]> CalDAV Managed Attachments Apple Inc.
1 Infinite Loop Cupertino CA 95014 USA cyrus@daboo.name http://www.apple.com/
Oracle Corporation
180, Avenue de l'Europe Saint Ismier cedex 38334 France arnaud.quillaud@oracle.com http://www.oracle.com/
Applications CalDAV calendaring attachment ATTACH 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.
The iCalendar data format is used to represent calendar data and is used with iTIP to handle scheduling operations between calendar users. defines the CalDAV Calendar Access protocol, based on HTTP, 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.
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 . The notation used in this memo is the ABNF notation of as used by iCalendar. Any syntax elements shown below that are not explicitly defined in this specification come from iCalendar.
There are four main operations a client needs to do with attachments for calendar data: add, update, remove, and retrieve. The first 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.
A server that supports the features described in this specification is REQUIRED to support the CalDAV "calendar-access" features. In addition, such a server MUST support the "return=representation" Prefer header value 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.
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.
An HTTP POST request is used to add, update, or remove attachments. The request-URI will contain various query parameters to specify the behavior.
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: Indicates an operation that is adding an attachment to a calendar object resource. See for more details. Indicates an operation that is updating an existing attachment on a calendar object resource. See for more details. Indicates an operation that is removing an attachment on a calendar object resource. See for more details.
Example: http://calendar.example.com/events/1.ics?action=attachment-add
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: 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. 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. 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 ).
Example: http://calendar.example.com/events/1.ics? action=attachment-add&rid=M,20111022T160000
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
To add an attachment to an existing calendar object resource, the following occurs: The client issues a POST request targeted at the calendar object resource. The request-URI will include an "action" query parameter with the value "attachment-add" (see ). 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 ). The body of the request contains the data for the attachment. The client MUST include a valid Content-Type HTTP header describing the media type of the attachment (as required by HTTP). The client SHOULD include a Content-Disposition HTTP header with a "type" parameter set to "attachment", and a "filename" parameter that indicates the name of the attachment. When the server receives the POST request it does the following: Validates that any recurrence instances referred to via the "rid" query parameter are valid for the calendar object resource being targeted. Stores the supplied attachment data into a resource and generates an appropriate URI for clients to access the resource. 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 . 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. 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 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 HTTP header) to return the updated version of that event in the response.
>> Request <<

Agenda

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>> Response <<
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: The client issues a POST request targeted at the calendar object resource. The request-URI will include an "action" query parameter with the value "attachment-update" (see ). 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 ). The body of the request contains the updated data for the attachment. The client MUST include a valid Content-Type header describing the media type of the attachment (as required by HTTP). The client SHOULD include a Content-Disposition header with a "type" parameter set to "attachment", and a "filename" parameter that indicates the name of the attachment. When the server receives the POST request it does the following: Validates that the "managed-id" query parameter is valid for the calendar object resource. Updates the content of the attachment resource corresponding to that managed-id with the supplied attachment data. 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 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 . The "ATTACH" property SHOULD include a "SIZE" parameter whose value represents the size in octets of the updated attachment. 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 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 ). In the following example, the client updates an existing attachment and asks the server (via the Prefer HTTP header) to return the updated version of that event in the response.
>> Request <<

