Network Working Group                                           A. Niemi
Internet-Draft                                                   K. Kiss
Intended status: Standards Track                                   Nokia
Expires: August 26, 2009                                       S. Loreto
                                                                Ericsson
                                                            Feb 22, 2009


   Session Initiation Protocol (SIP) Event Notification Extension for
                        Notification Throttling
                 draft-niemi-sipping-event-throttle-08

Status of this Memo

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   This Internet-Draft will expire on August 26, 2009.

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Abstract

   This memo specifies the throttle, forge and average mechanisms for
   adjusting the rate of Session Initiation Protocol (SIP) event
   notifications.  These mechanisms can be applied in subscriptions to
   all SIP event packages, but in particular the throttle mechanism is
   especially designed to be used in combination with a subscription to
   a Resource List Server (RLS).











































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Definitions and Document Conventions . . . . . . . . . . . . .  5
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Throttle Use Case  . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Force Use Case . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Average Use Case . . . . . . . . . . . . . . . . . . . . .  7
     3.4.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  7
     3.5.  Event Throttle Model for Resource List Server  . . . . . .  8
     3.6.  Basic Operation  . . . . . . . . . . . . . . . . . . . . . 10
     3.7.  Usage of Throttle, Force and Average in a subscription . . 11
   4.  Operation of Event Throttles . . . . . . . . . . . . . . . . . 12
     4.1.  Negotiating the Use of Throttle  . . . . . . . . . . . . . 12
     4.2.  Setting the Throttle . . . . . . . . . . . . . . . . . . . 12
       4.2.1.  Subscriber Behavior  . . . . . . . . . . . . . . . . . 12
       4.2.2.  Notifier Behavior  . . . . . . . . . . . . . . . . . . 13
     4.3.  Selecting the Throttle Interval  . . . . . . . . . . . . . 13
     4.4.  Buffer Policy Description  . . . . . . . . . . . . . . . . 14
       4.4.1.  Partial State Notifications  . . . . . . . . . . . . . 14
       4.4.2.  Full State Notifications . . . . . . . . . . . . . . . 14
     4.5.  Estimated Bandwidth Savings  . . . . . . . . . . . . . . . 14
   5.  Operation of Event Force . . . . . . . . . . . . . . . . . . . 15
     5.1.  Negotiating the Use of Force . . . . . . . . . . . . . . . 15
     5.2.  Setting the Force  . . . . . . . . . . . . . . . . . . . . 16
       5.2.1.  Subscriber Behavior  . . . . . . . . . . . . . . . . . 16
       5.2.2.  Notifier Behavior  . . . . . . . . . . . . . . . . . . 16
   6.  Operation of Event Average . . . . . . . . . . . . . . . . . . 17
     6.1.  Negotiating the Use of Average . . . . . . . . . . . . . . 17
     6.2.  Calculating the Average Interval . . . . . . . . . . . . . 17
     6.3.  Setting the Average  . . . . . . . . . . . . . . . . . . . 18
       6.3.1.  Subscriber Behavior  . . . . . . . . . . . . . . . . . 18
       6.3.2.  Notifier Behavior  . . . . . . . . . . . . . . . . . . 18
   7.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     7.1.  "throttle", "force" and "average" Header Field
           Parameters . . . . . . . . . . . . . . . . . . . . . . . . 19
     7.2.  Augmented BNF Definitions  . . . . . . . . . . . . . . . . 19
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 21
     11.2. Informative References . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22







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1.  Introduction

   The SIP events framework [RFC3265] defines a generic framework for
   subscriptions to and notifications of events related to SIP systems.
   This framework defines the methods SUBSCRIBE and NOTIFY, and
   introduces the concept of an event package, which is a concrete
   application of the SIP events framework to a particular class of
   events.

   One of the things the SIP events framework mandates is that each
   event package specification defines an absolute maximum on the rate
   at which notifications are allowed to be generated by a single
   notifier.  Such a limit is provided in order to reduce network
   congestion.

   All of the existing event package specifications include a maximum
   notification rate recommendation, ranging from once in every five
   seconds [RFC3856], [RFC3680], [RFC3857] to once per second [RFC3842].

   Per the SIP events framework, each event package specification is
   also allowed to define additional throttle mechanisms which allow the
   subscriber to further limit the rate of event notification.  So far
   none of the event package specifications have defined such a
   mechanism.

