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Versions: (draft-niemi-sipping-event-throttle) 00 01 02 03 04 05 06 07 08 09 RFC 6446

Network Working Group                                           A. Niemi
Internet-Draft                                                   K. Kiss
Intended status: Standards Track                                   Nokia
Expires: January 10, 2011                                      S. Loreto
                                                                Ericsson
                                                           July 09, 2010


   Session Initiation Protocol (SIP) Event Notification Extension for
                       Notification Rate Control
                draft-ietf-sipcore-event-rate-control-04

Abstract

   This document specifies mechanisms for adjusting the rate of Session
   Initiation Protocol (SIP) event notifications.  These mechanisms can
   be applied in subscriptions to all SIP event packages.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 10, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as



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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
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   it for publication as an RFC or to translate it into languages other
   than English.






































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Definitions and Document Conventions . . . . . . . . . . . . .  5
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Use Case for limiting the maximum rate of notifications  .  5
     3.2.  Use Case for setting a minimum rate for notifications  . .  6
     3.3.  Use Case for specifying the average rate of
           notifications  . . . . . . . . . . . . . . . . . . . . . .  7
     3.4.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  7
     3.5.  The maximum rate mechanism for Resource List Server  . . .  8
     3.6.  Basic Operation  . . . . . . . . . . . . . . . . . . . . . 10
   4.  Operation of the maximum rate mechanism  . . . . . . . . . . . 11
     4.1.  Subscriber Behavior  . . . . . . . . . . . . . . . . . . . 11
     4.2.  Notifier Behavior  . . . . . . . . . . . . . . . . . . . . 11
     4.3.  Selecting the maximum rate . . . . . . . . . . . . . . . . 12
     4.4.  Buffer Policy Description  . . . . . . . . . . . . . . . . 13
       4.4.1.  Partial State Notifications  . . . . . . . . . . . . . 13
       4.4.2.  Full State Notifications . . . . . . . . . . . . . . . 14
     4.5.  Estimated Bandwidth Savings  . . . . . . . . . . . . . . . 14
   5.  Operation of the minimum rate mechanism  . . . . . . . . . . . 14
     5.1.  Subscriber Behavior  . . . . . . . . . . . . . . . . . . . 15
     5.2.  Notifier Behavior  . . . . . . . . . . . . . . . . . . . . 15
   6.  Operation of the average rate mechanism  . . . . . . . . . . . 16
     6.1.  Subscriber Behavior  . . . . . . . . . . . . . . . . . . . 16
     6.2.  Notifier Behavior  . . . . . . . . . . . . . . . . . . . . 17
     6.3.  Calculating the timeout  . . . . . . . . . . . . . . . . . 18
   7.  Usage of "min-interval", "max-interval" and
       "average-interval" in a combination  . . . . . . . . . . . . . 19
   8.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     8.1.  "min-interval", "max-interval" and "average-interval"
           Header Field Parameters  . . . . . . . . . . . . . . . . . 20
     8.2.  Augmented BNF Definitions  . . . . . . . . . . . . . . . . 20
     8.3.  Event header field usage in responses to the NOTIFY
           request  . . . . . . . . . . . . . . . . . . . . . . . . . 21
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 22
     12.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24









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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 load.

   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 heterogeneous 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:

   min-interval:  specifies a minimum notification time period between
      two notifications, in seconds.







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   max-interval:  specifies a maximum notification time period between
      two notifications, in seconds.

   average-interval:  specifies an average cadence at which
      notifications are desired, in seconds.

   The requirements and model are further discussed in Section 3.  All
   these mechanisms are simply timer values that indicate the minimum,
   maximum and average time period allowed between two notifications.
   As a result of these mechanisms, 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.  Use Case for limiting the maximum rate of notifications

   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
   notification of the presence state of the resource list.

