Network Working Group A. Niemi
Internet-Draft K. Kiss
Intended status: Standards Track Nokia
Expires: April 25, 2009 S. Loreto
Ericsson
Oct 22, 2008
Session Initiation Protocol (SIP) Event Notification Extension for
Notification Throttling
draft-niemi-sipping-event-throttle-07
Status of this Memo
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Abstract
This memo specifies the throttle, forge and average mechanisms for
adjusting the rate of Session Initiation Protocol (SIP) event
notifications. These mechanisms can be applied in subscriptions to
all SIP event packages, but in particular the throttle mechanism is
especially designed to be used in combination with a subscription to
a Resource List Server (RLS).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions and Document Conventions . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Throttle Use Case . . . . . . . . . . . . . . . . . . . . 4
3.2. Force Use Case . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Average Use Case . . . . . . . . . . . . . . . . . . . . . 6
3.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
3.5. Event Throttle Model for Resource List Server . . . . . . 7
3.6. Basic Operation . . . . . . . . . . . . . . . . . . . . . 9
3.7. Usage of Throttle, Force and Average in a subscription . . 10
4. Operation of Event Throttles . . . . . . . . . . . . . . . . . 10
4.1. Negotiating the Use of Throttle . . . . . . . . . . . . . 10
4.2. Setting the Throttle . . . . . . . . . . . . . . . . . . . 11
4.2.1. Subscriber Behavior . . . . . . . . . . . . . . . . . 11
4.2.2. Notifier Behavior . . . . . . . . . . . . . . . . . . 11
4.3. Selecting the Throttle Interval . . . . . . . . . . . . . 12
4.4. Buffer Policy Description . . . . . . . . . . . . . . . . 12
4.4.1. Partial State Notifications . . . . . . . . . . . . . 12
4.4.2. Full State Notifications . . . . . . . . . . . . . . . 13
4.5. Estimated Bandwidth Savings . . . . . . . . . . . . . . . 13
5. Operation of Event Force . . . . . . . . . . . . . . . . . . . 13
5.1. Negotiating the Use of Force . . . . . . . . . . . . . . . 14
5.2. Setting the Force . . . . . . . . . . . . . . . . . . . . 14
5.2.1. Subscriber Behavior . . . . . . . . . . . . . . . . . 14
5.2.2. Notifier Behavior . . . . . . . . . . . . . . . . . . 14
6. Operation of Event Average . . . . . . . . . . . . . . . . . . 15
6.1. Negotiating the Use of Average . . . . . . . . . . . . . . 15
6.2. Calculating the Average Interval . . . . . . . . . . . . . 15
6.3. Setting the Average . . . . . . . . . . . . . . . . . . . 16
6.3.1. Subscriber Behavior . . . . . . . . . . . . . . . . . 16
6.3.2. Notifier Behavior . . . . . . . . . . . . . . . . . . 16
7. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1. "throttle", "force" and "average" Header Field
Parameters . . . . . . . . . . . . . . . . . . . . . . . . 17
7.2. Augmented BNF Definitions . . . . . . . . . . . . . . . . 18
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
9. Security Considerations . . . . . . . . . . . . . . . . . . . 18
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
11.1. Normative References . . . . . . . . . . . . . . . . . . . 19
11.2. Informative References . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . . . 22
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1. Introduction
The SIP events framework [RFC3265] 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 [RFC3856], [RFC3680], [RFC3857] to once per second [RFC3842].
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.
The resource list extension [RFC4662] to the SIP events framework
also deals with rate limiting of event notifications. The extension
allows a subscriber to subscribe to a heterogenous list of resources
with a single SUBSCRIBE request, rather than having to install a
subscription for each resource separately. The event list
subscription also allows rate limiting, or throttling of
notifications, by means of the Resource List Server (RLS) buffering
notifications of resource state changes, and sending them in batches.
However, the event list mechanism provides no means for the
subscriber to set the interval for the throttling; it is strictly an
implementation decision whether batching of notifications is
supported, and by what means.
