Internet DRAFT - draft-douglass-serverside-subscriptions
draft-douglass-serverside-subscriptions
Network Working Group M. Douglass
Internet-Draft Spherical Cow Group
Updates: 2518 (if approved) September 4, 2018
Intended status: Standards Track
Expires: March 8, 2019
Serverside Subscriptions
draft-douglass-serverside-subscriptions-00
Abstract
This specification provides a mechanism whereby subscriptions to
external resources can be handled by the server.
This specification updates [RFC4791] to add new properies for the
MKCOL request.
Status of This Memo
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Copyright Notice
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions Used in This Document . . . . . . . . . . . . 3
2. CalDAV Subscriptions . . . . . . . . . . . . . . . . . . . . 3
2.1. Request . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. New DAV and CALDAV properties . . . . . . . . . . . . . . . . 4
3.1. DAV:subscription . . . . . . . . . . . . . . . . . . . . 4
3.2. DAV:subscription-href . . . . . . . . . . . . . . . . . . 4
3.3. DAV:subscription-deletions-suppressed . . . . . . . . . . 5
3.4. DAV:subscription-disabled . . . . . . . . . . . . . . . . 5
3.5. DAV:subscription-next-refresh . . . . . . . . . . . . . . 6
4. Refreshing and Reenabling the subscription . . . . . . . . . 6
5. Response Delays . . . . . . . . . . . . . . . . . . . . . . . 6
6. CalDAV service Considerations . . . . . . . . . . . . . . . . 7
6.1. Deleted events . . . . . . . . . . . . . . . . . . . . . 7
6.2. CalDAV restrictions . . . . . . . . . . . . . . . . . . . 7
6.3. Invitations in Subscriptions . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
11. Normative References . . . . . . . . . . . . . . . . . . . . 8
Appendix A. Open issues . . . . . . . . . . . . . . . . . . . . 10
Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The motivation for this specification was initially to handle
external subscriptions to calendar data. However, any resource which
allows subscriptions might make use of this specification.
Currently subscriptions to calendar feeds are handled by calendar
clients. There are a number of disadvantages to this approach: users
have to subscribe from multiple devices and the subscription cannot
affect scheduling handled by the server.
This specification defines a mechanism whereby the server will
subscribe to the feed and make it visible in the user's home.
The advantages are popular feeds can be cached by the server and the
user only has to make a single subscription.
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1.1. 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].
2. CalDAV Subscriptions
2.1. Request
A client will subscribe to a URL by performing a MKCOL request with
resource type elements of at least DAV:collection and
DAV:subscription. For a calendar subscription there will also be a
caldav calendar element.
This is an example of the MKCOL request and response from a server
that supports extended MKCOL.
>> Request <<
POST /caldav/user/mike/calendars/parrots HTTP/1.1
Host: example.com
Content-Type: text/calendar; component=VEVENT; method=REQUEST
Content-Length: xxxx
<?xml version="1.0" encoding="utf-8" ?>
<D:mkcol xmlns:D="DAV:"
xmlns:C="urn:ietf:params:xml:ns:caldav">
<D:set>
<D:prop>
<D:resourcetype>
<D:collection/>
<C:calendar/>
<D:subscription/>
</D:resourcetype>
<D:displayname>Parrot Events</D:displayname>
<D:subscription-href>http://example.org/parrot-events.ics</D:subscription-href>
<D:subscription-deletions-suppressed>true</D:subscription-deletions-suppressed>
</D:prop>
</D:set>
</D:mkcol>
>> Response <<
HTTP/1.1 200 OK
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3. New DAV and CALDAV properties
3.1. DAV:subscription
Name: subscription
Namespace: DAV
Purpose: To indicate that the resource is a subscription to an
external resource which is managed by the server.
Conformance: When this is specified the request MUST also contain at
least a DAV:subscription-href element as defined in this
specification.
Description: The DAV:specification resource type element is used to
indicate a collection that is a subscription. A subscription MUST
report the DAV:subscription XML element in the value of the DAV:
resourcetype property.
Definition:
<!ELEMENT subscription empty>
3.2. DAV:subscription-href
Name: subscription-href
Namespace: DAV
Purpose: Provides the url for the external subscription.
Conformance: This property MUST be defined on any collection which
has a resource-type containing a DAV:subscription element.
Definition:
<!ELEMENT vpoll-max-items (#PCDATA)>
PCDATA value: a url
Example:
<D:subscription-href xmlns:D="DAV"
>https://example.com/events.ics</D:subscription-href>
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3.3. DAV:subscription-deletions-suppressed
Name: subscription-deletions-suppressed
Namespace: DAV
Purpose: To indicate that resources that no longer appear in the
feed should be retained by the server.
Conformance: This property MAY be defined on any subscription.
Description: Many feeds provide only the current active set of
resources. For example, a calendar feed may only contain events
from the current date onwards - while many subscribers would like
to retain a copy of all events received over time.
This property indicates that the server SHOULD retain resources
that disappear from the feed. Services MAY define some mechanism
to indicate that a particular resource SHOULD be removed. For
example this specification suggests setting a status of DELETED on
a calendar event.
Definition:
<!ELEMENT subscription-deletions-suppressed empty>
3.4. DAV:subscription-disabled
Name: subscription-disabled
Namespace: DAV
Purpose: To indicate that subscription has been disabled.
Conformance: This property MUST be reported for any disabled
subscription.