   The resource list extension [RFC4662] to the SIP events framework
   also deals with rate limiting of event notifications.  The extension
   allows a subscriber to subscribe to a heterogenous list of resources
   with a single SUBSCRIBE request, rather than having to install a
   subscription for each resource separately.  The event list
   subscription also allows rate limiting, or throttling of
   notifications, by means of the Resource List Server (RLS) buffering
   notifications of resource state changes, and sending them in batches.
   However, the event list mechanism provides no means for the
   subscriber to set the interval for the throttling; it is strictly an
   implementation decision whether batching of notifications is
   supported, and by what means.

   This document defines an extension to the SIP events framework
   defining the following three "Event" header field parameters that
   allow a subscriber to set a Minimum, a Maximum and an Average rate of
   event notifications generated by the notifier:

   Throttle:  specifies a minimum notification time period between two
      notifications, in seconds.






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   Force:  specifies a maximum notification time period between two
      notifications, in seconds.  Whenever the time since the most
      recent notification exceeds the value in the "force" parameter,
      then the current state would be sent in its entirety (just like
      after a subscription refresh).



   Average:  specifies an average cadence at which notifications are
      desired, in seconds.  It works similar to the "force" parameter,
      except that it will reduce the frequency at which notifications
      are sent if several have already been sent recently.



   The requirements and model are further discussed in Section 3.  All
   those mechanisms are simply timer values that indicates the minimum,
   maximum and average time period allowed between two notifications.
   As a result of those mechanism, a compliant notifier will adjust the
   rate at which it generates notifications.

   These mechanisms are applicable to any event subscription, both
   single event subscription and event list subscription.


2.  Definitions and Document Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119] and
   indicate requirement levels for compliant implementations.

      Indented passages such as this one are used in this document to
      provide additional information and clarifying text.  They do not
      contain normative protocol behavior.


3.  Overview

3.1.  Throttle Use Case

   A presence client in a mobile device contains a list of 100 buddies
   or presentities.  In order to decrease the processing and network
   load of watching 100 presentities, the presence client has employed a
   Resource List Server (RLS) with the list of buddies, and therefore
   only needs a single subscription to the RLS in order to receive



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   notification of the presence state of the resource list.

   In order to control the buffer policy of the RLS, the presence client
   sets a throttle interval via the event throttle extension.
   Alternatively, the presence client could set a default throttle for
   the resource list, via a list manipulation interface, e.g., using the
   XML Configuration Access Protocol (XCAP) [RFC4825].

   The RLS will buffer notifications that do not comply with the
   throttle interval, and batch all of the buffered state changes
   together in a single notification when allowed by the throttle.  The
   throttle applies to the overall resource list, which means that there
   is a hard cap imposed by the throttle to the amount of traffic the
   presence client can expect to receive.

   For example, with a throttle of 20 seconds, the presence application
   can expect to receive a notification every 20 seconds at a maximum.

   The presence client can also modify the throttle during the lifetime
   of the subscription.  For example, if the User Interface (UI) of the
   application shows inactivity for a period of time, it can simply
   pause the notifications by setting the throttle interval to the
   subscription expiration time, while still keeping the subscription
   alive.  When the user becomes active again, the presence client can
   resume the stream of notifications by re-setting the throttle to the
   earlier used value.

      Currently, a subscription refresh is needed in order to update the
      throttle interval.  However, this is highly inefficient, since
      each refresh automatically generates a (full-state) notification
      carrying the latest resource state.  There is work
      [I-D.ietf-sip-subnot-etags] ongoing to solve these inefficiencies.

3.2.  Force Use Case

   A location application is monitoring the movement of a target.

   In order to decrease the processing and network load, the location
   application has made a subscription with a set of location filters
   [I-D.ietf-geopriv-loc-filters] that specify, for example, to send an
   update only when the target has moved at least 10 meters.  However
   the application is interested in receiving an update periodically
   even if the target has not moved more than 10 meters in a second.

   The application is interested in discovering if the state is changed,
   even when it has not changed enough to satisfy any of the 'trigger'
   criteria




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   In order to control the actual state, the location application sets a
   force interval via the event force extension.  The force triggers a
   notification that is exactly and precisely like a notification after
   a subscription refresh.

   The location application can also modify the force during the
   lifetime of the subscription.

3.3.  Average Use Case

   The throttle and force mechanisms introduce a static and
   instantaneous rate control.  However there are some applications that
   would work better with an adaptive rate control (i.e. an average
   rate).  This section illustrates the tracking scenario.