   In order to control the buffer policy of the RLS, the presence client
   sets a maximum rate ("min-interval" parameter), i.e. a minimum time
   interval between two notifications.  Alternatively, the presence
   client could set the maximum rate 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 maximum
   rate and batch all of the buffered state changes together in a single



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   notification only when allowed by the maximum rate.  The maximum rate
   applies to the overall resource list, which means that there is a
   hard cap imposed by the maximum rate to the number of notifications
   the presence client can expect to receive.

   For example, with a "min-interval" of 20 seconds, the presence
   application can expect to receive a notification at a minimum of
   every 20 seconds.

   The presence client can also modify the "min-interval" parameter
   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 "min-
   interval" parameter 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-
   subscribing with a "min-interval" parameter set to the earlier used
   value.  Application of the mechanism defined by RFC 5839 [RFC5839]
   can also eliminate for the presence client to receive a (full-state)
   notification carrying the latest resource state after the
   subscription refresh.

3.2.  Use Case for setting a minimum rate for notifications

   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 trigger criterias, for
   example, to send an update only when the target has moved at least n
   meters.  However, the application is also interested in receiving the
   current state periodically even if the state of the target has not
   changed enough to satisfy any of the trigger criteria, i.e. has not
   moved at least n meters within the period.

   In order to control the actual state, the location application sets a
   minimum rate ("max-interval" parameter), i.e. a maximum time interval
   between two notifications.

   The location application can also modify the "max-interval" parameter
   during the lifetime of the subscription.  When the subscription to
   the movement of a target is made, the notifier may not have the
   location information available.  Thus, the first notification might
   be empty, or certain values might be absent.  An important use case
   is placing constraints on when complete state should be provided
   after creating the subscription.  The "max-interval" parameter
   indicates the time to the notifier when a complete state information
   should be notified.  Once state is acquired and the second



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   notification is sent, the subscriber updates or changes the "max-
   interval" parameter to a more sensible value.  This update can be
   performed in the 200 OK response to the NOTIFY request that contains
   the complete state information.

3.3.  Use Case for specifying the average rate of notifications

   The previous mechanisms introduce a static and instantaneous rate
   control.  However there are some applications that would work better
   with an adaptive rate control.  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 increases
   the interval between notifications if the target has sent out several
   notifications recently.

   In order to set an adaptive rate control, the application defines a
   average cadence ("average-interval" parameter) at which notifications
   are desired.  The "average-interval" parameter value is used by the
   notifier to dynamically calculate the maximum time allowed between
   two notifications.  In order to dynamically calculate the maximum,
   the Notifier takes into consideration the frequency at which
   notifications have been sent recently.

   This type of rate control allows the notifier to dynamically increase
   or decrease the Notification frequency.

   The tracking application can also modify the "average-interval"
   parameter during the lifetime of the subscription.

3.4.  Requirements

   REQ1:   The subscriber must be able to set the minimum time period
           ("min-interval" parameter) between two notifications in a
           specific subscription.

   REQ2:   The subscriber must be able to set the maximum time period
           ("max-interval" parameter) between two notifications in a
           specific subscription.

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





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   REQ4:   It must be possible to apply all together, or in any
           combination, the "min-interval", "max-interval" and "average-
           interval" mechanisms in a specific subscription.

   REQ5:   It must be possible to use of the different rate control
           mechanisms in subscriptions to any events.

   REQ6:   It must be possible to use the different rate control
           mechanisms together with any other event filtering
           mechanisms.

   REQ7:   The notifier must be allowed to use a 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 policy that in
              effect increases the subscriber-configured minimum time
              period between two notifications.  In another example, the
              notifier can decrease the proposed minimum time by the
              subscriber to match it with the remaining expiry time left
              for the subscription.

   REQ8:   The different rate control mechanisms must discuss corner
           cases for setting the time periods between two notifications.
           At a minimum, the mechanisms must include discussion of the
           situation resulting from a minimum, maximum or average time
           period which exceeds the subscription duration, and should
           provide mechanisms for avoiding this situation.

