SIMPLE WG K. Kiss
Internet-Draft E. Leppanen
Expires: July 26, 2004 H. Khartabil
Nokia
January 26, 2004
Requirements for Filtering of Watcher Information
draft-ietf-simple-winfo-filter-reqs-01
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document defines a set of structured requirements whereby a
watcher information subscriber (client) may select specific
information to be received in the watcher information notification
sent by the notifier (server). The purpose is to limit the content so
that only essential information is delivered by the server.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Event Filtering Model . . . . . . . . . . . . . . . . . . . . 4
4. Requirements for Specification of Filters . . . . . . . . . . 4
5. Notification Content Limiting . . . . . . . . . . . . . . . . 5
6. Requirements for Uploading Rules (Operational Rules) . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Example Applications for Notification Filtering . . . . . . . 7
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
10. Changes from previous versions . . . . . . . . . . . . . . . . 7
10.1 Main changes from version 00 . . . . . . . . . . . . . . . . . 7
References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . 9
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1. Introduction
SIP event notification is described in [4]. It defines a general
framework for subscriptions and notifications for SIP event packages.
Concrete applications of the general event framework to a specific
group of events are described in [3] (user presence) and [5] (watcher
information).
The watcher information refers to the set of users subscribed to a
particular resource within a particular event package. Watcher
information changes dynamically as users subscribe, unsubscribe, are
approved, or rejected. A client can subscribe to this information.
As the inherent usage of event packages grows, the client needs some
mechanisms for controlling the event notifications at the source.
Evidence of this need is found in [2].
The Internet Draft describing the watcher information template
package [5] mentions the possibility for filtering. Accordingly, the
SUBSCRIBE request may contain a body for filtering the watcher
information subscription. However, the definition of filtering has
been left out of the scope of the Internet Draft.
These mechanisms are expected to be particularly valuable to users of
wireless devices. The characteristics of these devices typically
include low bandwidth, low data processing capabilities, small
display and limited battery power. Such devices can benefit from the
ability to filter the amount of information generated at the source
of the event notification.
However, it is expected that the control mechanisms for event
notifications add value for all users irrespectively of their device
or network access characteristics.
Section 4 and Section 6 of this draft propose a set of requirements
whereby a client may specify which notifications it is interested in.
That is, a means to specify filtering rules to be executed by the
server. Section 8 provides a few example applications of notification
filtering.
2. Conventions
In this document, the key words 'MUST', 'MUST NOT', 'REQUIRED',
'SHALL', 'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY',
and 'OPTIONAL' are to be interpreted as described in RFC 2119 [1]
and indicate requirement levels for compliant implementations.
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3. Event Filtering Model
There are two parts to the event filtering model. From a Watcher
Information service view point, Watchers subscribe for information
about a resource. That resource, called watcherinfo subscriber,
sometimes needs to collect information about the watchers. The first
part of the model enables the watcherinfo subscriber to limit the
watcher information delivered to it. Allowing the watcherinfo
subscriber to select the information of interest to it results in the
ability to limit the contents of a watcher information document,
therefore reducing the size of a notification message.
+--+
--| |Watcher
Watcherinfo Server --- +--+
Subscriber +---------+ ----
+-----+ | | ---
| | | |-- +--+
| |--------------| |--------------| |Watcher
| | | |-- +--+
+-----+ | | ---
+---------+ ----
--- +--+
--| |Watcher
+--+
The second part of the model defines the triggering. In a filter-less
subscription, it might be a server's default policy for the first
NOTIFY after a subscription to carry full state, and subsequent
notifications to carry partial state. This model enables the
watcherinfo subscriber to select the events or changes in watcher
information that trigger notifications to be sent. Other changes in
watcher state that are not defined as triggers in a filter do not
result in a notification message being delivered to the watcher.
4. Requirements for Specification of Filters
The following requirements relate to the creation of filters (rules).
REQ A1: A common set of constructs MUST be defined for the creation
of rules. There MUST be a common set of operations that follow a
common syntax. It MUST be possible for the client to define different
rules for different purposes using a common filtering mechanism.
REQ A2: It MUST be possible for the client to indicate the event
package to which the filter applies.
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REQ A3: It MUST be possible for the client to indicate the target
resource to which a certain filter is applied.
5. Notification Content Limiting
This chapter presents requirements for specifying the content to be
sent in the notifications.
REQ B1: It MUST be possible for the client to specify the XML
elements in winfo format [6] to be delivered in the notification.
REQ B2: It MUST be possible to define a set of conditions for the
values of certain elements in a winfo document that determine which
elements to send in notifications.
REQ B3: It MUST be possible to construct expressions that combine
multiple tests.
REQ B4: The client MUST be able to instruct the server to include, in
the notifications, only those watchers that are in a specific state.
