One document matched: draft-khartabil-simple-filter-format-00.txt
SIMPLE H. Khartabil
Internet-Draft M. Lonnfors
Expires: September 30, 2003 J. Costa-Requena
E. Leppanen
Nokia
April 2003
An Extensible Markup Language (XML) Based Format for Event
Notification Filtering
draft-khartabil-simple-filter-format-00
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on September 30, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
The SIP event notification framework describes the usage of the
Session Initiation Protocol (SIP) for subscriptions and notifications
of changes to a state of a resource. The document does not describe a
mechanism of how filtering of event notification information can be
achieved.
In order to enable this, a format is needed to enable the subscriber
to choose when notifications are to be sent to it and what they are
to contain. This document presents a solution in the form of an XML
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document format.
Table of Contents
1. Conventions . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Motivation for Using XML . . . . . . . . . . . . . . . . . 4
3. Structure of XML-Encoded Filter Criteria . . . . . . . . . 4
3.1 The <ev-filter-set> Root Element . . . . . . . . . . . . . 5
3.2 The <ev-filter> Element . . . . . . . . . . . . . . . . . 5
3.3 The <what> Element . . . . . . . . . . . . . . . . . . . . 5
3.3.1 Defining the Delivered Content . . . . . . . . . . . . . . 5
3.4 The <trigger> Element . . . . . . . . . . . . . . . . . . 6
3.4.1 The <changed> Element . . . . . . . . . . . . . . . . . . 6
3.4.1.1 The "changed-from" Attribute . . . . . . . . . . . . . . . 7
3.4.1.2 The "changed-to" Attribute . . . . . . . . . . . . . . . . 7
3.4.1.3 The "changed-by" Attribute . . . . . . . . . . . . . . . . 7
3.4.1.4 Combination of Attributes . . . . . . . . . . . . . . . . 7
3.4.2 The <added> Element . . . . . . . . . . . . . . . . . . . 7
3.4.3 The <removed> Element . . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . 8
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1 Filter Using <what> Element . . . . . . . . . . . . . . . 8
5.2 Filter Using <trigger> Element . . . . . . . . . . . . . . 8
5.3 Filter Using <what> and <trigger> Elements . . . . . . . . 9
6. XML Schema for Filter Criteria . . . . . . . . . . . . . . 9
7. Security Requirements . . . . . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 11
References . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . 14
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1. 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.
2. Introduction
SIP event notification is described in [5]. It defines a general
framework for sending subscriptions and receiving notifications in
SIP based systems. It introduces the concept of event packages,
which are concrete applications of the general event framework to a
specific usage of events.
Filtering is a mechanism for controlling the content of event
notifications and for specifying the rules for when a notification
should be sent. See requirements in [8] and [9].
The filtering mechanism is expected to be particularly valuable to
users of mobile wireless access devices. The characteristics of the
devices typically include high latency, 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.
It is stated in [5] that the notifier may send a NOTIFY at any time,
but typically it is sent when the state of the resource changes. It
also states that the notifications would contain the complete and
current state of the resource authorized for a certain subscriber to
see. The format of such resource state information is package
specific. In this memo, we assume that the NOTIFY for any package
contains an XML document.
There is a separate approach for the rate limiting of SIP event
notifications, namely the throttling mechanism [10]. This throttling
mechanism allows the subscriber to specify a maximum rate at which
event notifications are generated by a single notifier. The solution
for such requirements is out of scope for this document.
This document presents an XML format whereby a subscriber describes
its preference when notifications are to be sent to it and what they
are to contain. The scope of the "when" part is triggering.
Triggering is defined as enabling a subscriber to specify triggering
rules for notifications where the criteria are based on changes of
the package specific state information, e.g., for the presence
information document [4] the change in the value of the <status>
element.
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2.1 Motivation for Using XML
When trying to select the schema and protocol for defining the filter
criteria, it can be found that there are several alternatives
available. The best practice is to modularise the problem and work on
smaller modules (the protocol and the filter information schema). The
selection of the schema for defining the filter criteria requires a
readable syntax with enough flexibility to define the semantics.
