing calendar data from another source, clients SHOULD
   remove any "MANAGED-ID" property parameters on "ATTACH" properties
   (failure to do so will likely result in the server removing those
   properties automatically).


4.  Modifications to iCalendar Syntax

4.1.  SIZE Property Parameter

   Parameter Name:  SIZE

   Purpose:  To specify the size of an attachment.

   Format Definition:  This property parameter is defined by the
      following notation:

   sizeparam = "SIZE" "=" paramtext
   ; positive integers







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   Description:  This property parameter MAY be specified on "ATTACH"
      properties.  It indicates the size in octets of the corresponding
      attachment data.  Since iCalendar integer values are restricted to
      a maximum value of 2147483647, the current parameter is defined as
      text to allow an extended range to be used.

   Example:

   ATTACH;SIZE=1234:http://attachments.example.com/abcd.txt

4.2.  FILENAME Property Parameter

   Parameter Name:  FILENAME

   Purpose:  To specify the file name of a managed attachment.

   Format Definition:  This property parameter is defined by the
      following notation:

   filenameparam = "FILENAME" "=" paramtext

   Description:  This property parameter MAY be specified on "ATTACH"
      properties corresponding to managed attachments.  Its value
      provides information on how to construct a filename for storing
      the attachment data.  This parameter is very similar in nature to
      the Content-Disposition HTTP header "filename" parameter and
      exposes the same security risks.  As a consequence, clients MUST
      follow the guidelines expressed in Section 4.3 of [RFC6266] when
      consuming this parameter value.  Similarly, servers MUST follow
      those same guidelines before storing a value.

   Example:

   ATTACH;FILENAME=agenda.html:http://attachments.example.c
    om/rt452S

4.3.  MANAGED-ID Property Parameter

   Parameter Name:  MANAGED-ID

   Purpose:  To uniquely identify a managed attachment.

   Format Definition:  This property parameter is defined by the
      following notation:

   managedidparam = "MANAGED-ID" "=" paramtext





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   Description:  This property parameter MUST be specified on "ATTACH"
      properties corresponding to managed attachments.  Its value is
      generated by the server and uniquely identifies a managed
      attachment.  This property parameter MUST NOT be present in the
      case of non managed attachments.

   Example:

   ATTACH;MANAGED-ID=aUNhbGVuZGFy:http://attachments.example.c
    om/abcd.txt


5.  Additional Message Header Fields

5.1.  Cal-Managed-ID Response Header

   The Cal-Managed-ID response header provides the value of the
   MANAGED-ID parameter corresponding to a newly added ATTACH property.
   It MUST be sent only in response to a successful POST request with an
   action set to attachment-add.

   Cal-Managed-ID = "Cal-Managed-ID" ":" paramtext
   ; "paramtext" is defined in Section 3.1 of [RFC5545]

   Example:

   Cal-Managed-ID:aUNhbGVuZGFy


6.  Additional WebDAV properties

6.1.  CALDAV:managed-attachments-server-URL property

   Name:  managed-attachments-server-URL

   Namespace:  urn:ietf:params:xml:ns:caldav

   Purpose:  Specifies the server base URI to use when retrieving
      managed attachments.

   Protected:  This property MUST be protected as only the server can
      update the value.

   COPY/MOVE behavior:  This property is only defined on a calendar home
      collection which cannot be moved or copied.






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   allprop behavior:  SHOULD NOT be returned by a PROPFIND DAV:allprop
      request.

   Description:  This property MAY be defined on a calendar home
      collection.  If present, it contains zero or one DAV:href XML
      elements.

      When one DAV:href element is present, its value MUST be an
      absolute HTTP URI containing only the scheme (i.e. "https") and
      authority (i.e. host and port) parts .  Whenever a managed
      attachment is to be retrieved via an HTTP GET, the client MUST
      construct the actual URL of the attachment by substituting the
      scheme and authority parts of the attachment URI (as stored in the
      iCalendar "ATTACH" property) with the present WebDAV property
      value.

      When no DAV:href element is present, the client MUST substitute
      the scheme and authority parts of the attachment URI with the
      scheme and authority part of the calendar home collection absolute
      URI.

      In the absence of this property, the client can consider the
      attachment URI as its actual URL.