   REQ9:   The different rate control mechanisms must be possible to be
           installed, modified, or removed in the course of an active
           subscription.

   REQ10:  The different rate control mechanisms must allow for the
           application of authentication and integrity protection
           mechanisms to subscriptions invoking that mechanism.

3.5.  The maximum rate mechanism for Resource List Server

   When applied to a list subscription [RFC4662], the maximum rate
   mechanism has some additional considerations.  Specifically, the
   maximum rate applies to the aggregate notification stream resulting
   from the list subscription, rather than explicitly controlling the
   notification of each of the implied constituent events.  Moreover,
   the RLS can use the maximum rate mechanism on its own to control the
   rate of the back-end subscriptions to avoid overflowing its buffer.




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   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|<======'min-interval|
                        |      |       `------------'
                        )--.---(
                           |
                           |
                       .---+---.
                       | 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 back end 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 maximum rate mechanism.  When
   a set of notifications are batched together, the way in which
   overlapping resource state is handled depends on the type of the
   resource state:





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      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 "min-interval"
   Event header parameter as part of the SUBSCRIBE request.  The "min-
   interval" value indicates the minimum time allowed between
   transmission of two consecutive notifications in a subscription.

      Note that the witnessed time between two consecutive received
      notifications may not conform to the "min-interval" value for a
      number of reasons.  For example, network jitter and
      retransmissions may result in the subscriber receiving the
      notifications with smaller intervals than the "min-interval" value
      recommends.

   A subscriber that wants to have a maximum notification time period in
   a specific event subscription does so by including a "max-interval"
   Event header parameter as part of the SUBSCRIBE request.  The "max-
   interval" value indicates the maximum time allowed between
   transmission of two consecutive notifications in a subscription.

   A subscriber that wants to have an average cadence for the
   notifications in a specific event subscription does so by including a
   "average-interval" Event header parameter as part of the SUBSCRIBE
   request.

   A subscriber that wants to update a previously agreed event rate
   control parameter does so by including the updated "min-interval",
   "max-interval" or "average-interval" Event header parameter as part
   of a subsequent SUBSCRIBE request or a 200-class response to the
   NOTIFY request.




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   A notifier that supports the different rate control mechanisms will
   comply with the value given in "min-interval", "max-interval" and
   "average-interval" parameters and adjust its rate of notification
   accordingly.  However, if the notifier needs to lower the
   subscription expiration value or a local policy or other
   implementation-determined constraint at the notifier can not satisfy
   the rate control request, then the notifier can adjust (i.e. increase
   or decrease) opportunely the subscriber requested rate control.


4.  Operation of the maximum rate mechanism

4.1.  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 rate to notifications in
   a subscription MUST construct a SUBSCRIBE request that includes a
   minimum time interval between two consecutive notifications included
   in the "min-interval" Event header field parameter.  The value of
   this parameter is an integral number of seconds in decimal.

   A subscriber that wishes to update the previously agreed maximum rate
   of notifications MUST include the updated "min-interval" Event header
   field parameter in a subsequent SUBSCRIBE request or a 200-class
   response to the NOTIFY request.  If the Event header field of the
   SUBSCRIBE request did not include the "min-interval" parameter, the
   subscriber MUST NOT include an initial value of the "min-interval"
   Event header field parameter in a 200-class response to the NOTIFY
   request.

   A subscriber that wishes to remove the maximum rate control from
   notifications MUST indicate so by not including a "min-interval"
   Event header field parameter in a subsequent SUBSCRIBE request or a
   200-class response to the NOTIFY request.

   There are two main consequences for the subscriber when applying the
   maximum rate 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
   notifications, developers are instructed not to use the mechanism.

