Network Working Group A. Niemi
Internet-Draft K. Kiss
Intended status: Standards Track Nokia
Expires: May 1, 2010 S. Loreto
Ericsson
October 28, 2009
Session Initiation Protocol (SIP) Event Notification Extension for
Notification Rate Control
draft-ietf-sipcore-event-rate-control-01
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Abstract
This document specifies mechanisms for adjusting the rate of Session
Initiation Protocol (SIP) event notifications. These mechanisms can
be applied in subscriptions to all SIP event packages.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Definitions and Document Conventions . . . . . . . . . . . . . 5
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Use Case for limiting the maximum rate of notifications . 5
3.2. Use Case for setting a minimum rate for notifications . . 6
3.3. Use Case for specifying the average rate of
notifications . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7
3.5. The maximum rate mechanism for Resource List Server . . . 9
3.6. Basic Operation . . . . . . . . . . . . . . . . . . . . . 10
4. Operation of the maximum rate mechanism . . . . . . . . . . . 11
4.1. Subscriber Behavior . . . . . . . . . . . . . . . . . . . 11
4.2. Notifier Behavior . . . . . . . . . . . . . . . . . . . . 12
4.3. Selecting the maximum rate . . . . . . . . . . . . . . . . 13
4.4. Buffer Policy Description . . . . . . . . . . . . . . . . 14
4.4.1. Partial State Notifications . . . . . . . . . . . . . 14
4.4.2. Full State Notifications . . . . . . . . . . . . . . . 14
4.5. Estimated Bandwidth Savings . . . . . . . . . . . . . . . 14
5. Operation of the minimum rate mechanism . . . . . . . . . . . 15
5.1. Subscriber Behavior . . . . . . . . . . . . . . . . . . . 15
5.2. Notifier Behavior . . . . . . . . . . . . . . . . . . . . 16
6. Operation of the average rate mechanism . . . . . . . . . . . 16
6.1. Subscriber Behavior . . . . . . . . . . . . . . . . . . . 16
6.2. Notifier Behavior . . . . . . . . . . . . . . . . . . . . 17
6.3. Calculating the timeout . . . . . . . . . . . . . . . . . 18
7. Usage of "min-interval", "max-interval" and
"average-interval" in a combination . . . . . . . . . . . . . 19
8. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.1. "min-interval", "max-interval" and "average-interval"
Header Field Parameters . . . . . . . . . . . . . . . . . 20
8.2. Augmented BNF Definitions . . . . . . . . . . . . . . . . 20
8.3. Event header field usage in responses to the NOTIFY
request . . . . . . . . . . . . . . . . . . . . . . . . . 21
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
10. Security Considerations . . . . . . . . . . . . . . . . . . . 22
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
12.1. Normative References . . . . . . . . . . . . . . . . . . . 22
12.2. Informative References . . . . . . . . . . . . . . . . . . 22
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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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 heterogeneous 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:
min-interval: specifies a minimum notification time period between
two notifications, in seconds.
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max-interval: specifies a maximum notification time period between
two notifications, in seconds. Whenever the time since the most
recent notification exceeds the value in the "max-interval"
parameter, then the current state would be sent in its entirety,
just like after a subscription refresh.
average-interval: specifies an average cadence at which
notifications are desired, in seconds. It works similar to the
"max-interval" 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, both
single event subscription and 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. Use Case for limiting the maximum rate of notifications
A presence client 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 client has employed a
Resource List Server (RLS) with the list of buddies, 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 client
sets a maximum rate ("min-interval" parameter), i.e. a minimum time
interval between two notifications. Alternatively, the presence
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client could set the maximum rate 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 maximum
rate and batch all of the buffered state changes together in a single
notification only when allowed by the maximum rate. The maximum rate
applies to the overall resource list, which means that there is a
hard cap imposed by the maximum rate to the amount of traffic the
presence client can expect to receive.
For example, with a "min-interval" of 20 seconds, the presence
application can expect to receive a notification at a minimum of
every 20 seconds.