Description: A server MAY choose to disable a subscription if there
is an excessive number of errors when attempting to synchronize
with the target This property indicates to the client that the
subscription has been disabled.
There is no explicit action that can be taken to reenable a
subscription. However, on subsequent requests a client may
indicate a refresh is desired which MAY have the effect of
reenabling the subscription.
Definition:
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<!ELEMENT subscription-enabled empty>
3.5. DAV:subscription-next-refresh
Name: subscription-next-refresh
Namespace: DAV
Purpose: To indicate the next refresh time for a subscription.
Conformance: This property MUST be reported for any active
subscription.
Description: This provides the date and time of the next refresh.
It uses the format defined in [RFC3986] appendix 2, that is date
parts are separated by "-" and time parts by ":".
Definition:
<!ELEMENT subscription-next-refresh (#PCDATA)>
PCDATA value: a UTC date-time value
Example:
<D:subscription-next-refresh xmlns:D="DAV"
>2019-03-15T11:00:23Z</D:subscription-next-refresh>
4. Refreshing and Reenabling the subscription
The client may make use of the "Prefer" header field defined in
[RFC7240] with a preference of "subscription-refresh" to indicate to
the server that a refresh of th data as desired.
A server MAY choose to always ignore the header or to ignore the
header if it appears too often.
If the server decides to initiate a refresh it MAY choose to respond
with a 102 HTTP status indicating that it is still waiting for the
data or a 202 HTTP status to indicate the request was accepted.
5. Response Delays
Implementations of this feature may have an outboard or background
process handling the actaul synchronization of the data. The target
may be hosted on a slow service or the data may be very large
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All these factors may lead to a significant delay in having data
ready for delivery to the client.
The following approaches are more or less appropriate for handling
requests:
Return with available data: This is the normal behavior. The
subscription looks like a regular collection so the server can
respond to the normal requests with whatever data is available.
Wait for completion: If the synchronization process is active the
server may just choose to wait. This risks a request timeout if
the data synchronization takes asignificant amount of time.
Return 102 status(es): The server may choose to wait but
periodically send a 102 response to keep the connection alive.
(Is this how it's supposed to work - the spec is unclear). Only
one 102 response would still cause a timeout - so every 20 secs?
Return 202 status: This is probably the best response. There is no
need to indicate where the client should go to retrieve the data.
All it needs to do is retry the operation after an appropriate
delay.
If the server decides to initiate a refresh it MAY choose to respond
with a 102 HTTP status indicating that it is still waiting for the
data or a 202 HTTP status to indicate the request was accepted
6. CalDAV service Considerations
As mentioned above, this feature is particularly useful for CalDAV
servers and clients. There are some specific considerations.
6.1. Deleted events
If subscription-deletions-suppressed is specified then the server
SHOULD retain all events. However, they server MAY choose to remove
old events once they become older than the CALDAV:min-date-time
property as specified in [RFC4791] section 5.2.6.
6.2. CalDAV restrictions
A server SHOULD apply all appropriate restrictions on events obtained
from a subscription. In particular the CALDAV:min-date-time and
CALDAV:max-date-time properties as specified in [RFC4791] sections
5.2.6 and 5.2.7 SHOULD be applied.
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Additionally the CALDAV:max-resource-size property restrics the size
of events and the CALDAV:max-instances property the number of
instances.
6.3. Invitations in Subscriptions
Any reason not to allow them?
7. Security Considerations
Servers implementing this feature need to be aware of the risks
entailed in using the URIs provided as values to subscription-href.
See [RFC3986] for a discussion of the security considerations
relating to URIs.
8. Privacy Considerations
Properties with a "URI" value type can expose their users to privacy
leaks as any network access of the URI data can be tracked. Clients
SHOULD NOT automatically download data referenced by the URI without
explicit instruction from users. This specification does not
introduce any additional privacy concerns beyond those described in
[RFC5545].
9. IANA Considerations
10. Acknowledgements
The author would also like to thank the members of the Calendaring
and Scheduling Consortium Calendar Sharing technical committee and
the following individuals for contributing their ideas and support:
...
The authors would also like to thank the Calendaring and Scheduling
Consortium for advice with this specification.
11. Normative References
[I-D.ietf-calext-extensions]
Daboo, C., "New Properties for iCalendar", draft-ietf-
calext-extensions-05 (work in progress), August 2016.
[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>.
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[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 2434,
DOI 10.17487/RFC2434, October 1998,
<https://www.rfc-editor.org/info/rfc2434>.
[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>.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<https://www.rfc-editor.org/info/rfc3688>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[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>.
[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>.
[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>.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
<https://www.rfc-editor.org/info/rfc5988>.
[RFC7240] Snell, J., "Prefer Header for HTTP", RFC 7240,
DOI 10.17487/RFC7240, June 2014,
<https://www.rfc-editor.org/info/rfc7240>.
[W3C.REC-xml-20060816]
Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fourth
Edition)", World Wide Web Consortium Recommendation REC-
xml-20060816, August 2006,
<http://www.w3.org/TR/2006/REC-xml-20060816>.
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Appendix A. Open issues
invitations: Any reason not to allow them?
Appendix B. Change log
v00 2018-06-26 MD
o First pass
Author's Address
Michael Douglass
Spherical Cow Group
226 3rd Street
Troy, NY 12180
USA
Email: mdouglass@sphericalcowgroup.com
URI: http://sphericalcowgroup.com
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