4.2.  Notifier Behavior

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



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   A notifier that supports the maximum rate mechanism MUST extract the
   value of the "min-interval" Event header parameter from a SUBSCRIBE
   request or a 200-class response to the NOTIFY request and use it as
   the suggested time allowed between two notifications.  This value can
   be adjusted by the notifier, as defined in Section 4.3.  If the Event
   header field of the SUBSCRIBE request did not include the "min-
   interval" parameter, the notifier MUST ignore an initial value of the
   "min-interval" Event header field parameter in a 200-class response
   to the NOTIFY request, if present.

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

   A notifier that does not understand this extension will not reflect
   the "min-interval" Subscription-State header field parameter in the
   NOTIFY requests; the absence of this parameter indicates to the
   subscriber that no rate control is supported by the notifier.

   A compliant notifier MUST NOT generate notifications more frequently
   than the maximum rate 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 timer for the next
   notification, even though they do not need to abide by it.

   When a local policy dictates a maximum rate for notifications, a
   notifier will not generate notifications more frequently than the
   local policy maximum rate, even if the subscriber is not asking for
   maximum rate control.  The notifier MAY inform the subscriber about
   such local policy maximum rate using the "min-interval" Subscription-
   State header field parameter included in the subsequent NOTIFY
   requests.

   Retransmissions of NOTIFY requests are not affected by the maximum
   rate mechanism, i.e., the maximum rate mechanism only applies to the
   generation of new transactions.  In other words, the maximum rate
   mechanism does not in any way break or modify the normal
   retransmission mechanism specified in RFC 3261 [RFC3261].

4.3.  Selecting the maximum rate

   Special care needs to be taken when selecting the "min-interval"
   value.  Using the "min-interval" syntax it is possible to insist both
   very short and very long intervals between notifications.



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   For example, the maximum rate 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.  If the
   subscriber requests a "min-interval" value greater than the
   subscription expiration, the notifier MUST lower the "min-interval"
   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.  If the subscription expiry is
   shortened during an active subscription, the notifier MUST also lower
   the "min-interval" value and set it to the reduced expiration time.

   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 "min-interval" value to the
   remaining subscription expiration value.  This results in receiving
   no further notifications until the subscription expires or the
   subscriber sends a SUBSCRIBE request resuming notifications.

   The notifier MAY decide to increase or decrease the proposed maximum
   rate value by the subscriber based on its local policy, static
   configuration or other implementation-determined constraints.  The
   notifier MUST include the adjusted "min-interval" value in the
   Subscription-State header field's "min-interval" parameter in each of
   the NOTIFY requests.  In addition, different event packages MAY
   define additional constraints to the allowed "min-interval"
   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 difference of the two states,
   and generating a notification that contains that difference.

   When the maximum rate mechanism is applied to the subscription, it is
   important that F' is replaced with F only when the difference of F
   and F' was already included in a partial state notification to the
   subscriber allowed by the maximum rate mechanism.  Additionally, the
   notifier implementation SHOULD check to see that the size of an
   accumulated partial state notification is smaller than the full



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   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 the maximum rate mechanism is applied to 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 maximum rate mechanism over a subscription, since such
   estimates will vary depending on the usage scenarios.  However, it is
   easy to see that given a subscription where several full state
   notifications would have normally been sent in any given interval set
   by the "min-interval" parameter, only a single notification is sent
   during the same interval when using the maximum rate mechanism,
   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 rate
   controlled 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 header fields of the NOTIFY request.

   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
   maximum rate 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 the minimum rate mechanism






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5.1.  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 minimum rate to notifications in
   a subscription MUST construct a SUBSCRIBE request that includes a
   maximum time interval between two consecutive notifications included
   in the "max-interval" Event header field parameter.  The value of
   this parameter is an integral number of seconds in decimal.

   A subscriber that wishes to update the previously agreed minimum rate
   of notifications MUST include the updated "max-interval" Event header
   field parameter in a subsequent SUBSCRIBE request or a 200-class
   response to the NOTIFY request.  If the Event header field of the
   SUBSCRIBE request did not include the "max-interval" parameter, the
   subscriber MUST NOT include an initial value of the "max-interval"
   Event header field parameter in a 200-class response to the NOTIFY
   request.