The presence client can also modify the "min-interval" parameter
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 "min-
interval" parameter to the subscription expiration time, while still
keeping the subscription alive. When the user becomes active again,
the presence client can resume the stream of notifications by re-
setting the "min-interval" parameter to the earlier used value.
Currently, a subscription refresh is needed in order to update the
maximum rate. 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-sipcore-subnot-etags] ongoing to solve these
inefficiencies.
3.2. Use Case for setting a minimum rate for notifications
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 location filters
[I-D.ietf-geopriv-loc-filters] that specify trigger criterias, for
example, to send an update only when the target has moved at least n
meters. However, the application is also interested to receive the
current state periodically even if the state of the target has not
changed enough to satisfy any of the trigger criteria, i.e. has not
moved at least n meters within the period.
In order to control the actual state, the location application sets a
minimum rate ("max-interval" parameter), i.e. a maximum time interval
between two notifications.
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The location application can also modify the "max-interval" parameter
during the lifetime of the subscription. When the subscription to
the movement of a target is made, the notifier does not typically
have the location information available. Thus, the first
notification might be empty, or certain values might be absent. An
important use case is placing constraints on when complete state
should be provided after creating the subscription. The "max-
interval" parameter indicates to the notifier the time when to
generate a notification containing complete state information. Once
state is acquired and the second notification is sent, the subscriber
updates or changes the "max-interval" parameter to a more sensible
value. This update can be performed in the 200 OK response to the
NOTIFY request that contains the complete state information.
3.3. Use Case for specifying the average rate of notifications
The previous mechanisms introduce a static and instantaneous rate
control. However there are some applications that would work better
with an adaptive rate control. 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 increases
the interval between notifications if the target has sent out several
notifications recently.
In order to set an adaptive rate control, the application defines a
average cadence ("average-interval" parameter) at which notifications
are desired. The "average-interval" parameter value is used by the
notifier to dynamically calculate the maximum time allowed between
two subscriptions. In order to dynamically calculate the maximum,
the Notifier takes into consideration the frequency at which
notifications have been sent recently.
This type of rate control allows the notifier to dynamically increase
or decrease the Notification frequency.
The tracking application can also modify the "average-interval"
parameter during the lifetime of the subscription.
3.4. Requirements
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REQ1: The subscriber must be able to set the minimum time period
("min-interval" parameter) between two notifications in a
specific subscription.
REQ2: The subscriber must be able to set the maximum time period
("max-interval" parameter) between two notifications in a
specific subscription.
REQ3: The subscriber must be able to set an average cadence
("average-interval" parameter) at which notifications are
desired in a specific subscription.
REQ4: It must be possible to apply all together, or in any
combination, the "min-interval", "max-interval" and "average-
interval" mechanisms in a specific subscription.
REQ5: It must be possible to use of the different rate control
mechanisms in subscriptions to any events.
REQ6: It must be possible to use the different rate control
mechanisms together with any other event filtering
mechanisms.
REQ7: The notifier must be allowed to use a 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 policy that in
effect increases the subscriber-configured minimum time
period between two notifications.
REQ8: The different rate control mechanisms must discuss corner
cases for setting the time periods between two notifications.
At a minimum, the mechanisms must include discussion of the
situation resulting from a minimum, maximum or average time
period which exceeds the subscription duration, and should
provide mechanisms for avoiding this situation.
REQ9: The different rate control mechanisms must be possible to be
installed, modified, or removed in the course of an active
subscription.
REQ10: The different rate control mechanisms must allow for the
application of authentication and integrity protection
mechanisms to subscriptions invoking that mechanism.
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Note that Section 10 contains further discussion on the security
implications of the different rate control mechanisms.
3.5. The maximum rate mechanism for Resource List Server
When applied to a list subscription, the maximum rate mechanism has
some additional considerations. Specifically, the maximum rate
applies to the aggregate notification stream resulting from the list
subscription, rather than explicitly controlling the notification of
each of the implied constituent events. Moreover, the list event
notifier can use the maximum rate mechanism on its own to control the
rate of the individual subscriptions to avoid overflowing its buffer.
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.