This document defines an extension to the SIP events framework
defining the following three "Event" header field parameters that
allow a subscriber to set a Minimum, a Maximum and an Average rate of
event notifications generated by the notifier:
Throttle: specifies a minimum notification time period between two
notifications, in seconds.
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Force: specifies a maximum notification time period between two
notifications, in seconds. Whenever the time since the most
recent notification exceeds the value in the "force" parameter,
then the current state would be sent in its entirety (just like
after a subscription refresh).
Average: specifies an average cadence at which notifications are
desired, in seconds. It works similar to the "force" parameter,
except that it will reduce the frequency at which notifications
are sent if several have already been sent recently.
The requirements and model are further discussed in Section 3. All
those mechanisms are simply timer values that indicates the minimum,
maximum and average time period allowed between two notifications.
As a result of those mechanism, a compliant notifier will adjust the
rate at which it generates notifications.
These mechanisms are applicable to any event subscription, but the
throttle mechanism is mainly intended for use with an event list
subscription.
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 [RFC2119] 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
3.1. Throttle Use Case
A presence application in a mobile device contains a list of 100
buddies or presentities. In order to decrease the processing and
network load of watching 100 presentities, the presence application
has employed a Resource List Server (RLS) with the list of buddies,
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and therefore only needs a single subscription to the RLS in order to
receive notification of the presence state of the resource list.
In order to control the buffer policy of the RLS, the presence
application sets a throttle interval via the event throttle
extension. Alternatively, the presence application could set a
default throttle for the resource list, via a list manipulation
interface, e.g., using the XML Configuration Access Protocol (XCAP)
[RFC4825].
The RLS will buffer notifications that do not comply with the
throttle interval, and batch all of the buffered state changes
together in a single notification when allowed by the throttle. The
throttle applies to the overall resource list, which means that there
is a hard cap imposed by the throttle to the amount of traffic the
presence application can expect to receive.
For example, with a throttle of 20 seconds, the presence application
can expect to receive a notification every 20 seconds at a maximum.
The presence application can also modify the throttle during the
lifetime of the subscription. For example, if the User Interface
(UI) of the application shows inactivity for a period of time, it can
simply pause the notifications by setting the throttle interval to
the subscription expiration time, while still keeping the
subscription alive. When the user becomes active again, the presence
application can resume the stream of notifications by re-setting the
throttle to the earlier used value.
Currently, a subscription refresh is needed in order to update the
throttle interval. However, this is highly inefficient, since
each refresh automatically generates a (full-state) notification
carrying the latest resource state. There is work
[I-D.ietf-sip-subnot-etags] ongoing to solve these inefficiencies.
3.2. Force Use Case
A location application is monitoring the movement of a target.
In order to decrease the processing and network load, the location
application has made a subscription with a set of filters that
specify, for example, to send an update only when the target has
moved at least 10 meters. However the application is interested in
receiving an update periodically even if the target has not moved
more than 10 meters in a second.
The application is interested in discovering if the state is changed,
even when it has not changed enough to satisfy any of the 'trigger'
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criteria
In order to control the actual state, the location application sets a
force interval via the event force extension. The force triggers a
notification that is exactly and precisely like a notification after
a subscription refresh.
The location application can also modify the force during the
lifetime of the subscription.
3.3. Average Use Case
The throttle and force mechanisms introduce a static and
instantaneous rate control. However there are some applications that
would work better with an adaptive rate control (i.e. an average
rate). This section illustrates the tracking scenario.
A tracking application is monitoring a target.
In order to decrease the processing and network load, the tracking
application wants to make a subscription that dynamically reduces the
frequency at which notifications are sent if the target has started
to move sending out already several notifications recently.
In order to set an average rate control, the application defines a
average interval via the event average extension. The average value
is used by the notifier to dynamically calculate the maximum time
allowed between two subscriptions. In order to dinamically calculate
the maximum, the Notifer takes into consideration the frequency at
which notifications have been sent recently.
The average rate control allows the notifier to dynamically increase
or decrease the Notification frequency.
The tracking application can also modify the average during the
lifetime of the subscription.