   A tracking application is monitoring a target.

   In order to decrease the processing and network load, the tracking
   application wants to make a subscription that dynamically reduces the
   frequency at which notifications are sent if the target has started
   to move sending out already several notifications recently.

   In order to set an average rate control, the application defines a
   average interval via the event average extension.  The average value
   is used by the notifier to dynamically calculate the maximum time
   allowed between two subscriptions.  In order to dinamically calculate
   the maximum, the Notifer takes into consideration the frequency at
   which notifications have been sent recently.

   The average rate control allows the notifier to dynamically increase
   or decrease the Notification frequency.

   The tracking application can also modify the average interval during
   the lifetime of the subscription by setting the event average
   extension to a different value.

3.4.  Requirements

   REQ1:   The subscriber must be able to set the minimum time
           (throttle) period between two notifications in a specific
           subscription.

   REQ2:   The subscriber must be able to set the maximum time period
           (force) between two notifications in a specific subscription.







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   REQ3:   The subscriber must be able to set an average cadence
           (average) at which notifications are desired in a specific
           subscription.

   REQ4:   It must be possible to apply all together, or in any
           combination, the throttle, force and average mechanisms in a
           specific subscription.

   REQ5:   It must be possible to use of the throttle, force and average
           mechanisms in subscriptions to any events.

   REQ6:   It must be possible to use the throttle, force and average
           mechanisms together with any other event filtering
           mechanisms.

   REQ7:   The notifier must be allowed to use a throttling policy in
           which the minimum time period between two notifications is
           adjusted from the value given by the subscriber.

              For example, due to congestion reasons, local policy at
              the notifier could temporarily dictate a throttling policy
              that in effect increases the subscriber-configured minimum
              time period between two notifications.

   REQ8:   The throttle mechanism must discuss corner cases for setting
           the minimum period between two notifications.  At a minimum,
           the throttling mechanism must include discussion of the
           situation resulting from a minimum time period which exceeds
           the subscription duration, and should provide mechanisms for
           avoiding this situation.

   REQ9:   A throttle, force and average must be possible to be
           installed, modified, or removed in the course of an active
           subscription.

   REQ10:  A throttle, force and average mechanism must allow for the
           application of authentication and integrity protection
           mechanisms to subscriptions invoking that mechanism.

      Note that Section 9 contains further discussion on the security
      implications of the throttle mechanism.

3.5.  Event Throttle Model for Resource List Server

   When applied to a list subscription, the event throttle mechanism has
   some additional considerations.  Specifically, the throttle applies
   to the aggregate notification stream resulting from the list
   subscription, rather than explicitly controlling the notification of



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   each of the implied constituent events.  Moreover, the list event
   notifier can use the throttle mechanism on its own to control the
   rate of the individual subscriptions to avoid overflowing its buffer.

   The notifier is responsible for sending out event notifications upon
   state changes of the subscribed resource.We can model the notifier as
   consisting of three components: the event state resource(s), the
   Resource List Server (RLS) (or any other notifier), a notification
   buffer, and finally the subscriber, or watcher of the event state, as
   shown in Figure 1.

                       +--------+
                       | Event  |
        +--------+     |Resource|     +--------+
        | Event  |     +--------+     | Event  |
        |Resource|         |          |Resource|
        +---.=---+         |          +---=----+
              `-..         |         _.--'
                  ``-._    |    _.--'
                       +'--'--'-+
                       |Resource|
                       |  List  |
                       | Server |
                       +---.----+
                           |
                           |
                        )--+---(
                        |      |       .--------.
                        |Buffer|<======'Throttle|
                        |      |       `--------'
                        )--.---(
                           |
                           |
                       .---+---.
                       | Event |
                       |Watcher|
                       `-------'


       Figure 1: Model for the Resource List Server (RLS) Supporting
                                Throttling

   In short, the RLS reads event state changes from the event state
   resource, either by creating a backend subscription, or by other
   means; it packages them into event notifications, and submits them
   into the output buffer.  The rate at which this output buffer drains
   is controlled by the subscriber via the event throttle mechanism.
   When a set of notifications are batched together, the way in which



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   overlapping resource state is handled depends on the type of the
   resource state:

      In theory, there are many buffer policies that the notifier could
      implement.  However, we only concentrate on two practical buffer
      policies in this specification, leaving additional ones for
      further study and out of the scope of this work.  These two buffer
      policies depend on the mode in which the notifier is operating.