   A subscriber that wishes to remove the minimum rate control from
   notifications MUST indicate so by not including a "max-interval"
   Event header field parameter in a subsequent SUBSCRIBE request or a
   200-class response to the NOTIFY request.

   The main consequence for the subscriber when applying the minimum
   rate mechanism is that it can receive a notification even if nothing
   has changed in the current state of the notifier.  However, RFC 5839
   [RFC5839] defines a mechanism that allows sending only an etag
   instead of the full resource state in a notification if the state has
   not changed.

5.2.  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 minimum rate mechanism MUST extract the
   value of the "max-interval" Event header parameter from a SUBSCRIBE
   request or a 200-class response to the NOTIFY request and use it as
   the suggested maximum time allowed between two notifications.  If the
   Event header field of the SUBSCRIBE request did not include the "max-
   interval" parameter, the notifier MUST ignore an initial value of the
   "max-interval" Event header field parameter in a 200-class response
   to the NOTIFY request, if present.

   The notifier MAY decide to increase or decrease the proposed minimum



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   rate value based on its local policy, static configuration or other
   implementation-determined constraints.  A compliant notifier MUST
   reflect back the possibly adjusted maximum time interval in a "max-
   interval" Subscription-State header field parameter of the subsequent
   NOTIFY requests.  The indicated "max-interval" value is adopted by
   the notifier, and the notification rate is adjusted accordingly.

   A notifier that does not understand this extension, will not reflect
   the "max-interval" Subscription-State header field parameter in the
   NOTIFY requests; the absence of this parameter indicates to the
   subscriber that no rate control is supported by the notifier.

   A compliant notifier MUST generate notifications whenever the time
   since the most recent notification exceeds the value in the "max-
   interval" parameter.  Depending on the event package and subscriber
   preferences indicated in the SUBSCRIBE request, the NOTIFY request
   sent as a result of a max-interval expiration MUST contain either the
   current full state or the partial state showing the difference
   between the current state and the last successfully communicated
   state.

   Retransmissions of NOTIFY requests are not affected by the minimum
   rate mechanism, i.e., the minimum rate mechanism only applies to the
   generation of new transactions.  In other words, the minimum rate
   mechanism does not in any way break or modify the normal
   retransmission mechanism.


6.  Operation of the average rate mechanism

6.1.  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 rate to notifications in
   a subscription MUST construct a SUBSCRIBE request that includes a
   proposed average time interval between two consecutive notifications
   included in a "average-interval" Event header field parameter.  The
   value of this parameter is an integral number of seconds in decimal.

   A subscriber that wishes to update the previously agreed average rate
   of notifications MUST include the updated "average-interval" Event
   header field parameter in a subsequent SUBSCRIBE request or a 200-
   class response to the NOTIFY request.  If the Event header field of
   the SUBSCRIBE request did not include the "average-interval"
   parameter, the subscriber MUST NOT include an initial value of the



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   "average-interval" Event header field parameter in a 200-class
   response to the NOTIFY request.

   A subscriber that wishes to remove the average rate control from
   notifications MUST indicate so by not including a "average-interval"
   Event header field parameter in a subsequent SUBSCRIBE request or a
   200-class response to the NOTIFY request.

   The main consequence for the subscriber when applying the average
   rate mechanism is that it can receive a notification even if nothing
   has changed in the current state of the notifier.  However, RFC 5839
   [RFC5839] defines a mechanism that allows sending only an etag
   instead of the full resource state in a notification if the state has
   not changed.