   Definition:

   <!ELEMENT managed-attachments-server-URL (DAV:href?)>

   Example:

   <C:managed-attachments-server-URL xmlns:D="DAV:"
       xmlns:C="urn:ietf:params:xml:ns:caldav">
     <D:href>https://attachstore.example.com</D:href>
   </C:managed-attachments-server-URL>

6.2.  CALDAV:max-attachment-size property

   Name:  max-attachment-size

   Namespace:  urn:ietf:params:xml:ns:caldav

   Purpose:  Provides a numeric value indicating the maximum attachment
      size, in octets, that the server is willing to accept when a
      managed attachment is stored on the server.







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   Protected:  MUST be protected as it indicates limits provided by the
      server.

   COPY/MOVE behavior:  This property value MUST be preserved in COPY
      and MOVE operations.

   allprop behavior:  SHOULD NOT be returned by a PROPFIND DAV:allprop
      request.

   Description:  The CALDAV:max-attachment-size property is used to
      specify a numeric value that represents the maximum attachment
      size, in octets, that the server is willing to accept when a
      managed attachment is stored on the server.  The property is
      defined on the parent collection of the calendar object resource
      to which the attachment is associated.  Any attempt to store a
      managed attachment exceeding this size MUST result in an error,
      with the CALDAV:max-attachment-size precondition (Section 3.11.1)
      being violated.  In the absence of this property, the client can
      assume that the server will allow storing an attachment of any
      reasonable size.

   Definition:

   <!ELEMENT max-attachment-size (#PCDATA)>
   <!-- PCDATA value: a numeric value (positive decimal integer) -->

   Example:

   <C:max-attachment-size xmlns:C="urn:ietf:params:xml:ns:caldav"
        >102400000</C:max-attachment-size>

6.3.  CALDAV:max-attachments-per-resource property

   Name:  max-attachments-per-resource

   Namespace:  urn:ietf:params:xml:ns:caldav

   Purpose:  Provides a numeric value indicating the maximum number of
      managed attachments across all instances of a calendar object
      resource stored in a calendar collection.

   Protected:  MUST be protected as it indicates limits provided by the
      server.

   COPY/MOVE behavior:  This property value MUST be preserved in COPY
      and MOVE operations.





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   allprop behavior:  SHOULD NOT be returned by a PROPFIND DAV:allprop
      request.

   Description:  The CALDAV:max-attachments-per-resource property is
      used to specify a numeric value that represents the maximum number
      of managed attachments across all instances of a calendar object
      resource stored in a calendar collection.  Non managed attachments
      are not counted toward that limit.  The property is defined on the
      parent collection of the calendar object resource to which the
      attachment is associated.  Any attempt to add a managed attachment
      that would cause the calendar resource to exceed this limit MUST
      result in an error, with the CALDAV:max-attachments-per-resource
      precondition (Section 3.11.1) being violated.  In the absence of
      this property, the client can assume that the server can handle
      any number of managed attachments per calendar resource.

   Definition:

   <!ELEMENT max-attachments-per-resource (#PCDATA)>
   <!-- PCDATA value: a numeric value (positive decimal integer) -->

   Example:

   <C:max-attachments-per-resource
       xmlns:C="urn:ietf:params:xml:ns:caldav"
        >12</C:max-attachments-per-resource>


7.  Security Considerations

   Malicious content could be introduced into the Calendar Server by way
   of a managed attachment, and propagated to many end users via
   scheduling.  Servers SHOULD check managed attachments for malicious
   or inappropriate content.  Upon detecting of such content, servers
   SHOULD remove the attachment, following the rules described in
   Section 3.11.5.


8.  IANA Considerations

8.1.  Parameter Registrations

   This specification defines the following new iCalendar property
   parameters to be added to the registry defined in Section 8.2.3 of
   [RFC5545]:






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          +--------------------+---------+----------------------+
          | Property Parameter | Status  | Reference            |
          +--------------------+---------+----------------------+
          | FILENAME           | Current | RFCXXXX, Section 4.2 |
          | SIZE               | Current | RFCXXXX, Section 4.1 |
          | MANAGED-ID         | Current | RFCXXXX, Section 4.3 |
          +--------------------+---------+----------------------+

8.2.  Message Header Field Registrations

   The message header fields below should be added to the Permanent
   Message Header Field Registry (see [RFC3864]).

8.2.1.  Cal-Managed-ID

   Header field name: Cal-Managed-ID

   Applicable protocol: http

   Status: standard

   Author/Change controller: IETF

   Specification document(s): this specification (Section 5.1)

   Related information: none


9.  Acknowledgments

   This specification came about via discussions at the Calendaring and
   Scheduling Consortium.  Thanks in particular to Mike Douglass, Ken
   Murchison, and Eric York.


