Functional Description of Event Notification Filtering
RFC 4660
| Document | Type |
RFC - Proposed Standard
(September 2006; Errata)
Updated by RFC 6665
|
|
|---|---|---|---|
| Authors | Jose Costa-Requena , Hisham Khartabil , Eva Leppanen , Mikko Lonnfors | ||
| Last updated | 2020-01-21 | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text html pdf htmlized with errata bibtex | ||
| Stream | WG state | (None) | |
| Document shepherd | No shepherd assigned | ||
| IESG | IESG state | RFC 4660 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus Boilerplate | Unknown | ||
| Telechat date | |||
| Responsible AD | Ted Hardie | ||
| Send notices to | hisham.khartabil@nokia.com | ||
Network Working Group H. Khartabil
Request for Comments: 4660 Telio
Category: Standards Track E. Leppanen
M. Lonnfors
J. Costa-Requena
Nokia
September 2006
Functional Description of Event Notification Filtering
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The SIP event notification framework describes the usage of the
Session Initiation Protocol (SIP) for subscriptions and notifications
of changes to the state of a resource. The document does not
describe a mechanism whereby filtering of event notification
information can be achieved.
This document describes the operations a subscriber performs in order
to put filtering rules associated with a subscription to event
notification information in place. The handling, by the subscriber,
of responses to subscriptions carrying filtering rules and the
handling of notifications with filtering rules applied to them are
also described. Furthermore, the document conveys how the notifier
behaves when receiving such filtering rules and how a notification is
constructed.
Khartabil, et al. Standards Track [Page 1]
RFC 4660 Functional Description of Filtering September 2006
Table of Contents
1. Introduction ....................................................3
2. Conventions .....................................................3
3. Client Operation ................................................4
3.1. Transport Mechanism ........................................4
3.2. SUBSCRIBE Bodies ...........................................4
3.3. Subscriber Generating of SUBSCRIBE Requests ................4
3.3.1. Defining the Filtering Rules ........................4
3.3.2. Request-URI vs. Filter URI ..........................5
3.3.3. Changing Filters within a Dialog ....................5
3.3.4. Subscriber Interpreting of SIP Responses ............6
3.4. Subscriber Processing of NOTIFY Requests ...................6
4. Resource List Server Behaviour ..................................7
4.1. Request-URI vs. Filter URI .................................7
4.2. Changing Filters within a Dialog ...........................9
5. Server Operation ................................................9
5.1. NOTIFY Bodies ..............................................9
5.2. Notifier Processing of SUBSCRIBE Requests ..................9
5.2.1. Request-URI vs. Filter URI .........................10
5.2.2. Changing Filters within a Dialog ...................11
5.3. Notifier Generating of NOTIFY Requests ....................11
5.3.1. Generation of NOTIFY Contents ......................12
5.3.2. Handling of Notification Triggering Rules ..........13
5.4. Handling Abnormal Cases ...................................13
6. XML Document Validation ........................................14
7. Examples .......................................................14
7.1. Presence Specific Examples ................................14
7.1.1. Subscriber Requests Messaging-Related Information ..15
7.1.2. Subscriber Fetches Information about "Open"
Communication Means ................................16
7.1.3. Subscriber Requests Notifications When
Presentity's Status Changes ........................18
7.2. Watcher Information Specific Examples .....................21
7.2.1. Watcher Subscriber Makes Subscription to
Get All the Information about Active Watchers ......22
7.2.2. Watcher Subscriber Requests Information of
Watchers with Specific Subscription Duration
Conditions .........................................23
7.2.3. Watcher Subscriber Requests Specific
Watcher Info on Specific Triggers ..................24
8. Security Considerations ........................................27
9. IANA Considerations ............................................28
10. Acknowledgements ..............................................28
11. References ....................................................28
11.1. Normative References .....................................28
11.2. Informative References ...................................28
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RFC 4660 Functional Description of Filtering September 2006
1. Introduction
SIP event notification is described in [3]. 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. Additionally, the subscriber may specify the rules
for when a notification should be sent to it. 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. However,
implementers need to be aware of the computational burden on the
source of the event notification. This is discussed further in
Section 8.
It is stated in [3] 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.
This document, together with [5], presents a mechanism for filtering
whereby a subscriber describes its preference of when notifications
are to be sent to it and what they are to contain. It also describes
how the notifier functions when generating notifications by taking
into account filters and default functionality of the package/
service.
The XML format for defining the filter is described in [5].
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.
"Content" refers to the XML document that appears in a notification
reflecting the state of a resource.
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3. Client Operation
3.1. Transport Mechanism
Transportation of the filter to the server is achieved by inserting
the XML document, as defined in [5], in the body of the SUBSCRIBE
request. Alternatively, the XML document can be uploaded to the
server using means outside the scope of this document.
3.2. SUBSCRIBE Bodies
SIP entities compliant with this specification MUST support the
content type 'application/simple-filter+xml'.
3.3. Subscriber Generating of SUBSCRIBE Requests
This section presents additional functionality required from the
subscriber when filters are used in the bodies of the SUBSCRIBE
requests. Normal operations of services (e.g., as defined in [8],
[10], and [4]) are otherwise followed.
As defined in [3], the SUBSCRIBE message MAY contain a body. This
body would carry filtering information. Honouring those filters is
at the discretion of the notifier and might depend on local policies.
No content in the body of a SUBSCRIBE indicates to the notifier that
no filter is being requested, so the notifier is instructed to send
all the NOTIFY requests using the notifier's own or service-specific
policy. Note that, for example, in the list case [4], the filter
might have been uploaded to the server beforehand (by means outside
the scope of this document).
If the body of the SUBSCRIBE includes the filter, the body MUST be of
the MIME-Type 'application/simple-filter+xml'.
3.3.1. Defining the Filtering Rules
Multiple filters MAY be included in one SUBSCRIBE. This is achieved
by including multiple <filter> elements in the filter [5]. Each
<filter> element may include a 'uri' attribute.
