Network Working Group A. Niemi
Internet-Draft Nokia Research Center
Expires: August 25, 2005 February 21, 2005
Session Initiation Protocol (SIP) Event Notification Throttle
Mechanism
draft-niemi-sipping-event-throttle-02
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Copyright Notice
Copyright (C) The Internet Society (2005).
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions and Document Conventions . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1 Use Case . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1.1 Pre-conditions . . . . . . . . . . . . . . . . . . . . 4
3.1.2 Normal Flow . . . . . . . . . . . . . . . . . . . . . 4
3.1.3 Alternative Flow I . . . . . . . . . . . . . . . . . . 4
3.1.4 ALternative Flow II . . . . . . . . . . . . . . . . . 5
3.1.5 Post-conditions . . . . . . . . . . . . . . . . . . . 5
3.2 Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
3.3 Event Throttle Model . . . . . . . . . . . . . . . . . . . 7
3.4 Basic Operation . . . . . . . . . . . . . . . . . . . . . 8
4. Operation of Event Throttles . . . . . . . . . . . . . . . . . 8
4.1 Negotiating the Use of Throttle . . . . . . . . . . . . . 8
4.2 Setting the Throttle . . . . . . . . . . . . . . . . . . . 9
4.2.1 Subscriber Behavior . . . . . . . . . . . . . . . . . 9
4.2.2 Notifier Behavior . . . . . . . . . . . . . . . . . . 9
4.3 Selecting the Throttle Interval . . . . . . . . . . . . . 10
4.4 Buffer Policy Description . . . . . . . . . . . . . . . . 10
4.4.1 Partial State Notifications . . . . . . . . . . . . . 10
4.4.2 Full State Notifications . . . . . . . . . . . . . . . 11
4.5 Estimated Bandwidth Savings . . . . . . . . . . . . . . . 11
5. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1 "event-throttle" SIP Option-tag . . . . . . . . . . . . . 12
5.2 "throttle" Header Field Parameter . . . . . . . . . . . . 12
5.3 Augmented BNF Definitions . . . . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1 Normative References . . . . . . . . . . . . . . . . . . . 13
9.2 Informative References . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . 15
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1. Introduction
The SIP events framework [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 [5], [6], [7] to once per second [8].
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 3. 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.
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 [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.
3. Overview
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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 mobile device uses the event throttling mechanism to
limit the amount of traffic it may expect to receive.
3.1.1 Pre-conditions
A presence application in Lisa's mobile device contains a list of 100
buddies or 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, limiting 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
o Heikki publishes a presence status of "red", which results in a
presence notification to be sent to Lisa.
o In 10 seconds, Heikki publishes a presence status of "blue". As
the throttling policy set by Lisa only allows the presence agent
to generate notifications at a maximum of once per 20 seconds, the
notification is put on hold.
o After another 10 seconds, the notification is allowed to be sent
to Lisa.
o Lisa receives a presence update conforming to the set throttling
policy.
3.1.3 Alternative Flow I
o Heikki publishes a presence status of "red", which results in a
presence notification to be sent to Lisa.
o In 10 seconds, Heikki publishes a presence status of "blue". As
the throttling policy set by Lisa only allows the presence agent
to generate notifications at a maximum rate of once per 20
seconds, the notification is put on hold.
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o After another 5 seconds, Heikki publishes a presence status of
"green". The resulting notification is not conformant to the
throttling policy set by Lisa and is therefore put on hold,
replacing the earlier queued notification (the one containing the
status of "blue").
o Lisa receives a presence update conforming to the set throttling
policy and containing the "green" status.
3.1.4 ALternative Flow II
Instead of full state, the notifications now contain partial state.
o Heikki publishes a presence status of "red", which results in a
presence notification to be sent to Lisa.
o In 10 seconds, Heikki publishes a changed presence status of
"blue". As the throttling policy set by Lisa only allows the
presence agent to generate notifications at a maximum rate of once
per 20 seconds, the notification is put on hold.
o After another 5 seconds, Heikki publishes an additional presence
status of "bitter". The resulting notification is not conformant
to the throttling policy set by Lisa and is therefore put on hold.