   Full-state:   Last (most recent) full state notification of each
      resource is sent out, and all others in the buffer are discarded.
      This policy applies to those event packages that carry full-state
      notifications.

   Partial-state:   The state deltas of each buffered partial
      notification per resource are merged, and the resulting
      notification is sent out.  This policy applies to those event
      packages that carry partial-state notifications.

3.6.  Basic Operation

   A subscriber that wants to limit the rate of event notification in a
   specific event subscription does so by including a throttle as part
   of the SUBSCRIBE request.  The throttle indicating the minimum time
   allowed between transmission of two consecutive notifications in a
   subscription is given as an Event header parameter in the SUBSCRIBE
   request.

      Note that the witnessed time between two consecutive received
      notifications may not conform to the set throttle for a number of
      reasons.  For example, network jitter and retransmissions may
      result in the subscriber receiving the notifications in lesser
      intervals than what the throttle recommends.

   A subscriber that wants to have a maximum notification time period in
   a specific event subscription does so by including a force as part of
   the SUBSCRIBE request.  The force indicating the maximum time allowed
   between transmission of two consecutive notifications in a
   subscription is given as an Event header parameter in the SUBSCRIBE
   request.

   A subscriber that wants to have an average cadence at which
   notifications are desired in a specific event subscription does so by
   including an average as part of the SUBSCRIBE request.  The average
   is given as an Event header parameter in the SUBSCRIBE request.

   A notifier that supports the throttle, force and average mechanisms
   will comply with value given in the throttle, force and average and



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   adjust its rate of notification accordingly.  However, if the
   notifier needs to lower the subscription expiration value or a local
   policy at the notifier can not meet the requested throttle value,
   then the notifier can adjust opportunely the received throttle value.

   Throttled, forced and averaged notifications will have exactly the
   same properties as the ones the un-throttled, un-forced and un-
   averaged, with the exception that they will be generated with the
   frequency that has been requested.

3.7.  Usage of Throttle, Force and Average in a subscription

   Applications can subscribe to an event package using all the
   throttle, force and average mechanisms singly, or in combination; in
   fact there is no technical incompatibility among them.  However there
   are some combinations that make little sense to be used together.
   This section lists all the possible combinations that is possible to
   insert in a subscription; the utility to use each combination in a
   subscription is also analyzed.

   Throttle and Force:  this combination let possible to reduce the
      notification frequence rate, but at same time assures the
      reception of a notification every time the most recent
      notification exceeds a specified interval.

      A subscriber SHOULD choose a "force" value higher than the
      "throttle" value, otherwise the notifier MUST adjust the
      subscriber provided "force" value to a value equivalent or higher
      than the "throttle" value.

   Throttle and Average:  it works in a similar way as the combination
      above, but with the difference that the interval at which
      notifications are assured changes dynamically.

      A subscriber SHOULD choose an "average" value higher than the
      "throttle" value, otherwise the notifier MUST adjust the
      subscriber provided "average" value to a value equivalent or
      higher than the "throttle" value.

   Force and Average:  as both the parameters are designed to force an
      update, this combination makes sense only in some corner cases.

      A subscriber SHOULD choose a "force" value higher than the
      "average" value, otherwise the notifier MUST not consider the
      "force" value.






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   Throttle, Force and Average:  this combination makes little sense to
      be used.


4.  Operation of Event Throttles

4.1.  Negotiating the Use of Throttle

   A subscriber that wishes to apply a throttle to notifications in a
   subscription constructs a SUBSCRIBE request that includes a throttle
   interval in a "throttle" Event header field parameter.

   A compliant notifier will reflect back the possibly adjusted throttle
   interval in a "throttle" Subscription-State header field parameter of
   the subsequent NOTIFY requests.  The indicated throttle value is
   adopted by the notifier, and the notification rate is adjusted
   accordingly.

   A notifier that does not understand the event-throttle extension,
   will not reflect the "throttle" parameter in the NOTIFY requests; the
   absence of this parameter serves as a hint to the subscriber that no
   throttling is supported by the notifier.

   A subscriber that wishes to remove a throttle from notifications
   constructs a SUBSCRIBE request that does not include a "throttle"
   Event header field parameter.