A SUBSCRIBE request destined to a list URI [4] MAY include multiple
filters specific to individual resources. This is achieved by
including multiple <filter> elements with different URIs of resources
in each of those elements. This resource specific resource-specific
filter are processed first before any list specific list-specific
filter, if any. The list specific list-specific filter may or may
not include a URI.
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Furthermore, regardless of whether the SUBSCRIBE is destined to a
list URI, there can only be one filter applicable to a single
resource or domain within a single SUBSCRIBE. That is, each filter
within a subscription MUST uniquely identify one resource or one
domain.
A filter can be enabled and disabled using the 'enabled' attribute in
the <filter> element, as described in [5].
3.3.2. Request-URI vs. Filter URI
The URI in the filter defines the target resource. For example, in
the Presence service case, it is the presentity's presence
information to which the filter is applied. The subscriber MAY
choose to leave the URI in the filter undefined. If the URI is not
defined within the filter, the filter applies to the resource
identified in the Request-URI. Similarly, the subscriber MAY define
a filter URI. If the Request-URI is a list URI [4], the filter URI
MUST be the list URI, a sub-list URI, or resource whose URI is one of
the URIs that result from a lookup, by a Resource List Server (RLS),
on the Request-URI. If it is not, the filter may be ignored or may
be rejected. URI matching is done according to the matching rules
defined for a particular scheme (SIP URI matching rules are defined
in RFC 3261 [2]).
A filter may also be addressed to a domain using the 'domain'
attribute instead of the 'uri' attribute. In this case, the filter
applies to resources in that domain. This can be used when a
subscription is for a resource that is an event list with many
resources from differing domains. If an individual resource-specific
filter is present along with the domain filter, this
resource-specific filter overrides any domain-specific filter, if
any.
3.3.3. Changing Filters within a Dialog
The subscriber MAY reset or change the filter by re-issuing a new
SUBSCRIBE request within the existing dialog. A SUBSCRIBE within the
exiting dialog that does not contain a filter is assumed to maintain
existing filters. This means that filters are persistent within a
dialog and are only explicitly removed.
A subscriber requiring removal of a filter may do so by using the
'remove="true"' attribute, as defined in [5].
In the case where the URI in the filter is that of a list, a
subscriber may override the existing filter with a filter for an
individual resource that is part of the list subscribed to earlier by
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issuing a new SUBSCRIBE within the existing dialog and including a
filter, specific for that individual resource, using a new filter ID.
The new filter need not include the original filter since a filter is
only removed in the manner indicated above.
A filter is replaced by the subscriber re-issuing the filter using
the same filter ID and replacing the contents of the filter.
Replacing a filter by changing the filter ID and keeping the resource
URI is considered an error since this causes the server to assume
that two filters are placed for the same resource.
Again, a filter can be disabled and re-enabled using the 'enabled'
attribute in the <filter> element, as described in [5].
3.3.4. Subscriber Interpreting of SIP Responses
The SUBSCRIBE request will be confirmed with a final response. A
200-class response indicates that the subscription has been accepted
and that a NOTIFY will be sent immediately. A "200" response
indicates that the subscription has been accepted and that the filter
is accepted. A "202" response merely indicates that the subscription
has been understood, that the content type has been accepted, and
that authorization may or may not have been granted. A "202"
response also indicates that the filter has not been accepted yet.
The acceptance of the filter MAY arrive in a subsequent NOTIFY.
A non-200 class final response indicates that no subscription or
dialog has been created, and no subsequent NOTIFY message will be
sent. All non-200 class final responses have the same meanings and
handling as described in [2] and [3].
Specifically, a "415" response indicates that the MIME type
'application/simple-filter+xml' is not understood by the notifier. A
"488" response indicates that the content type (filter) is understood
but some aspects of it were either not understood or not accepted.
3.4. Subscriber Processing of NOTIFY Requests
If the 2xx response was returned for the SUBSCRIBE, the NOTIFY that
follows MAY contain a body that describes the present state of the
resource after the filters have been applied.
If the NOTIFY indicates that a subscription has been terminated [3],
the subscription is assumed to be terminated. Behaviour in such
events is also described in [3].
If the subscription is indicated as active, NOTIFY requests are
handled as described in package-specific documents and in [3].
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4. Resource List Server Behaviour
The Resource List Server is defined in [4]. This section describes
how such an entity behaves in the presence of a filter in a
subscription to a list.
4.1. Request-URI vs. Filter URI
If the URI is not defined within the filter, the filter applies to
the resource list identified in the Request-URI of the SUBSCRIBE
request. This results in the filters being applied to all the
notifications that the RLS issues to this subscription. The same
processing applies to a filter that defines a URI that matches the
request-URI of the SUBSCRIBE request. That is, the filter applies to
all notifications that the RLS issues to this subscription.
If the URI indicated by the filter is for one resource whose URI is
one of the URIs that result from a lookup by the RLS on the
Request-URI, the filter for that particular resource is extracted and
propagated in the SUBSCRIBE request sent to that resource. It is
possible to have more than one filter in a SUBSCRIBE request body,
and therefore a filter specific to a resource MUST be extracted and
only that one is propagated. For example, if the Request-URI in a
SUBSCRIBE has the value "sip:mybuddies@example.com", where
"bob@example.com" is a resource belonging to that list, and the URI
in a filter is "sip:bob@example.com", the filter specific for Bob is
extracted and placed in the body of the SUBSCRIBE sent to
"bob@example.com".
If the URI indicated by the filter is for one resource whose URI is
NOT under the RLS administrative control, the RLS propagates the
filter to all the fanned out subscriptions. This is to accommodate
the scenario where the subscriber knows that there are sub-lists in
the event list that are under a different administrative domain from
that where the original subscription was sent, and the subscriber
wishes to set a filter for a resource in that sub-list.