10.  References

10.1.  Normative References

   [I-D.snell-http-prefer]
              Snell, J., "Prefer Header for HTTP",
              draft-snell-http-prefer-18 (work in progress),
              January 2013.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,



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              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [RFC4331]  Korver, B. and L. Dusseault, "Quota and Size Properties fo
              r Distributed Authoring and Versioning (DAV) Collections",
              RFC 4331, February 2006.

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

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5545]  Desruisseaux, B., "Internet Calendaring and Scheduling
              Core Object Specification (iCalendar)", RFC 5545,
              September 2009.

   [RFC6266]  Reschke, J., "Use of the Content-Disposition Header Field
              in the Hypertext Transfer Protocol (HTTP)", RFC 6266,
              June 2011.

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

10.2.  Informative References

   [RFC5546]  Daboo, C., "iCalendar Transport-Independent
              Interoperability Protocol (iTIP)", RFC 5546,
              December 2009.


Appendix A.  Change History (To be removed by RFC Editor before
             publication)

   Changes in -03:

   1.  Fixed some examples.

   2.  Fixed return-representation -> return=representation.

   3.  Added statement that servers must not allow clients to DELETE
       attachments directly.




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   4.  Added new preconditions for valid managed-id values.

   5.  Filled out Access Control section.

   6.  Allow servers to not support per-instance attachments and
       advertise that fact to clients.

   Changes in -02:

   1.  MANAGED-ID changes on PUT.

   2.  MTAG has been removed.

   3.  Error pre-conditions added.

   4.  Interaction with WebDAV QUOTA discussed.

   5.  max-attachment-* limits added.

   6.  Updated references.

   7.  Removed MUST for specific 2xx codes in favor of generic success
       code.

   Changes in -01:

   1.   Tweaked OPTIONS capability wording.

   2.   Added section on clients expecting 100-Continue for delayed
        response.

   3.   Added text for clean-up and use of HTTP 410 on orphans.

   4.   Added text on removing "MANAGED-ID" when exporting/importing
        calendar data.

   5.   Added protocol examples.

   6.   Added MTAG property parameter on ATTACH property

   7.   Added FILENAME property parameter on ATTACH property

   8.   "id" query parameter is now "managed-id".

   9.   Use of Cal-Managed-ID header instead of Location header in
        responses.





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   10.  rid query param MUST contain RECURRENCE-ID without any
        conversion to UTC (case of floating events).

   11.  Introduced CALDAV:managed-attachments-server-URL property

   12.  Made support for Prefer header a MUST for servers.


Appendix B.  Example Involving Recurring Events

   In the following example, the organizer of a recurring meeting adds
   an agenda (HTML attachment) to the corresponding calendar resource.
   Attendees of the meeting are granted read access to the newly created
   attachment resource.  Their own copy of the meeting is updated to
   include the new ATTACH property pointing to the attachment resource
   and they are notified of the change via their scheduling inbox.

   >> Request <<


   POST /events/65.ics?action=attachment-add HTTP/1.1
   Host: cal.example.com
   Content-Type: text/html; charset="utf-8"
   Content-Disposition:attachment;filename=agenda.html
   Content-Length: xxxx
   Prefer: return=representation

   <html>
     <body>
       <h1>Agenda</h1>
       <p>As usual</p>
     </body>
   </html>



   >> Response <<














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   HTTP/1.1 201 Created
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: yyyy
   Content-Location: http://cal.example.com/events/65.ics
   ETag: "123456789-000-111"
   Cal-Managed-ID: 97S

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Server//EN
   BEGIN:VTIMEZONE
   LAST-MODIFIED:20040110T032845Z
   TZID:America/Montreal
   BEGIN:DAYLIGHT
   DTSTART:20000404T020000
   RRULE:FREQ=YEARLY;BYDAY=1SU;BYMONTH=4
   TZNAME:EDT
   TZOFFSETFROM:-0500
   TZOFFSETTO:-0400
   END:DAYLIGHT
   BEGIN:STANDARD
   DTSTART:20001026T020000
   RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=10
   TZNAME:EST
   TZOFFSETFROM:-0400
   TZOFFSETTO:-0500
   END:STANDARD
   END:VTIMEZONE
   BEGIN:VEVENT
   UID:20010712T182145Z-123401@example.com
   DTSTAMP:20120201T203412Z
   DTSTART;TZID=America/Montreal:20120206T100000
   DURATION:PT1H
   RRULE:FREQ=WEEKLY
   SUMMARY:Planning Meeting
   ORGANIZER:mailto:cyrus@example.com
   ATTENDEE;CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:mailto:cyrus@exampl
    e.com
   ATTENDEE;CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:mailto:arnaudq@exam
    ple.com
   ATTENDEE;CUTYPE=INDIVIDUAL;PARTSTAT=NEEDS-ACTION:mailto:mike@exa
    mple.com
   ATTACH;MANAGED-ID=97S;FMTTYPE=text/html;SIZE=xxxx;
    FILENAME=agenda.html:https://cal.example.com/attach/65/34X22R
   END:VEVENT
   END:VCALENDAR