+--------+
| Event |
+--------+ |Resource| +--------+
| Event | +--------+ | Event |
|Resource| | |Resource|
+---.=---+ | +---=----+
`-.. | _.--'
``-._ | _.--'
+'--'--'-+
|Resource|
| List |
| Server |
+---.----+
|
|
)--+---(
| | .------------.
|Buffer|<======'min-interval|
| | `------------'
)--.---(
|
|
.---+---.
| Event |
|Watcher|
`-------'
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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 back end 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 maximum rate 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.
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 "min-interval"
Event header parameter as part of the SUBSCRIBE request. The "min-
interval" value indicates the minimum time allowed between
transmission of two consecutive notifications in a subscription.
Note that the witnessed time between two consecutive received
notifications may not conform to the "min-interval" value for a
number of reasons. For example, network jitter and
retransmissions may result in the subscriber receiving the
notifications with smaller intervals than the "min-interval" value
recommends.
A subscriber that wants to have a maximum notification time period in
a specific event subscription does so by including a "max-interval"
Event header parameter as part of the SUBSCRIBE request. The "max-
interval" value indicates the maximum time allowed between
transmission of two consecutive notifications in a subscription.
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A subscriber that wants to have an average cadence for the
notifications in a specific event subscription does so by including a
"average-interval" Event header parameter as part of the SUBSCRIBE
request.
A subscriber that wants to update a previously agreed event rate
control parameter does so by including the updated "min-interval",
"max-interval" or "average-interval" Event header parameter as part
of a subsequent SUBSCRIBE request or a 200-class response to the
NOTIFY request.
A notifier that supports the different rate control mechanisms will
comply with the value given in "min-interval", "max-interval" and
"average-interval" parameters and adjust its rate of notification
accordingly. However, if the notifier needs to lower the
subscription expiration value or a local policy or other
implementation-determined constraint at the notifier can not satisfy
the rate control request, then the notifier can adjust opportunely
the subscriber requested rate control.
Rate controlled notifications will have exactly the same properties
as the ones without rate control, with the exception that they will
be generated within the timing constraints requested.
4. Operation of the maximum rate mechanism
4.1. 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 rate to notifications in
a subscription MUST construct a SUBSCRIBE request that includes a
minimum time interval between two consecutive notifications included
in the "min-interval" Event header field parameter. The value of
this parameter is an integral number of seconds in decimal.
Subscribers implementing the maximum rate mechanism MUST include an
Event header field in any 200-class responses to NOTIFY requests.
A subscriber that wishes to update the previously agreed maximum rate
of notifications MUST include the updated "min-interval" Event header
field parameter in a subsequent SUBSCRIBE request or a 200-class
response to the NOTIFY request.
A subscriber that wishes to remove the maximum rate control from
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notifications MUST indicate so by not including a "min-interval"
Event header field parameter in a subsequent SUBSCRIBE request or a
200-class response to the NOTIFY request.
There are two main consequences for the subscriber when applying the
maximum rate 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
notifications, developers are instructed not to use the mechanism.
4.2. 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 maximum rate mechanism MUST extract the
value of the "min-interval" Event header parameter from a SUBSCRIBE
request or a 200-class response to the NOTIFY request and use it as
the suggested time allowed between two notifications. This value can
be adjusted by the notifier, as defined in Section 4.3.
A compliant notifier MUST reflect back the possibly adjusted minimum
time interval in a "min-interval" Subscription-State header field
parameter of the subsequent NOTIFY requests. The indicated "min-
interval" value is adopted by the notifier, and the notification rate
is adjusted accordingly.
A notifier that does not understand this extension will not reflect
the "min-interval" Subscription-State header field parameter in the
NOTIFY requests; the absence of this parameter serves as a hint to
the subscriber that no rate control is supported by the notifier.
A compliant notifier MUST NOT generate notifications more frequently
than the maximum rate 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 timer for the next
notification, even though they do not need to abide by it.
When a local policy dictates a maximum rate for notifications, a
notifier will not generate notifications more frequently than the
local policy maximum rate, even if the subscriber is not asking for
maximum rate control. The notifier MAY inform the subscriber about
such local policy maximum rate using the "min-interval" Subscription-
State header field parameter included in the subsequent NOTIFY
requests.