Agenda

Discuss attachment draft

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>> Response <<
To remove an existing attachment from a calendar object, the following occurs: The client issues a POST request targeted at the calendar object resource. The request-URI will include an "action" query parameter with the value "attachment-remove" (see ). 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 ). 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 ). The body of the request will be empty. When the server receives the POST request it does the following: Validates that any recurrence instances referred to via the "rid" query parameter are valid for the calendar object resource being targeted. Validates that the "managed-id" query parameter is valid for the calendar object resource and specific instances being targeted. 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. 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. 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 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 HTTP header is not set in the request so the calendar object resource data is not returned in the response.
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Clients can make use of existing managed attachments by adding the corresponding "ATTACH" property to calendar object resources (subject to the restrictions described in ). 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.
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.
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.
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 ).
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 () 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 () 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 ) (DAV:quota-not-exceeded) (from Section 6 of ) (DAV:sufficient-disk-space) (from Section 6 of ) 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).
The WebDAV Quotas 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.
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.
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 . Using this mechanism ensures that, if a redirect does occur, the client does not needlessly send the attachment data.
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.
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 .
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 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).
SIZE To specify the size of an attachment. This property parameter is defined by the following notation:
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.
FILENAME To specify the file name of a managed attachment. This property parameter is defined by the following notation:
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 when consuming this parameter value. Similarly, servers MUST follow those same guidelines before storing a value.
MANAGED-ID To uniquely identify a managed attachment. This property parameter is defined by the following notation:
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.
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.
Example:
managed-attachments-server-URL urn:ietf:params:xml:ns:caldav Specifies the server base URI to use when retrieving managed attachments. This property MUST be protected as only the server can update the value. This property is only defined on a calendar home collection which cannot be moved or copied. SHOULD NOT be returned by a PROPFIND DAV:allprop request. 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.
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https://attachstore.example.com ]]>
max-attachment-size urn:ietf:params:xml:ns:caldav 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. MUST be protected as it indicates limits provided by the server. This property value MUST be preserved in COPY and MOVE operations. SHOULD NOT be returned by a PROPFIND DAV:allprop request. 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 () being violated. In the absence of this property, the client can assume that the server will allow storing an attachment of any reasonable size.
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102400000 ]]>
max-attachments-per-resource urn:ietf:params:xml:ns:caldav Provides a numeric value indicating the maximum number of managed attachments across all instances of a calendar object resource stored in a calendar collection. MUST be protected as it indicates limits provided by the server. This property value MUST be preserved in COPY and MOVE operations. SHOULD NOT be returned by a PROPFIND DAV:allprop request. 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 () 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.
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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 .
This specification defines the following new iCalendar property parameters to be added to the registry defined in Section 8.2.3 of : Property Parameter Status Reference FILENAME Current RFCXXXX, SIZE Current RFCXXXX, MANAGED-ID Current RFCXXXX,
The message header fields below should be added to the Permanent Message Header Field Registry (see ).
Header field name: Cal-Managed-ID Applicable protocol: http Status: standard Author/Change controller: IETF Specification document(s): this specification () Related information: none
This specification came about via discussions at the Calendaring and Scheduling Consortium. Thanks in particular to Mike Douglass, Ken Murchison, and Eric York.
&rfc2119; &rfc2616; &rfc3864; &rfc4331; &rfc4791; &rfc5234; &rfc5545;&rfc6266; &rfc6638;&idPreferHeader; &rfc5546;
Changes in -03: Fixed some examples. Fixed return-representation -> return=representation. Added statement that servers must not allow clients to DELETE attachments directly. Added new preconditions for valid managed-id values. Filled out Access Control section. Allow servers to not support per-instance attachments and advertise that fact to clients. Changes in -02: MANAGED-ID changes on PUT. MTAG has been removed. Error pre-conditions added. Interaction with WebDAV QUOTA discussed. max-attachment-* limits added. Updated references. Removed MUST for specific 2xx codes in favor of generic success code. Changes in -01: Tweaked OPTIONS capability wording. Added section on clients expecting 100-Continue for delayed response. Added text for clean-up and use of HTTP 410 on orphans. Added text on removing "MANAGED-ID" when exporting/importing calendar data. Added protocol examples. Added MTAG property parameter on ATTACH property Added FILENAME property parameter on ATTACH property "id" query parameter is now "managed-id". Use of Cal-Managed-ID header instead of Location header in responses. rid query param MUST contain RECURRENCE-ID without any conversion to UTC (case of floating events). Introduced CALDAV:managed-attachments-server-URL property Made support for Prefer header a MUST for servers.
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 <<

Agenda

As usual

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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 <<

Agenda

Something different, for a change

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