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Internet Engineering Task Force (IETF) A. Niemi Request for Comments: 6446 K. Kiss Updates: 3265 Nokia Category: Standards Track S. Loreto ISSN: 2070-1721 Ericsson

                                                          January 2012
   Session Initiation Protocol (SIP) Event Notification Extension
                   for Notification Rate Control

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.  This document
 updates RFC 3265.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6446.

Niemi, et al. Standards Track [Page 1] RFC 6446 Event Rate Control January 2012

Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 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
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Niemi, et al. Standards Track [Page 2] RFC 6446 Event Rate Control January 2012

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 an Adaptive Minimum Rate of
         Notifications  . . . . . . . . . . . . . . . . . . . . . .  6
   3.4.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  7
 4.  Basic Operations . . . . . . . . . . . . . . . . . . . . . . .  8
   4.1.  Subscriber Behavior  . . . . . . . . . . . . . . . . . . .  8
   4.2.  Notifier Behavior  . . . . . . . . . . . . . . . . . . . .  9
 5.  Operation of the Maximum Rate Mechanism  . . . . . . . . . . .  9
   5.1.  Subscriber Behavior  . . . . . . . . . . . . . . . . . . .  9
   5.2.  Notifier Behavior  . . . . . . . . . . . . . . . . . . . . 10
   5.3.  Selecting the Maximum Rate . . . . . . . . . . . . . . . . 11
   5.4.  The Maximum Rate Mechanism for the Resource List Server  . 11
   5.5.  Buffer Policy Description  . . . . . . . . . . . . . . . . 13
     5.5.1.  Partial-State Notifications  . . . . . . . . . . . . . 13
     5.5.2.  Full-State Notifications . . . . . . . . . . . . . . . 13
   5.6.  Estimated Bandwidth Savings  . . . . . . . . . . . . . . . 14
 6.  Operation of the Minimum Rate Mechanism  . . . . . . . . . . . 14
   6.1.  Subscriber Behavior  . . . . . . . . . . . . . . . . . . . 14
   6.2.  Notifier Behavior  . . . . . . . . . . . . . . . . . . . . 15
   6.3.  Selecting the Minimum Rate . . . . . . . . . . . . . . . . 16
 7.  Operation of the Adaptive Minimum Rate Mechanism . . . . . . . 16
   7.1.  Subscriber Behavior  . . . . . . . . . . . . . . . . . . . 16
   7.2.  Notifier Behavior  . . . . . . . . . . . . . . . . . . . . 17
   7.3.  Selecting the Adaptive Minimum Rate  . . . . . . . . . . . 18
   7.4.  Calculating the Timeout  . . . . . . . . . . . . . . . . . 18
 8.  Usage of the Maximum Rate, Minimum Rate, and Adaptive
     Minimum Rate Mechanisms in a Combination . . . . . . . . . . . 19
 9.  Protocol Element Definitions . . . . . . . . . . . . . . . . . 20
   9.1.  "max-rate", "min-rate", and "adaptive-min-rate" Header
         Field Parameters . . . . . . . . . . . . . . . . . . . . . 21
   9.2.  Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . 21
   9.3.  Event Header Field Usage in Responses to the NOTIFY
         Request  . . . . . . . . . . . . . . . . . . . . . . . . . 21
 10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
 11. Security Considerations  . . . . . . . . . . . . . . . . . . . 22
 12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23
 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
   13.1. Normative References . . . . . . . . . . . . . . . . . . . 23
   13.2. Informative References . . . . . . . . . . . . . . . . . . 24

Niemi, et al. Standards Track [Page 3] RFC 6446 Event Rate Control January 2012

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
 recommendation for the maximum notification rate, 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
 allowed to define additional throttle mechanisms that 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.
 Some event packages are also interested in specifying an absolute or
 an adaptive minimum rate at which notifications need to be generated
 by a notifier.  This helps the subscriber to effectively use
 different trigger criteria within a subscription to eliminate
 unnecessary notifications but at the same time make sure that the
 current event state is periodically received.
 This document defines an extension to the SIP events framework by
 defining the following three Event header field parameters that allow
 a subscriber to set a maximum, a minimum, and an adaptive minimum
 rate of notifications generated by the notifier:

