Internet Engineering Task Force (IETF) A. Niemi
Request for Comments: 6446 K. Kiss
Updates: 3265 Nokia
Category: Standards Track S. Loreto
ISSN: 2070-1721 Ericsson
January 2012
Session Initiation Protocol (SIP) Event Notification Extension
for Notification Rate Control
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. This document
updates RFC 3265.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6446.
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Copyright Notice
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document authors. All rights reserved.
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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 an Adaptive Minimum Rate of
Notifications . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7
4. Basic Operations . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Subscriber Behavior . . . . . . . . . . . . . . . . . . . 8
4.2. Notifier Behavior . . . . . . . . . . . . . . . . . . . . 9
5. Operation of the Maximum Rate Mechanism . . . . . . . . . . . 9
5.1. Subscriber Behavior . . . . . . . . . . . . . . . . . . . 9
5.2. Notifier Behavior . . . . . . . . . . . . . . . . . . . . 10
5.3. Selecting the Maximum Rate . . . . . . . . . . . . . . . . 11
5.4. The Maximum Rate Mechanism for the Resource List Server . 11
5.5. Buffer Policy Description . . . . . . . . . . . . . . . . 13
5.5.1. Partial-State Notifications . . . . . . . . . . . . . 13
5.5.2. Full-State Notifications . . . . . . . . . . . . . . . 13
5.6. Estimated Bandwidth Savings . . . . . . . . . . . . . . . 14
6. Operation of the Minimum Rate Mechanism . . . . . . . . . . . 14
6.1. Subscriber Behavior . . . . . . . . . . . . . . . . . . . 14
6.2. Notifier Behavior . . . . . . . . . . . . . . . . . . . . 15
6.3. Selecting the Minimum Rate . . . . . . . . . . . . . . . . 16
7. Operation of the Adaptive Minimum Rate Mechanism . . . . . . . 16
7.1. Subscriber Behavior . . . . . . . . . . . . . . . . . . . 16
7.2. Notifier Behavior . . . . . . . . . . . . . . . . . . . . 17
7.3. Selecting the Adaptive Minimum Rate . . . . . . . . . . . 18
7.4. Calculating the Timeout . . . . . . . . . . . . . . . . . 18
8. Usage of the Maximum Rate, Minimum Rate, and Adaptive
Minimum Rate Mechanisms in a Combination . . . . . . . . . . . 19
9. Protocol Element Definitions . . . . . . . . . . . . . . . . . 20
9.1. "max-rate", "min-rate", and "adaptive-min-rate" Header
Field Parameters . . . . . . . . . . . . . . . . . . . . . 21
9.2. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.3. Event Header Field Usage in Responses to the NOTIFY
Request . . . . . . . . . . . . . . . . . . . . . . . . . 21
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
11. Security Considerations . . . . . . . . . . . . . . . . . . . 22
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
13.1. Normative References . . . . . . . . . . . . . . . . . . . 23
13.2. Informative References . . . . . . . . . . . . . . . . . . 24
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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 load.
All of the existing event package specifications include a
recommendation for the maximum notification rate, 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
allowed to define additional throttle mechanisms that 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.
Some event packages are also interested in specifying an absolute or
an adaptive minimum rate at which notifications need to be generated
by a notifier. This helps the subscriber to effectively use
different trigger criteria within a subscription to eliminate
unnecessary notifications but at the same time make sure that the
current event state is periodically received.
This document defines an extension to the SIP events framework by
defining the following three Event header field parameters that allow
a subscriber to set a maximum, a minimum, and an adaptive minimum
rate of notifications generated by the notifier:
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max-rate: specifies a maximum number of notifications per second.
min-rate: specifies a minimum number of notifications per second.
adaptive-min-rate: specifies an adaptive minimum number of
notifications per second.
These mechanisms are applicable to any event subscription, both
single event subscription and event list subscription. A notifier
compliant to this specification will adjust 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 [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
an RLS with the list of buddies, and therefore only needs a single
subscription to the RLS to receive notifications 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 of notifications. The RLS will buffer
notifications that are generated faster than they are allowed to be
sent due to the maximum rate and batch all of the buffered state
changes together in a single notification. The maximum rate applies
to the overall resource list, which means that there is a hard cap
imposed by the maximum rate to the number of notifications per second
that the presence client can expect to receive.
The presence client can also modify the maximum rate of notifications
during the lifetime of the subscription. For example, if the mobile
device detects inactivity from the user for a period of time, the
presence client can simply pause notifications by choosing a "max-
rate" parameter that allows only a single notification for the
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remainder of the subscription lifetime. When the user becomes active
again, the presence client can resume the stream of notifications by
re-subscribing with a "max-rate" parameter set to the earlier-used
value. Application of the mechanism defined by RFC 5839 [RFC5839]
can also eliminate the transmission of a (full-state) notification
carrying the latest resource state to the presence client after a
subscription refresh.