6.2.  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 average rate mechanism MUST extract the
   value of the "average-interval" Event header parameter from a
   SUBSCRIBE request or a 200-class response to the NOTIFY request and
   use it to calculate the maximum time allowed between two transactions
   as defined in Section 6.3.  If the Event header field of the
   SUBSCRIBE request did not include the "average-interval" parameter,
   the notifier MUST ignore an initial value of the "average-interval"
   Event header field parameter in a 200-class response to the NOTIFY
   request, if present.

   The notifier MAY decide to increase or decrease the proposed average
   time interval based on its local policy, static configuration or
   other implementation-determined constraints.  A compliant notifier
   MUST reflect back the possibly adjusted average time interval in an
   "average-interval" Subscription-State header field parameter of the
   subsequent NOTIFY requests.  The indicated "average-interval" value
   is adopted by the notifier, and the notification rate is adjusted
   accordingly.

   A notifier that does not understand this extension will not reflect
   the "average-interval" Subscription-State header parameter in the
   NOTIFY requests; the absence of this parameter indicates to the
   subscriber that no rate control is supported by the notifier.

   A compliant notifier SHOULD generate notifications whenever the time
   since the most recent notification exceeds the value calculated using
   the formula defined in Section 6.3.




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   The average rate 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-interval"
       seconds for the "period".

   2)  The "timeout" value is calculated according to the equation given
       in Section 6.3.

   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 does not in any way break or modify the
   normal retransmission mechanism specified in RFC 3261 [RFC3261].

6.3.  Calculating the timeout

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

   timeout = (average-interval ^ 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-interval:  The value of the "average-interval" parameter
      conveyed in the Subscription-State header field, in seconds.

   period:  The rolling average period, in seconds.  The value of the
      "period" parameter MUST be greater than the value of the "average-
      interval" parameter.

   count:  The number of notifications that have been sent during the
      last "period" of seconds not including any retransmissions of
      requests.

   In case both the maximum rate and the average rate mechanisms are



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


timeout = MAX[min-interval, (average-interval ^ 2) * count / period] (2)


   min-interval:  The value of the "min-interval" parameter conveyed in
      the Event header field, in seconds.

   The formula in (2) makes sure that for all the possible values of the
   "min-interval" and "average-interval" parameters, with "average-
   interval" > "min-interval", the timeout never results in a lower
   value than the value of the "min-interval" parameter.

   In some situation it may be beneficial for the Notifier to achieve an
   average rate in a different way than the algorithm detailed in this
   document allows.  However, the Notifier MUST comply with any "min-
   interval" or "max-interval" parameters that have been negotiated.


7.  Usage of "min-interval", "max-interval" and "average-interval" in a
    combination

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

   min-interval and max-interval:  this combination allows to reduce the
      notification frequency rate, but at the same time assures the
      reception of a notification every time the most recent
      notification exceeds a specified interval.

      A subscriber SHOULD choose a "max-interval" value higher than the
      "min-interval" value, otherwise the notifier MUST adjust the
      subscriber provided "max-interval" value to a value equivalent or
      higher than the "min-interval" value.

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






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

   max-interval and average-interval:  as both the parameters are
      designed as minimum rate mechanisms, this combination makes sense
      only in some corner cases.

      A subscriber SHOULD choose a "max-interval" value higher than the
      "average-interval" value, otherwise the notifier MUST NOT consider
      the "max-interval" value.

   min-interval, max-interval and average-interval:  this combination
      makes little sense to be used although not forbidden.

      A subscriber SHOULD choose a "max-interval" and "average-interval"
      values higher than the "min-interval" value, otherwise the
      notifier MUST adjust the subscriber provided "max-interval" and
      "average-interval" values to a value equivalent or higher than the
      "min-interval" value.

      A subscriber SHOULD choose a "max-interval" value higher than the
      "average-interval" value, otherwise the notifier MUST NOT consider
      the "max-interval" value.


8.  Syntax

   This section describes the syntax extensions required for the
   different rate control mechanisms.

