Network Working Group A. Niemi
Internet-Draft Nokia
Expires: August 28, 2003 February 27, 2003
Requirements for Limiting the Rate of Event Notifications
draft-niemi-sipping-event-throttle-reqs-01
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Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
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
5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Normative References . . . . . . . . . . . . . . . . . . . . . . 6
Informative References . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
A. Changes to draft-niemi-sipping-event-throttle-reqs-00 . . . . . 7
B. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . 8
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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
congestion.
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [1].
3. Event Throttle Model
The model assumed for the event throttle mechanism is similar to the
one described in [6]. In it, the notifier employs a quarantine for
outgoing notifications, in which outgoing notifications are kept for
a certain amount of time before they are actually sent out to the
subscriber. In general, the policy by which queueing notifications
inside the quarantine are treated is not specified. For example, an
incoming notification might simply replace an existing notification
[6], or the quarantine could merge the states of all of the
quarantined notifications.
The throttle mechanism is intended to enable the quarantine period to
be configured by the subscriber. Further, the quarantine can be
strict, or can also employ a leaky-bucket style algorithm, in which
on average the quarantine period might hold, but occasional bursts
might also be allowed.
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OPEN ISSUE: Do we want to consider both models for event
throttles, or is only the strict limit really usable?
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 throttled rate
defines; and the latter will affect notifications of "stateless"
nature, e.g., location updates within a quarantine period will be
lost.
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 mobile device uses the event throttling mechanism to
limit the amount of traffic it may receive.
A mobile application is watching the state of 100 presentities each
generating notifications at a maximum rate of once per five seconds.
Assuming that the arrival times of notifications are evenly
distributed, this will result in a maximum notification frequency of:
f = 100 * (1 / 5s) = 100 / 5 Hz = 20 Hz
experienced by the mobile. The same watcher subscribing using a
throttle mechanism to limit the maximum rate at which notifications
are to be generated to once per 20 seconds can expect a maximum
notification frequency of:
f = 100 * (1 / 20s) = 100 / 20 Hz = 5 Hz
thus resulting in 75% reduction in the maximum rate of incoming
presence notifications.
Note that the actual rate of notification is the sum of many
factors, and this example only makes a very broad assumption on
the absolute maximum rate at which the notifications might be
generated.
5. Requirements
REQ1: The subscriber MUST be able to limit using a throttle mechanism
the maximum rate at which the notifier is allowed to generate
notifications in a subscription.
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REQ2: The subscriber MUST be able to indicate that it requires the
use of a throttle mechanism in the 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 maximum rate lower than
the one given by the subscriber. For example, local policy
could dictate an even lower rate of notification than what the
subscriber requires.
REQ7: Authentication and integrity protection SHOULD be applied to
subscriptions that apply the throttle 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 introduces some new security issues to consider:
The throttle mechanism might allow a subscriber to set a very low
maximum notification rate - one that possibly exceeds the
subscription expiration. Such a limit inserted by a malicious
third party would result in very few if any notifications to be
generated, which could be perceived as theft of service to the
subscriber.
Similarly, the throttle mechanism might allow the subscriber to
set a very high maximum rate of notification that possibly is
higher than the default recommended rate of notification. Such a
high rate inserted by a malicious third party could result in
denial of service of the notifier due to performance issues.
Using the throttle mechanism potentially allows a subscriber to
increase the number of active subscriptions due to the decrease in
the maximum rate of notifications generated by a single notifier.
If a malicious third party is able to remove the throttle from the
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subscriptions, the subscriber might be flooded with notifications.
All of the above problems can be avoided by ensuring that the
integrity and authenticity of subscriptions is protected by applying
relevant security measures.
7. Open Issues
This chapter lists the main open issues within this document.
ISSUE1: Is the offered model accurate and appropriate?
ISSUE2: Within the model, do we have a need to enable both the
leaky-bucket and the strict rate limiting models at the same
time, or is only one of them enough?
ISSUE3: Is it enough to leave the handling of notifications in the
qarantine out-of-scope, or are there requirements for that
as well that should be captured by this document
ISSUE4: Are the threats listed in the security section really valid
ISSUE5: Is the added value of event throttles enough to merit their
standardization?
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
2003.
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[6] Mahy, R., "A Message Summary and Message Waiting Indication
Event Package for the Session Initiation Protocol (SIP)",
draft-ietf-sipping-mwi-01 (work in progress), November 2002.
Author's Address
Aki Niemi
Nokia
P.O. Box 321
NOKIA GROUP, FIN 00045
Finland
Phone: +358 50 389 1644
EMail: aki.niemi@nokia.com
Appendix A. Changes to draft-niemi-sipping-event-throttle-reqs-00
Changes to this 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
Appendix B. Acknowledgements
The author would like to thank Tim Moran, Jonathan Rosenberg, Hisham
Khartabil, Juha Kalliokulju, Paul Kyzivat and Henning Schulzrinne for
their valuable comments.
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