wdiff rfc8607.original rfc8607.txt

Calendering Extensions
Internet Engineering Task Force (IETF) C. Daboo
Internet-Draft
Request for Comments: 8607 Apple
Intended status:
Category: Informational A. Quillaud
Expires: September 6, 2019
ISSN: 2070-1721 Oracle
K. Murchison, Ed.
FastMail
March 5,
June 2019

CalDAV

Calendaring Extensions to WebDAV (CalDAV): Managed Attachments
draft-ietf-calext-caldav-attachments-04

Abstract

This specification defines adds an extension to the calendar access
protocol 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 Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents valid
approved by the IESG are candidates for a maximum any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained 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 September 6, 2019.
https://www.rfc-editor.org/info/rfc8607.

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
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described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Rationale for Informational Status . . . . . . . . . . . 4 3
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 . . . . . . . . . . 5
3.3.1. action= action Query Parameter . . . . . . . . . . . . . . . 6 5
3.3.2. rid= rid Query Parameter . . . . . . . . . . . . . . . . . 6
3.3.3. managed-id= managed-id Query Parameter . . . . . . . . . . . . . 7
3.4. Adding attachments 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 . . . . . . . . . . . . . . . . . 16
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 Clean-Up 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 Property . . . . . 21
6.2. CALDAV:max-attachment-size property Property . . . . . . . . . . . 22
6.3. CALDAV:max-attachments-per-resource property Property . . . . . . 23
7. Implementation Status . . . . . . . . . . . . . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . . 25
9. 23
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
9.1. 24
8.1. Parameter Registrations . . . . . . . . . . . . . . . . . 26
9.2. 24
8.2. Message Header Field Registrations . . . . . . . . . . . 26
9.2.1. 24
8.2.1. Cal-Managed-ID . . . . . . . . . . . . . . . . . . . 26
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
11. 24
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
11.1. 25
9.1. Normative References . . . . . . . . . . . . . . . . . . 27
11.2. 25
9.2. Informative References . . . . . . . . . . . . . . . . . 28
11.3. URIs . . . . . . . . . . . . . . . . . . . . . 26
Appendix A. Example Involving Recurring Events . . . . . 29
Appendix A. Change History (To be removed by RFC Editor before
publication) . . . . 27
Acknowledgments . . . . . . . . . . . . . . . . 29
Appendix B. Example Involving Recurring Events . . . . . . . . . 32 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39 33

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 example, a copy of a presentation, presentation or the
meeting agenda). iCalendar provides an "ATTACH" property whose value
is either the inline Base64 encoded attachment data, 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 - 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 takes care of adding add appropriate "ATTACH" properties in the
iCalendar data as well as managing 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,
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 [1] [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 do 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. 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, 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 to not to support the storing of managed
attachments on a
per-recurrence instance 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 Request-URI will contain various query parameters
to specify the behavior.

3.3.1. action= 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

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

attachment-update

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

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-add

3.3.2. rid= 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). (denoted in ASCII as "0x2C"). The item values
can be in one of two forms:

Master instance instance: The value "M" (case-insensitive) (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 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 "RECURRENCE-ID" value as stored in the
calendar object resource (i.e. (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,
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 3.1). 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= 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 Attachments

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

1. The client issues a POST request targeted at the calendar object
resource.
resource structured as follows:

A. The request-URI 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. Request-URI.
If one or more specific recurrence instances are targeted,
then the request-URI 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 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, 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 an a "FILENAME" property
parameter whose value matches the Content-Disposition 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, the
server 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" "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 non
recurring
nonrecurring event and asks the server (via the Prefer [RFC7240] header
field) 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: xxxx 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: yyyy 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=xxxx;
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 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, value or the "MANAGED-ID"
property parameter value.

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

1. The client issues a POST request targeted at the calendar object
resource.
resource structured as follows:

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

B. The request-URI 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.

2. When the server receives the POST request 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 "managed-id" value 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" 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 Content-
Type header field value from the request - 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, the
server it
MUST return an appropriate HTTP success status
response, 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 which 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 [RFC7240] header field) 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: xxxx 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: yyyz 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=xxxy;
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 occurs: needs to occur:

1. The client issues a POST request targeted at the calendar object
resource.
resource structured as follows:

A. The request-URI 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. Request-URI.
If one or more specific recurrence instances are targeted,
then the request-URI Request-URI will include a "rid" query parameter
containing the list of instances (see Section 3.3.2).