Niemi, et al. Standards Track [Page 4] RFC 6446 Event Rate Control January 2012

 max-rate:  specifies a maximum number of notifications per second.
 min-rate:  specifies a minimum number of notifications per second.
 adaptive-min-rate:  specifies an adaptive minimum number of
    notifications per second.
 These mechanisms are applicable to any event subscription, both
 single event subscription and event list subscription.  A notifier
 compliant to this specification will adjust the rate at which it
 generates notifications.

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
 an RLS with the list of buddies, and therefore only needs a single
 subscription to the RLS to receive notifications 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 of notifications.  The RLS will buffer
 notifications that are generated faster than they are allowed to be
 sent due to the maximum rate and batch all of the buffered state
 changes together in a single notification.  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 per second
 that the presence client can expect to receive.
 The presence client can also modify the maximum rate of notifications
 during the lifetime of the subscription.  For example, if the mobile
 device detects inactivity from the user for a period of time, the
 presence client can simply pause notifications by choosing a "max-
 rate" parameter that allows only a single notification for the

Niemi, et al. Standards Track [Page 5] RFC 6446 Event Rate Control January 2012

 remainder of the subscription lifetime.  When the user becomes active
 again, the presence client can resume the stream of notifications by
 re-subscribing with a "max-rate" parameter set to the earlier-used
 value.  Application of the mechanism defined by RFC 5839 [RFC5839]
 can also eliminate the transmission of a (full-state) notification
 carrying the latest resource state to the presence client after a
 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 to a Location Server with a set
 of location filters [RFC6447] that specify trigger criteria, e.g., 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, e.g., has not moved at
 least n meters within the period.
 The location application sets a minimum rate of notifications and
 includes it in the subscription sent to the Location Server. The
 "min-rate" parameter indicates the minimum number of notifications
 per second the notifier needs to generate.
 The location application can modify the minimum rate of notifications
 during the lifetime of the subscription.  For example, 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.  Once state is
 acquired and the second notification is sent, the subscriber updates
 or changes the "min-rate" parameter to a more sensible value.  This
 update can be performed in the response to the notification that
 contains the complete state information.

3.3. Use Case for Specifying an Adaptive Minimum Rate of Notifications

 The minimum rate mechanism introduces a static and instantaneous rate
 control without the functionality to increase or decrease the
 notification rate adaptively.  However, there are some applications
 that would work better with an adaptive minimum rate control.
 A location application is monitoring the movement of a target.  In
 order to decrease the processing in the application, the location
 application wants to make a subscription that dynamically decreases
 the minimum rate of notifications if the target has sent out several

Niemi, et al. Standards Track [Page 6] RFC 6446 Event Rate Control January 2012

 notifications recently.  However, if there have been only few recent
 notifications by the target, the location application wants the
 minimum rate of notifications to increase.
 The location application sets an adaptive minimum rate of
 notifications and includes it in the subscription sent to the
 Location Server.  The "adaptive-min-rate" parameter value is used by
 the notifier to dynamically calculate the actual maximum time between
 two notifications.  In order to dynamically calculate the maximum
 time, the notifier takes into consideration the rate at which
 notifications have been sent recently.  In the adaptive minimum rate
 mechanism, the notifier can increase or decrease the notification
 rate compared to the minimum rate mechanism based on the recent
 number of notifications sent out in the last period.
 The location application can also modify the "adaptive-min-rate"
 parameter during the lifetime of the subscription.

