RFC 8607
Calendaring Extensions to WebDAV (CalDAV): Managed Attachments
Pages: 34
Informational
Informational
Part 1 of 2 – Pages 1 to 18
Internet Engineering Task Force (IETF) C. Daboo Request for Comments: 8607 Apple Category: Informational A. Quillaud ISSN: 2070-1721 Oracle K. Murchison, Ed. FastMail June 2019 Calendaring Extensions to WebDAV (CalDAV): Managed AttachmentsAbstract
This specification adds an extension to the Calendaring Extensions to WebDAV (CalDAV) to allow attachments associated with iCalendar data to be stored and managed on the server. This specification documents existing code deployed by multiple vendors. It is published as an Informational specification rather than Standards Track due to its noncompliance with multiple best current practices of HTTP. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are candidates for any level of Internet Standard; see Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8607.
Copyright Notice Copyright (c) 2019 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 (https://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 to this document. Code Components extracted from this document must 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.Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Rationale for Informational Status . . . . . . . . . . . 4 2. Conventions Used in This Document . . . . . . . . . . . . . . 4 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . 5 3.2. Discovering Support for Managed Attachments . . . . . . . 5 3.3. POST Request for Managing Attachments . . . . . . . . . . 6 3.3.1. action Query Parameter . . . . . . . . . . . . . . . 6 3.3.2. rid Query Parameter . . . . . . . . . . . . . . . . . 6 3.3.3. managed-id Query Parameter . . . . . . . . . . . . . 7 3.4. Adding Attachments . . . . . . . . . . . . . . . . . . . 7 3.5. Updating Attachments . . . . . . . . . . . . . . . . . . 10 3.6. Removing Attachments via POST . . . . . . . . . . . . . . 13 3.7. Adding Existing Managed Attachments via PUT . . . . . . . 15 3.8. Updating Attachments via PUT . . . . . . . . . . . . . . 15 3.9. Removing Attachments via PUT . . . . . . . . . . . . . . 15 3.10. Retrieving Attachments . . . . . . . . . . . . . . . . . 15 3.11. Error Handling . . . . . . . . . . . . . . . . . . . . . 16 3.12. Additional Considerations . . . . . . . . . . . . . . . . 17 3.12.1. Quotas . . . . . . . . . . . . . . . . . . . . . . . 17 3.12.2. Access Control . . . . . . . . . . . . . . . . . . . 17 3.12.3. Redirects . . . . . . . . . . . . . . . . . . . . . 18 3.12.4. Processing Time . . . . . . . . . . . . . . . . . . 18 3.12.5. Automatic Cleanup by Servers . . . . . . . . . . . . 18 3.12.6. Sending Scheduling Messages with Attachments . . . . 18 3.12.7. Migrating Calendar Data . . . . . . . . . . . . . . 18 4. Modifications to iCalendar Syntax . . . . . . . . . . . . . . 19 4.1. SIZE Property Parameter . . . . . . . . . . . . . . . . . 19 4.2. FILENAME Property Parameter . . . . . . . . . . . . . . . 19 4.3. MANAGED-ID Property Parameter . . . . . . . . . . . . . . 20 5. Additional Message Header Fields . . . . . . . . . . . . . . 20
5.1. Cal-Managed-ID Response Header Field . . . . . . . . . . 20
6. Additional WebDAV Properties . . . . . . . . . . . . . . . . 21
6.1. CALDAV:managed-attachments-server-URL Property . . . . . 21
6.2. CALDAV:max-attachment-size Property . . . . . . . . . . . 22
6.3. CALDAV:max-attachments-per-resource Property . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
8.1. Parameter Registrations . . . . . . . . . . . . . . . . . 24
8.2. Message Header Field Registrations . . . . . . . . . . . 25
8.2.1. Cal-Managed-ID . . . . . . . . . . . . . . . . . . . 25
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
9.1. Normative References . . . . . . . . . . . . . . . . . . 25
9.2. Informative References . . . . . . . . . . . . . . . . . 27
Appendix A. Example Involving Recurring Events . . . . . . . . . 28
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction
The iCalendar [RFC5545] data format is used to represent calendar
data and is used with iCalendar Transport-independent
Interoperability Protocol (iTIP) [RFC5546] to handle scheduling
operations between calendar users.
[RFC4791] defines the Calendaring Extensions to WebDAV (CalDAV),
based on HTTP [RFC7230], 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 made, 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
having the server add appropriate "ATTACH" properties in the
iCalendar data as well as manage 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.