Among the multiple possibilities (i.e. XML, Perl, TCL, CPL, etc), XML
provides a generic framework for defining an XML schema for the
filter criteria. The XML schema provides an extensible mechanism
where the logic required for the filtering functionality can be
built. A globally unique URI is associated to the XML schema
specified for the filtering. The XML schema is shown as a namespace
in a filter criteria.
The XML also enables the use of XPath [16] for referencing items in
an XML document (which is in the case of Presence [4] in the format
of the PIDF).
Some of the content-types of the event notifications are extensible
XML documents, e.g. PIDF [2]. Since there is a need to reference the
extensions in filters, a solution where fixed data element names of
an XML document are referenced is not appropriate.
In addition, by using XML the need for reliance on the transport
protocol in order to interpret the filter criteria can be eliminated.
3. Structure of XML-Encoded Filter Criteria
The filter criteria is an XML document [15] that MUST be well-formed
and SHOULD be valid. The filter criteria XML documents MUST have the
XML declaration and it SHOULD contain an encoding declaration in the
XML declaration, for example; "<?XML version='1.0'
encoding='UTF-8'?>". This specification makes use of XML namespaces
for identifying the XML schema of the filter criteria documents and
document fragments.
The namespace identifier for elements defined by this specification
is a URN [12], using the namespace identifier 'ietf' defined by [13]
and extended by [11]. This urn is:
urn:ietf:params:xml:ns:simple-filter+xml.
This namespace declaration indicates the namespace on which the
filter criteria are based on.
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3.1 The <ev-filter-set> Root Element
The root element of the filter criteria is <ev-filter-set>.
The <ev-filter-set> root element MUST contain the namespace mentioned
above. This element MUST contain one or more <ev-filter> elements.
3.2 The <ev-filter> Element
The <ev-filter> element is used to specify the content of an
individual filter.
<ev-filter> MUST have an 'id' attribute. The value of the 'id'
attribute MUST be unique within the <ev-filter-set> root element.
<ev-filter> MAY have an 'uri' attribute. The value of the 'uri'
attribute is the URI of the resource that is being subscribed to.
The URI of the resource is useful in cases where the 'eventlist'
extension [6] is used with a package and different filter criteria
are defined for one or more members of the list. The 'uri' attribute
in the <ev-filter> element identifies the member within list to whom
that specific filter criteria apply. If the <ev-filter> does not
contain the 'uri' attribute, the filter criteria apply to all the
members in the list unless a member of the list has its own filter
criteria attributed by the 'uri' attribute.
The <ev-filter> element MAY contain a <what> element and MAY contain
one or more <trigger> elements, but MUST contain either the <what>
element or the <trigger> element.
3.3 The <what> Element
The <what> element is used to specify the content to be delivered to
the subscriber. It does not specify the exact content but the rules
that are used to construct that information.
3.3.1 Defining the Delivered Content
It must be possible in the filter criteria to be able to select XML
attributes and/or elements of the default information format specific
to an event package. It must also be possible to reference any
extensions to the default information formats that are valid for that
event package.
A general way to reference any items (XML elements and attributes) of
the NOTIFY information document is specified. The referencing
mechanism support hierarchy of the items since there may be several
nested XML elements and attributes in the NOTIFY document.
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The preferred elements are indicated using the capabilities of XPath
[16]. The XPath clause is stated in the value of the <what> element.
The items addressed with the XPath are indicated with the expanded
name, which means that the URI of the namespace referenced in the
XPath expression is included in addition to the reference to the
information element or attribute.
The XPath expression needs to specifically request values of elements
and attributes to be included in the newly constructed XML document.
