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Internet Engineering Task Force (IETF)
Request for Comments: 8607
Category: Informational
ISSN: 2070-1721
C. Daboo
Apple
A. Quillaud
Oracle
K. Murchison, Ed.
FastMail
June 2019

Calendaring Extensions to WebDAV (CalDAV): Managed Attachments

Abstract

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 © 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:

  • 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.
  • 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-add

3.3.2. rid Query Parameter

The "rid" query parameter is used to identify which recurrence instances are being targeted by the client for the attachment operation. This query parameter MUST contain one or more items, separated by commas (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.

  1. 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.

  1. 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.

  1. 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: 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).

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.
  1. 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).

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 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 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.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 unmanaged
      attachments.

Example:

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" property parameter corresponding to a newly added "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" query parameter set to "attachment-add" or "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

   Name:  managed-attachments-server-URL
   
   Namespace:  urn:ietf:params:xml:ns:caldav
   
   Purpose:  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, 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 either a single DAV:href XML
      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., "https") and authority (i.e., host and port) parts. Whenever a managed attachment is to be retrieved via an HTTP GET, the client MUST construct the actual URL of the attachment by substituting the scheme and authority parts of the attachment URI (as stored in the iCalendar "ATTACH" property) with the present WebDAV property value.

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

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

Definition: 

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

Example: 

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

6.2. CALDAV:max-attachment-size Property

   Name:  max-attachment-size
   
   Namespace:  urn:ietf:params:xml:ns:caldav
   
   Purpose:  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" 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)>
   <!-- PCDATA value: a numeric value (positive decimal integer) -->

Example: 

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

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

   Name:  max-attachments-per-resource
   
   Namespace:  urn:ietf:params:xml:ns:caldav
   
   Purpose:  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" 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.  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)>
   <!-- PCDATA value: a numeric value (positive decimal integer) -->

Example:

<C:max-attachments-per-resource

xmlns:C="urn:ietf:params:xml:ns:caldav" 

        >12</C:max-attachments-per-resource>

7. Security Considerations

The security considerations in [RFC4791] and [RFC4918] apply to this extension. Additionally, servers need 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, and deleting attachments.

Malicious content could be introduced into the calendar server by way of a managed attachment, and propagated to many end users via scheduling. Servers SHOULD check managed attachments for malicious or inappropriate content. Upon detecting such content, servers SHOULD remove the attachment following the rules described in Section 3.12.5.

8. IANA Considerations

8.1. Parameter Registrations

This specification defines the following new iCalendar property parameters to be added to the "Parameters" registry defined in Section 8.2.4 of [RFC5545]: 

             +------------+---------+-----------------------+
             | Parameter  | Status  | Reference             |
             +------------+---------+-----------------------+
             | SIZE       | Current | RFC 8607, Section 4.1 |
             | FILENAME   | Current | RFC 8607, Section 4.2 |
             | MANAGED-ID | Current | RFC 8607, Section 4.3 |
             +------------+---------+-----------------------+

8.2. Message Header Field Registrations

The message header fields below should be added to the "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 specification (Section 5.1)
   
   Related information: none

9. References

9.1. 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>.
   
   [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 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>.

9.2. 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>.
   
   [RFC8144]  Murchison, K., "Use of the Prefer 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 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 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: 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: 929
   Content-Location: https://cal.example.com/events/65.ics
   ETag: "123456789-000-000"
   
   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
   END:VEVENT
   END:VCALENDAR

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" property pointing to the attachment resource, and they are notified of the change via their scheduling inbox. 

   >> Request <<
   
   POST /events/65.ics?action=attachment-add HTTP/1.1
   Host: cal.example.com
   Content-Type: text/html; charset="utf-8"
   Content-Disposition: attachment;filename=agenda.html
   Content-Length: 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: 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=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" query parameter. Note that an overridden instance is created with the "RECURRENCE-ID" property value matching the value of the "rid" query parameter in the request. 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: 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: 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=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=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
   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
   United States of America
   
   Email: murch@fastmailteam.com
   URI:   http://www.fastmail.com/