4.2.  Setting the Throttle

4.2.1.  Subscriber Behavior

   In general, the way in which a subscriber generates SUBSCRIBE
   requests and processes NOTIFY requests is according to RFC 3265
   [RFC3265].

   A subscriber that wishes to throttle the notifications in a
   subscription includes a "throttle" Event header parameter in the
   SUBSCRIBE request, indicating in seconds the desired throttle value.
   The value of this parameter is an integral number of seconds in
   decimal.

   There are two main consequences for the subscriber when applying the
   throttle mechanism: state transitions may be lost, and event
   notifications may be delayed.  If either of these side effects
   constitute a problem to the application that is to utilize event
   throttles, developers are instructed not to use the mechanism.





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4.2.2.  Notifier Behavior

   In general, the way in which a notifier processes SUBSCRIBE requests
   and generates NOTIFY requests is according to RFC 3265 [RFC3265].

   A notifier that supports the event-throttle extension extracts the
   value of the "throttle" Event header parameter, and uses it as the
   suggested minimum time allowed between two notifications.  This value
   can be adjusted by the notifier, as defined in Section 4.3.

   The notifier MUST reflect back the possibly adjusted throttle
   interval in a "throttle" Subscription-State header field parameter of
   the subsequent NOTIFY requests.

   A compliant notifier MUST NOT generate notifications more frequent
   than what the throttle allows for, except when generating the
   notification either upon receipt of a SUBSCRIBE request (the first
   notification), when the subscription state is changing from "pending"
   to "active" state or upon termination of the subscription (the last
   notification).  Such notifications reset the throttle timer, even
   though they do not need to abide by it.

   Retransmissions of NOTIFY requests are not affected by the throttle,
   i.e., the throttle only applies to the generation of new
   transactions.  In other words, the throttle is reset only after the
   previous transaction has completed.

4.3.  Selecting the Throttle Interval

   Special care needs to be taken when selecting the throttle value.
   Using the throttle syntax it is possible to insist both very short
   and very long throttles to be applied to the subscription.  For
   example, a throttle could potentially set a minimum time value
   between notifications that exceeds the subscription expiration value.
   Such a configuration would effectively quench the notifier, resulting
   in exactly two notifications to be generated.

   In some cases it makes sense to pause the notification stream on an
   existing subscription dialog on a temporary basis without terminating
   the subscription, e.g. due to inactivity on the application UI.
   Whenever a subscriber discovers the need to perform the notification
   pause operation, it SHOULD set the throttle interval to the remaining
   subscription expiration value.  This results in receiving no further
   notifications until the subscription expires, renewed or
   notifications are resumed by the subscriber.

   The notifier is responsible for adjusting the proposed throttle value
   based on its local policy or other properties.



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   If the subscriber requests a throttle greater than the subscription
   expiration,the notifier MUST lower the throttle value and set it to
   the expiration time left.  According to RFC 3265 [RFC3265] the
   notifier may also shorten the subscription expiry anytime during an
   active subscription.  For such cases, the notifier MUST also lower
   the throttle value and set it to the reduced expiration time.

   The notifier MAY also choose a higher throttle value, e.g., because
   of static throttle value configuration given by local policy.  The
   notifier MUST include the adjusted throttle value in the
   Subscription-State header field's "throttle" parameter in each of the
   NOTIFY requests.  In addition, different event packages MAY define
   additional constraints to the allowed throttle intervals.  Such
   constraints are out of the scope of this specification.

4.4.  Buffer Policy Description

4.4.1.  Partial State Notifications

   With partial notifications, the notifier will always need to keep
   both a copy of the current full state of the resource F, as well as
   the last successfully communicated full state view F' of the resource
   in a specific subscription.  The construction of a partial
   notification then involves creating a diff of the two states, and
   generating a notification that contains that diff.

   When a throttle is applied to the subscription, it is important that
   F' is replaced with F only when the throttle is reset.  Additionally,
   the notifier implementation SHOULD check to see that the size of an
   accumulated partial state notification is smaller than the full
   state, and if not, the notifier SHOULD send the full state
   notification instead.

4.4.2.  Full State Notifications

   With full state notifications, the notifier only needs to keep the
   full state of the resource, and when that changes, send the resulting
   notification over to the subscriber.

   When a throttle is applied in the subscription, the notifier receives
   the state changes of the resource, and generates a notification.  If
   there is a pending notification, the notifier simply replaces that
   notification with the new notification, discarding the older state.