8.1.  "min-interval", "max-interval" and "average-interval" Header Field
      Parameters

   The "min-interval", "max-interval" and "average-interval" parameters
   are added to the rule definitions of the Event header field and the
   Subscription-State header field in RFC 3265 [RFC3265] grammar.  Usage
   of this parameter is described in Section 4, Section 5 and Section 6.

8.2.  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 RFC 3265 [RFC3265], adding additional alternatives to the
   alternative sets of "event-param" and "subexp-params" defined
   therein.




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      event-param    =/  min-interval-param
      subexp-params  =/  min-interval-param
      min-interval-param =   "min-interval" EQUAL delta-seconds

      event-param    =/  max-interval-param
      subexp-params  =/  max-interval-param
      max-interval-param =   "max-interval" EQUAL delta-seconds

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

8.3.  Event header field usage in responses to the NOTIFY request

   This table expands the table described in Section 7.2 of RFC 3265
   [RFC3265] allowing the Event header field to appear in a 200-class
   response to a NOTIFY request.  A UA that wishes to update the value
   for minimum, maximum or average rate of notifications can do so by
   including all desired values for the rate control parameters in an
   Event header field of the 200-class response to a NOTIFY request.

      Header field      where proxy ACK BYE CAN INV OPT REG PRA SUB NOT
      -----------------------------------------------------------------
      Event             2xx          -   -   -   -   -   -   -   -   o


9.  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                min-interval          No           [RFCxxxx]
      Subscription-State   min-interval          No           [RFCxxxx]
      Event                max-interval          No           [RFCxxxx]
      Subscription-State   max-interval          No           [RFCxxxx]
      Event                average-interval      No           [RFCxxxx]
      Subscription-State   average-interval      No           [RFCxxxx]

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






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10.  Security Considerations

   Naturally, the security considerations listed in RFC 3265 [RFC3265],
   which the rate control mechanisms described in this document extends,
   apply in entirety.  In particular, authentication and message
   integrity SHOULD be applied to subscriptions with this extension.

   RFC 3265 [RFC3265] recommends the integrity protection of the Event
   header field of SUBSCRIBE requests.  Implementations of this
   extension SHOULD also provide integrity protection for the Event
   header field included in the 200-class response to the NOTIFY
   request.

   When the maximum rate mechanism involves partial state notifications,
   the security considerations listed in RFC 5263 [RFC5263] apply in
   entirety.


11.  Acknowledgments

   Thanks to Pekka Pessi, Dean Willis, Eric Burger, Alex Audu, Alexander
   Milinski, Jonathan Rosenberg, Cullen Jennings, Adam Roach, Hisham
   Khartabil, Dale Worley, Martin Thomson, Byron Campen, Alan Johnston,
   Michael Procter and Janet Gunn for support and/or review of this
   work.

   Thanks to Brian Rosen for the idea of the minimum and average rate
   mechanisms, and Adam Roach for the work on the averaging algorithm
   and other feedback.


12.  References

12.1.  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



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              Resource Lists", RFC 4662, August 2006.

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

   [RFC5263]  Lonnfors, M., Costa-Requena, J., Leppanen, E., and H.
              Khartabil, "Session Initiation Protocol (SIP) Extension
              for Partial Notification of Presence Information",
              RFC 5263, September 2008.

12.2.  Informative References

   [I-D.ietf-geopriv-loc-filters]
              Mahy, R., Rosen, B., and H. Tschofenig, "Filtering
              Location Notifications in the Session Initiation Protocol
              (SIP)", draft-ietf-geopriv-loc-filters-11 (work in
              progress), March 2010.

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

   [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.

   [RFC5839]  Niemi, A. and D. Willis, "An Extension to Session
              Initiation Protocol (SIP) Events for Conditional Event
              Notification", RFC 5839, May 2010.





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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
   323 Fairchild Dr
   Mountain View, CA  94043
   US

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


   Salvatore Loreto
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: salvatore.loreto@ericsson.com






















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