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Retransmissions of NOTIFY requests are not affected by the maximum
rate mechanism, i.e., the maximum rate mechanism only applies to the
generation of new transactions. In other words, the maximum rate
mechanism does not in any way break or modify the normal
retransmission mechanism.
4.3. Selecting the maximum rate
Special care needs to be taken when selecting the "min-interval"
value. Using the "min-interval" syntax it is possible to insist both
very short and very long intervals between notifications.
For example, the maximum rate 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. If the
subscriber requests a "min-interval" value greater than the
subscription expiration, the notifier MUST lower the "min-interval"
value and set it to the expiration time left. According to RFC 3265
[RFC3265] the notifier may also shorten the subscription expiry
anytime during an active subscription. For such cases, the notifier
MUST also lower the "min-interval" value and set it to the reduced
expiration time.
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 "min-interval" value 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 MAY decide to adjust the proposed maximum rate value
based on its local policy or other implementation-determined
constraints. The notifier MAY also choose a higher "min-interval"
value than the subscriber proposed one, e.g., because of static
configuration given by local policy.
The notifier MUST include the adjusted "min-interval" value in the
Subscription-State header field's "min-interval" parameter in each of
the NOTIFY requests. In addition, different event packages MAY
define additional constraints to the allowed "min-interval"
intervals. Such constraints are out of the scope of this
specification.
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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 difference of the two states,
and generating a notification that contains that difference.
When the maximum rate mechanism is applied to the subscription, it is
important that F' is replaced with F only when the difference of F
and F' was already included in a partial state notification to the
subscriber allowed by the maximum rate mechanism. 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
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 the maximum rate mechanism is applied to 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 maximum rate 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
notifications would have normally been sent in any given interval set
by the "min-interval" parameter, only a single notification is sent
during the same interval when using the maximum rate mechanism,
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 rate
controlled notification merging all of these n partial states
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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 header fields of the NOTIFY request.
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
maximum rate 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 the minimum rate mechanism
5.1. 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 minimum rate to notifications in
a subscription MUST construct a SUBSCRIBE request that includes a
maximum time interval between two consecutive notifications included
in the "max-interval" Event header field parameter. The value of
this parameter is an integral number of seconds in decimal.
Subscribers implementing the minimum rate mechanism MUST include an
Event header field in any 200-class responses to NOTIFY requests.
A subscriber that wishes to update the previously agreed minimum rate
of notifications MUST include the updated "max-interval" Event header
field parameter in a subsequent SUBSCRIBE request or a 200-class
response to the NOTIFY request.
A subscriber that wishes to remove the minimum rate control from
notifications MUST indicate so by not including a "max-interval"
Event header field parameter in a subsequent SUBSCRIBE request or a
200-class response to the NOTIFY request.
The main consequence for the subscriber when applying the minimum
rate 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-sipcore-subnot-etags] ongoing to only send a
reference in a notification if nothing has changed.
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5.2. 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 minimum rate mechanism MUST extract the
value of the "max-interval" Event header parameter from a SUBSCRIBE
request or a 200-class response to the NOTIFY request and use it as
the suggested maximum time allowed between two notifications.
The notifier MAY decide to adjust the proposed minimum rate value
based on its local policy or other implementation-determined
constraints. A compliant notifier MUST reflect back the possibly
adjusted maximum time interval in a "max-interval" Subscription-State
header field parameter of the subsequent NOTIFY requests. The
indicated "max-interval" value is adopted by the notifier, and the
notification rate is adjusted accordingly.
A notifier that does not understand this extension, will not reflect
the "max-interval" Subscription-State header field parameter in the
NOTIFY requests; the absence of this parameter serves as a hint to
the subscriber that no rate control is supported by the notifier.
A compliant notifier MUST generate notifications whenever the time
since the most recent notification exceeds the value in the "max-
interval" parameter. Depending on the event package and subscriber
preferences indicated in the SUBSCRIBE request, the NOTIFY request
MUST contain either the current full state or the partial state
showing the difference between the current state and the last
successfully communicated state.