4.5.  Estimated Bandwidth Savings

   It is difficult to estimate the total bandwidth savings accrued by
   using the throttle mechanism over a subscription, since such



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   estimates will vary depending on the usage scenarios.  However, it is
   easy to see that given a subscription where several full state
   notification would have normally been sent in any given throttle
   interval, a throttled subscription would only send a single
   notification during the same interval, yielding bandwidth savings of
   several times the notification size.

   With partial-state notifications, drawing estimates is further
   complicated by the fact that the states of consecutive updates may or
   may not overlap.  However, even in the worst case scenario, where
   each partial update is to a different part of the full state, a
   throttled notification merging all of these n partial states together
   should at a maximum be the size of a full-state update.  In this
   case, the bandwidth savings are approximately n times the size of the
   NOTIFY header.

   It is also true that there are several compression schemes available
   that have been designed to save bandwidth in SIP, e.g., SigComp
   [RFC3320] and TLS compression [RFC3943].  However, such compression
   schemes are complementary rather than competing mechanisms to the
   throttle mechanism.  After all, they can both be applied
   simultaneously, and in such a way that the compound savings are as
   good as the sum of applying each one alone.


5.  Operation of Event Force

5.1.  Negotiating the Use of Force

   A subscriber that wishes to apply a maximum notification time period
   between two notifications in a subscription constructs a SUBSCRIBE
   request that includes a proposed maximum interval in a "force" Event
   header field parameter.

   A compliant notifier will reflect back the possibly adjusted forced
   interval in a "force" Subscription-State header field parameter of
   the subsequent NOTIFY requests.  The indicated force value is adopted
   by the notifier, and the notification rate is adjusted accordingly.

   A notifier that does not understand the event-force extension, will
   not reflect the "force" parameter in the NOTIFY requests; the absence
   of this parameter serves as a hint to the subscriber that no forcing
   is supported by the notifier.

   A subscriber that wishes to remove a force from notifications
   constructs a SUBSCRIBE request that does not include a "force" Event
   header field parameter.




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5.2.  Setting the Force

5.2.1.  Subscriber Behavior

   In general, the way in which a subscriber generates SUBSCRIBE
   requests and processes NOTIFY requests is according to RFC 3265
   [RFC3265].

   A subscriber that wishes to apply a maximum notification time period
   between the notifications in a subscription includes a "force" Event
   header parameter in the SUBSCRIBE request, indicating in seconds the
   desired force value.  The value of this parameter is an integral
   number of seconds in decimal.

   The main consequence for the subscriber when applying the force
   mechanism is that it can receive a notification even if nothing has
   changed in the current state of the notifier.

   There is work [I-D.ietf-sip-subnot-etags] ongoing to only send a
   reference in a notification if nothing has changed.

5.2.2.  Notifier Behavior

   In general, the way in which a notifier processes SUBSCRIBE requests
   and generates NOTIFY requests is according to RFC 3265 [RFC3265].

   A notifier that supports the event-force extension extracts the value
   of the "force" Event header parameter, and uses it as the suggested
   maximum time allowed between two notifications.  This value can be
   adjusted by the notifier based on its local policy or other
   properties.

   The notifier MUST reflect back the possibly adjusted force value in a
   "force" Subscription-State header field parameter of the subsequent
   NOTIFY requests.

   A compliant notifier MUST generate notifications whenever the time
   since the most recent notification exceeds the value in the "force"
   parameter.  The NOTIFY request then MUST contain the current state in
   its entirety, just like after a subscription refresh.

   Retransmissions of NOTIFY requests are not affected by the force,
   i.e., the force only applies to the generation of new transactions.
   In other words, the force is reset only after the previous
   transaction has completed.






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6.  Operation of Event Average

6.1.  Negotiating the Use of Average

   A subscriber that wishes to apply an average cadence at which
   notifications are desired in a subscription constructs a SUBSCRIBE
   request that includes a proposed average interval in an "average"
   Event header field parameter.

   A compliant notifier will reflect back the possibly adjusted average
   interval in an "average" Subscription-State header field parameter of
   the subsequent NOTIFY requests.  The indicated average value is
   adopted by the notifier, and the notification rate is adjusted
   accordingly.

   A notifier that does not understand the event-average extension will
   not reflect the "average" parameter in the NOTIFY requests; the
   absence of this parameter serves as a hint to the subscriber that no
   averaging is supported by the notifier.