C. The request-URI 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 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 an overridden component with the
appropriate "RECURRENCE-ID" property, property and the matching
"ATTACH" property removed. This later 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, the server 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, the
server 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 "managed-id" value in the request. The Prefer [RFC7240] 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: 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 3.12.2) 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 - 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 re-writing 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, 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 "CALDAV:max-attachment-size" property value (Section 6.2) on
the calendar collection of the target calendar resource; 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 "CALDAV:max-attachments-per-resource"
property value (Section 6.3) on the calendar collection of the
target calendar resource; resource.

(CALDAV:valid-action): The action "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 "rid" query parameter in the POST request
MUST NOT be present for with an attachment-update action, "action=attachment-update" query
parameter and MUST contain the value "M" and/or values
corresponding to "RECURRENCE-
ID" "RECURRENCE-ID" property values in the iCalendar
data targeted by the request.

(CALDAV:valid-managed-id): The managed-id "managed-id" query parameter in the
POST request MUST NOT be present for with an attachment-add action, "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 and an existing calendar object resource
can make reference to an existing managed attachment. The following
new preconditions 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), (Forbidden)
if the request should not be repeated because it will always
fail, or 409 (Conflict), (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 [RFC4331] 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 attachments attachment sizes when
calculating the amount of storage used by a particular resource.

3.12.2. Access Control

Access to the managed attachments store referenced in a calendar object
resource SHOULD be restricted to only those calendar users who have
access to that calendar object either directly, 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 updating, or removing managed attachments. In
addition, the server MUST prevent a calendar user from re-using reusing a
managed attachment (based on its managed-id "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 re-issue reissue the
POST request using a different request-URI. 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)" 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 Clean-Up Cleanup by Servers

Servers MAY automatically remove attachment data, for example example, to
regain the storage taken by unused attachments, attachments or as the result of a
virus scanning. When doing so so, they SHOULD NOT modify calendar data
referencing those attachments. Instead Instead, they SHOULD respond with "410
(Gone)" 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 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 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

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 property parameter is
defined as text to allow an extended range to be used.

Example:

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

4.2. FILENAME Property Parameter

Parameter Name: FILENAME

Purpose: To specify the file name filename 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 field "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 property parameter value. Similarly, servers MUST
follow those same guidelines before storing a value.

Example:

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

ATTACH;FILENAME=agenda.html:
https://attachments.example.com/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

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 within the scope of the CalDAV server. This property
parameter MUST NOT be present in the case of non-managed unmanaged
attachments.

Example:

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

ATTACH;MANAGED-ID=aUNhbGVuZGFy:
https://attachments.example.com/abcd.txt

5. Additional Message Header Fields

5.1. Cal-Managed-ID Response Header Field

The Cal-Managed-ID response header field provides the value of the
MANAGED-ID
"MANAGED-ID" property parameter corresponding to a newly added ATTACH
"ATTACH" property.

ABNF:

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

Example:

Cal-Managed-ID:aUNhbGVuZGFy

The Cal-Managed-ID header field MUST only be sent by an origin server
in response to a successful POST request with an action "action" query
parameter set to
attachment-add "attachment-add" or attachment-update. "attachment-update". It MUST
only appear once in a response and MUST NOT appear in trailers.

The Cal-Managed-ID header field is end to end and MUST be forwarded
by intermediaries. Intermediaries MUST NOT insert, delete, or modify
a Cal-Managed-ID header field.

6. Additional WebDAV Properties

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

Name: managed-attachments-server-URL

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

Purpose: Specifies This property 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
collection, which cannot be moved or copied.

allprop behavior: This property 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 either a single DAV:href XML
elements.
element or none at all.

When one DAV:href element is present, its value MUST be an
absolute HTTP URI containing only the scheme (i.e. (i.e., "https") and
authority (i.e. (i.e., host and port) parts . 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 Property

Name: max-attachment-size

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

Purpose: Provides This property 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.