This means that specifying only the XML element or attribute path is
not enough for receiving the values. This is achieved using the XPath
"text()" function. This way of constructing XPath expressions makes
it possible for the subscribers to retrieve the structure of an XML
document with or without element or attribute values. For example, to
request a tuple from a presence document that has a <basic> element
with value 'open', and to request that the values of all the
sub-elements in that tuple to be included in the constructed XML
document, you need to specify an XPath expression that look like:
//*[status/basic="open"]/descendant::* | //*[status/basic="open"]/
ancestor-or-self::* | //*[status/basic="open"]/descendant::*/text() |
//*[status/basic="open"]/ancestor-or-self::*/text()
3.4 The <trigger> Element
The <trigger> element is used to identify the package specific
changes that a resource has to encounter before a notification is
delivered to the subscriber. It can appear more than once in a filter
document. Multiple appearances of this element denotes the "OR"
operation. This means that updates to a resource that satisfy any of
the <trigger> elements criteria constitute a notification to be
delivered.
The <trigger> element MAY contain the <changed> element, the <added>
element or the <removed>, but MUST contain at least one of the three
elements. Any combination of the 3 elements is possible. Multiple
appearances of those element within a <trigger> element denotes the
"AND" operation. This means that updates to a resource that satisfy
ALL of the <changed>, <added> and <removed> elements' criteria within
the <trigger> element constitute a notification to be delivered.
The XPath expression in all the elements inside the <trigger> element
may comtain a reference to an element/attribute or to the value of
the element/attribute. Both are interpreted as references to the
value.
3.4.1 The <changed> Element
The <changed> element is used to identify the XML element or
attribute, from the package specific XML document, that must change,
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compared to the previous XML document, in order to activate the
trigger and cause the NOTIFY to be delivered. The XML element or
attribute is indicated using the capabilities of XPath. The XPath
expression MUST only reference one element or attribute. [16].
The <changed> element MAY contain the "changed-from" attribute,
"changed-to" attribute, "changed-by" attribute, or any combination of
the three.
3.4.1.1 The "changed-from" Attribute
A trigger is active when the element or attribute identified with the
<changed> element has changed from the value indicated by this
attribute to a different value.
3.4.1.2 The "changed-to" Attribute
A trigger is active when the element or attribute identified with the
<changed> element has changed to the value indicated by this
attribute from a different value.
3.4.1.3 The "changed-by" Attribute
A trigger is active when the element or attribute identified with the
<changed> element has changed by the value indicated by this
attribute from a different value.
3.4.1.4 Combination of Attributes
Any combination of the "changed-from", "changed-to", and "changed-by"
attributes in the <changed> element is possible. For example, if the
"changed-from" attribute was combined with the "changed-to" attribute
it is interpreted as: the trigger is active when the XML element or
attribute identified with the <changed> element has changed from the
"changed-from" value to the "changed-to" value. Note that if the
"changed-by" attribute is used in combination with the other
attributes, the other attribute types MUST match the "changed-by"
type of decimal.
3.4.2 The <added> Element
The <added> element is used to trigger a NOTIFY when the element or
attribute, identified in it, has been added to the new XML document
in comparison to the previous XML document. It can be used, for
example, by subscribers to learn of new services and/or capabilities
subscribed to by the notifier, or services and/or capabilities that
the notifier has now allowed the subscriber to see.
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3.4.3 The <removed> Element
The <removed> element is used to trigger a NOTIFY when the element or
attribute, identified in it, has been removed from the new XML
document in comparison to the previous XML document.
4. IANA Considerations
A new content type "application/simple-filter+xml" is defined to
represent an XML MIME for the filter criteria.
This specification follows the guidelines of RFC3023 [14]
OPEN ISSUE: IANA registration template must be added later.
5. Examples
5.1 Filter Using <what> Element
This XPath expression selects all the upper, lower and parallel level
presence information elements that match the criteria (this means the
whole tuple and the base element). That criteria are: select <label>
elements that have a value beginning with "MMS", "SMS" or "IM". The
<label> element is defined in [3] as extension to PIDF [2].