If the URI indicated by the filter is for one resource whose URI is
under the RLS administrative control but is not part of the resource
list that the subscription was addressed to, the filter is not
propagated. In this case, it is the RLS's responsibility to make
sure that this filter is applied to notifications issued, if
information about that resource is present.
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For example: If we have 2 lists, each located on its own RLS:
List1 (list1@example.com) on RLS1 has: bob@example.com
list2@biloxi.com
List2 on RLS2 has: alice@biloxi.com sarah@example.com
(Note: list2 is a resource in list1)
RLS1 receives the following SUBSCRIBE request (the SUBSCRIBE is
addressed to list1 and contains 2 filters: one for sarah@example.com
and the other for alice@biloxi.com):
SUBSCRIBE sip:List1@example.com SIP/2.0
...
<?xml version="1.0" encoding="UTF-8"?>
<filter-set xmlns="urn:ietf:params:xml:ns:simple-filter">
<ns-bindings>
<ns-binding prefix="pidf" urn="urn:ietf:params:xml:ns:pidf"/>
</ns-bindings>
<filter id="999" uri="sip:sarah@example.com">
<what>
<include type="namespace">
urn:ietf:params:xml:ns:pidf</include>
<exclude>
//pidf:tuple/pidf:note</exclude>
</what>
</filter>
<filter id="8439" uri="sip:alice@biloxi.com">
<what>
<include>
//pidf:tuple/pidf:status/pidf:basic</include>
</what>
</filter>
</filter-set>
RLS1 fans out subscriptions to resources on list1. The text above
suggests that if a filter is destined to a resource that is not part
of the list and is outside the administrative domain of an RLS, then
that filter is propagated. The rest are consumed. In our example,
only the filter to alice@biloxi.com is propagated since biloxi.com is
not under the administrative domain of RLS1. The filter to
sarah@example.com is consumed, and RLS1 needs to apply that filter to
notifications it receives.
URI matching is done according to the matching rules defined for a
particular scheme (SIP URI matching rules are defined in RFC 3261
[2]).
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A filter may also be addressed to a domain using the 'domain'
attribute instead of the 'uri' attribute. In this case, the filter
applies to resources in that domain, and the RLS MUST NOT apply
filters to any notifications it sends. Instead, it MUST forward the
filter with all fanned-out subscriptions to the notifiers.
As indicated in Section 3.3.1, multiple filters can be present in a
SUBSCRIBE request. Filters can also be added or modified as
indicated in Section 3.3.3. In such circumstances, an RLS MUST check
that there are no filters addressed to the same resource or domain,
and if there are, it MUST reject the SUBSCRIBE request with a "488"
error response.
4.2. Changing Filters within a Dialog
If an RLS receives a subscription refresh request with no filters
specified (empty payload), the RLS assumes that the client does not
wish to update the filters. If an RLS receives a subscription
refresh with a filter containing the 'remove="true"' attribute, as
defined in [5], the RLS assumes that the client is removing that
filter identified by the filter ID.
If an RLS receives a subscription refresh request with a filter that
already exists (i.e., having the same filter ID), the RLS interprets
it as a replacement of the existing filter. Replacing a filter by
changing the filter ID and keeping the resource URI is considered an
error since this causes the RLS to assume that two filters are in
place for the same resource.
A filter can be disabled and re-enabled using the 'enabled' attribute
in the <filter> element, as described in [5].
5. Server Operation
5.1. NOTIFY Bodies
SIP entities compliant with this specification MUST support
content-type 'application/simple-filter+xml'.
5.2. Notifier Processing of SUBSCRIBE Requests
This section presents additional functionality required from the
notifier when filters are used in the bodies of the SUBSCRIBE
requests. Normal package-specific functionality is otherwise
followed.
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The notifier will examine the Content-Type header field and will
return a 415 response if it does not understand the content type
'application/simple-filter+xml'.
A 200-class response indicates that the subscription has been
accepted, and the NOTIFY will be sent immediately. A "200" response
indicates that the subscription has been accepted, the user is
authorized, and the filter is accepted. A "202" response merely
indicates that the subscription has been understood, but that the
authorization may or may not have been granted. A "202" response
also indicates that the filters have not been accepted yet. The
acceptance of the filters MAY arrive in a subsequent NOTIFY.
Procedures described in Section 5.4 are followed if an error is
encountered.
As indicated in Section 3.3.1, multiple filters can be present in a
SUBSCRIBE request. Filters can also be added or modified as
indicated in Section 3.3.3. In such circumstances, a server MUST
check that there are no filters addressed to the same resource or
domain, and if they are, it MUST reject the SUBSCRIBE request with a
"488" error response.
5.2.1. Request-URI vs. Filter URI
The subscriber may have chosen to leave the URI in the filter
undefined. If the URI is not defined within the filter, the filter
applies to the resource identified in the Request-URI.
Similarly, the subscriber may have chosen to include a URI in the
filter. In this case, the filter applies to all notifications sent
with content associated with the resource with that URI for this
subscription. If the Request-URI and the URI in the filter do not
match, the filter may be ignored or rejected. URI matching is done
according to the matching rules defined for a particular scheme (SIP
URI matching rules are defined in RFC 3261 [2]).
A filter may also be addressed to a domain using the 'domain'
attribute instead of the 'uri' attribute. In this case, the filter
applies to resources in that domain. A notifier MUST ignore any
filter using a 'domain' attribute containing a domain for which this
notifier is not responsible. The notifier MUST NOT apply such a
filter to any notification it sends. Notifiers belonging to the
domain MUST apply the filter to all notifications it sends for that
subscription, unless policy dictates otherwise.
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5.2.2. Changing Filters within a Dialog
If a server receives a subscription refresh request with no filters
specified (empty payload), it assumes that the client does not wish
to update the filters. If it receives a subscription refresh with a
filter containing the 'remove="true"' attribute, as defined in [5],
the server assumes that the client is removing the filter identified
by the filter ID.