Retransmissions of NOTIFY requests are not affected by the minimum
rate mechanism, i.e., the minimum rate mechanism only applies to the
generation of new transactions. In other words, the minimum rate
mechanism does not in any way break or modify the normal
retransmission mechanism.
6. Operation of the average rate mechanism
6.1. 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 rate to notifications in
a subscription MUST construct a SUBSCRIBE request that includes a
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proposed average time interval between two consecutive notifications
included in a "average-interval" Event header field parameter. The
value of this parameter is an integral number of seconds in decimal.
Subscribers implementing the minimum rate mechanism MUST include an
Event header field in any 200-class responses to NOTIFY requests.
A subscriber that wishes to update the previously agreed average rate
of notifications MUST include the updated "average-interval" Event
header field parameter in a subsequent SUBSCRIBE request or a 200-
class response to the NOTIFY request.
A subscriber that wishes to remove the average rate control from
notifications MUST indicate so by not including a "average-interval"
Event header field parameter in a subsequent SUBSCRIBE request or a
200-class response to the NOTIFY request.
The main consequence for the subscriber when applying the average
rate 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-sipcore-subnot-etags] ongoing to only send a
reference in a notification if nothing has changed.
6.2. 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 average rate mechanism MUST extract the
value of the "average-interval" Event header parameter from a
SUBSCRIBE request or a 200-class response to the NOTIFY request and
use it to calculate the maximum time allowed between two transactions
as defined in Section 6.3.
The notifier MAY decide to adjust the proposed average time interval
based on its local policy or other implementation-determined
constraints. A compliant notifier MUST reflect back the possibly
adjusted average time interval in an "average-interval" Subscription-
State header field parameter of the subsequent NOTIFY requests. The
indicated "average-interval" value is adopted by the notifier, and
the notification rate is adjusted accordingly.
A notifier that does not understand this extension will not reflect
the "average-interval" Subscription-State header parameter in the
NOTIFY requests; the absence of this parameter serves as a hint to
the subscriber that no rate control is supported by the notifier.
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A compliant notifier MUST generate notifications whenever the time
since the most recent notification exceeds the value calculated using
the formula defined in Section 6.3.
The average rate 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-interval"
seconds for the "period".
2) The "timeout" value is calculated according to the equation given
in Section 6.3.
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 does not in any way break or modify the
normal retransmission mechanism.
6.3. Calculating the timeout
The formula used to vary the absolute pacing in a way that will meet
the average rate requested over the period is given in equation (1):
timeout = (average-interval ^ 2) * count / period (1)
The output of the formula, "timeout", is the time to the next
notification, expressed in seconds. The formula has three inputs:
average-interval: The value of the "average-interval" parameter
conveyed in the Event header field, in seconds.
period: The rolling average period, in seconds. A suggested
reasonable period is 60 seconds.
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[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 case both the maximum rate and the average rate mechanisms are
used in the same subscription the formula used to dynamically
calculate the timeout is given in equation (2):
timeout = MAX[min-interval, (average-interval ^ 2) * count / period] (2)
min-interval: The value of the "min-interval" parameter conveyed in
the Event header field, in seconds.
The formula in (2) makes sure that for all the possible values of the
"min-interval" and "average-interval" parameters, with "average-
interval" > "min-interval", the timeout never results in a lower
value than the value of the "min-interval" parameter.
7. Usage of "min-interval", "max-interval" and "average-interval" in a
combination
Applications can subscribe to an event package using all the rate
control mechanisms individually, or in combination; in fact there is
no technical incompatibility among them. However there are some
combinations of the different rate control mechanisms that make
little sense to be used together. This section lists all the
combinations that are possible to insert in a subscription; the
utility to use each combination in a subscription is also analyzed.
min-interval and max-interval: this combination allows to reduce the
notification frequency rate, but at the same time assures the
reception of a notification every time the most recent
notification exceeds a specified interval.