3.4. Requirements
REQ1: The subscriber must be able to set the minimum time period
between two notifications in a specific subscription.
REQ2: The subscriber must be able to set the maximum time period
between two notifications in a specific subscription.
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REQ3: The subscriber must be able to set an average cadence at
which notifications are desired in a specific subscription.
REQ4: It must be possible to apply all together, or in any
combination, the throttle, force and average mechanisms in a
specific subscription.
REQ5: It must be possible to use of the throttle, force and average
mechanisms in subscriptions to all events.
REQ6: It must be possible to use the throttle, force and average
mechanisms together with any event filtering mechanism.
REQ7: The notifier must be allowed to use a throttling policy in
which the minimum time period between two notifications is
adjusted from the value 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.
REQ8: 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.
REQ9: A throttle, force and average must be possible to be
installed, adapted, or removed in the course of an active
subscription.
REQ10: A throttle, force and average mechanism must allow for the
application of authentication and integrity protection
mechanisms to subscriptions invoking that mechanism.
Note that Section 9 contains further discussion on the security
implications of the throttle mechanism.
3.5. Event Throttle Model for Resource List Server
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 resource(s), the
Resource List Server (RLS) (or any other notifier), a notification
buffer, and finally the subscriber, or watcher of the event state, as
shown in Figure 1.
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+--------+
| Event |
+--------+ |Resource| +--------+
| Event | +--------+ | Event |
|Resource| | |Resource|
+---.=---+ | +---=----+
`-.. | _.--'
``-._ | _.--'
+'--'--'-+
|Resource|
| List |
| Server |
+---.----+
|
|
)--+---(
| | .--------.
|Buffer|<======'Throttle|
| | `--------'
)--.---(
|
|
.---+---.
| Event |
|Watcher|
`-------'
Figure 1: Model for the Resource List Server (RLS) Supporting
Throttling
In short, the RLS reads event state changes from the event state
resource, either by creating a backend subscription, or by other
means; it 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.
When a set of notifications are batched together, the way in which
overlapping resource state is handled depends on the type of the
resource state:
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 (most recent) full state notification of each
resource 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 state deltas of each buffered partial
notification per resource are merged, and the resulting
notification is sent out. This policy applies to those event
packages that carry partial-state notifications.
3.6. Basic Operation
A subscriber that wants to limit the rate of event notification in a
specific event subscription does so by including a throttle as part
of the SUBSCRIBE request. 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 recommends.
A subscriber that wants to have a maximum notification time period in
a specific event subscription does so by including a force as part of
the SUBSCRIBE request. The force indicating the maximum time allowed
between transmission of two consecutive notifications in a
subscription is given as an Event header parameter in the SUBSCRIBE
request.
A subscriber that wants to have an average cadence at which
notifications are desired in a specific event subscription does so by
including an average as part of the SUBSCRIBE request. The average
is given as an Event header parameter in the SUBSCRIBE request.
A notifier that supports the throttle, force and average mechanisms
will comply with value given in the throttle, force and average and
adjust its rate of notification accordingly.
Throttled, forced and averaged notifications will have exactly the
same properties as the ones the un-throttled, un-forced and un-
averaged, with the exception that they will be generated with the
frequency that has been requested.
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3.7. Usage of Throttle, Force and Average in a subscription
Applications can subscribe to an event package using all the
throttle, force and average mechanisms singly, or in combination; in
fact there is no technical incompatibility among them. However there
are some combinations that make little sense to be used together.
This section lists all the possible combinations that is possible to
insert in a subscription; the utility to use each combination in a
subscription is also analyzed.
Throttle and Force: this combination let possible to reduce the
notification frequence rate, but at same time assures the
reception of a notification every time the most recent
notification exceeds a specified interval.
A subscriber SHOULD choose a "force" value higher than the
"throttle" value, otherwise the notifier MUST adjust the
subscriber provided "force" value to a value equivalent or higher
than the "throttle" value.
Throttle and Average: it works in a similar way as the combination
above, but with the difference that the interval at which
notifications are assured changes dynamically.