If the server receives a subscription refresh request with a filter
that already exists (i.e., having the same filter ID), it interprets
it as a replacement of the existing filter. Replacing a filter by
changing the filter ID and keeping the resource URI is considered an
error since this causes the server to assume that two filters are
placed for the same resource.
5.3. Notifier Generating of NOTIFY Requests
Upon receiving the SUBSCRIBE with the filter, the notifier SHOULD
retain the filter as long as the subscription persists. The filter
MAY be incorporated within an existing subscription (in an active
dialog) by sending a re-SUBSCRIBE that includes the filter in the
body.
If the response sent to the SUBSCRIBE was a "202" and the "202" was
chosen because the filter could not be accepted that time, the NOTIFY
MAY be used to terminate the subscription if the filter is found
unacceptable.
As described in [3], the NOTIFY message MAY contain a body that
describes the state of the resource. This body is in one of the
formats listed in the Accept header field of the SUBSCRIBE, or in the
package-specific default if the Accept header field is omitted.
Based on the contents of a filter, the following processing occurs:
o A filter with only a <what> element will result in sending the
requested resource state information in that <what> element
whenever there is a change in the resource state.
o A filter with only a <trigger> element will result in sending all
resource state information whenever there is a change in the
resource state that matches the triggers.
o A filter with <what> and <trigger> elements will result in sending
the requested resource state information in that <what> element
whenever there is a change in the resource state that matches the
triggers.
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When a filter is disabled (by setting the 'enabled' attribute to
"false"), it means the same thing as the absence of that filter.
That is, all state and state changes are reported by issuing a
notification to the subscriber (assuming there are no other filters).
When a filter is re-enabled (by setting the 'enabled' attribute to
"true" or by omitting the 'enabled' attribute), the notifier behaves
as if the filter has just been placed by the SUBSCRIBE request
enabling it. Immediate NOTIFY rules, as stated in Section 5.3.1,
apply.
5.3.1. Generation of NOTIFY Contents
If the NOTIFY being sent is the one sent immediately after a 2xx
response to the original SUBSCRIBE, its contents MUST be populated
according to the filter <what> element, unless the processing of the
filters will take too long or the NOTIFY request is following a "202"
response to the SUBSCRIBE request and is terminating the
subscription. In the case that the filter is taking too long to
process, the NOTIFY request being sent may be empty or may be
populated with a pre-configured value as authorised to that
subscriber. If applying the filter results in no content to be
delivered, the NOTIFY MUST be sent with empty contents. If the
filter contains <trigger> elements, the notifier ignores the trigger
values when generating the first NOTIFY request.
The input to the content filter is a package-specific XML document
(e.g., [7] and [9]) derived according to the package-specific
specifications, (e.g., [8] and [10]).
The content is filtered according to the expressions in the <what>
element of the filter. The expression indicates the delivered XML
elements and/or attributes. Prefixes of the namespaces of the items
of the XML document to be filtered must be expanded before applying
the filter to the items.
The expression directly states the XML elements and attributes to be
delivered in the NOTIFY, along with their values. In addition to the
selected contents, the namespaces of all the selected items are also
included in the NOTIFY. The XML elements and/or attributes indicated
by the expression in the <what> element must be items that the
subscriber is authorised to see. If they are not, the notifier
policy dictates the behaviour of the notifier (which can ignore the
filter, parts of the filter, or reject the filter completely).
Implementers need to carefully consider such an implementation
decision; the subscriber may not be aware of the authorised contents
and therefore most likely will include a filter requesting
unauthorised contents. It is therefore RECOMMENDED that notifiers
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just ignore the parts of the filter that are requesting unauthorised
info (i.e., the filter in the <filter> element where the unauthorised
contents are requested is ignored). If polite blocking is used by
the notifier, the notifier may choose to deliver notifications
containing bogus information in the unauthorised elements or
attributes and applying the filter afterwards.
The resultant XML document MUST be well formed and valid according to
the XML schema. This means that all mandatory elements and
attributes, along with their values, MUST be included in the XML
document regardless of the expression. In other words, if the result
of applying a filter on an XML document is a non-valid XML document,
the notifier MUST add elements and attributes, along with their
values, from the original XML document into the newly formulated one
in order for it to be valid.
5.3.2. Handling of Notification Triggering Rules
There can be several <trigger> elements inside one <filter> element.
If the criteria for any of the <trigger> elements are satisfied, a
NOTIFY SHOULD be generated.
The items (XML elements and/or attributes) indicated by the
expression in the <changed> element, <added> element, or <removed>
element must be items that the subscriber is authorised to access.
If they are not, the notifier policy dictates the behaviour of the
notifier (which can ignore the filter, parts of the filter, or reject
the filter completely).
5.4. Handling Abnormal Cases
In case of an invalid filter definition where the XML document of the
filter is not aligned with the XML schema of the filter format [5],
the notifier rejects the SUBSCRIBE request with a "488" response. A
Warning header field in the response may give a better indication as
to why the filters were not accepted. If the subscription was
accepted with a "202" response but the invalid filter was discovered
after that, a NOTIFY with a subscription-state of value 'terminated'
is sent. An event-reason-value "badfilter", introduced here, of
subexp-params [3] MAY be included.
In case of an erroneous expression in the filter definition, the
notifier either ignores the filter definition or terminates the
subscription.
If a <what> or <trigger> element is empty, the notifier proceeds as
if the element did not exist.
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6. XML Document Validation
The subscriber of the filter MUST ensure that the XML document
inserted as the SUBSCRIBE request body is well formed and valid. The
subscriber MUST NOT insert any extension elements or attributes into
the XML document unless it has access to the extension schema and can
validate the XML document. The XML document notifier MAY validate
the XML document according to the schemas, including extension
schemas, to which it has access that are applicable to this XML
document.