A subscriber SHOULD choose a "max-interval" value higher than the
"min-interval" value, otherwise the notifier MUST adjust the
subscriber provided "max-interval" value to a value equivalent or
higher than the "min-interval" value.
min-interval and average-interval: it works in a similar way as the
combination above, but with the difference that the interval at
which notifications are assured changes dynamically.
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A subscriber SHOULD choose a "average-interval" value higher than
the "min-interval" value, otherwise the notifier MUST adjust the
subscriber provided "average-interval" value to a value equivalent
or higher than the "min-interval" value.
max-interval and average-interval: as both the parameters are
designed as minimum rate mechanisms, this combination makes sense
only in some corner cases.
A subscriber SHOULD choose a "max-interval" value higher than the
"average-interval" value, otherwise the notifier MUST NOT consider
the "max-interval" value.
min-interval, max-interval and average-interval: this combination
makes little sense to be used.
8. Syntax
This section describes the syntax extensions required for the
different rate control mechanisms.
8.1. "min-interval", "max-interval" and "average-interval" Header Field
Parameters
The "min-interval", "max-interval" and "average-interval" 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 4, Section 5 and
Section 6.
8.2. 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 SIP Events [RFC3265], adding additional alternatives to
the alternative sets of "event-param" and "subexp-params" defined
therein.
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event-param =/ min-interval-param
subexp-params =/ min-interval-param
min-interval-param = "min-interval" EQUAL delta-seconds
event-param =/ max-interval-param
subexp-params =/ max-interval-param
max-interval-param = "max-interval" EQUAL delta-seconds
event-param =/ average-interval-param
subexp-params =/ average-interval-param
average-interval-param = "average-interval" EQUAL delta-seconds
8.3. Event header field usage in responses to the NOTIFY request
Implementations using the extensions described in this document MUST
include an Event header field in any 200-class responses to NOTIFY
requests. This table expands the table described in Section 7.2 of
SIP Events [RFC3265] allowing the Event header field to appear in a
200-class response to a NOTIFY request.
Header field where proxy ACK BYE CAN INV OPT REG PRA SUB NOT
-----------------------------------------------------------------
Event 2xx - - - - - - - - m
9. 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 min-interval No [RFCxxxx]
Subscription-State min-interval No [RFCxxxx]
Event max-interval No [RFCxxxx]
Subscription-State max-interval No [RFCxxxx]
Event average-interval No [RFCxxxx]
Subscription-State average-interval No [RFCxxxx]
(Note to the RFC Editor: please replace "xxxx" with the RFC number of
this specification, when assigned.)
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10. Security Considerations
Naturally, the security considerations listed in SIP events
[RFC3265], which the rate control mechanisms described in this
document extends, apply in entirety. In particular, authentication
and message integrity SHOULD be applied to subscriptions with this
extension.
11. Acknowledgments
Thanks to Pekka Pessi, Dean Willis, Eric Burger, Alex Audu, Alexander
Milinski, Jonathan Rosenberg, Cullen Jennings, Adam Roach, Hisham
Khartabil, Dale Worley, Martin Thomson and Byron Campen for support
and/or review of this work.
Thanks to Brian Rosen for the idea of the minimum and average rate
mechanisms, and Adam Roach for the work on the averaging algorithm
and other feedback.
12. References
12.1. 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.
12.2. Informative References
[I-D.ietf-geopriv-loc-filters]
Mahy, R., Rosen, B., and H. Tschofenig, "Filtering
Location Notifications in the Session Initiation Protocol
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Internet-Draft Event Rate Control October 2009
(SIP)", draft-ietf-geopriv-loc-filters-07 (work in
progress), October 2009.
[I-D.ietf-sipcore-subnot-etags]
Niemi, A., "An Extension to Session Initiation Protocol
(SIP) Events for Conditional Event Notification",
draft-ietf-sipcore-subnot-etags-02 (work in progress),
April 2009.
[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.
[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
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Krisztian Kiss
Nokia
313 Fairchild Dr
Mountain View, CA 94043
US
Phone: +1 650 391 5969
Email: krisztian.kiss@nokia.com
Salvatore Loreto
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: salvatore.loreto@ericsson.com
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