TOC 
Network Working GroupA. Niemi
Internet-DraftNokia
Expires: January 17, 2005July 19, 2004

Session Initiation Protocol (SIP) Event Notification Throttles

draft-niemi-sipping-event-throttle-01

Status of this Memo

By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.

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."

The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.

This Internet-Draft will expire on January 17, 2005.

Copyright Notice

Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

This memo specifies a throttle mechanism for limiting the rate of Session Initiation Protocol (SIP) event notifications. This mechanism can be applied in subscriptions to all SIP event packages.



Table of Contents

1.  Introduction
2.  Definitions and Document Conventions
3.  Overview
    3.1  Use Case
        3.1.1  Pre-conditions
        3.1.2  Normal Flow
        3.1.3  Alternative Flow I
        3.1.4  ALternative Flow II
        3.1.5  Post-conditions
    3.2  Requirements
    3.3  Event Throttle Model
    3.4  Basic Operation
4.  Operation of Event Throttles
    4.1  Negotiating the Use of Throttle
    4.2  Setting the Throttle
        4.2.1  Subscriber Behavior
        4.2.2  Notifier Behavior
    4.3  Selecting the Throttle Interval
5.  Syntax
    5.1  "event-throttle" SIP Option Tag
    5.2  "throttle" Header Parameter
    5.3  Augmented BNF Definitions
6.  IANA Considerations
7.  Security Considerations
8.  Acknowledgements
§.  Normative References
§.  Informative References
§  Author's Address
§  Intellectual Property and Copyright Statements




 TOC 

1. Introduction

The SIP events frameworkRoach, A., Session Initiation Protocol (SIP)-Specific Event Notification, June 2002.[1] defines a generic framework for subscriptions to and notifications of events related to SIP systems. This framework defines the methods SUBSCRIBE and NOTIFY, and introduces the concept of an event package, which is a concrete application of the SIP events framework to a particular class of events.

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

All of the existing event package specifications include a maximum notification rate recommendation, ranging from once in every five seconds [4]Rosenberg, J., A Presence Event Package for the Session Initiation Protocol (SIP), January 2003., [5]Rosenberg, J., A Session Initiation Protocol (SIP) Event Package for Registrations, October 2002., [6]Rosenberg, J., A Watcher Information Event Template-Package for the Session Initiation Protocol (SIP), January 2003. to once per second [7]Mahy, R., A Message Summary and Message Waiting Indication Event Package for the Session Initiation Protocol (SIP), December 2003..

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.

This document defines an extension to the SIP events framework that allows a subscriber to set a throttle to event notifications generated by the notifier. The requirements and model for generic event throttles are further discussed in Section 3Overview. A throttle is simply a timer value that indicates the minimum time period allowed between two notifications. As a result of this throttle, a compliant notifier will limit the rate at which it generates notifications.



 TOC 

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 2119Bradner, S., Key words for use in RFCs to Indicate Requirement Levels, March 1997.[2] 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.



 TOC 

3. Overview

3.1 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.

3.1.1 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 seconds.

Heikki is one of the presentities Lisa is watching. Heikki's presence agent conforms to the throttling policy requested by Lisa's presence application. The event package includes only full-state notifications.

3.1.2 Normal Flow

3.1.3 Alternative Flow I

3.1.4 ALternative Flow II

Instead of full state, the notifications now contain partial-state.

3.1.5 Post-conditions

Lisa receives notifications of Heikki's presence at a maximum of once per 20 seconds. Only newest notifications containing full-state are ever sent to Lisa. With partial-notifications, the notifier merges the states of all notifications generated within a single 20 second period.

3.2 Requirements

REQ1:
The subscriber MUST be able to set using a throttle mechanism the minimum time period between two notifications in a specific subscription.
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.
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 7Security Considerations contains further discussion on the security implications of the throttle mechanism.

3.3 Event Throttle Model

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 1Notifier modeled as a statistical multiplexer illustrates the model.



                   C1 |\
           1 ---------|  \
             ---------|    \        C
             ---------| ||||O---------- 1
             ---------|    /           
           n ---------|  /  
                   Cn |/
      
      
 Notifier modeled as a statistical multiplexer 

The output connection has a default rate that is generally dictated by each individual event package. The rate can also be set using the throttle mechanism described in this document. A notifer that accepts a subscriber-defined throttle, adjusts its output rate accordingly.

There is typically only a single input connection, characterized by the event package, and consisting of a stream of event notifications. In general, applying a throttle MUST NOT alter the properties of the buffer. I.e., the event notifications are expected to deliver consistent information even when throttled.

In practice, there are only two viable buffer policies for SIP event notifiers:

Full-state:
Last one in 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 states of buffered notifications are merged, and the resulting notification is sent out. This policy applies to those event packages that carry partial-state notifications.

3.4 Basic Operation

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

The subscriber also indicates that it requires the throttle to be applied to the subscription. This is done using the SIP option-tag mechanism, by insisting that the notifier applies the event throttle extension when processing the request. A notifier that does not support the event throttle extension will reject the subscription.

A notifier that supports the throttle mechanism will comply with value given in the throttle, and adjust its rate of notification accordingly.

Throttled notifications will have exactly the same properties as the un-throttled ones, with the exception that they will not be generated more frequent than what the throttle allows.



