Network Working Group C. Daboo
Internet-Draft Apple
Intended status: Standards Track October 24, 2011
Expires: April 26, 2012
CalDAV Managed Attachments
draft-daboo-caldav-attachments-00
Abstract
This document defines how CalDAV servers can provide server managed
collections to allow attachments associated with iCalendar data, to
be stored and managed on the server.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Open Issues (to be removed prior to publication as a RFC) . . 4
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Discovering Support for Managed Attachments . . . . . . . 5
4.2. POST Request for Managing Attachments . . . . . . . . . . 5
4.3. Adding attachments . . . . . . . . . . . . . . . . . . . . 6
4.4. Updating Attachments . . . . . . . . . . . . . . . . . . . 7
4.5. Removing Attachments via POST . . . . . . . . . . . . . . 9
4.6. Adding Existing Managed Attachments via PUT . . . . . . . 10
4.7. Updating Attachments via PUT . . . . . . . . . . . . . . . 10
4.8. Removing Attachments via PUT . . . . . . . . . . . . . . . 10
4.9. Retrieving Attachments . . . . . . . . . . . . . . . . . . 10
4.10. Additional Considerations . . . . . . . . . . . . . . . . 10
5. Modifications to iCalendar Syntax . . . . . . . . . . . . . . 11
5.1. SIZE Property Parameter . . . . . . . . . . . . . . . . . 11
5.2. MANAGED-ID Property Parameter . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7.1. Paramater Registrations . . . . . . . . . . . . . . . . . 12
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
The iCalendar [RFC5545] data format is used to represent calendar
data and is used with iTIP [RFC5546] to handle scheduling operations
between calendar users.
[RFC4791] defines the CalDAV Calendar Access protocol, based on HTTP
[RFC2616], for accessing calendar data stored on a server.
Calendar users often want to include attachments with their calendar
data events or tasks (for example a copy of a presentation, or the
meeting agenda). iCalendar provides an "ATTACH" property whose value
is either the inline Base64 encoded attachment data, or a URL
specifying the location of the attachment data.
Use of inline attachment data is not ideal with CalDAV because the
data would need to be uploaded to the server each time a change to
the calendar data is done - even minor changes such as a change to
the summary. Whilst a client could choose to use a URL value
instead, the problem then becomes where and how the client discovers
an appropriate URL to use and how it ensures that only those
attendees listed in the event or task are able to access it.
This specification provides a solution to this problem whereby the
client sends the attachment to the server separately from the
iCalendar data and the server takes care of adding appropriate
"ATTACH" properties in the iCalendar data as well as managing
privileges . The server can also provide additional information to
the client about each attachment, such as the size and an identifier,
so clients can manage their list of attachments directly from the
calendar data.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
The notation used in this memo is the ABNF notation of [RFC5234] as
used by iCalendar [RFC5545]. Any syntax elements shown below that
are not explicitly defined in this specification come from iCalendar
[RFC5545].
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3. Open Issues (to be removed prior to publication as a RFC)
1. Since clients need to reload the calendar object resource after a
POST operation, should we provide a way for the client to request
the server to return the calendar data in the successful POST
response?
2. Do we need a way to allow for multiple attachments to be added,
updated or removed in a single POST (batching)?
3. Do we really need POST to remove given that a client PUT is
pretty easy to do? Only benefit for POST is the request has no
body.
4. SIZE uses iCalendar integer - but that is limited to 2^31.
However attachments could exceed that size.
5. Do we need a REPORT to allow clients to detect the largest
attachments affecting a user's quota? Should there be a request
to purge all attachments older than some cut-off?
6. Do we need a way for the server to indicate limits for managed
attachments (individual size, total size, total number etc).
Also a way for a server to indicate it does not allow inline
attachments?