A subscriber SHOULD choose an "average" value higher than the
"throttle" value, otherwise the notifier MUST adjust the
subscriber provided "average" value to a value equivalent or
higher than the "throttle" value.
Force and Average: as both the parameter are designed to force an
update, this combination makes little sense to be used.
Throttle, Force and Average: this combination makes little sense to
be used.
4. Operation of Event Throttles
4.1. Negotiating the Use of Throttle
A subscriber that wishes to apply a throttle to notifications in a
subscription constructs a SUBSCRIBE request that includes a throttle
interval in a "throttle" Event header field parameter.
A compliant notifier will reflect back the possibly adjusted throttle
interval in a "throttle" Subscription-State header field parameter of
the subsequent NOTIFY requests.
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A notifier that does not understand the event-throttle extension,
will not reflect the "throttle" parameter in the NOTIFY requests; the
absence of this parameter serves as a hint to the subscriber that no
throttling is supported by the notifier.
Otherwise, the indicated throttle value is adopted 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
[RFC3265].
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.
There are two main consequences 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 [RFC3265].
A notifier that supports the event-throttle extension extracts the
value of the "throttle" Event header parameter, and uses it as the
suggested minimum time allowed between two notifications. This value
can be adjusted by the notifier, as defined in Section 4.3.
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), when the subscription state is changing from "pending"
to "active" state or upon termination of the subscription (the last
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
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transactions. In other words, the throttle is reset only after the
previous transaction has completed.
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.
In some cases it makes sense to pause the notification stream on an
existing subscription dialog on a temporary basis without terminating
the subscription, e.g. due to inactivity on the application UI.
Whenever a subscriber discovers the need to perform the notification
pause operation, it SHOULD set the throttle interval to the remaining
subscription expiration value. This results in receiving no further
notifications until the subscription expires, renewed or
notifications are resumed by the subscriber.
The notifier is responsible for adjusting the proposed throttle value
based on its local policy or other properties. The notifier MAY
lower the throttle value, e.g., because of lowering the subscription
expiration. The notifier MAY also choose a higher throttle value,
e.g., because of static throttle value configuration given by local
policy. 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
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state, and if not, the notifier SHOULD send the full state
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
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 usage 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 consecutive 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
should at a maximum be 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 schemes available
that have been designed to save bandwidth in SIP, e.g., SigComp
[RFC3320] and TLS compression [RFC3943]. However, such compression
schemes are complementary rather than competing mechanisms to the
throttle mechanism. After all, they can both be applied
simultaneously, and in such a way that the compound savings are as
good as the sum of applying each one alone.
5. Operation of Event Force
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5.1. Negotiating the Use of Force
A subscriber that wishes to apply a maximum notification time period
between two notifications in a subscription constructs a SUBSCRIBE
request that includes a proposed maximum interval in a "force" Event
header field parameter.
A compliant notifier will reflect back the possibly adjusted forced
interval in a "force" Subscription-State header field parameter of
the subsequent NOTIFY requests.
A notifier that does not understand the event-force extension, will
not reflect the "force" parameter in the NOTIFY requests; the absence
of this parameter serves as a hint to the subscriber that no forcing
is supported by the notifier.
Otherwise, the indicated force value is adopted by the notifier, and
the notification rate is adjusted accordingly.
5.2. Setting the Force
5.2.1. Subscriber Behavior
In general, the way in which a subscriber generates SUBSCRIBE
requests and processes NOTIFY requests is according to RFC 3265
[RFC3265].
A subscriber that wishes to apply a maximum notification time period
between the notifications in a subscription includes a "force" Event
header parameter in the SUBSCRIBE request, indicating in seconds the
desired force value. The value of this parameter is an integral
number of seconds in decimal.
The main consequence for the subscriber when applying the force
mechanism is that it can receive a notification even if nothing has
changed in the current state of the notifier.
There is work [I-D.ietf-sip-subnot-etags] ongoing to only send a
reference in a notification if nothing has changed.