7. Examples
The following sections include filtering examples for Presence and
Watcher Information. The format of filter is according to [5].
7.1. Presence Specific Examples
This section describes three use cases where the presence information
filtering solution is utilised [8]. In the first use case, the
watcher is interested in getting messaging-specific information of a
certain presentity. In the second use case, the watcher is
interested in getting information about the communication means and
contact addresses on which the presentity is currently available for
communication. The third case shows how a presentity can request
triggers to receive notifications.
Below is the presentity's presence information in PIDF [7]. It
includes two tuples: one for the instant messaging and another for
the voice-related information.
<?xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:rpid="urn:ietf:params:xml:ns:pidf:rpid"
entity="sip:presentity@example.com">
<tuple id="432sd">
<status>
<basic>closed</basic>
</status>
<rpid:class>IM</rpid:class>
<contact>im:presentity@example.com</contact>
</tuple>
<tuple id="thr76jk">
<status>
<basic>open</basic>
</status>
<rpid:class>voice</rpid:class>
<contact>tel:2224055555@example.com</contact>
Khartabil, et al. Standards Track [Page 14]
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</tuple>
</presence>
7.1.1. Subscriber Requests Messaging-Related Information
The subscriber initiates a subscription to the presentity's messaging
(MMS, IM, and SMS) related presence information. The subscription
includes the content limiting filter.
The filtered content is indicated with an expression. This
expression selects the <basic> element and all the parent elements
(i.e., the <status>, the <tuple>, and its root element), the <class>
element, and the <contact> element. The filter matches if the
<class> element contains "MMS", "SMS", or "IM".
In this case, the notification includes the contents of the tuple
that has the value "IM" in its <class> element.
SUBSCRIBE request from the subscriber including filter:
SUBSCRIBE sip:presentity@example.com
Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKxjfdsjfk
To: <sip:presentity@example.com>
From: <sip:watcher@example.com>;tag:12341111
Call-ID: 32432udfidfjmk342
Cseq: 1 SUBSCRIBE
Expires: 3600
Event: Presence
Contact: <sip:watcher@client.example.com>
Content-Type: application/simple-filter+xml
Content-Length: ...
<?xml version="1.0" encoding="UTF-8"?>
<filter-set xmlns="urn:ietf:params:xml:ns:simple-filter">
<ns-bindings>
<ns-binding prefix="pidf" urn="urn:ietf:params:xml:ns:pidf"/>
<ns-binding prefix="rpid"
urn="urn:ietf:params:xml:ns:pidf:rpid"/>
</ns-bindings>
<filter id="123" uri="sip:presentity@example.com">
<what>
<include type="xpath">
//pidf:tuple[rpid:class="IM" or rpid:class="SMS"
or rpid:class="MMS"]/pidf:status/pidf:basic
</include>
<include type="xpath">
//pidf:tuple[rpid:class="IM" or rpid:class="SMS"
or rpid:class="MMS"]/rpid:class
Khartabil, et al. Standards Track [Page 15]
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</include>
<include type="xpath">
//pidf:tuple[rpid:class="IM" or rpid:class="SMS"
or rpid:class="MMS"]/pidf:contact
</include>
</what>
</filter>
</filter-set>
Notification to the subscriber:
NOTIFY sip:watcher@client.example.com SIP/2.0
Via: SIP/2.0/TCP presence.example.com:5060;branch=z9hG4bKxjfder
To: <sip:watcher@example.com>;tag:12341111
From: <sip:presentity@example.com>;tag:232321
Call-ID: 32432udfidfjmk342
Cseq: 1 NOTIFY
Event: Presence
Subscription-State: active; expires=3599
Contact: sip:presentity@server.example.com
Content-Type: application/pidf+xml
Content-Length: ...
<?xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:rpid="urn:ietf:params:xml:ns:pidf:rpid"
entity="sip:presentity@example.com">
<tuple id="432sd">
<status>
<basic>closed</basic>
</status>
<rpid:class>IM</rpid:class>
<contact>im:presentity@example.com</contact>
</tuple>
</presence>
7.1.2. Subscriber Fetches Information about "Open" Communication Means
The subscriber makes a subscription to the presentity's available
communication means. The subscription includes the content-limiting
filter.
The filtered content is indicated with an expression. This
expression selects the <basic> element and all the parent elements
(i.e., the <status>, the <tuple>, and its root element), the <class>
element, and the <contact> element. The filter matches if the
<basic> element's value is "open".
Khartabil, et al. Standards Track [Page 16]
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In this case, the notification returns the contents of the tuple that
has the value "open" inside the <status> element.
SUBSCRIBE request from the subscriber including filter:
SUBSCRIBE sip:presentity@example.com SIP/2.0
Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKxjfdsjfk
To: <sip:presentity@example.com>
From: <sip:watcher@example.com>;tag:12341111
Call-ID: 32432udfidfjmk342
Cseq: 1 SUBSCRIBE
Expires: 3600
Event: Presence
Contact: <sip:watcher@client.example.com>
Content-Type: application/simple-filter+xml
Content-Length: ...
<?xml version="1.0" encoding="UTF-8"?>
<filter-set xmlns="urn:ietf:params:xml:ns:simple-filter">
<ns-bindings>
<ns-binding prefix="pidf" urn="urn:ietf:params:xml:ns:pidf"/>
<ns-binding prefix="rpid"
urn="urn:ietf:params:xml:ns:pidf:rpid"/>
</ns-bindings>
<filter id="123" uri="sip:presentity@example.com">
<what>
<include type="xpath">
//pidf:tuple/pidf:status[pidf:basic="open"]/pidf:basic
</include>
<include type="xpath">
//pidf:tuple[pidf:status/pidf:basic="open"]/rpid:class
</include>
<include type="xpath">
//pidf:tuple[pidf:status/pidf:basic="open"]/pidf:contact
</include>
</what>
</filter>
</filter-set>
Notification to the subscriber:
NOTIFY sip:watcher@client.example.com SIP/2.0
Via: SIP/2.0/TCP presence.example.com:5060;branch=z9hG4bKxjfder
To: <sip:watcher@example.com>;tag:12341111
From: <sip:presentity@example.com>;tag:232321
Call-ID: 32432udfidfjmk342
Cseq: 1 NOTIFY
Event: Presence
Khartabil, et al. Standards Track [Page 17]
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Subscription-State: active; expires=3599
Contact: sip:presentity@server.example.com
Content-Type: application/pidf+xml
Content-Length: ...