Since there already exists a queued notification (that of the
"blue" status), the notifier merges the two notifications into a
single notification (containing both "blue" and "bitter"
statuses).
o Lisa receives a presence update conforming to the set throttling
policy and containing the "blue" and "bitter" status.
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.
Given a default maximum notification rate of once per 5 seconds, and
the average notification header size of X and payload size of Y
bytes, the total maximum bandwidth consumption for full state
notifications during a single 60 minute subscription is:
100 * (X + Y) * 1 / 5 s * 3600 s = 72000 (X + Y)
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The total maximum bandwidth consumption using a throttle value of 20
seconds:
100 * (X + Y) * 1 / 20 s * 3600 = 18000 (X + Y)
Yielding a total bandwidth savings of 75%. With partial
notifications, using a worst-case scenario of totally non-overlapping
partial updates, the total maximum bandwidth consumption for partial
state notifications with an average payload size of Y' within a 60
minute subscription is:
100 * (X + Y') * 1 / 5 s * 3600 = 72000 (X + Y')
And with throttled partial state notifications using a 20 second
throttle interval, the maximum bandwidth consumption within a 60
minute subscription is:
(100 * X * 1 / 20 s * 3600) + (100 * Y' * 1 / 5 s * 3600)
= 18000 X + 72000 Y'
Yielding the same 75% bandwidth savings but only in terms of header
size. These can still be regarded as considerable savings, depending
of course on what the exact average sizes of partial notifications
and their headers are.
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.
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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 7 contains further discussion on the security
implications of the throttle mechanism.
3.3 Event Throttle Model
The notifier is responsible for sending out event notifications upon
state changes of the subscribed resource. We can model the notifier
as consisting of three components: the event state source S, the
packetizer P and the output buffer Bo, as shown in Figure 5.
,-.
,' `. Bo
+-----+ ; : +-+-+-+
| S |----->| P |---->| | | |
+-----+ : ; +-+-+-+
`. ,'
`-'
Figure 5: Model for the notifier
In short, the notifier reads event state changes from the event state
source, 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 event throttle mechanism.
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 work. These two buffer policies
depend on the mode in which the notifier is operating:
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Full-state: Last notification 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 transmission of two consecutive notifications in a
subscription is given as an Event header parameter in the SUBSCRIBE
request.
Note that the witnessed time between two consecutive received
notifications may not conform to the set throttle for a number of
reasons. For example, network jitter and retransmissions may
result in the subscriber receiving the notifications in lesser
intervals than what the throttle allows for.
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.
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) response are according to SIP [3].
A subscriber that wishes to apply a throttle to notifications in a
subscription contsructs a SUBSCRIBE request such that it includes a
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Require header field containing an "event-throttle" option-tag.
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 3265 [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 desired 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 throttle
extension present, unless doing so would overly burden the
subscriber.
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.
There are two main consequencies for the subscriber when applying the
throttle 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 is to utilize event
throttles, developers are instructed not to use the mechanism.
4.2.2 Notifier Behavior
In general, the way in which a notifier processes SUBSCRIBE requests
and generates NOTIFY requests is according to RFC 3265 [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 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
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notification). Such notifications reset the throttle timer, even
though they do not need to abide by it.
Retransmissions of NOTIFY requests are not affected by the throttle,
i.e., the throttle only applies to the generation of new
transactions. In other words, the throttle is reset only after the
previous transaction has completed.
As specified in RFC 3261 [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.
The notifier is responsible for adjusting the proposed throttle value
based on its local policy. The notifier MAY lower the throttle
value, e.g., because of lowering the subscription expiration. The
notifier MUST include the adjusted throttle value in the
Subscription-State header field's "throttle" parameter in each of the
NOTIFY requests. In addition, different event packages MAY define
additional constraints to the allowed throttle intervals. Such
constraints are out of the scope of this specification.
4.4 Buffer Policy Description
4.4.1 Partial State Notifications
With partial 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 diff of the two states, and
generating a notification that contains that diff.
When a throttle is applied to the subscription, it is important that
F' is replaced with F only when the throttle is reset. 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
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notification instead.
4.4.2 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 over to the subscriber.
When a throttle is applied in 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.