Protected: This property 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: This property SHOULD NOT be returned by a PROPFIND
DAV:allprop request.

Description: The CALDAV:max-attachment-size "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)
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)>

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 Property

Name: max-attachments-per-resource

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

Purpose: Provides This property 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: This property 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: This property SHOULD NOT be returned by a PROPFIND
DAV:allprop request.

Description: The CALDAV:max-attachments-per-resource "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 Unmanaged 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) 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)>

Example:

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

7. Implementation Status

< RFC Editor: before publication please remove Security Considerations

The security considerations in [RFC4791] and [RFC4918] apply to this section, the
reference
extension. Additionally, servers need to [RFC7942], be aware that a client
could attack underlying storage by POSTing extremely large
attachments and any resulting "URIs" references sub-
section. >
This section records the status of known implementations of the
protocol defined by this specification at the could attack processing time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, uploading a recurring
event with a catalog large number of available implementations or their
features. Readers are advised to note that other implementations may
exist.

According to [RFC7942], "this will allow reviewers overrides and working groups
to assign due consideration to documents that have then repeatedly adding,
updating, and deleting attachments.

Malicious content could be introduced into the benefit of
running code, which may serve as evidence calendar server by way
of valuable experimentation a managed attachment, and feedback that have made the implemented protocols more mature.
It is up to the individual working groups propagated to use this information as
they see fit".

7.1. Calendar and Contacts Server

The open source Calendar and Contacts Server [2] project is a
standards-compliant server implementing many end users via
scheduling. Servers SHOULD check managed attachments for malicious
or inappropriate content. Upon detecting such content, servers
SHOULD remove the CalDAV protocol. This
production level implementation supports all of attachment following the requirements rules described in this document and successfully interoperates with the
Apple Calendar, BusyCal, 2Do, and CalDAVTester client implementations
described below.
Section 3.12.5.

8. IANA Considerations

8.1. Parameter Registrations

This implementation is freely distributable under
the terms of specification defines the Apache License, Version 2.0 [3].

7.2. Cyrus Server

The open source Cyrus Server [4] project is a highly scalable
enterprise mail system which also supports calendaring. This
production level CalDAV implementation supports all of following new iCalendar property
parameters to be added to the
requirements described "Parameters" registry defined in this document and successfully
interoperates with the Apple Calendar and CalDAVTester client
implementations described below. This implementation is freely
distributable under a BSD style license from Computing Services at
Carnegie Mellon University [5].

7.3. Oracle Communications Calendar Server

The Oracle Communications Calendar Server [6] project is a standards-
compliant, scalable, enterprise-ready calendaring solution. This
production level CalDAV implementation supports all
Section 8.2.4 of [RFC5545]:

8.2. Message Header Field Registrations

The message header fields below should be added to the
requirements described in "Permanent
Message Header Field Names" registry (see [RFC3864]).

8.2.1. Cal-Managed-ID

Header field name: Cal-Managed-ID

Protocol: http

Status: standard

Author/Change controller: IETF

Reference: this document and successfully
interoperates with the Apple Calendar and CalDAVTester client
implementations described below. This implementation is proprietary
and available specification (Section 5.1)

Related information: none

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for a free trial and/or purchase from the vendor.

7.4. Apple Calendar

The widely used Apple Calendar [7] client is a standards-compliant
client implementing the CalDAV protocol. This production level
implementation supports all the requirements described use in this
document RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S., and successfully interoperates with the
Calendar D.
Jensen, "HTTP Extensions for Distributed Authoring --
WEBDAV", RFC 2518, DOI 10.17487/RFC2518, February 1999,
<https://www.rfc-editor.org/info/rfc2518>.

[RFC3864] Klyne, G., Nottingham, M., and Contacts Server, Cyrus Server, J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.

[RFC4331] Korver, B. and
Oracle Communications Calendar Server implementations described
above. This client implementation is proprietary L. Dusseault, "Quota and is distributed
as part of Apple's desktop operating systems.