<?xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:rpid="urn:ietf:params:xml:ns:pidf:rpid"
entity="sip:presentity@example.com">
<tuple id="thr76jk">
<status>
<basic>open</basic>
</status>
<rpid:class>voice</rpid:class>
<contact>tel:2224055555@example.com</contact>
</tuple>
</presence>
7.1.3. Subscriber Requests Notifications When Presentity's Status
Changes
The subscriber subscribes to the presentity, specifying in the filter
that it wants notifications only when the <basic> element has changed
to value "open".
SUBSCRIBE request from the subscriber including filter:
SUBSCRIBE sip:presentity@example.com
Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKxjfdsjfk
To: <sip:presentity@example.com>
From: <sip:watcher@example.com>;tag:12341111
Call-ID: 32432udfidfjmk342
Cseq: 1 SUBSCRIBE
Expires: 3600
Event: Presence
Contact: <sip:watcher@client.example.com>
Content-Type: application/simple-filter+xml
Content-Length: ...
<?xml version="1.0" encoding="UTF-8"?>
<filter-set xmlns="urn:ietf:params:xml:ns:simple-filter">
<ns-bindings>
<ns-binding prefix="pidf" urn="urn:ietf:params:xml:ns:pidf"/>
</ns-bindings>
<filter id="123" uri="sip:presentity@example.com">
<trigger>
<changed from="closed" to="open">
/pidf:presence/pidf:tuple/pidf:status/pidf:basic
Khartabil, et al. Standards Track [Page 18]
RFC 4660 Functional Description of Filtering September 2006
</changed>
</trigger>
</filter>
</filter-set>
At some point during the subscription, a second PIDF document is
created with both tuples having a status of "closed":
<?xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:rpid="urn:ietf:params:xml:ns:pidf:rpid"
entity="sip:presentity@example.com">
<tuple id="432sd">
<status>
<basic>closed</basic>
</status>
<rpid:class>IM</rpid:class>
<contact>im:presentity@example.com</contact>
</tuple>
<tuple id="thr76jk">
<status>
<basic>closed</basic>
</status>
<rpid:class>voice</rpid:class>
<contact>tel:2224055555@example.com</contact>
</tuple>
</presence>
A NOTIFY is not sent to the subscriber in this case.
Now, a third PIDF document is created when the IM status changes to
"open":
<?xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:rpid="urn:ietf:params:xml:ns:pidf:rpid"
entity="sip:presentity@example.com">
<tuple id="432sd">
<status>
<basic>open</basic>
</status>
<rpid:class>IM</rpid:class>
<contact>im:presentity@example.com</contact>
</tuple>
<tuple id="thr76jk">
<status>
<basic>closed</basic>
</status>
Khartabil, et al. Standards Track [Page 19]
RFC 4660 Functional Description of Filtering September 2006
<rpid:class>voice</rpid:class>
<contact>tel:2224055555@example.com</contact>
</tuple>
</presence>
Notification containing both tuples is sent to the subscriber in this
case:
NOTIFY sip:watcher@client.example.com SIP/2.0
Via: SIP/2.0/TCP presence.example.com:5060;branch=z9hG4bKxjfder
To: <sip:watcher@example.com>;tag:12341111
From: <sip:presentity@example.com>;tag:232321
Call-ID: 32432udfidfjmk342
Cseq: 1 NOTIFY
Event: Presence
Subscription-State: active; expires=3599
Contact: sip:presentity@server.example.com
Content-Type: application/pidf+xml
Content-Length: ...
<?xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:rpid="urn:ietf:params:xml:ns:pidf:rpid"
entity="sip:presentity@example.com">
<tuple id="432sd">
<status>
<basic>closed</basic>
</status>
<rpid:class>IM</rpid:class>
<contact>im:presentity@example.com</contact>
</tuple>
<tuple id="thr76jk">
<status>
<basic>open</basic>
</status>
<rpid:class>voice</rpid:class>
<contact>tel:2224055555@example.com</contact>
</tuple>
</presence>
Khartabil, et al. Standards Track [Page 20]
RFC 4660 Functional Description of Filtering September 2006
7.2. Watcher Information Specific Examples
The examples in this section use the winfo template-package with the
presence event package [10].
Watcher information to a Presentity:
<?xml version="1.0"?>
<watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
version="0" state="full">
<watcher-list resource="sip:presentity@example.com"
package="presence">
<watcher status="active"
id="sr8fdsj"
duration-subscribed="509"
expiration="20"
event="approved">sip:watcherA@example.com"</watcher>
<watcher status="pending"
id="sr8fdsj"
duration-subscribed="501"
expiration="100"
event="subscribe">sip:watcherB@example.com"</watcher>
<watcher status="terminated"
id="sr8fdsj"
duration-subscribed="500"
expiration="0"
event="rejected">sip:watcherC@example.com"</watcher>
<watcher status="active"
id="sr8fdsj"
duration-subscribed="20"
expiration="30"
event="approved">sip:watcherD@example.com"</watcher>
</watcher-list>
</watcherinfo>
Khartabil, et al. Standards Track [Page 21]
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7.2.1. Watcher Subscriber Makes Subscription to Get All the Information
about Active Watchers
SUBSCRIBE request from the presentity including the filter:
SUBSCRIBE sip:presentity@example.com
Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKxjfdsjfk
To: <sip:presentity@example.com>
From: <sip:presentity@example.com>;tag:12341111
Call-ID: 32432udfidfjmk342
Cseq: 1 SUBSCRIBE
Expires: 3600
Event: Presence.winfo
Contact: sip:presentity@client.example.com
Content-Type: application/simple-filter+xml
Content-Length: ...