4.5 Estimated Bandwidth Savings
As can already be seen from Section 3.1.5, it is difficult to
estimate the total bandwidth savings accrued by using the throttle
mechanism over a subscription, since such estimates will vary
depending on the useage scenarios. However, it is easy to see that
given a subscription where several full state notification would have
normally been sent in any given throttle interval, a throttled
subscription would only send a single notification during the same
interval, 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 consequtive 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
throttled notification merging all of these n partial states together
is at a maximum the size of a full-state update. In this case, the
bandwidth savings are approximately n times the size of the NOTIFY
header.
It is also true that there are several compression shcemes available
that have been designed to save bandwidth in SIP, e.g., SigComp [9]
and TLS compression [10]. However, such comression schemes are
complementary rather than competing mechanisms to the throttle
mechanism. After all, they can both be applied simultaniously, and
in such a way that the compound savings are as good as the sum of
applying each one alone.
5. Syntax
This section describes the syntax extensions required for the
throttle mechanism.
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5.1 "event-throttle" SIP Option-tag
The "event-throttle" option-tag is added to the rule definition of
the SIP option-tag in the SIP [3] grammar. Usage of this option-tag
is defined in Section 4.1.
5.2 "throttle" Header Field Parameter
The "throttle" parameter is added to the rule definitions of the
Event header field and the Subscription-State header field in the SIP
Events [1] grammar. Usage of this parameter is described in section
Section 4.2.
5.3 Augmented BNF Definitions
This section describes the Augmented BNF [4] definitions for the new
syntax elements. Note that we derive here from the ruleset present
in both SIP Events [1] and SIP [3], adding additional alternatives to
the alternative sets of "event-param", "subexp-params" and
"option-tag" defined therein.
event-param =/ throttle-param
subexp-params =/ throttle-param
option-tag =/ throttle-tag
throttle-param = "throttle" EQUAL delta-seconds
throttle-tag = "event-throttle"
6. IANA Considerations
This specification registers a new SIP option tag, defined by the
following information which is to be added to the Option Tags
sub-registry under http://www.iana.org/assignments/sip-parameters.
Name: event-throttle
Description: This option tag indicates support for the Session
Initiation Protocol event notification throttle mechanism. Its
use with the Supported header field indicates support for the
throttle mechanism, and its use with the Require header field
indicates that the user agent client requires the use of the
throttle mechanism in its subscription.
This specification also registers a new SIP header field parameter,
defined by the following information which is to be added to the
Header Field Parameters and Parameter Values sub-registry under
http://www.iana.org/assignments/sip-parameters.
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Header Field Parameter Name Values Reference
-------------------- --------------- --------- ---------
Event throttle No [RFCxxxx]
Subscription-State throttle No [RFCxxxx]
(Note to the RFC Editor: please replace "xxxx" with the RFC number of
this specification, when assigned.)
7. Security Considerations
Naturally, the security considerations listed in SIP events [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.
8. Acknowledgements
Thanks to Pekka Pessi, Dean Willis, Eric Burger, Alex Audu, Alexander
Milinski, Jonathan Rosenberg, Cullen Jennings and Adam Roach for
support and/or review of this work.
9. References
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.
[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.
[4] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
9.2 Informative References
[5] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.
[6] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Registrations", RFC 3680, March 2004.
[7] Rosenberg, J., "A Watcher Information Event Template-Package
for the Session Initiation Protocol (SIP)", RFC 3857, August
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2004.
[8] Mahy, R., "A Message Summary and Message Waiting Indication
Event Package for the Session Initiation Protocol (SIP)",
RFC 3842, August 2004.
[9] Price, R., Bormann, C., Christoffersson, J., Hannu, H., Liu, Z.
and J. Rosenberg, "Signaling Compression (SigComp)", RFC 3320,
January 2003.
[10] Friend, R., "Transport Layer Security (TLS) Protocol
Compression Using Lempel-Ziv-Stac (LZS)", RFC 3943, November
2004.
Author's Address
Aki Niemi
Nokia Research Center
P.O. Box 407
NOKIA GROUP, FIN 00045
Finland
Phone: +358 50 389 1644
Email: aki.niemi@nokia.com
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