7.5. BusyCal

BusyCal [8] is a standards-compliant calendar client Size Properties
for MacOS
implementing the CalDAV protocol. This implementation supports all
of the requirements described in this document and successfully
interoperates with the Calendar Distributed Authoring and Contacts Server Versioning (DAV)
Collections", RFC 4331, DOI 10.17487/RFC4331, February
2006, <https://www.rfc-editor.org/info/rfc4331>.

[RFC4791] Daboo, C., Desruisseaux, B., and Cyrus Server
implementations described above. This implementation is proprietary
and available for a free trial and/or purchase from the vendor.

7.6. CalDAVTester

CalDAVTester [9] is an open source test and performance application
designed L. Dusseault,
"Calendaring Extensions to work with CalDAV servers WebDAV (CalDAV)", RFC 4791,
DOI 10.17487/RFC4791, March 2007,
<https://www.rfc-editor.org/info/rfc4791>.

[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
Authoring and tests various aspects of
their protocol handling as well as performance. This widely used
implementation supports all of the requirements described in this
document Versioning (WebDAV)", RFC 4918,
DOI 10.17487/RFC4918, June 2007,
<https://www.rfc-editor.org/info/rfc4918>.

[RFC5234] Crocker, D., Ed. and successfully interoperates with the server
implementations described above. This implementation is freely
distributable under the terms of the Apache License, Version 2.0
[10].

7.7. 2Do

2Do [11] is a standards-complient calendar client for iOS which uses
the CalDAV standard P. Overell, "Augmented BNF for communication. This implementation supports
all Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.

[RFC5545] Desruisseaux, B., Ed., "Internet Calendaring and
Scheduling Core Object Specification (iCalendar)",
RFC 5545, DOI 10.17487/RFC5545, September 2009,
<https://www.rfc-editor.org/info/rfc5545>.

[RFC6266] Reschke, J., "Use of the requirements described Content-Disposition Header Field
in this document and successfully
interoperates with the Calendar and Contacts Server implementation
described above. This implementation is proprietary and available
for purchase from the vendor.

8. Security Considerations

The security considerations in [RFC4791] Hypertext Transfer Protocol (HTTP)", RFC 6266,
DOI 10.17487/RFC6266, June 2011,
<https://www.rfc-editor.org/info/rfc6266>.

[RFC6638] Daboo, C. and [RFC4918] apply to this
extension. Additionally, servers need B. Desruisseaux, "Scheduling Extensions to be aware that a client
could attack underlying storage by POSTing extremely large
attachments and could attack processing time by uploading a recurring
event with a large number of overrides and then repeatedly adding,
updating,
CalDAV", RFC 6638, DOI 10.17487/RFC6638, June 2012,
<https://www.rfc-editor.org/info/rfc6638>.

[RFC7230] Fielding, R., Ed. and deleting attachments.

Malicious content could be introduced into the calendar server by way
of a managed attachment, J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax 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 3.12.5.

9. IANA Considerations

9.1. 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]:

9.2. Message Header Field Registrations

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

9.2.1. 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

10. Acknowledgments

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

11. References

11.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D.
Jensen, "HTTP Extensions for Distributed Authoring --
WEBDAV", RFC 2518, DOI 10.17487/RFC2518, February 1999,
<https://www.rfc-editor.org/info/rfc2518>.

[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.

[RFC4331] Korver, B. and L. Dusseault, "Quota and Size Properties
for Distributed Authoring and Versioning (DAV)
Collections", RFC 4331, DOI 10.17487/RFC4331, February
2006, <https://www.rfc-editor.org/info/rfc4331>.

[RFC4791] Daboo, C., Desruisseaux, B., and L. Dusseault,
"Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
DOI 10.17487/RFC4791, March 2007,
<https://www.rfc-editor.org/info/rfc4791>.

[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918,
DOI 10.17487/RFC4918, June 2007,
<https://www.rfc-editor.org/info/rfc4918>.

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.

[RFC6266] Reschke, J., "Use of the Content-Disposition Header Field
in the Hypertext Transfer Protocol (HTTP)", RFC 6266,
DOI 10.17487/RFC6266, June 2011,
<https://www.rfc-editor.org/info/rfc6266>.

[RFC6638] Daboo, C. and B. Desruisseaux, "Scheduling Extensions to
CalDAV", RFC 6638, DOI 10.17487/RFC6638, June 2012,
<https://www.rfc-editor.org/info/rfc6638>.