<?xml version="1.0" encoding="UTF-8"?>
<ev-filter-set xmlns="urn:ietf:params:xml:ns:simple-filter">
<ev-filter id = "mess" uri="sip:presentity@domain.com">
<what>
//*[rpids:label="im" or rpids:label="sms" or rpids:label="mms"]/descendant-or-self::* | //*[rpids:label="im" or rpids:label="sms" or rpids:label="mms"]/ancestor::*
</what>
</ev-filter>
</ev-filter-set>
5.2 Filter Using <trigger> Element
This filter selects notifications to be sent to the subscriber when a
certain PIDF [2] tuple changes its <basic> status from CLOSED to
OPEN.
<?xml version="1.0" encoding="UTF-8"?>
<ev-filter-set xmlns="urn:ietf:params:xml:ns:simple-filter">
<ev-filter id = "open_mean" uri="sip:presentity@domain.com">
<trigger>
<changed changed-from="CLOSED" changed-to="OPEN">//urn:ietf:params:xml:ns:cpim-pidf/presence/tuple/status/basic</changed>
</trigger>
</ev-filter>
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</ev-filter-set>
5.3 Filter Using <what> and <trigger> Elements
This filter selects watcher information notifications [7] to be sent
when the pending subscription status has changed from "pending" to
"terminated". In the notification, only the watchers that have a
status of "terminated" and an event of "rejected" are included.
<?xml version="1.0" encoding="UTF-8"?>
<ev-filter-set xmlns="urn:ietf:params:xml:ns:simple-winfo-filter"
pkgdef="winfo">
<ev-filter id = "filter_X" uri="sip:presentity@domain.com">
<what>
//*[(../@package="presence" and ((@status="terminated" and @event="rejected"))]/ancestor-or-self::*
</what>
<trigger>
<changed changed-from="pending" changed-to="terminated">//*[@status] </changed>
</trigger>
</ev-filter>
</ev-filter-set>
6. XML Schema for Filter Criteria
XML Schema Implementation (Normative)
<!-- *********************************************************** -->
<?xml version="1.0" encoding="UTF-8"?>
<!-- *********************************************************** -->
<xsd:schema targetNamespace="urn:ietf:params:xml:ns:simple-pres-filter"
xmlns:xsd=http://www.w3.org/2001/XMLSchema/>
<xsd:import namespace="http://www.w3.org/XML/1998/namespace"
schemaLocation="http://www.w3.org/2001/xml.xsd"> <!--lang -->
<xsd:annotation>
<xsd:documentation xml:lang="en">
XML Schema Definition for SIP Event Filtering.
</xsd:documentation>
</xsd:annotation>
<!-- *********************************************************** -->
<xsd:element name="ev-filter-set" type="EventFilterSetType"/>
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<xsd:complexType name="EventFilterSetType">
<xsd:sequence>
<xsd:element name="ev-filter" type="EventFilterType"
minOccurs="1" maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
<!-- An event filter corresponds to the SIP URI indicated by its "uri" attribute. The URI defines the presentity or a list of presentities. -->
<xsd:complexType name="EventFilterType">
<xsd:sequence>
<xsd:element name="what" type="xsd:string" minOccurs="0" maxOccurs="1"/>
<xsd:element name="trigger" type="TriggerType" minOccurs="0" maxOccurs="unbounded"/>
</xsd:sequence>
<xsd:attribute name="id" type="xsd:string" use="required"/>
<xsd:attribute name="uri" type="xsd:anyURI" use="optional/>
</xsd:complexType>
<xsd:complexType name="TriggerType">
<xsd:sequence>
<xsd:element name="changed" type="ChangedType" minOccurs="0" maxOccurs="unbounded"/>
<xsd:element name="added" type="xsd:string" minOccurs="0" maxOccurs="unbounded"/>
<xsd:element name="removed" type="xsd:string" minOccurs="0" maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
<xs:complexType name="ChangedType">
<xs:simpleContent>
<xs:extension base="xds:string">
<xs:attribute name="changed-from" type="xsd:anySimpleType" use="optional"/>
<xs:attribute name="changed-to" type="xsd:anySimpleType" use="optional"/>
<xs:attribute name="changed-by" type="xsd:decimal" use="optional"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
<!-- *********************************************************** -->
</xsd:schema>
<!-- *********************************************************** -->
7. Security Requirements
The filters in the body in a SIP message has a significant effect on
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the ways in which the request is handled at a server. As a result, it
is especially important that messages containing this extension be
authenticated and authorised.