3.4. Requirements

 REQ1:   The subscriber must be able to set a maximum rate of
         notifications in a specific subscription.
 REQ2:   The subscriber must be able to set a minimum rate of
         notifications in a specific subscription.
 REQ3:   The subscriber must be able to set an adaptive minimum rate
         of notifications in a specific subscription, which adjusts
         the minimum rate of notifications based on a moving average.
 REQ4:   It must be possible to apply the maximum rate, the minimum
         rate, and the adaptive minimum rate mechanisms all together,
         or in any combination, in a specific subscription.
 REQ5:   It must be possible to use any of the different rate control
         mechanisms in subscriptions to any events.
 REQ6:   It must be possible to use any of 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
         maximum rate, minimum rate, and adaptive minimum rate
         parameters are adjusted from the value given by the
         subscriber.

Niemi, et al. Standards Track [Page 7] RFC 6446 Event Rate Control January 2012

            For example, due to congestion, local policy at the
            notifier could temporarily dictate a policy that in effect
            further decreases the maximum rate of notifications.  In
            another example, the notifier could increase the
            subscriber-proposed maximum rate so that at least one
            notification is generated during the remainder of the
            subscription lifetime.
 REQ8:   The different rate control mechanisms must address corner
         cases for setting the notification rates appropriately.  At a
         minimum, the mechanisms must address the situation in which
         the time between two notifications exceeds the subscription
         duration and should provide procedures for avoiding this
         situation.
 REQ9:   It must be possible to invoke, modify, or remove the
         different rate control mechanisms 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.

4. Basic Operations

4.1. Subscriber Behavior

 In general, a subscriber generates SUBSCRIBE requests and processes
 NOTIFY requests as described in RFC 3265 [RFC3265].
 A subscriber that wants to have a maximum, minimum, or adaptive
 minimum rate of event notifications in a specific event subscription
 does so by including a "max-rate", "min-rate", or "adaptive-min-rate"
 Event header field parameter(s) 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 "max-rate", "min-
 rate", or "adaptive-min-rate" Event header field parameter(s) as part
 of a subsequent SUBSCRIBE request or a 2xx response to the NOTIFY
 request.  If the subscriber does not include at least one of the
 "max-rate", "min-rate", or "adaptive-min-rate" header field
 parameters in the most recent SUBSCRIBE request in a given dialog,
 the subscriber MUST NOT include an Event header field with any of
 those parameters in a 2xx response to a NOTIFY request in that
 dialog.

Niemi, et al. Standards Track [Page 8] RFC 6446 Event Rate Control January 2012

4.2. Notifier Behavior

 In general, a notifier processes SUBSCRIBE requests and generates
 NOTIFY requests as described in RFC 3265 [RFC3265].
 A notifier that supports the different rate control mechanisms MUST
 adjust its rate of notification according to the rate control values
 agreed with the subscriber.  If the notifier needs to lower the
 subscription expiration value, or if a local policy or other
 implementation-determined constraint at the notifier cannot satisfy
 the rate control request, then the notifier can adjust (i.e.,
 increase or decrease) appropriately the subscriber-requested rate
 control values.  The notifier MUST reflect back the possibly adjusted
 rate control values in a "max-rate", "min-rate", or "adaptive-min-
 rate" Subscription-State header field parameter of the subsequent
 NOTIFY requests.

5. Operation of the Maximum Rate Mechanism

5.1. Subscriber Behavior

 A subscriber that wishes to apply a maximum rate to notifications in
 a subscription MUST construct a SUBSCRIBE request that includes the
 "max-rate" Event header field parameter.  This parameter specifies
 the requested maximum number of notifications per second.  The value
 of this parameter is a positive real number given by a finite decimal
 representation.
    Note that the grammar in section 9.2 constrains this value to be
    between 0.0000000001 and 99.9999999999.  Zero is not an allowed
    value.
    Note that the witnessed notification rate may not conform to the
    "max-rate" value for a number of reasons.  For example, network
    jitter and retransmissions may result in the subscriber receiving
    the notifications more frequently than the "max-rate" value
    recommends.
 A subscriber that wishes to update the previously agreed maximum rate
 of notifications MUST include the updated "max-rate" Event header
 field parameter in a subsequent SUBSCRIBE request or a 2xx response
 to the NOTIFY request.
 A subscriber that wishes to remove the maximum rate control from
 notifications MUST indicate so by not including a "max-rate" Event
 header field parameter in a subsequent SUBSCRIBE request or a 2xx
 response to the NOTIFY request.

