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.












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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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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.