4. Overview
There are four main operations a client needs to do with attachments
for calendar data: add, update, remove, and retrieve. The first
three operations are carried out by the client issuing an HTTP POST
request on the calendar object resource for which the attachment is
required. One exception to this is the delete operation, which
additionally allows for attachment removal by the client simply
updating the calendar object resource and removing any "ATTACH"
properties. The retrieve operation is accomplished by simply issuing
an HTTP GET request targeting the attachment URI.
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 updating
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 query parameters on the POST request-URI.
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4.1. Discovering Support for Managed Attachments
Servers advertise support for this extension by including the token
"calendar-managed-attachments" in the DAV response header to an
OPTIONS request on a calendar home collection. Clients MUST check
for the presence of that token before using any of the features
defined by this specification.
4.2. POST Request for Managing Attachments
An HTTP POST request is used to add, update, or remove attachments.
The request-URI will contain various query parameters to specify the
behavior.
4.2.1. action= Query Parameter
The "action" query parameter is used to identify which behavior the
client is requesting. This parameter MUST be present once on each
POST request used to manage attachments. One of three values MUST be
used:
attachment-add Indicates an operation that is adding an attachment
to a calendar object resource. See Section 4.3 for more details.
attachment-update Indicates an operation that is updating an
existing attachment on a calendar object resource. See
Section 4.4 for more details.
attachment-remove Indicates an operation that is removing an
attachment on a calendar object resource. See Section 4.5 for
more details.
Example:
http://calendar.example.com/events/1.ics?action=attachment-add
4.2.2. rid= Query Parameter
The "rid" query parameter is used to identify which recurrence
instance is being targeted by the client for the attachment
operation. The query parameter MUST contain one or more items,
separated by commas (0x2C). The item values can be in one of two
forms:
Master instance The value "M" (case-insensitive) refers to the
"master" recurrence instance, i.e., the component that does not
include a "RECURRENCE-ID" property. This item MUST only be
present once.
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Specific instance A specific iCalendar instance is targeted by using
its "RECURRENCE-ID" value as the query parameter value. If
multiple items of this form are used, they MUST be unique values.
If the "rid" query parameter is not present, all recurrence instances
in the calendar object resource are targeted.
Example:
http://calendar.example.com/events/1.ics?
action=attachment-add&rid=M,20111022T160000Z
4.2.3. id= Query Parameter
The "id" query parameter is used to identify which "ATTACH" property
is being updated or removed. The value of this query parameter MUST
match the "MANAGED-ID" property parameter value on the "ATTACH"
property in the calendar object resource instance(s) targeted by the
request.
Example:
http://calendar.example.com/events/1.ics?
action=attachment-update&id=aUNhbGVuZGFy
4.3. Adding attachments
To add an attachment to an existing calendar object resource, the
following occurs:
1. The client issues a POST request targeted at the calendar object
resource.
A. The request-URI will include an "action" query parameter with
the value "attachment-add" (see Section 4.2.1).
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 4.2.2).
C. The body of the request contains the data for the attachment.
D. The client MUST include a valid Content-Type HTTP header
describing the media type of the attachment (as required by
HTTP).
E. The client SHOULD include a Content-Disposition HTTP header
[RFC6266] with a "type" of "attachment", and a "filename"
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parameter that indicates the name of the attachment.
2. When the server receives the POST request it does the following:
A. Validates that any recurrence instances referred to via the
"rid" query parameter are valid for the calendar object
resource being targeted.
B. Stores the supplied attachment data into a resource and
generates an appropriate URI for clients to access the
resource.
C. For each affected recurrence instance in the calendar object
resource targeted by the request, the server adds an "ATTACH"
property, whose value is the URI of the stored attachment.
The "ATTACH" property MUST contain a "MANAGED-ID" 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 value from the request. The
"ATTACH" property SHOULD include a "SIZE" property parameter
whose value represents the size in octets of the attachment.
Note that if a specified recurrence instance does not have a
matching component in the calendar object resource, then the
server MUST modify the calendar object resource to include
the overridden component with the appropriate "RECURRENCE-ID"
property included.