Niemi, et al. Standards Track [Page 9] RFC 6446 Event Rate Control January 2012

 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 utilizes the event
 notifications, developers are instructed not to use the mechanism.

5.2. Notifier Behavior

 A notifier that supports the maximum rate mechanism MUST extract the
 value of the "max-rate" Event header parameter from a SUBSCRIBE
 request or a 2xx response to the NOTIFY request and use it as the
 suggested maximum number of notifications per second.  This value can
 be adjusted by the notifier, as defined in Section 5.3.
 A compliant notifier MUST reflect back the possibly adjusted maximum
 rate of notifications in a "max-rate" Subscription-State header field
 parameter of the subsequent NOTIFY requests.  The indicated "max-
 rate" 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-rate" 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 a notification if the interval
 since the most recent notification is less than the reciprocal value
 of the "max-rate" parameter, except when generating the notification
 either upon receipt of a SUBSCRIBE request, when the subscription
 state is changing from "pending" to "active" state, or upon
 termination of the subscription (the last notification).
 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 a local policy maximum rate using the "max-rate" Subscription-
 State header field parameter included in 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].

Niemi, et al. Standards Track [Page 10] RFC 6446 Event Rate Control January 2012

5.3. Selecting the Maximum Rate

 Special care needs to be taken when selecting the maximum rate.  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 being generated.  If the subscriber
 requests a maximum rate that would result in no notification before
 the subscription expiration, the notifier MUST increase the maximum
 rate and set it to the reciprocal value of the remaining subscription
 expiration time.  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 increase the "max-rate"
 value and set it to the reciprocal value of the reduced subscription
 expiration time.
 In some cases, it makes sense to temporarily pause the notification
 stream on an existing subscription dialog without terminating the
 subscription, e.g., due to inactivity on the application user
 interface.  Whenever a subscriber discovers the need to perform the
 notification pause operation, it SHOULD set the maximum rate to the
 reciprocal value of 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 "max-
 rate" value by the subscriber based on its local policy, static
 configuration, or other implementation-determined constraints.  In
 addition, different event packages MAY define other constraints for
 the allowed maximum rate ranges.  Such constraints are out of the
 scope of this specification.

5.4. The Maximum Rate Mechanism for the 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.
 The notifier is responsible for sending event notifications upon
 state changes of the subscribed resource.  We can model the notifier
 as consisting of four components: the event state resource(s), the

Niemi, et al. Standards Track [Page 11] RFC 6446 Event Rate Control January 2012

 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|<======'max-rate|
                      |      |       `--------'
                      )--.---(
                         |
                         |
                     .---+---.
                     | Event |
                     |Watcher|
                     `-------'
       Figure 1: Model for the RLS Supporting Event Rate Control
 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:
    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 specification.  These
    two buffer policies depend on the mode in which the notifier is
    operating.

Niemi, et al. Standards Track [Page 12] RFC 6446 Event Rate Control January 2012

 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.

5.5. Buffer Policy Description

5.5.1. Partial-State Notifications

 With partial notifications, the notifier needs to maintain a separate
 buffer for each subscriber since each subscriber may have a different
 value for the maximum rate of 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' be replaced with F only when the difference of F
 and F' is 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
 state, and if not, the notifier SHOULD send the full-state
 notification instead.

5.5.2. 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 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.

Niemi, et al. Standards Track [Page 13] RFC 6446 Event Rate Control January 2012

5.6. 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 "max-rate" 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.