<?xml version="1.0" encoding="UTF-8"?>
<filter-set xmlns="urn:ietf:params:xml:ns:simple-filter">
<ns-bindings>
<ns-binding prefix="wi"
urn="urn:ietf:params:xml:ns:watcherinfo"/>
</ns-bindings>
<filter id="123" uri="sip:presentity@example.com">
<what>
<include>
/wi:watcherinfo/wi:watcher-list[@package="presence"]/
wi:watcher[@status="active"]
</include>
</what>
</filter>
</filter-set>
Notification to the subscriber:
NOTIFY sip:presentity@client.example.com SIP/2.0
Via: SIP/2.0/TCP presence.example.com:5060;branch=z9hG4bKxjfder
To: sip:presentity@example.com;tag:12341111
From: sip:presentity@example.com;tag:232321
Call-ID: 32432udfidfjmk342
Cseq: 1 NOTIFY
Contact: sip:presentity@server.example.com
Event: Presence.winfo
Content-Type: application/watcherinfo+xml
Content-Length: ...
Khartabil, et al. Standards Track [Page 22]
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<?xml version="1.0"?>
<watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
version="0" state="full">
<watcher-list resource="sip:presentity@example.com"
package="presence">
<watcher status="active"
id="sr8fdsj"
duration-subscribed="509"
expiration="20"
event="approved">sip:watcherA@example.com"</watcher>
<watcher status="active"
id="sr8fdsj"
duration-subscribed="20"
expiration="30"
event="approved">sip:watcherD@example.com"</watcher>
</watcher-list>
</watcherinfo>
7.2.2. Watcher Subscriber Requests Information of Watchers with
Specific Subscription Duration Conditions
SUBSCRIBE request from the presentity including the filter:
SUBSCRIBE sip:presentity@example.com
Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKxjfdsjfk
To: <sip:presentity@example.com>;tag:12341111
From: <sip:presentity@example.com>
Call-ID: 32432udfidfjmk342
Cseq: 1 SUBSCRIBE
Expires: 0
Event: Presence.winfo
Contact: <sip:presentity@client.example.com>
Content-Type: application/simple-filter+xml
Content-Length: ...
<?xml version="1.0" encoding="UTF-8"?>
<filter-set xmlns="urn:ietf:params:xml:ns:simple-filter">
<ns-bindings>
<ns-binding prefix="wi"
urn="urn:ietf:params:xml:ns:watcherinfo"/>
</ns-bindings>
<filter id="123" uri="sip:presentity@example.com">
<what>
<include>
/wi:watcherinfo/wi:watcher-list[@package="presence"]/
wi:watcher[@duration-subscribed>500]
</include>
</what>
Khartabil, et al. Standards Track [Page 23]
RFC 4660 Functional Description of Filtering September 2006
</filter>
</filter-set>
Notification to the subscriber:
NOTIFY sip:presentity@client.example.com SIP/2.0
Via: SIP/2.0/TCP presence.example.com:5060;branch=z9hG4bKxjfder
To: sip:presentity@example.com;tag:12341111
From: sip:presentity@example.com;tag:232321
Call-ID: 32432udfidfjmk342
Cseq: 1 NOTIFY
Contact: sip:presentity@server.example.com
Event: Presence.winfo
Content-Type: application/watcherinfo+xml
Content-Length: ...
<?xml version="1.0"?>
<watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
version="0" state="full">
<watcher-list resource="sip:presentity@example.com"
package="presence">
<watcher status="active"
id="sr8fdsj"
duration-subscribed="509"
expiration="20"
event="approved">sip:watcherA@example.com"</watcher>
<watcher status="pending"
id="sr8fdsj"
duration-subscribed="501"
expiration="100"
event="subscribe">sip:watcherB@example.com"</watcher>
</watcher-list>
</watcherinfo>
7.2.3. Watcher Subscriber Requests Specific Watcher Info on Specific
Triggers
This filter selects watcher information notifications [9] 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.
SUBSCRIBE request from the Watcher Subscriber including the filter:
SUBSCRIBE sip:presentity@example.com
Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKxjfdsjfk
To: <sip:presentity@example.com>;tag:12341111
Khartabil, et al. Standards Track [Page 24]
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From: <sip:presentity@example.com>
Call-ID: 32432udfidfjmk342
Cseq: 1 SUBSCRIBE
Expires: 0
Event: Presence.winfo
Contact: <sip:presentity@client.example.com>
Content-Type: application/simple-filter+xml
Content-Length: ...