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   The organizer has a more specific agenda for the 20th of February
   meeting.  It is added to that particular instance of the meeting by
   specifying the rid parameter.

   >> Request <<


  POST /events/65.ics?action=attachment-add&rid=20120220T100000 HTTP/1.1
  Host: cal.example.com
  Content-Type: text/html; charset="utf-8"
  Content-Disposition:attachment;filename=agenda0220.html
  Content-Length: xxxx
  Prefer: return=representation

  <html>
    <body>
      <h1>Agenda</h1>
      <p>Something different, for a change</p>
    </body>
  </html>



   >> Response <<


   HTTP/1.1 201 Created
   Content-Type: text/calendar; charset="utf-8"
   Content-Length: yyyy
   Content-Location: http://cal.example.com/events/65.ics
   ETag: "123456789-000-222"
   Cal-Managed-ID: 33225

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//Example Corp.//CalDAV Server//EN
   BEGIN:VTIMEZONE
   LAST-MODIFIED:20040110T032845Z
   TZID:America/Montreal
   BEGIN:DAYLIGHT
   DTSTART:20000404T020000
   RRULE:FREQ=YEARLY;BYDAY=1SU;BYMONTH=4
   TZNAME:EDT
   TZOFFSETFROM:-0500
   TZOFFSETTO:-0400
   END:DAYLIGHT
   BEGIN:STANDARD
   DTSTART:20001026T020000



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   RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=10
   TZNAME:EST
   TZOFFSETFROM:-0400
   TZOFFSETTO:-0500
   END:STANDARD
   END:VTIMEZONE
   BEGIN:VEVENT
   UID:20010712T182145Z-123401@example.com
   DTSTAMP:20120201T203412Z
   DTSTART;TZID=America/Montreal:20120206T100000
   DURATION:PT1H
   RRULE:FREQ=WEEKLY
   SUMMARY:Planning Meeting
   ORGANIZER:mailto:cyrus@example.com
   ATTENDEE;CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:mailto:cyrus@exampl
    e.com
   ATTENDEE;CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:mailto:arnaudq@exam
    ple.com
   ATTENDEE;CUTYPE=INDIVIDUAL;PARTSTAT=NEEDS-ACTION:mailto:mike@exa
    mple.com
   ATTACH;MANAGED-ID=97S;FMTTYPE=text/html;SIZE=xxxx;
    FILENAME=agenda.html:https://cal.example.com/attach/65/34X22R
   END:VEVENT
   BEGIN:VEVENT
   UID:20010712T182145Z-123401@example.com
   RECURRENCE-ID;TZID=America/Montreal:20120220T100000
   DTSTAMP:20120201T203412Z
   DTSTART;TZID=America/Montreal:20120220T100000
   DURATION:PT1H
   SUMMARY:Planning Meeting
   ORGANIZER:mailto:cyrus@example.com
   ATTENDEE;CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:mailto:cyrus@example.
    com
   ATTENDEE;CUTYPE=INDIVIDUAL;PARTSTAT=ACCEPTED:mailto:arnaudq@exampl
    e.com
   ATTENDEE;CUTYPE=INDIVIDUAL;PARTSTAT=NEEDS-ACTION:mailto:mike@examp
    le.com
   ATTACH;MANAGED-ID=33225;FMTTYPE=text/html;SIZE=xxxx;
    FILENAME=agenda0220.html:https://cal.example.com/attach/65/FGZ225
   END:VEVENT
   END:VCALENDAR










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Authors' Addresses

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

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


   Arnaud Quillaud
   Oracle Corporation
   180, Avenue de l'Europe
   Saint Ismier cedex,   38334
   France

   Email: arnaud.quillaud@oracle.com
   URI:   http://www.oracle.com/































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