6. Operation of the Minimum Rate Mechanism

6.1. Subscriber Behavior

 A subscriber that wishes to apply a minimum rate to notifications in
 a subscription MUST construct a SUBSCRIBE request that includes the
 "min-rate" Event header field parameter.  This parameter specifies
 the requested minimum number of notifications per second.  The value
 of this parameter is a positive real number given by a finite decimal
 representation.
    Note that the grammar in section 9.2 constrains this value to be
    between 0.0000000001 and 99.9999999999.  Zero is not an allowed
    value.
 A subscriber that wishes to update the previously agreed minimum rate
 of notifications MUST include the updated "min-rate" Event header
 field parameter in a subsequent SUBSCRIBE request or a 2xx response
 to the NOTIFY request.

Niemi, et al. Standards Track [Page 14] RFC 6446 Event Rate Control January 2012

 A subscriber that wishes to remove the minimum rate control from
 notifications MUST indicate so by not including a "min-rate" Event
 header field parameter in a subsequent SUBSCRIBE request or a 2xx
 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 suppression of a NOTIFY
 request or a NOTIFY request body if the state has not changed.

6.2. Notifier Behavior

 A notifier that supports the minimum rate mechanism MUST extract the
 value of the "min-rate" Event header field parameter from a SUBSCRIBE
 request or a 2xx response to the NOTIFY request and use it as the
 suggested minimum number of notifications per second.  This value can
 be adjusted by the notifier, as defined in Section 6.3.
 A compliant notifier MUST reflect back the possibly adjusted minimum
 rate of notifications in a "min-rate" Subscription-State header field
 parameter of the subsequent NOTIFY requests.  The indicated "min-
 rate" 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-rate" 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 when state changes
 occur or when the time since the most recent notification exceeds the
 reciprocal value of the "min-rate" parameter.  Depending on the event
 package and subscriber preferences indicated in the SUBSCRIBE
 request, the NOTIFY request sent as a result of a minimum rate
 mechanism MUST contain either the current full state or the partial
 state showing the difference between the current state and the last
 successfully communicated state.  If the subscriber and the notifier
 support the procedures in RFC 5839 [RFC5839], the complete NOTIFY
 request or the NOTIFY request body can be suppressed if the state has
 not changed from the previous notification.
 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.

Niemi, et al. Standards Track [Page 15] RFC 6446 Event Rate Control January 2012

6.3. Selecting the Minimum Rate

 The minimum rate mechanism can be used to generate a lot of
 notifications, creating additional processing load for the notifier.
 Some of the notifications may also be unnecessary possibly repeating
 already known state information to the subscriber.  It is difficult
 to provide generic guidelines for the acceptable minimum rate value
 ranges; however, the subscriber SHOULD request the lowest possible
 minimum rate.  Different event packages MAY define other constraints
 for the allowed minimum rate values.  Such constraints are out of the
 scope of this specification.
 The notifier MAY decide to increase or decrease the proposed "min-
 rate" value by the subscriber based on its local policy, static
 configuration, or other implementation-determined constraints.

7. Operation of the Adaptive Minimum Rate Mechanism

7.1. Subscriber Behavior

 A subscriber that wishes to apply an adaptive minimum rate to
 notifications in a subscription MUST construct a SUBSCRIBE request
 that includes the "adaptive-min-rate" Event header field parameter.
 This parameter specifies an adaptive minimum number of notifications
 per second.  The value of this parameter is a positive real number
 given by a finite decimal representation.
    Note that the grammar in section 9.2 constrains this value to be
    between 0.0000000001 and 99.9999999999.  Zero is not an allowed
    value.
 A subscriber that wishes to update the previously agreed adaptive
 minimum rate of notifications MUST include the updated "adaptive-min-
 rate" Event header field parameter in a subsequent SUBSCRIBE request
 or a 2xx response to the NOTIFY request.
 A subscriber that wishes to remove the adaptive minimum rate control
 from notifications MUST indicate so by not including an "adaptive-
 min-rate" Event header field parameter in a subsequent SUBSCRIBE
 request or a 2xx response to the NOTIFY request.
 The main consequence for the subscriber when applying the adaptive
 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 suppression of a
 NOTIFY request or a NOTIFY request body if the state has not changed.

