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Versions: 00 01 02                                                      
Network Working Group                                           A. Niemi
Internet-Draft                                                     Nokia
Expires: December 29, 2003                                 June 30, 2003

       Requirements for Limiting the Rate of Event Notifications

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   All event packages are required to specify a maximum rate at which
   event notifications are generated by a single notifier. Such a limit
   is provided in order to reduce network congestion. In addition to the
   fixed limits introduced by specific event packages, further
   mechanisms for limiting the rate of event notification are also
   allowed to be defined by event package specifications but none have
   been specified so far. This memo discusses the requirements for a
   throttle mechanism that allows a subscriber to further limit the rate
   of event notification.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Event Throttle Model . . . . . . . . . . . . . . . . . . . . .  3
   4.  Example Use Case . . . . . . . . . . . . . . . . . . . . . . .  4
   4.1 Pre-conditions . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.2 Normal Flow  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.3 Post-conditions  . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  5
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   7.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   8.  Changes to "draft-niemi-sipping-event-throttle-reqs-01"  . . .  6
   9.  Changes to "draft-niemi-sipping-event-throttle-reqs-00"  . . .  7
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
       Normative References . . . . . . . . . . . . . . . . . . . . .  7
       Informative References . . . . . . . . . . . . . . . . . . . .  7
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  8
       Intellectual Property and Copyright Statements . . . . . . . .  9

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1. Introduction

   The SIP events framework described in RFC 3265 [2] mandates 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

   All of the existing event package specifications include a maximum
   notification rate recommendation, ranging from once in every five
   seconds [3], [4], [5] to once per second [6].

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

   This memo discusses the requirements for a generic throttling
   mechanism, which allows the subscriber to limit the rate of event
   notifications. It is intended that the throttle mechanism is not
   event package specific, but commonly available to be used with all
   event subscriptions.

2. Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119 [1],
   and indicate requirement priorities.

3. Event Throttle Model

   A throttle is defined as a protocol element that establishes a
   throttling policy at the notifier. This policy simply indicates the
   minimum time period allowed between two notifications. In practice,
   this throttling policy only extends the default policy of each event
   package, making it subscriber-configurable.

   Using notations from traffic theory, we can model the notifier as a
   statistical multiplexer with an input rate of Ci (i = 1,...,n), and
   an output rate of C <= C1 + ... + Cn. Typically, the statistical
   multiplexer is lossy, with a finite buffer size. The loss probability
   of the statistical multiplexer can be decreased by enlarging this
   buffer. Figure 1 illustrates the model.

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                      C1 |\
              1 ---------|  \
                ---------|    \        C
                ---------| ||||O---------- 1
                ---------|    /
              n ---------|  /
                      Cn |/

        Figure 1: Notifier modeled as a statistical multiplexer

   In event notification, there is typically only a single input
   connection, characterized by the event package, and consisting of a
   stream of event notification packets. Properties of the buffer, such
   as buffer size, policy (e.g., FIFO, LIFO), and packet treatment in
   lossy conditions, are all implementation and event package specific.

      A valid buffer model is a LIFO (Last In First Out) buffer with a
      size of one notification. Out of all buffered notifications, only
      the latest one is ever sent to the subscriber. Another equally
      valid buffer model might be one that has a near infinite buffer
      size. In that case, it is enough that the output rate C exceeds
      the aggregate average rate of all the inputs. Under lossy
      conditions, notifications might be dropped or their state merged,
      depending on the event package.

   The main implication of this model for event throttles is that they
   are lossy. Either some state changes are lost, or some level of
   accuracy in notifications is lost. The former will affect state
   changes that occur more frequent than what the throttling policy
   allows; and the latter will affect notifications of "stateless"
   nature, e.g., accuracy of buffered location updates decreases.

4. Example Use Case

   There are many applications that potentially would make use of a
   throttle mechanism. This chapter only illustrates one possible use
   case, in which a device uses the event throttling mechanism to limit
   the amount of traffic it may receive.