1.1. Rationale for Informational Status
Although this extension to CalDAV has wide deployment, its design does not comply with some of the best current practices of HTTP, namely: o All operations on attachments are modeled as HTTP POST operations, where the actual type of operation is specified using a query parameter instead of using separate HTTP POST, PUT, and DELETE methods where appropriate. o Specific query strings are hardwired into the protocol in violation of Section 2.4 of [RFC7320]. Additionally, this extension misuses the Content-Disposition header field [RFC6266] as a request header field to convey a filename for an attachment rather than using an existing request header field suitable for that purpose, such as "Slug" (see Section 9.7 of [RFC5023]). Rather than creating interoperability problems with deployed code by updating the design of this extension to be compliant with best current practices and to allow this specification to be placed on the Standards Track, it was decided to simply document how existing implementations interoperate and to publish the document as Informational.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 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 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 perform 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 (see Section 3.3). In the case of the remove
operation, the client can alternatively directly update the calendar object resource and remove the relevant "ATTACH" properties (see Section 3.9). The retrieve operation is accomplished by simply issuing an HTTP GET request targeting the attachment URI specified by the calendar resource's "ATTACH" property (see Section 3.10). 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 SHOULD support the "return=representation" Prefer header field [RFC7240] preference 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 field 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 token in the DAV response header field (as defined in Section 10.1 of [RFC4918]) from an OPTIONS request on a calendar home collection. A server might choose not to support the storing of 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 also include "calendar-managed-attachments-no-recurrence" as a token in the DAV response header field 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.
3.3. POST Request for Managing Attachments
An HTTP POST request is used to add, update, or remove attachments. These requests are subject to the preconditions listed in Section 3.11. 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 from a calendar object resource. See Section 3.6 for more details. Example: https://calendar.example.com/events/1.ics?action=attachment-add3.3.2. 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 (denoted in ASCII as "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. For example, to target a recurrence defined by property
RECURRENCE-ID;TZID=America/Montreal:20111022T160000, the query
parameter rid=20111022T160000 would be used.
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 Section 3.2).
Example (targeting the master instance and a specific overridden
instance):
https://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" (Section 4.3) 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:
https://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 needs to occur:
1. The client issues a POST request targeted at the calendar object
resource structured as follows:
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 header field
describing the media type of the attachment (as required by
HTTP).
E. The client SHOULD include a Content-Disposition header field
[RFC6266] with a "type" parameter set to "attachment", and a
"filename" parameter that indicates the name of the
attachment. Note that the use of Content-Disposition as a
request header field is nonstandard and specific to this
protocol.
F. The client MAY include a Prefer header field [RFC7240] with
the "return=representation" preference to request that the
modified calendar object resource be returned as the body of
a successful response to the POST request.
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, adds an "ATTACH" property
whose value is the URI of the stored attachment. The
"ATTACH" property MUST contain a "MANAGED-ID" property
parameter whose value is a unique identifier (within the
context of the server as a whole). The "ATTACH" property
SHOULD contain an "FMTTYPE" property parameter whose value
matches the Content-Type header field value from the request.
The "ATTACH" property SHOULD contain a "FILENAME" property
parameter whose value matches the value of the Content-
Disposition header field "filename" parameter value from the
request, taking into account the restrictions expressed in
Section 4.2. The "ATTACH" property SHOULD include a "SIZE"
property 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 an overridden component with the
appropriate "RECURRENCE-ID" property.
D. Upon successful creation of the attachment resource, and
modification of the targeted calendar object resource, it
MUST return an appropriate HTTP success status response and
include a "Cal-Managed-ID" header field containing the
"MANAGED-ID" property parameter value of the newly created
"ATTACH" property. The client can use the "Cal-Managed-ID"
header field value to correlate the attachment with "ATTACH"
properties added to the calendar object resource. If the
client included a Prefer header field with the
"return=representation" preference in the request, the server
SHOULD return the modified calendar object resource as the
body of the response. Otherwise, the server can expect that
the client will reload the calendar object resource with a
subsequent GET request to refresh any local cache.
In the following example, the client adds a new attachment to a
nonrecurring event and asks the server (via the Prefer header field
[RFC7240]) to return the modified 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: 59
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: 371
Content-Location: https://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=59;
FILENAME=agenda.html:https://cal.example.com/attach/64/34X22R
END:VEVENT
END:VCALENDAR
3.5. Updating Attachments
When an attachment is updated, the server MUST change the associated
"MANAGED-ID" property 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"
property parameter value.
To change the data of an existing managed attachment in a calendar
object resource, the following needs to occur:
1. The client issues a POST request targeted at the calendar object
resource structured as follows:
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" property
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 field
describing the media type of the attachment (as required by
HTTP).
E. The client SHOULD include a Content-Disposition header field
[RFC6266] with a "type" parameter set to "attachment", and a
"filename" parameter that indicates the name of the
attachment.
F. The client MAY include a Prefer header field [RFC7240] with
the "return=representation" preference to request that the
modified calendar object resource be returned as the body of
a successful response to the POST request.
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" value with the supplied attachment data.