Niemi, et al. Standards Track [Page 16] RFC 6446 Event Rate Control January 2012

7.2. Notifier Behavior

 A notifier that supports the adaptive minimum rate mechanism MUST
 extract the value of the "adaptive-min-rate" Event header parameter
 from a SUBSCRIBE request or a 2xx response to the NOTIFY request and
 use it to calculate the actual maximum time between two
 notifications, as defined in Section 7.4.
 The "adaptive-min-rate" value can be adjusted by the notifier, as
 defined in Section 7.3.
 A compliant notifier MUST reflect back the possibly adjusted adaptive
 minimum rate of notifications in an "adaptive-min-rate" Subscription-
 State header field parameter of the subsequent NOTIFY requests.  The
 indicated "adaptive-min-rate" 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 "adaptive-min-rate" 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 MUST generate notifications when state changes
 occur or when the time since the most recent notification exceeds the
 value calculated using the formula defined in Section 7.4.  Depending
 on the event package and subscriber preferences indicated in the
 SUBSCRIBE request, the NOTIFY request sent as a result of a minimum
 rate mechanism MUST contain either the current full state or the
 partial state showing the difference between the current state and
 the last successfully communicated state.  If the subscriber and the
 notifier support the procedures in RFC 5839 [RFC5839], the complete
 NOTIFY request or the NOTIFY request body can be suppressed if the
 state has not changed from the previous notification.
 The adaptive minimum rate mechanism is implemented as follows:
 1)  When a subscription is first created, the notifier creates a
     record ("count" parameter) that keeps track of the number of
     notifications that have been sent in the "period".  The "count"
     parameter is initialized to contain a history of having sent a
     "period * adaptive-min-rate" number of notifications for the
     "period".
 2)  The "timeout" value is calculated according to the equation given
     in Section 7.4.

Niemi, et al. Standards Track [Page 17] RFC 6446 Event Rate Control January 2012

 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" event or a state change), the notifier updates the
     notification history record stored in the "count" parameter,
     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].

7.3. Selecting the Adaptive Minimum Rate

 The adaptive minimum rate mechanism can be used to generate a lot of
 notifications, creating additional processing load for the notifier.
 Some of the notifications may also be unnecessary, possibly repeating
 already known state information to the subscriber.  It is difficult
 to provide generic guidelines for the acceptable adaptive minimum
 rate value ranges; however, the subscriber SHOULD request the lowest
 possible adaptive minimum rate value.  Different event packages MAY
 define other constraints for the allowed adaptive minimum rate
 values.  Such constraints are out of the scope of this specification.
 The notifier MAY decide to increase or decrease the proposed
 "adaptive-min-rate" value based on its local policy, static
 configuration, or other implementation-determined constraints.

7.4. Calculating the Timeout

 The formula used to vary the absolute pacing in a way that will meet
 the adaptive minimum rate requested over the period is given in
 equation (1):
 timeout = count / ((adaptive-min-rate ^ 2) * period)              (1)
 The output of the formula, "timeout", is the time to the next
 notification, expressed in seconds.  The formula has three inputs:
 adaptive-min-rate:  The value of the "adaptive-min-rate" parameter
    conveyed in the Subscription-State header field.