REQ B5: The client MUST be able to indicate to the server to include
only those watchers in the notifications which are transitioned in
their current status because of a specific event.
REQ B6: The client MUST be able to indicate to the server to include
only those watchers in the notifications which have subscription
lifetime higher than (less than) a specific amount of seconds.
REQ B7: The client MUST be able to indicate to the server to include
only those watchers in the notifications which are subscribed for a
duration higher than (less than) a specific amount of seconds.
REQ B8: The client MUST be able to indicate to the server to include
only certain watchers (e.g. watchers from a particular domain,
specific watchers defined by the filters) in the notifications.
6. Requirements for Uploading Rules (Operational Rules)
REQ C1: It MUST be possible for the client to upload the rules to the
server and know the status - accepted or rejected - based on server
policy.
REQ C2: Placing filtering rules in the body of the subscription MUST
be supported.
REQ C3: The server MUST retain the uploaded filter setting for the
duration of the subscription.
REQ C4: It MUST be possible to change the filter settings during a
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subscription.
REQ C5: It MUST be possible for the client to reset the filter
settings to the service (server) defined default.
REQ C6 It MUST be possible for a server not supporting filtering to
inform the watcherinfo subscriber of the failure.
REQ C7: It MUST be possible for a server not understanding a
filtering to inform the watcherinfo subscriber of the failure.
REQ C8: It MUST be possible for a server not accepting a filter to
inform the watcherinfo subscriber of the reasons for not accepting
the filter.
REQ C9: It MUST be possible for the server to terminate a
subscription if a filter is no longer acceptable, e.g., due to policy
change or server load.
7. Security Considerations
Security requirements specified for [3] also applies to winfo
filtering. Additional security considerations are described as
follows.
REQ D1 It SHOULD be possible for the server to hide the fact that a
filter was not acceptable.
REQ D2: The presence of a filter in the body in a SIP message has a
significant effect on the way in which the request is handled at a
server. As a result, it MUST be possible to authenticate messages
carrying filters and authorise the watcherinfo subscriber to upload
filters.
REQ D3 Modification to filters by an intermediary could also result
in the watcherinfo subscriber either not receiving notifications of
watcher information they are interested in or receiving a very large
watcher info document. Therefore filters SHOULD be integrity
protected between.
REQ D4: Processing of requests and looking up filters requires some
amount of computation. This enables a DoS attack whereby a user can
send requests with substantial numbers messages with large contents,
in the hopes of overloading the server. To prevent this the number of
filters allowed in a request SHOULD be limited.
REQ D5: Requests containing filters can reveal sensitive information
about a UA's capabilities. If this information is sensitive, it
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SHOULD be encrypted using methods that allow it to be read by those
nodes that need to do so.
REQ D6: Authorization SHOULD occur irrespective of the filtering.
8. Example Applications for Notification Filtering
o A presentity wishes to see who has subscribed to their presence.
The presentity only wishes to see information for subscribers who
are co-workers.
o A presentity makes subscription to get information about active
watchers.
o A presentity makes subscription to get information about defined,
new or unauthorised watchers.
o A presentity requests information on watchers that have their
status as "waiting", for authorization purposes.
9. Acknowledgements
The authors would like to thank Jonathan Rosenberg, Tim Moran, Juha
Kalliokulju, Paul Kyzivat and Aki Niemi for their valuable input.
10. Changes from previous versions
10.1 Main changes from version 00
o Added filtering model section.
o Rephrased some requirements for clarity.
o Rearranged requirements into more appropriate sections.
References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Kiss, K., "Requirements for Presence Service based on 3GPP
specifications and wireless environment characteristics",
draft-kiss-simple-presence-wireless-reqs-02, February 2003.
[3] Rosenberg, J., "Session Initiation Protocol (SIP) Extensions for
Presence", draft-ietf-simple-presence-10.txt, January 2003.
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[4] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[5] Rosenberg, J., "A Watcher Information Event Template-Package for
the Session Initiation Protocol (SIP)",
draft-ietf-simple-winfo-package-05.txt, January 2003.
[6] Rosenberg, J., "An Extensible Markup Language (XML) Based Format
for Watcher Information",
draft-ietf-simple-winfo-format-04.txt, January 2003.
Authors' Addresses
Krisztian Kiss
Nokia
P.O. Box 100
33721 Tampere
Finland
Phone: + 358 50 4835363
EMail: krisztian.kiss@nokia.com
Eva Leppanen
Nokia
P.O BOX 785
Tampere
Finland
Phone: +358 7180 77066
EMail: eva-maria.leppanen@nokia.com
Hisham Khartabil
Nokia
P.O. Box 321
Helsinki
Finland
Phone: +358 7180 76161
EMail: hisham.khartabil@nokia.com
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