C. For each affected recurrence instance in the calendar object
resource targeted by the request, updates the "ATTACH"
property whose "MANAGED-ID" property parameter value matches
the "managed-id" query parameter. The "MANAGED-ID" property
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" property parameter whose value matches the Content-
Type header field 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" property parameter whose value matches
the Content-Disposition header field "filename" parameter
value from the request, taking into account the restrictions
expressed in Section 4.2. The "ATTACH" property SHOULD
include a "SIZE" property 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, it
MUST return an appropriate HTTP success status response and
include a "Cal-Managed-ID" header field containing the new
value of the "MANAGED-ID" property parameter. The client can
use the "Cal-Managed-ID" header field value to correlate the
attachment with "ATTACH" properties added to the calendar
object resource. If the client included a Prefer header
field with the "return=representation" preference in the
request, the server SHOULD return the modified calendar
object resource as the body of the response. Otherwise, the
server can expect that the client will reload the calendar
object resource with a subsequent GET request to refresh any
local cache.
The update operation does not take a "rid" query parameter and does
not add, or remove, any "ATTACH" property in the targeted 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 that 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 header field [RFC7240]) to return
the updated version of that event in the response.
>> 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: 96
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: 371
Content-Location: https://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=96;
FILENAME=agenda.html:https://cal.example.com/attach/64/34X22R
END:VEVENT
END:VCALENDAR
3.6. Removing Attachments via POST
To remove an existing attachment from a calendar object, the
following needs to occur:
1. The client issues a POST request targeted at the calendar object
resource structured as follows:
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.
E. The client MAY include a Prefer header field [RFC7240] with
the "return=representation" preference to request that the
modified calendar object resource be returned as the body of
a successful response to the POST request.
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, 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 an overridden component with the
appropriate "RECURRENCE-ID" property and the matching
"ATTACH" property removed. This latter 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, it can delete the
attachment resource to free up storage space.
E. Upon successful removal of the attachment resource and
modification of the targeted calendar object resource, it
MUST return an appropriate HTTP success status response. If
the client included a Prefer header field with the
"return=representation" preference in the request, the server
SHOULD return the modified calendar object resource as the
body of the response. Otherwise, the server can expect that
the client will reload the calendar object resource with a
subsequent GET request to refresh any local cache.
In the following example, the client deletes an existing attachment
by passing its "managed-id" value in the request. The Prefer header
field [RFC7240] 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: 03.7. 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 3.12.2). If a managed attachment is used in more than calendar resource, servers SHOULD NOT change either the "MANAGED-ID" property parameter value or the "ATTACH" property value for these attachments; this ensures that clients do not have to download the attachment data again if they already have it cached. Additionally, servers SHOULD validate "SIZE" property parameter values and replace incorrect values with the actual sizes of existing attachments. These PUT requests are subject to the preconditions listed in Section 3.11.3.8. 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 updated via use of POST requests on the calendar data.3.9. Removing Attachments via PUT
Clients can remove attachments by simply rewriting 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. 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-action): The "action" query parameter in the POST request MUST contain only one of the following three values: "attachment-add", "attachment-update", or "attachment-remove". (CALDAV:valid-rid): The "rid" query parameter in the POST request MUST NOT be present with an "action=attachment-update" query parameter and MUST contain the value "M" and/or values corresponding to "RECURRENCE-ID" property values in the iCalendar data targeted by the request. (CALDAV:valid-managed-id): The "managed-id" query parameter in the POST request MUST NOT be present with an "action=attachment-add" query parameter and 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]) (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 an existing calendar object resource
can make reference to an existing managed attachment. The following
new precondition 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).
If a precondition for a request is not satisfied:
1. The response status of the request MUST either be 403 (Forbidden)
if the request should not be repeated because it will always
fail, or 409 (Conflict) if it is expected that the user might be
able to resolve the conflict and resubmit the request.
2. The appropriate XML element MUST be returned as the child of a
top-level DAV:error element in the response body.
3.12. Additional Considerations
3.12.1. Quotas
The WebDAV Quotas specification [RFC4331] 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 attachment sizes when
calculating the amount of storage used by a particular resource.
3.12.2. Access Control
Access to the managed attachments referenced 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 reusing a
managed attachment (based on its "managed-id" value), unless that
user is the one who originally created the managed attachment.
3.12.3. Redirects
For POST requests that add or update attachment data, the server MAY issue a 307 (Temporary Redirect) [RFC7231] or 308 (Permanent Redirect) [RFC7538] response to require the client to reissue the POST request using a different Request-URI. As a result, clients SHOULD use the "100-continue" expectation defined in Section 5.1.1 of [RFC7231]. Using this mechanism ensures that, if a redirect does occur, the client does not needlessly send the attachment data.3.12.4. Processing Time
Clients can expect servers to take a while to respond to POST requests that include large attachment bodies. Servers SHOULD use the 102 (Processing) interim response defined in Section 10.1 of [RFC2518] to keep the client connection alive if the POST request will take significant time to complete.3.12.5. Automatic Cleanup 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 respond with 410 (Gone) to any request on the removed attachment URI.3.12.6. 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].3.12.7. Migrating Calendar Data
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).
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