4.1 Pre-conditions

   A presence application in Lisa's device contains a list of 100
   presentities. In order to decrease the processing and network load of
   watching 100 presentities, Lisa's presence application has included
   an event throttle to each of the subscriptions, to limit the maximum
   rate at which notifications are to be generated to once per 20

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4.2 Normal Flow

   o  Heikki is one of the presentities Lisa is wathcing. Heikki's
      presence agent conforms to the throttling policy requested by
      Lisa's presence application.

   o  Heikki changes his location, which results in a presence
      notification to be sent to Lisa.

   o  Heikki's location changes again, and now very fast. His presence
      agent receives outgoing presence notification much more frequently
      than what the throttling policy allows it to generate
      notifications out to Lisa. The notifications are buffered.

   o  Lisa receives presence updates conforming to the set throttling

   o  Now Heikki's movements stabilize, and his location remains stable.

4.3 Post-conditions

   The throttled subscriptions even out rapid changes in presence
   status. Lisa still receives all of the buffered presence
   notifications. Her understanding of Heikki's presence status is
   temporarily different from Heikki's current real-time status, but as
   the buffered notifications get exhausted, will eventually converge to
   the real-time status.

5. Requirements

   REQ1: The subscriber MUST be able to set using a throttle mechanism
         the minimum time period between two notifications in a specific

   REQ2: The subscriber MUST be able to indicate that it requires the
         notifier to comply with the suggested throttling policy in a
         specific subscription.

   REQ3: The notifier MUST be able to indicate that it does not support
         the use of a throttle mechanism in the subscription.

   REQ4: It MUST be possible to use the throttle mechanism in
         subscriptions to all events.

   REQ5: It MUST be possible to use the throttle mechanism together with
         any event filtering mechanism.

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   REQ6: The notifier MUST be allowed to use a throttling policy in
         which the minimum time period between two notifications is
         longer than the one given by the subscriber.

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

   REQ7: The throttle mechanism MUST provide a reasonable resolution for
         setting the minimum period between two notifications. At a
         minimum, the throttling mechanism MUST include discussion of
         the situation resulting from a minimum time period which
         exceeds the subscription duration, and SHOULD provide
         mechanisms for avoiding this situation.

   REQ8: A throttle mechanism MUST allow for the application of
         authentication and integrity protection mechanisms to
         subscriptions invoking that mechanism.

      Note that Section 6 contains further discussion on the security
      implications of the throttle mechanism.

6. Security Considerations

   Naturally all of the security considerations for event subscriptions
   and notifications also apply to subscriptions and notifications that
   use the throttle mechanism. In addition, using the event throttle
   mechanism may introduce some new security issues to consider.
   However, there are no additional requirements regarding security at
   this stage.

7. Open Issues

   This chapter lists the main open issues within this document.

   o  Is the model comprehensive?

   o  Is this work mature enough to be handed a WG work item status?

8. Changes to "draft-niemi-sipping-event-throttle-reqs-01"

   Changes from the last version were:

   o  Refined the model based on feedback.

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   o  Clarified language and terminology used in the requirements, based
      on feedback.

9. Changes to "draft-niemi-sipping-event-throttle-reqs-00"

   Changes from the previous version include:

   o  Added the chapter describing the model for event throttles.

   o  Reworded the requirements to reflect the model discussion

   o  Added acknowledgements, changelog, and open issues sections

10. Acknowledgements

   The author would like to thank Tim Moran, Jonathan Rosenberg, Hisham
   Khartabil, Juha Kalliokulju, Paul Kyzivat, Henning Schulzrinne and
   Dean Willis for their valuable comments.

Normative References

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

Informative References

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

   [3]  Rosenberg, J., "A Presence Event Package for the Session
        Initiation Protocol (SIP)", draft-ietf-simple-presence-10 (work
        in progress), January 2003.

   [4]  Rosenberg, J., "A Session Initiation Protocol (SIP) Event
        Package for Registrations", draft-ietf-sipping-reg-event-00
        (work in progress), October 2002.

   [5]  Rosenberg, J., "A Watcher Information Event Template-Package for
        the Session Initiation  Protocol (SIP)",
        draft-ietf-simple-winfo-package-05 (work in progress), January

   [6]  Mahy, R., "A Message Summary and Message Waiting Indication
        Event Package for the  Session Initiation Protocol (SIP)",
        draft-ietf-sipping-mwi-02 (work in progress), March 2003.

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Author's Address

   Aki Niemi
   P.O. Box 321
   NOKIA GROUP, FIN  00045

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

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