<?xml version="1.0" encoding="UTF-8"?>
<filter-set xmlns="urn:ietf:params:xml:ns:simple-winfo-filter">
<ns-bindings>
<ns-binding prefix="wi"
urn="urn:ietf:params:xml:ns:watcherinfo"/>
</ns-bindings>
<filter id="123" uri="sip:presentity@example.com">
<what>
<include>
/wi:watcherinfo/wi:watcher-list[@package="presence"]/
wi:watcher[@status="terminated" and @event="rejected"]
</include>
</what>
<trigger>
<changed from="pending"
to="terminated">
//@status
</changed>
</trigger>
</filter>
</filter-set>
At some point during the subscription, a second Winfo document is
created due to some change:
<?xml version="1.0"?>
<watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
version="0" state="full">
<watcher-list resource="sip:presentity@example.com"
package="presence">
<watcher status="active"
id="sr8fdsj"
duration-subscribed="509"
expiration="20"
event="approved">sip:watcherA@example.com"</watcher>
<watcher status="terminated"
id="sr8fdsj"
duration-subscribed="501"
expiration="100"
Khartabil, et al. Standards Track [Page 25]
RFC 4660 Functional Description of Filtering September 2006
event="rejected">sip:watcherB@example.com"</watcher>
<watcher status="terminated"
id="sr8fdsj"
duration-subscribed="500"
expiration="0"
event="rejected">sip:watcherC@example.com"</watcher>
<watcher status="active"
id="sr8fdsj"
duration-subscribed="20"
expiration="30"
event="approved">sip:watcherD@example.com"</watcher>
</watcher-list>
</watcherinfo>
Notification to the subscriber is created, taking into account the
<trigger> and <what> elements:
NOTIFY sip:presentity@client.example.com SIP/2.0
Via: SIP/2.0/TCP presence.example.com:5060;branch=z9hG4bKxjfder
To: sip:presentity@example.com;tag:12341111
From: sip:presentity@example.com;tag:232321
Call-ID: 32432udfidfjmk342
Cseq: 1 NOTIFY
Contact: sip:presentity@server.example.com
Event: Presence.winfo
Content-Type: application/watcherinfo+xml
Content-Length: ...
<?xml version="1.0"?>
<watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
version="0" state="full">
<watcher-list resource="sip:presentity@example.com"
package="presence">
<watcher status="terminated"
id="sr8fdsj"
duration-subscribed="501"
expiration="100"
event="rejected">sip:watcherB@example.com"</watcher>
<watcher status="terminated"
id="sr8fdsj"
duration-subscribed="500"
expiration="0"
event="rejected">sip:watcherC@example.com"</watcher>
</watcher-list>
</watcherinfo>
Khartabil, et al. Standards Track [Page 26]
RFC 4660 Functional Description of Filtering September 2006
8. Security Considerations
The presence of filters in the body of a SIP message has a
significant effect on 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 authorized.
Authentication can be achieved using the Digest Authentication
mechanism described in [2]. The authorisation decision is based on
the permissions that the resource (notifier) has given to the
watcher. An example of such auhorisation policy can be found in
[11].
Processing of requests and looking up filters requires set operations
and searches, which can require some amount of computation. This
enables a DoS attack whereby a user can send requests with
substantial numbers of messages with large contents, in the hopes of
overloading the server. To counter this, the server can establish a
limit on the number of occurrences of the <what>, <changed>, <added>,
and <removed> elements that are allowed in the filters. A default
limit of 40 is RECOMMENDED; however, servers may raise or lower the
limit depending upon their specific engineered capacity.
Requests can reveal sensitive information about a User Agent's (UA's)
capabilities. If this information is sensitive, it SHOULD be
encrypted using SIP S/MIME capabilities [6]. All package-specific
security measures MUST be followed.
Propagating filters in SUBSCRIBE requests to foreign domains reveals
sensitive information about a user's resource lists. It is therefore
required that an RLS does not forward a filter if that filter is
addressed to a resource that is under the administrative domain of
the RLS, but that is not on the resource list. Section 4.1 shows an
example where such a scenario can occur.
Note that a filtered document located at a subscriber may project
false reality. For example, if a subscriber asked to be notified
when a resource has changed his presence state from "closed" to
"open" but not from "open" to "closed", then the subscriber may
afterwards be under the false impression that the resource's presence
state is "open", even long after the resource has changed it to
"closed". Therefore, subscribers need to be sure what they put in a
filter, understand what they asked for, and be prepared to be out of
sync with the real state of a resource.
Khartabil, et al. Standards Track [Page 27]
RFC 4660 Functional Description of Filtering September 2006
9. IANA Considerations
A new event-reason-value "badfilter" is defined to represent the
event where the filter is not well formed and/or not accepted. No
IANA registration is required for this value.
10. Acknowledgements
The authors would like to thank George Foti, Tim Moran, Sreenivas
Addagatla, Juha Kalliokulju, Jari Urpalainen, and Mary Barnes for
their valuable input.
11. References
11.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[3] Roach, A.B., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[4] Roach, A.B., Campbell, B., and J. Rosenberg, "A Session
Initiation Protocol (SIP) Event Notification Extension for
Resource Lists", RFC 4663, September 2006.
[5] Khartabil, H., Leppanen, E., Lonnfors, M., and J. Costa-Requena,
"An Extensible Markup Language (XML)-Based Format for Event
Notification Filtering", RFC 4661, September 2006.
[6] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851, July
2004.
11.2. Informative References
[7] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and
J. Peterson, "Presence Information Data Format (PIDF)", RFC
3863, August 2004.
[8] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.
Khartabil, et al. Standards Track [Page 28]
RFC 4660 Functional Description of Filtering September 2006
[9] Rosenberg, J., "An Extensible Markup Language (XML) Based Format
for Watcher Information", RFC 3858, August 2004.
[10] Rosenberg, J., "A Watcher Information Event Template-Package for
the Session Initiation Protocol (SIP)", RFC 3857, August 2004.
[11] Rosenberg, J., "Presence Authorization Rules", Work in Progress,
June 2006.
Khartabil, et al. Standards Track [Page 29]
RFC 4660 Functional Description of Filtering September 2006
Authors' Addresses
Hisham Khartabil
Telio
P.O. Box 1203 Vika
Oslo
Norway
Phone: +47 2167 3544
EMail: hisham.khartabil@telio.no
Eva Leppanen
Nokia
P.O BOX 785
Tampere
Finland
Phone: +358 7180 77066
EMail: eva-maria.leppanen@nokia.com
Mikko Lonnfors
Nokia
P.O BOX 321
Helsinki
Finland
Phone: + 358 71800 8000
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
Khartabil, et al. Standards Track [Page 30]
RFC 4660 Functional Description of Filtering September 2006
Full Copyright Statement
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Khartabil, et al. Standards Track [Page 31]