   A subscriber that wishes to remove a average from notifications
   constructs a SUBSCRIBE request that does not include an "average"
   Event header field parameter.

6.2.  Calculating the Average Interval

   The formula used to vary the absolute pacing in a way that will meet
   the average requested over the period is given in equation (1):

           timeout = (average ^ 2) * count / period                (1)


   The output of the formula, "timeout", is the time to the next
   notification, expressed in seconds.  The formula has three inputs:

   average:  The value of the "average" parameter conveyed in the
      "Event" header field, in seconds.

   period:  The rolling average period, in seconds.  A suggested
      reasonable period is 60 seconds.

      [OPEN ISSUE]Is the period value something we should be able to
      tune, or we can simply specify a reasonable period?

   count:  The number of notifications that have been sent during the
      last "period" of seconds.

   In the case both the Throttle and the Average are used in the same



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   subscription the formula used to dynamically calculate the timeout is
   given in equation (2):

           timeout = MAX[throttle, (average ^ 2) * count / period]  (2)


   throttle:  The value of the "threshold" parameter conveyed in the
      "Event" header field, in seconds.

   The formula in (2) makes sure that for all the possible value of
   throttle and average, with average > throttle, the timeout never
   results in a lower value than throttle.

6.3.  Setting the Average

6.3.1.  Subscriber Behavior

   In general, the way in which a subscriber generates SUBSCRIBE
   requests and processes NOTIFY requests is according to RFC 3265
   [RFC3265].

   A subscriber that wishes to apply an average cadence at which
   notifications are desired in a subscription includes a "average"
   Event header parameter in the SUBSCRIBE request, indicating in
   seconds the desired average value.  The value of this parameter is an
   integral number of seconds in decimal.

   The main consequence for the subscriber when applying the average
   mechanism is that it can receive a notification even if nothing has
   changed in the current state of the notifier.

   There is work [I-D.ietf-sip-subnot-etags] ongoing to only send a
   reference in a notification if nothing has changed.

6.3.2.  Notifier Behavior

   In general, the way in which a notifier processes SUBSCRIBE requests
   and generates NOTIFY requests is according to RFC 3265 [RFC3265].

   A notifier that supports the event-average extension extracts the
   value of the "average" Event header parameter, and uses it to
   calculate the maximum time allowed between two transactions as
   defined in Section 6.2.  This value can be adjusted by the notifier
   based on its local policy or other properties.

   The notifier MUST reflect back the possibly adjusted average value in
   a "average" Subscription-State header field parameter of the
   subsequent NOTIFY requests.



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   A compliant notifier MUST generate notifications whenever the time
   since the most recent notification exceeds the value calculated using
   the formula defined in Section 6.2.

   The Average mechanism is implemented as follows:

   1)  When a subscription is first created, the notifier creates a
       record that keeps track of the number of notifications that have
       been sent in the "period".  This record is initialized to contain
       a history of having sent one message every "average" seconds for
       the "period".

   2)  The "timeout" value is calculated according to the equation given
       in section Section 6.2.

   3)  If the timeout period passes without a NOTIFY request being sent
       in the subscription, then the current resource state is sent
       (subject to any filtering associated with the subscription).

   4)  Whenever a NOTIFY request is sent (regardless of whether due to a
       timeout or a state change), the notifier updates the notification
       history record, recalculates the value of "timeout," and returns
       to step 3.

   Retransmissions of NOTIFY requests are not affected by the timeout,
   i.e., the timeout only applies to the generation of new transactions.
   In other words, the timeout is reset only after the previous
   transaction has completed.


7.  Syntax

   This section describes the syntax extensions required for the
   throttle, force and average mechanisms.

7.1.  "throttle", "force" and "average" Header Field Parameters

   The "throttle", "force" and "average" parameters are added to the
   rule definitions of the Event header field and the Subscription-State
   header field in the SIP Events [RFC3265] grammar.  Usage of this
   parameter is described in section Section 4.2.