[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.

[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.

[RFC7240] Snell, J., "Prefer Header for HTTP", RFC 7240,
DOI 10.17487/RFC7240, June 2014,
<https://www.rfc-editor.org/info/rfc7240>.

[RFC7538] Reschke, J., "The Hypertext Transfer Protocol Status Code
308 (Permanent Redirect)", RFC 7538, DOI 10.17487/RFC7538,
April 2015, <https://www.rfc-editor.org/info/rfc7538>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

11.2. Informative References

[RFC5023] Gregorio, J., Ed. and B. de hOra, Ed., "The Atom
Publishing Protocol", RFC 5023, DOI 10.17487/RFC5023,
October 2007, <https://www.rfc-editor.org/info/rfc5023>.

[RFC5546] Daboo, C., Ed., "iCalendar Transport-Independent
Interoperability Protocol (iTIP)", RFC 5546,
DOI 10.17487/RFC5546, December 2009,
<https://www.rfc-editor.org/info/rfc5546>.

[RFC7320] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 7320, DOI 10.17487/RFC7320, July 2014,
<https://www.rfc-editor.org/info/rfc7320>.

[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.

[RFC8144] Murchison, K., "Use of the Prefer Header Field in Web
Distributed Authoring and Versioning (WebDAV)", Routing",
RFC 8144, 7230, DOI 10.17487/RFC8144, April 2017,
<https://www.rfc-editor.org/info/rfc8144>.

11.3. URIs

[1] https://tools.ietf.org/html/bcp14

[2] http://calendarserver.org/

[3] http://www.apache.org/licenses/LICENSE-2.0.html

[4] http://www.cyrusimap.org/

[5] http://www.cmu.edu/computing/

[6] http://www.cyrusimap.org/

[7] http://www.apple.com/macos/

[8] http://www.busymac.com/busycal/

[9] http://calendarserver.org/wiki/CalDAVTester

[10] http://www.apache.org/licenses/LICENSE-2.0.html

[11] http://www.2doapp.com/

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

Changes in calext-04:

1. Added text explaining why this document is being published as
Informational.

2. Further specified Cal-Managed-ID per Section 8.3.1 of RFC 7231.

3. Specified that the MANAGED-ID parameter value is unique within
the scope of the server.

4. Added more text regarding preconditions.

5. Added text about specific DoS attack vectors.

6. Editorial changes from Gren Elliot and Phillip Kewisch.

7. Editorial changes from Adam Roach.

8. Editorial changes from Alexey Melnikov.

9. Added reference to RFC4918.

10. Minor editorial changes.

Changes in calext-03:

1. Changed to Informational based on feedback regarding non-standard
method of updating an existing resource.

2. Added references to sub-sections in Overview.

3. Made support for Prefer header field a SHOULD for servers.

4. Expanded recurring event examples to use conditional requests and
to include the Expect header field.

5. Minor editorial changes.

Changes in calext-02:

1. Moved "Error Handling" into its own sub-section.

2. Split "Other Client Considerations" into "Processing Time" and
"Migrating Calendar Data".

Changes in calext-01:

1. Changed all instances of "header" to "header field".

2. Reworked wording of Prefer header field handling.

3. Switched to recommending 102 (Processing) interim response to
keep the client connection alive.

4. Fixed description of Cal-Managed-ID response header field to
state that it is also required in responses to successful
attachment-update.

5. Minor editorial changes.

Changes in calext-00:

1. Added Murchison as editor.

2. Updated HTTP references to RFC7230 and RFC7231.

3. Updated Prefer header field references to RFC7240.

4. Added Implementation Status section.

5. Minor editorial changes.

Changes in daboo-03:

1. Fixed some examples.

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

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

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 daboo-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 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.

[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.

[RFC7240] Snell, J., "Prefer Header for specific 2xx codes in favor HTTP", RFC 7240,
DOI 10.17487/RFC7240, June 2014,
<https://www.rfc-editor.org/info/rfc7240>.

[RFC7538] Reschke, J., "The Hypertext Transfer Protocol Status Code
308 (Permanent Redirect)", RFC 7538, DOI 10.17487/RFC7538,
April 2015, <https://www.rfc-editor.org/info/rfc7538>.