 TOC 

4. Operation of Event Throttles

4.1 Negotiating the Use of Throttle

This specification uses the SIP option-tag mechanism for negotiating use of the throttle mechanism. Use of the "Require" header field and the 420 (Bad Extension) are according to SIPRosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, SIP: Session Initiation Protocol, June 2002.[3].

A subscriber that wishes to apply a throttle to notifications in a subscription insists that the notifier applies this throttle by including an "event-throttle" option-tag to the Require header field of the SUBSCRIBE request.

A notifier that does not understand the event-throttle extension, will respond with a 420 (Bad Extension) response. Otherwise, the throttle is processed by the notifier, and the notification rate is adjusted accordingly.

4.2 Setting the Throttle

4.2.1 Subscriber Behavior

In general, the way in which a subscriber generates SUBSCRIBE requests and processes NOTIFY requests is according to RFC 3265Roach, A., Session Initiation Protocol (SIP)-Specific Event Notification, June 2002.[1].

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

In case the notifier does not support the "event-throttle" extension, the subscriber SHOULD retry the subscription without that extension.

In this case the subscriber can resort to other means of limiting the notification rate. For example, instead of a subscription, it can fetch or poll the event state.

4.2.2 Notifier Behavior

In general, the way in which a notifier processes SUBSCRIBE requests and generates NOTIFY requests is according to RFC 3265Roach, A., Session Initiation Protocol (SIP)-Specific Event Notification, June 2002.[1].

A notifier that supports the "event-throttle" extension extracts the value of the "throttle" Event header parameter, and uses it as the minimum time allowed between two notifications.

A notifier MUST include the selected throttle value in a "throttle" parameter to the Subscription-State header field of the NOTIFY requests sent to the subscriber.

A compliant notifier MUST NOT generate notifications more frequent than what the throttle allows for, except when generating the notification either upon receipt of a SUBSCRIBE request (the first notification) or upon termination of the subscription (the last notification).

As specified in RFC 3261Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, SIP: Session Initiation Protocol, June 2002.[3] a notifier that supports event throttles SHOULD advertise its support by including the "event-throttle" option-tag in the Supported header field of a response to an OPTIONS request.

4.3 Selecting the Throttle Interval

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

OPEN ISSUE: Should we give recommendations to reasonable throttle resolutions, or define what behavior to exhibit if an unreasonable throttle value is given to the notifier?



 TOC 

5. Syntax

This section describes the syntax extensions required for watcherinfo history. Note that the formal syntax definitions described in this section are expressed in the Augmented BNF format used in SIPRosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, SIP: Session Initiation Protocol, June 2002.[3], and contain references to elements defined therein.

5.1 "event-throttle" SIP Option Tag

The "event-throttle" SIP option-tag is added to the "option-tag" definition in the SIP grammar. Usage of this option-tag is defined in Section 4.1Negotiating the Use of Throttle.

5.2 "throttle" Header Parameter

The "throttle" header parameter is added to the "generic-param" definition in the SIP grammar. Usage of this Event header parameter is described in section Section 4.2Setting the Throttle.

5.3 Augmented BNF Definitions

This section describes the Augmented BNF definitions for the new syntax element. The notation is as used in SIPRosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, SIP: Session Initiation Protocol, June 2002.[3] and the documents to which is refers.

   generic-param  =  throttle-param / token [ EQUAL gen-value ]
   throttle-param =  "throttle" EQUAL delta-seconds
   option-tag     =  throttle-tag / token
   throttle-tag   =  "event-throttle"  
          


 TOC 

6. IANA Considerations

TBD: New SIP option tag (event-throttle), and possibly new header parameter (throttle) need to be registered with IANA.



 TOC 

7. Security Considerations

Naturally, the security considerations listed in SIP eventsRoach, A., Session Initiation Protocol (SIP)-Specific Event Notification, June 2002.[1], which the throttle mechanism extends, apply in entirety. In particular, authentication and message integrity SHOULD be applied to subscriptions with the event-throttle extension.



 TOC 

8. Acknowledgements

Thanks to Pekka Pessi, Dean Willis, Eric Burger, Alex Audu , Alexander Milinski, and the SIPPING WG for support and review of this work.



 TOC 

9. References



 TOC 

9.1 Normative References

[1] Roach, A., "Session Initiation Protocol (SIP)-Specific Event Notification", RFC 3265, June 2002.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 (HTML, XML).
[3] 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.


 TOC 

9.2 Informative References

[4] Rosenberg, J., "A Presence Event Package for the Session Initiation Protocol (SIP)", draft-ietf-simple-presence-10 (work in progress), January 2003.
[5] Rosenberg, J., "A Session Initiation Protocol (SIP) Event Package for Registrations", draft-ietf-sipping-reg-event-00 (work in progress), October 2002.
[6] Rosenberg, J., "A Watcher Information Event Template-Package for the Session Initiation Protocol (SIP)", draft-ietf-simple-winfo-package-05 (work in progress), January 2003.
[7] Mahy, R., "A Message Summary and Message Waiting Indication Event Package for the Session Initiation Protocol (SIP)", draft-ietf-sipping-mwi-04 (work in progress), December 2003.


 TOC 

Author's Address

  Aki Niemi
  Nokia
  P.O. Box 100
  NOKIA GROUP, FIN 00045
  Finland
Phone:  +358 50 389 1644
EMail:  aki.niemi@nokia.com


 TOC 

Intellectual Property Statement

Disclaimer of Validity

Copyright Statement

Acknowledgment