SIPPING WG A. Niemi
Internet-Draft Nokia
Expires: March 8, 2004 September 8, 2003
Requirements for Limiting the Rate of Event Notifications
draft-ietf-sipping-event-throttle-reqs-00
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Copyright Notice
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
4.1 Pre-conditions . . . . . . . . . . . . . . . . . . . . . . . . 4
4.2 Normal Flow . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3 Post-conditions . . . . . . . . . . . . . . . . . . . . . . . 5
5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Document Change History . . . . . . . . . . . . . . . . . . . 6
7.1 Changes to "draft-niemi-sipping-event-throttle-reqs-02" . . . 6
7.2 Changes to "draft-niemi-sipping-event-throttle-reqs-01" . . . 6
7.3 Changes to "draft-niemi-sipping-event-throttle-reqs-00" . . . 7
8. 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
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],
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
seconds.
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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
policy.
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
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.
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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. Document Change History
This chapter lists the change history of this Internet Draft.
7.1 Changes to "draft-niemi-sipping-event-throttle-reqs-02"
Changes since the last version were:
o Submitted as SIPPING WG work item.
7.2 Changes to "draft-niemi-sipping-event-throttle-reqs-01"
Changes from the last version were:
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o Refined the model based on feedback.
o Clarified language and terminology used in the requirements, based
on feedback.
7.3 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
8. 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
2003.
[6] Mahy, R., "A Message Summary and Message Waiting Indication
Event Package for the Session Initiation Protocol (SIP)",
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draft-ietf-sipping-mwi-02 (work in progress), March 2003.
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
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