[RFC8174] Leiba, B., "Ambiguity of generic success
code.

Changes Uppercase vs Lowercase in daboo-01:

1. Tweaked OPTIONS capability wording.

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

3. Added text for clean-up RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2. Informative References

[RFC5023] Gregorio, J., Ed. 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.

10. rid query param MUST contain RECURRENCE-ID without any
conversion to UTC (case B. de hOra, Ed., "The Atom
Publishing Protocol", RFC 5023, DOI 10.17487/RFC5023,
October 2007, <https://www.rfc-editor.org/info/rfc5023>.

[RFC5546] Daboo, C., Ed., "iCalendar Transport-Independent
Interoperability Protocol (iTIP)", RFC 5546,
DOI 10.17487/RFC5546, December 2009,
<https://www.rfc-editor.org/info/rfc5546>.

[RFC7320] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 7320, DOI 10.17487/RFC7320, July 2014,
<https://www.rfc-editor.org/info/rfc7320>.

[RFC8144] Murchison, K., "Use of floating events).

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

12. Made support for the Prefer header a MUST for servers. Header Field in Web
Distributed Authoring and Versioning (WebDAV)", RFC 8144,
DOI 10.17487/RFC8144, April 2017,
<https://www.rfc-editor.org/info/rfc8144>.

Appendix B. A. Example Involving Recurring Events

In the following example, the organizer of a recurring meeting makes
an unsuccessful attempt to add an agenda (HTML attachment) to the
corresponding calendar resource with a conditional request. Note
that the client includes both the Expect and Prefer header fields in
the request, thereby preventing itself from needlessly sending the
attachment data, data and requesting that the current resource be returned
in the failure response (see Section 3.2 of [RFC8144]).

>> 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 80
If-Match: "abcdefg-000"
Expect: 100-continue
Prefer: return=representation
>> Final Response <<

HTTP/1.1 412 Precondition Failed
Content-Type: text/calendar; charset="utf-8"
Content-Length: yyyy 929
Content-Location: https://cal.example.com/events/65.ics
ETag: "123456789-000-000"

The organizer of a recurring meeting successfully 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 "ATTACH" property pointing to the attachment resource
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 80
If-Match: "123456789-000-000"
Expect: 100-continue
Prefer: return=representation

>> Interim Response <<

HTTP/1.1 100 Continue

>> Request Body <<

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

HTTP/1.1 201 Created
Content-Type: text/calendar; charset="utf-8"
Content-Length: yyyy 1043
Content-Location: https://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;
ATTACH;MANAGED-ID=97S;FMTTYPE=text/html;SIZE=80;
FILENAME=agenda.html:https://cal.example.com/attach/65/34X22R
END:VEVENT
END:VCALENDAR
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 "rid" query parameter. Note that an overridden
instance is created with the RECURRENCE-ID "RECURRENCE-ID" property value matching
the value of the "rid" query parameter in the request. Also Also, note
that the server takes significant time to complete the request and
notifies the client accordingly.

>> 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 105
If-Match: "123456789-000-111"
Expect: 100-continue
Prefer: return=representation

>> Interim Response <<

HTTP/1.1 100 Continue

>> Request Body <<

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

>> Interim Response <<

HTTP/1.1 102 Processing

>> Final Response <<

HTTP/1.1 201 Created
Content-Type: text/calendar; charset="utf-8"
Content-Length: yyyy 1661
Content-Location: https://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
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;
ATTACH;MANAGED-ID=97S;FMTTYPE=text/html;SIZE=80;
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;
ATTACH;MANAGED-ID=33225;FMTTYPE=text/html;SIZE=105;
FILENAME=agenda0220.html:https://cal.example.com/attach/65/FGZ225
END:VEVENT
END:VCALENDAR

Acknowledgments

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

Authors' Addresses

Cyrus Daboo
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
USA
United States of America

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/

Kenneth Murchison (editor)
FastMail US LLC
1429 Walnut St, Suite 1201
Philadephia, PA 19102
USA
United States of America

Email: murch@fastmailteam.com
URI: http://www.fastmail.com/