Niemi, et al. Standards Track [Page 18] RFC 6446 Event Rate Control January 2012

 period:  The rolling average period, in seconds.  The granularity of
    the values for the "period" parameter is set by local policy at
    the notifier; however, the notifier MUST choose a value greater
    than the reciprocal value of the "adaptive-min-rate" parameter.
    It is also RECOMMENDED that the notifier choose a "period"
    parameter several times larger than reciprocal value of the
    "adaptive-min-rate" parameter in order to maximize the
    effectiveness of equation (1).  It is an implementation decision
    whether the notifier uses the same value of the "period" parameter
    for all subscriptions or individual values for each subscription.
 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 adaptive minimum rate
 mechanisms are used in the same subscription, the formula used to
 dynamically calculate the timeout is given in equation (2):

timeout = MAX[(1/max-rate), count/((adaptive-min-rate ^ 2)*period)] (2)

 max-rate:  The value of the "max-rate" parameter conveyed in the
    Subscription-State header field.
 The formula in (2) makes sure that for all the possible values of the
 "max-rate" and "adaptive-min-rate" parameters, with "adaptive-min-
 rate" < "max-rate", the timeout never results in a lower value than
 the reciprocal value of the "max-rate" parameter.
 In some situations, it may be beneficial for the notifier to achieve
 an adaptive minimum rate in a different way than the algorithm
 detailed in this document allows.  However, the notifier MUST comply
 with any "max-rate" or "min-rate" parameters that have been
 negotiated.

8. Usage of the Maximum Rate, Minimum Rate, and Adaptive Minimum Rate

  Mechanisms 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
 ability to use each combination in a subscription is also analyzed.

Niemi, et al. Standards Track [Page 19] RFC 6446 Event Rate Control January 2012

 maximum rate and minimum rate:  This combination allows a reduced
    notification rate, but at the same time assures the reception of
    periodic notifications.
    A subscriber SHOULD choose a "min-rate" value lower than the "max-
    rate" value, otherwise, the notifier MUST adjust the subscriber
    provided "min-rate" value to a value equal to or lower than the
    "max-rate" value.
 maximum rate and adaptive minimum rate:  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 "adaptive-min-rate" value lower than
    the "max-rate" value, otherwise, the notifier MUST adjust the
    subscriber provided "adaptive-min-rate" value to a value equal to
    or lower than the "max-rate" value.
 minimum rate and adaptive minimum rate:  When using the adaptive
    minimum rate mechanism, frequent state changes in a short period
    can result in no notifications for a longer period following the
    short period.  The addition of the minimum rate mechanism ensures
    that the subscriber always receives notifications after a
    specified interval.
    A subscriber SHOULD choose a "min-rate" value lower than the
    "adaptive-min-rate" value, otherwise, the notifier MUST NOT
    consider the "min-rate" value.
 maximum rate, minimum rate, and adaptive minimum rate:  This
    combination makes little sense to be used, although it is not
    forbidden.
    A subscriber SHOULD choose a "min-rate" and "adaptive-min-rate"
    values lower than the "max-rate" value, otherwise, the notifier
    MUST adjust the subscriber provided "min-rate" and "adaptive-min-
    rate" values to a value equal to or lower than the "max-rate"
    value.
    A subscriber SHOULD choose a "min-rate" value lower than the
    "adaptive-min-rate" value, otherwise, the notifier MUST NOT
    consider the "min-rate" value.

9. Protocol Element Definitions

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

Niemi, et al. Standards Track [Page 20] RFC 6446 Event Rate Control January 2012

9.1. "max-rate", "min-rate", and "adaptive-min-rate" Header Field

    Parameters
 The "max-rate", "min-rate", and "adaptive-min-rate" 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 Sections 5, 6, and 7.

9.2. Grammar

 This section describes the Augmented BNF [RFC5234] definitions for
 the new header field parameters.  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.
    event-param     =  max-rate-param
                       / min-rate-param
                       / amin-rate-param
    subexp-params   =  max-rate-param
                       / min-rate-param
                       / amin-rate-param
    max-rate-param  =  "max-rate" EQUAL
                       (1*2DIGIT ["." 1*10DIGIT])
    min-rate-param  =  "min-rate" EQUAL
                       (1*2DIGIT ["." 1*10DIGIT])
    amin-rate-param =  "adaptive-min-rate" EQUAL
                       (1*2DIGIT ["." 1*10DIGIT])