D. Upon successful creation of the attachment resource and
modification of the targeted calendar object resource, the
server MUST return a 200 HTTP status response and include a
Location HTTP header containing the URI of the stored
attachment. The client can use the Location header value to
correlate the attachment with "ATTACH" properties added to
the calendar object resource. It is expected that the client
will immediately reload the calendar object resource to
refresh any local cache.
4.4. Updating Attachments
To change the data of an existing managed attachment in a calendar
object resource, the following occurs:
1. The client issues a POST request targeted at the calendar object
resource.
A. The request-URI will include an "action" query parameter with
the value "attachment-update" (see Section 4.2.1).
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B. If all recurrence instances are having an attachment updated,
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 4.2.2).
C. The request-URI will include an "id" query parameter with the
value matching that of the "MANAGED-ID" property parameter
for the "ATTACH" property being updated (see Section 4.2.3).
D. The body of the request contains the updated data for the
attachment.
E. The client MUST include a valid Content-Type header
describing the media type of the attachment (as required by
HTTP).
F. The client SHOULD include a Content-Disposition header
[RFC6266] with a "type" of "attachment", and a "filename"
parameter that indicates the name of the attachment.
2. When the server receives the POST request it does the following:
A. Validates that any recurrence instances referred to via the
"rid" query parameter are valid for the calendar object
resource being targeted.
B. Validates that the "id" query parameter is valid for the
calendar object resource and specific instances being
targeted.
C. Stores the supplied attachment data into a resource and
generates an appropriate URI for clients to access the
resource. The URI for the updated attachment data MUST be
different from the one for the attachment being updated.
This allows clients to use the URI value as a "change token".
D. For each affected recurrence instance in the calendar object
resource targeted by the request, the server updates the
"ATTACH" property, whose value is the URI of the updated
attachment. The "MANAGED-ID" property parameter value MUST
remain the same as it was for the attachment being updated.
The "ATTACH" property SHOULD contain an "FMTTYPE" property
parameter whose value matches the Content-Type header value
from the request - this could differ from the original value
if the media type of the updated attachment is different.
The "ATTACH" property SHOULD include a "SIZE" property
parameter whose value represents the size in octets of the
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updated attachment. Note that if a specified recurrence
instance does not have a matching component in the calendar
object resource, then the server MUST modify the calendar
object resource to include the overridden component with the
appropriate "RECURRENCE-ID" property included.
E. Upon successful update of the attachment resource and
modification of the targeted calendar object resource, the
server MUST return a 200 HTTP status response and include a
Location header containing the URI of the updated attachment.
It is expected that the client will immediately reload the
calendar object resource to refresh any local cache.
4.5. Removing Attachments via POST
To remove an attachment on an existing calendar object resource, the
following occurs:
1. The client issues a POST request targeted at the calendar object
resource.
A. The request-URI will include an "action" query parameter with
the value "attachment-remove" (see Section 4.2.1).
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 4.2.2).
C. The request-URI will include an "id" query parameter with the
value matching that of the "MANAGED-ID" property parameter
for the "ATTACH" property being removed (see Section 4.2.3).
D. The body of the request will be empty.
2. When the server receives the POST request it does the following:
A. Validates that any recurrence instances referred to via the
"rid" query parameter are valid for the calendar object
resource being targeted.
B. Validates that the "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
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matching "ATTACH" property. Note that if a specified
recurrence instance does not have a matching component in the
calendar object resource, then the server MUST modify the
calendar object resource to include the overridden component
with the appropriate "RECURRENCE-ID" property included, and
the matching "ATTACH" property removed.
D. Upon successful removal of the attachment resource and
modification of the targeted calendar object resource, the
server MUST return a 200 HTTP status response. It is
expected that the client will immediately reload the calendar
object resource to refresh any local cache.
4.6. 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 4.10.2).
4.7. 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).