Processing of requests and looking up filter criteria requires a set
of operations and searches, which can require some amount of
computation. This enables a DoS attack whereby a user can send
requests with substantial number of messages with large contents, in
the hopes of overloading the server. To counter this, the server
SHOULD only allow filters with no more than about 20 occurances of
the <what>, <changed>, <added> and <removed> elements.
Requests can reveal sensitive information about a UA's capabilities.
If this information is sensitive, it SHOULD be encrypted using SIP S/
MIME capabilities.
All filtering related security measures discussed in [5] MUST be
followed along with package specific security.
8. Acknowledgements
The authors would like to thank Tim Moran and Sreenivas Addagatla for
their valuable input.
References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Sugano, H., "CPIM Presence Information Data Format",
draft-ietf-impp-cpim-pidf-07.txt, December 2002.
[3] Schulzrinne, H., "RPIDS -- Rich Presence Information Data
Format for Presence Based on the Session Initiation Protocol
(SIP)", draft-schulzrinne-simple-rpids-01.txt, February 2003.
[4] Rosenberg, J., "Session Initiation Protocol (SIP) Extensions
for Presence", draft-ietf-simple-presence-10.txt, January
2003.
[5] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[6] Roach, A., "A Session Initiation Protocol (SIP) Event
Notification Extension for Collections",
draft-ietf-simple-event-list-01.txt, March 2003.
[7] Rosenberg, J., "An Extensible Markup Language (XML) Based
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Format for Watcher Information",
draft-ietf-simple-winfo-format-04.txt, January 2003.
[8] Moran, T., "Requirements for Presence Specific Event
Notification Filtering",
draft-ietf-simple-pres-filter-reqs-00.txt, April 2003.
[9] Kiss, K., "Requirements for Filtering of Watcher Information",
draft-ietf-simple-winfo-filter-reqs-00.txt, April 2003.
[10] Niemi, A., "Requirements for Limiting the Rate of Event
Notifications",
draft-niemi-sipping-event-throttle-reqs-01.txt, February 2003.
[11] Mealling, M., "The IETF XML Registry",
draft-mealling-iana-xmlns-registry-04.txt, June 2002.
[12] Moats, R., "The URN Syntax", RFC 2141, May 1997.
[13] Moats, R., "The URN Namespace for IETF Documents", RFC 2648,
August 1999.
[14] Murata, M., "XML Media Types", RFC 3023, March 1997.
[15] Bray, T., "Exensible Markup Language (XML) 1.0 (Second
Edition)", W3C CR CR-xml11-20011006, October 2000.
[16] Clark, J., "XML Path Language (XPath) Version 1.0", W3C REC
REC-xpath-19991116, November 1999.
Authors' Addresses
Hisham Khartabil
Nokia
P.O. Box 321
Helsinki
Finland
Phone: +358 7180 76161
EMail: hisham.khartabil@nokia.com
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Mikko Lonnfors
Nokia
Itamerenkatu 00180
Helsinki
Finland
Phone: + 358 50 4836402
EMail: mikko.lonnfors@nokia.com
Jose Costa-Requena
Nokia
P.O. Box 321
FIN-00045 NOKIA GROUP
FINLAND
Phone: +358 71800 8000
EMail: jose.costa-requena@nokia.com
Eva Leppanen
Nokia
P.O BOX 785
Tampere
Finland
Phone: +358 7180 77066
EMail: eva-maria.leppanen@nokia.com
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