5.2.2. Notifier Behavior
In general, the way in which a notifier processes SUBSCRIBE requests
and generates NOTIFY requests is according to RFC 3265 [RFC3265].
A notifier that supports the event-force extension extracts the value
of the "force" Event header parameter, and uses it as the suggested
maximum time allowed between two notifications.
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A compliant notifier MUST generate notifications whenever the time
since the most recent notification exceeds the value in the "force"
parameter. The NOTIFY request then MUST contain the current state in
its entirety, just like after a subscription refresh.
Retransmissions of NOTIFY requests are not affected by the force,
i.e., the force only applies to the generation of new transactions.
In other words, the force is reset only after the previous
transaction has completed.
6. Operation of Event Average
6.1. Negotiating the Use of Average
A subscriber that wishes to apply an average cadence at which
notifications are desired in a subscription constructs a SUBSCRIBE
request that includes a proposed average interval in an "average"
Event header field parameter.
A compliant notifier will reflect back the possibly adjusted average
interval in an "average" Subscription-State header field parameter of
the subsequent NOTIFY requests.
A notifier that does not understand the event-average extension will
not reflect the "average" parameter in the NOTIFY requests; the
absence of this parameter serves as a hint to the subscriber that no
averaging is supported by the notifier.
Otherwise, the indicated average value is adopted by the notifier,
and the notification rate is adjusted accordingly.
6.2. Calculating the Average Interval
The formula used to dynamically calculate the timeout is given in
equation (1):
timeout = (average ^ 2) * count / period (1)
The output of the formula, "timeout", is a timeout in seconds. The
formula has three inputs:
average: The value of the "average" parameter conveyed in the
"Event" header field, in seconds.
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period: The rolling average period, in seconds. A suggested
reasonable period is 60 seconds.
[OPEN ISSUE]Is the period value something we should be able to
tune, or we can simply specify a reasonable period?
count: The number of notifications that have been sent during the
last "period" of seconds.
In the case both the Throttle and the Average are used in the same
subscription the formula used to dynamically calculate the timeout is
given in equation (2):
timeout = MAX[throttle, (average ^ 2) * count / period] (2)
throttle: The value of the "threshold" parameter conveyed in the
"Event" header field, in seconds.
The formula in (2) makes sure that for all the possible value of
throttle and average, with average > throttle, the timeout never
results in a lower value than throttle.
6.3. Setting the Average
6.3.1. Subscriber Behavior
In general, the way in which a subscriber generates SUBSCRIBE
requests and processes NOTIFY requests is according to RFC 3265
[RFC3265].
A subscriber that wishes to apply an average cadence at which
notifications are desired in a subscription includes a "average"
Event header parameter in the SUBSCRIBE request, indicating in
seconds the desired average value. The value of this parameter is an
integral number of seconds in decimal.
The main consequence for the subscriber when applying the average
mechanism is that it can receive a notification even if nothing has
changed in the current state of the notifier.
There is work [I-D.ietf-sip-subnot-etags] ongoing to only send a
reference in a notification if nothing has changed.
6.3.2. Notifier Behavior
In general, the way in which a notifier processes SUBSCRIBE requests
and generates NOTIFY requests is according to RFC 3265 [RFC3265].
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A notifier that supports the event-average extension extracts the
value of the "average" Event header parameter, and uses it to
calculate the maximum time allowed between two transactions as
defined in Section 6.2.
A compliant notifier MUST generate notifications whenever the time
since the most recent notification exceeds the value calculated using
the formula defined in Section 6.2.
The Average mechanism is implemented as follows:
1) When a subscription is first created, the notifier creates a
record that keeps track of the number of notifications that have
been sent in the "period". This record is initialized to contain
a history of having sent one message every "average" seconds for
the "period".
2) The "timeout" value is calculated according to the equation given
in section Section 6.2.
3) If the timeout period passes without a NOTIFY request being sent
in the subscription, then the current resource state is sent
(subject to any filtering associated with the subscription).
4) Whenever a NOTIFY request is sent (regardless of whether due to a
timeout or a state change), the notifier updates the notification
history record, recalculates the value of "timeout," and returns
to step 3.