9.3. 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 2xx
 response to a NOTIFY request.  The use of the Event header field in
 responses other than 2xx to NOTIFY requests is undefined and out of
 scope of this specification.
    Header field      where proxy ACK BYE CAN INV OPT REG PRA SUB NOT
    -----------------------------------------------------------------
    Event             2xx          -   -   -   -   -   -   -   -   o
 A subscriber that wishes to update the previously agreed value for
 maximum, minimum, or adaptive minimum rate of notifications MUST
 include all desired values for the "max-rate", "min-rate", and
 "adaptive-min-rate" parameters in an Event header field of the 2xx
 response to a NOTIFY request.  Any of the other header field

Niemi, et al. Standards Track [Page 21] RFC 6446 Event Rate Control January 2012

 parameters currently defined for the Event header field by other
 specifications do not have a meaning if the Event header field is
 included in the 2xx response to the NOTIFY request.  These header
 field parameters MUST be ignored by the notifier, if present.
 The event type listed in the Event header field of the 2xx response
 to the NOTIFY request MUST match the event type of the Event header
 field in the corresponding NOTIFY request.

10. IANA Considerations

 This specification registers three new SIP header field parameters in
 the "Header Field Parameters and Parameter Values" sub-registry of
 the "Session Initiation Protocol (SIP) Parameters" registry.
                                             Predefined
    Header Field         Parameter Name        Values      Reference
    -------------------- ---------------     ----------    ---------
    Event                max-rate            No            [RFC6446]
    Subscription-State   max-rate            No            [RFC6446]
    Event                min-rate            No            [RFC6446]
    Subscription-State   min-rate            No            [RFC6446]
    Event                adaptive-min-rate   No            [RFC6446]
    Subscription-State   adaptive-min-rate   No            [RFC6446]
 This specification also updates the reference defining the Event
 header field in the "Header Fields" sub-registry of the "Session
 Initiation Protocol (SIP) Parameters" registry.
    Header Name  compact   Reference
    -----------  -------   ------------------
    Event          o       [RFC3265][RFC6446]

11. Security Considerations

 Naturally, the security considerations listed in RFC 3265 [RFC3265],
 which the rate control mechanisms described in this document extends,
 apply in their 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 2xx response to the NOTIFY request.
 Without integrity protection, an eavesdropper could see and modify
 the Event header field, or it could manipulate the transmission of a
 200 (OK) response to the NOTIFY request to suppress or flood
 notifications without the subscriber seeing what caused the problem.

Niemi, et al. Standards Track [Page 22] RFC 6446 Event Rate Control January 2012

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

12. 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, Janet Gunn, and Ari Keranen for support and/or
 review of this work.
 Thanks to Brian Rosen for the idea of the minimum and adaptive
 minimum rate mechanisms, and to Adam Roach for the work on the
 algorithm for the adaptive minimum rate mechanism and other feedback.

13. References

13.1. 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., Ed. 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.

Niemi, et al. Standards Track [Page 23] RFC 6446 Event Rate Control January 2012

13.2. Informative References

 [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.
 [RFC5839]  Niemi, A. and D. Willis, Ed., "An Extension to Session
            Initiation Protocol (SIP) Events for Conditional Event
            Notification", RFC 5839, May 2010.
 [RFC6447]  Mahy, R., Rosen, B., and H. Tschofenig, "Filtering
            Location Notifications in the Session Initiation Protocol
            (SIP)", RFC 6447, January 2012.

Niemi, et al. Standards Track [Page 24] RFC 6446 Event Rate Control January 2012

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
 200 South Mathilda Ave
 Sunnyvale, CA  94086
 US
 Phone: +1 650 391 5969
 EMail: krisztian.kiss@nokia.com
 Salvatore Loreto
 Ericsson
 Hirsalantie 11
 Jorvas  02420
 Finland
 EMail: salvatore.loreto@ericsson.com

Niemi, et al. Standards Track [Page 25]

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