7.2.  Augmented BNF Definitions

   This section describes the Augmented BNF [RFC5234] definitions for
   the new syntax elements.  Note that we derive here from the ruleset
   present in SIP Events [RFC3265], adding additional alternatives to
   the alternative sets of "event-param" and "subexp-params" defined



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   therein.

      event-param    =/  throttle-param
      subexp-params  =/  throttle-param
      throttle-param =   "throttle" EQUAL delta-seconds

      event-param    =/  force-param
      subexp-params  =/  force-param
      throttle-param =   "force" EQUAL delta-seconds

      event-param    =/  average-param
      subexp-params  =/  average-param
      throttle-param =   "average" EQUAL delta-seconds


8.  IANA Considerations

   This specification registers three new SIP header field parameters,
   defined by the following information which is to be added to the
   Header Field Parameters and Parameter Values sub-registry under
   http://www.iana.org/assignments/sip-parameters.

                                             Predefined
      Header Field         Parameter Name      Values     Reference
      -------------------- ---------------   ----------   ---------
      Event                throttle          No           [RFCxxxx]
      Subscription-State   throttle          No           [RFCxxxx]
      Event                force             No           [RFCxxxx]
      Subscription-State   force             No           [RFCxxxx]
      Event                average           No           [RFCxxxx]
      Subscription-State   average           No           [RFCxxxx]

   (Note to the RFC Editor: please replace "xxxx" with the RFC number of
   this specification, when assigned.)


9.  Security Considerations

   Naturally, the security considerations listed in SIP events
   [RFC3265], which the throttle mechanism extends, apply in entirety.
   In particular, authentication and message integrity SHOULD be applied
   to subscriptions with the event-throttle extension.


10.  Acknowledgements

   Thanks to Pekka Pessi, Dean Willis, Eric Burger, Alex Audu, Alexander
   Milinski, Jonathan Rosenberg, Cullen Jennings, Adam Roach, Hisham



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   Khartabil and Dale Worley for support and/or review of this work.

   Thanks to Brian Rosen for the idea of the "force" and "average"
   mechanisms, and to Adam Roach for the work on the averaging
   algorithm.


11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  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.

   [RFC3265]  Roach, A., "Session Initiation Protocol (SIP)-Specific
              Event Notification", RFC 3265, June 2002.

   [RFC4662]  Roach, A., Campbell, B., and J. Rosenberg, "A Session
              Initiation Protocol (SIP) Event Notification Extension for
              Resource Lists", RFC 4662, August 2006.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

11.2.  Informative References

   [I-D.ietf-geopriv-loc-filters]
              Mahy, R. and B. Rosen, "A Document Format for Filtering
              and Reporting Location Notications in the  Presence
              Information Document Format Location Object (PIDF-LO)",
              draft-ietf-geopriv-loc-filters-03 (work in progress),
              November 2008.

   [I-D.ietf-sip-subnot-etags]
              Niemi, A., "An Extension to Session Initiation Protocol
              (SIP) Events for Conditional  Event Notification",
              draft-ietf-sip-subnot-etags-03 (work in progress),
              July 2008.

   [RFC3320]  Price, R., Bormann, C., Christoffersson, J., Hannu, H.,
              Liu, Z., and J. Rosenberg, "Signaling Compression
              (SigComp)", RFC 3320, January 2003.




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   [RFC3680]  Rosenberg, J., "A Session Initiation Protocol (SIP) Event
              Package for Registrations", RFC 3680, March 2004.

   [RFC3842]  Mahy, R., "A Message Summary and Message Waiting
              Indication Event Package for the Session Initiation
              Protocol (SIP)", RFC 3842, August 2004.

   [RFC3856]  Rosenberg, J., "A Presence Event Package for the Session
              Initiation Protocol (SIP)", RFC 3856, August 2004.

   [RFC3857]  Rosenberg, J., "A Watcher Information Event Template-
              Package for the Session Initiation Protocol (SIP)",
              RFC 3857, August 2004.

   [RFC3943]  Friend, R., "Transport Layer Security (TLS) Protocol
              Compression Using Lempel-Ziv-Stac (LZS)", RFC 3943,
              November 2004.

   [RFC4825]  Rosenberg, J., "The Extensible Markup Language (XML)
              Configuration Access Protocol (XCAP)", RFC 4825, May 2007.


Authors' Addresses

   Aki Niemi
   Nokia
   P.O. Box 407
   NOKIA GROUP, FIN  00045
   Finland

   Phone: +358 50 389 1644
   Email: aki.niemi@nokia.com


   Krisztian Kiss
   Nokia
   313 Fairchild Dr
   Mountain View, CA  94043
   US

   Phone: +1 650 391 5969
   Email: krisztian.kiss@nokia.com









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   Salvatore Loreto
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: salvatore.loreto@ericsson.com












































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