4.8. Removing Attachments via PUT
Clients can remove attachments by simply re-writing the calendar
object resource data to remove the appropriate "ATTACH" properties.
4.9. Retrieving Attachments
Clients retrieve attachments by simply issuing an HTTP GET request
using the value of the corresponding "ATTACH" property as the
request-URI.
4.10. Additional Considerations
4.10.1. Error Handling
TODO: error codes for each request.
4.10.2. Access Control
TODO: talk about authentication for attachment URIs, access limited
to attendees only, or token based, only organizer able to re-use
"ATTACH" properties.
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4.10.3. Server Redirects
For POST requests that add or update attachment data, the server MAY
issue an HTTP redirect to require the client to re-issue the POST
request using a different request-URI. As a result, it is always
best for clients to use the "100 Continue" behavior define in Section
8.2.3 of [RFC2616]. That ensures that, if a redirect does occur, the
client does not needless send the attachment data.
When servers do redirect clients to a different request-URI, they
SHOULD ensure that the authentication/authorization for the
redirected URI falls within the same domain as that of the original
URI, so that the client can re-use credentials without having to
prompt the user.
4.10.4. Server Clean-up
TODO: describe reference counting of managed attachments - server can
remove actual attachment data once there are no more references to
the attachment URI. Also, servers can purge old attachment data -
return a 404 or 410.
4.10.5. 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
[I-D.desruisseaux-caldav-sched].
4.10.6. Attendees managing Attachments
TODO: how do we do this? Organizer adds a property to calendar data
to say attendees can add attachments, but how does scheduling happen?
5. Modifications to iCalendar Syntax
5.1. 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" "=" integer
; positive integers or -1
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Description: This property parameter MAY be specified on "ATTACH"
properties. It indicates the size in octets of the corresponding
attachment data. Since iCalendar integer values are restricted to
a maximum value of 2147483647, if the attachment size exceeds
that, the value -1 MUST instead be used.
Example:
ATTACH;SIZE=1234:http://attachments.example.com/abcd.txt
5.2. 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 MAY be specified on "ATTACH"
properties. Its value is generated by the server and uniquely
identifies a managed attachment.
Example:
ATTACH;MANAGED-ID=aUNhbGVuZGFy:http://attachments.example.c
om/abcd.txt
6. Security Considerations
TODO:servers need to enforce quotas, attachments could contain
malicious content (virus checking could be done)
7. IANA Considerations
7.1. Paramater Registrations
This documents defines the following new iCalendar properties to be
added to the registry defined in Section 8.2.3 of [RFC5545]:
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+--------------------+---------+----------------------+
| Property Parameter | Status | Reference |
+--------------------+---------+----------------------+
| SIZE | Current | RFCXXXX, Section 5.1 |
| MANAGED-ID | Current | RFCXXXX, Section 5.2 |
+--------------------+---------+----------------------+
8. Acknowledgments
This specification came about via discussions at the Calendaring and
Scheduling Consortium.
9. References
9.1. Normative References
[I-D.desruisseaux-caldav-sched]
Daboo, C. and B. Desruisseaux, "CalDAV Scheduling
Extensions to WebDAV", draft-desruisseaux-caldav-sched-10
(work in progress), September 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC4791] Daboo, C., Desruisseaux, B., and L. Dusseault,
"Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
March 2007.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5545] Desruisseaux, B., "Internet Calendaring and Scheduling
Core Object Specification (iCalendar)", RFC 5545,
September 2009.
[RFC6266] Reschke, J., "Use of the Content-Disposition Header Field
in the Hypertext Transfer Protocol (HTTP)", RFC 6266,
June 2011.
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9.2. Informative References
[RFC5546] Daboo, C., "iCalendar Transport-Independent
Interoperability Protocol (iTIP)", RFC 5546,
December 2009.
Author's Address
Cyrus Daboo
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
USA
Email: cyrus@daboo.name
URI: http://www.apple.com/
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