Retransmissions of NOTIFY requests are not affected by the timeout,
i.e., the timeout only applies to the generation of new transactions.
In other words, the timeout is reset only after the previous
transaction has completed.
7. Syntax
This section describes the syntax extensions required for the
throttle, force and average mechanisms.
7.1. "throttle", "force" and "average" Header Field Parameters
The "throttle", "force" and "average" parameters are added to the
rule definitions of the Event header field and the Subscription-State
header field in the SIP Events [RFC3265] grammar. Usage of this
parameter is described in section Section 4.2.
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7.2. Augmented BNF Definitions
This section describes the Augmented BNF [RFC5234] definitions for
the new syntax elements. Note that we derive here from the ruleset
present in both SIP Events [RFC3265] and SIP [RFC3261], adding
additional alternatives to the alternative sets of "event-param" and
"subexp-params" defined therein.
event-param =/ throttle-param
subexp-params =/ throttle-param
throttle-param = "throttle" EQUAL delta-seconds
event-param =/ force-param
subexp-params =/ force-param
throttle-param = "force" EQUAL delta-seconds
event-param =/ average-param
subexp-params =/ average-param
throttle-param = "average" EQUAL delta-seconds
8. IANA Considerations
This specification registers three new SIP header field parameters,
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.
Predefined
Header Field Parameter Name Values Reference
-------------------- --------------- ---------- ---------
Event throttle No [RFCxxxx]
Subscription-State throttle No [RFCxxxx]
Event force No [RFCxxxx]
Subscription-State force No [RFCxxxx]
Event average No [RFCxxxx]
Subscription-State average No [RFCxxxx]
(Note to the RFC Editor: please replace "xxxx" with the RFC number of
this specification, when assigned.)
9. Security Considerations
Naturally, the security considerations listed in SIP events
[RFC3265], which the throttle mechanism extends, apply in entirety.
In particular, authentication and message integrity SHOULD be applied
to subscriptions with the event-throttle extension.
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10. 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.
Thanks to Brian Rosen for the idea of the "force" and "average"
mechanisms, and to Adam Roach for the work on the averaging
algorithm.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] 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.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC4662] Roach, A., Campbell, B., and J. Rosenberg, "A Session
Initiation Protocol (SIP) Event Notification Extension for
Resource Lists", RFC 4662, August 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
11.2. Informative References
[I-D.ietf-sip-subnot-etags]
Niemi, A., "An Extension to Session Initiation Protocol
(SIP) Events for Conditional Event Notification",
draft-ietf-sip-subnot-etags-03 (work in progress),
July 2008.
[RFC3320] Price, R., Bormann, C., Christoffersson, J., Hannu, H.,
Liu, Z., and J. Rosenberg, "Signaling Compression
(SigComp)", RFC 3320, January 2003.
[RFC3680] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Registrations", RFC 3680, March 2004.
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[RFC3842] Mahy, R., "A Message Summary and Message Waiting
Indication Event Package for the Session Initiation
Protocol (SIP)", RFC 3842, August 2004.
[RFC3856] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.
[RFC3857] Rosenberg, J., "A Watcher Information Event Template-
Package for the Session Initiation Protocol (SIP)",
RFC 3857, August 2004.
[RFC3943] Friend, R., "Transport Layer Security (TLS) Protocol
Compression Using Lempel-Ziv-Stac (LZS)", RFC 3943,
November 2004.
[RFC4825] Rosenberg, J., "The Extensible Markup Language (XML)
Configuration Access Protocol (XCAP)", RFC 4825, May 2007.
Authors' Addresses
Aki Niemi
Nokia
P.O. Box 407
NOKIA GROUP, FIN 00045
Finland
Phone: +358 50 389 1644
Email: aki.niemi@nokia.com
Krisztian Kiss
Nokia
313 Fairchild Dr
Mountain View, CA 94043
US
Phone: +1 650 391 5969
Email: krisztian.kiss@nokia.com
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Salvatore Loreto
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: salvatore.loreto@ericsson.com
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