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 to a Location Server with a set
of location filters [RFC6447] that specify trigger criteria, e.g., to
send an update only when the target has moved at least n meters.
However, the application is also interested in receiving the current
state periodically, even if the state of the target has not changed
enough to satisfy any of the trigger criteria, e.g., has not moved at
least n meters within the period.
The location application sets a minimum rate of notifications and
includes it in the subscription sent to the Location Server. The
"min-rate" parameter indicates the minimum number of notifications
per second the notifier needs to generate.
The location application can modify the minimum rate of notifications
during the lifetime of the subscription. For example, when the
subscription to the movement of a target is made, the notifier may
not 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. Once state is
acquired and the second notification is sent, the subscriber updates
or changes the "min-rate" parameter to a more sensible value. This
update can be performed in the response to the notification that
contains the complete state information.
3.3. Use Case for Specifying an Adaptive Minimum Rate of Notifications
The minimum rate mechanism introduces a static and instantaneous rate
control without the functionality to increase or decrease the
notification rate adaptively. However, there are some applications
that would work better with an adaptive minimum rate control.
A location application is monitoring the movement of a target. In
order to decrease the processing in the application, the location
application wants to make a subscription that dynamically decreases
the minimum rate of notifications if the target has sent out several
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notifications recently. However, if there have been only few recent
notifications by the target, the location application wants the
minimum rate of notifications to increase.
The location application sets an adaptive minimum rate of
notifications and includes it in the subscription sent to the
Location Server. The "adaptive-min-rate" parameter value is used by
the notifier to dynamically calculate the actual maximum time between
two notifications. In order to dynamically calculate the maximum
time, the notifier takes into consideration the rate at which
notifications have been sent recently. In the adaptive minimum rate
mechanism, the notifier can increase or decrease the notification
rate compared to the minimum rate mechanism based on the recent
number of notifications sent out in the last period.
The location application can also modify the "adaptive-min-rate"
parameter during the lifetime of the subscription.
3.4. Requirements
REQ1: The subscriber must be able to set a maximum rate of
notifications in a specific subscription.
REQ2: The subscriber must be able to set a minimum rate of
notifications in a specific subscription.
REQ3: The subscriber must be able to set an adaptive minimum rate
of notifications in a specific subscription, which adjusts
the minimum rate of notifications based on a moving average.
REQ4: It must be possible to apply the maximum rate, the minimum
rate, and the adaptive minimum rate mechanisms all together,
or in any combination, in a specific subscription.
REQ5: It must be possible to use any of the different rate control
mechanisms in subscriptions to any events.
REQ6: It must be possible to use any of 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
maximum rate, minimum rate, and adaptive minimum rate
parameters are adjusted from the value given by the
subscriber.
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For example, due to congestion, local policy at the
notifier could temporarily dictate a policy that in effect
further decreases the maximum rate of notifications. In
another example, the notifier could increase the
subscriber-proposed maximum rate so that at least one
notification is generated during the remainder of the
subscription lifetime.
REQ8: The different rate control mechanisms must address corner
cases for setting the notification rates appropriately. At a
minimum, the mechanisms must address the situation in which
the time between two notifications exceeds the subscription
duration and should provide procedures for avoiding this
situation.
REQ9: It must be possible to invoke, modify, or remove the
different rate control mechanisms 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.
4. Basic Operations
4.1. Subscriber Behavior
In general, a subscriber generates SUBSCRIBE requests and processes
NOTIFY requests as described in RFC 3265 [RFC3265].
A subscriber that wants to have a maximum, minimum, or adaptive
minimum rate of event notifications in a specific event subscription
does so by including a "max-rate", "min-rate", or "adaptive-min-rate"
Event header field parameter(s) 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 "max-rate", "min-
rate", or "adaptive-min-rate" Event header field parameter(s) as part
of a subsequent SUBSCRIBE request or a 2xx response to the NOTIFY
request. If the subscriber does not include at least one of the
"max-rate", "min-rate", or "adaptive-min-rate" header field
parameters in the most recent SUBSCRIBE request in a given dialog,
the subscriber MUST NOT include an Event header field with any of
those parameters in a 2xx response to a NOTIFY request in that
dialog.
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4.2. Notifier Behavior
In general, a notifier processes SUBSCRIBE requests and generates
NOTIFY requests as described in RFC 3265 [RFC3265].
A notifier that supports the different rate control mechanisms MUST
adjust its rate of notification according to the rate control values
agreed with the subscriber. If the notifier needs to lower the
subscription expiration value, or if a local policy or other
implementation-determined constraint at the notifier cannot satisfy
the rate control request, then the notifier can adjust (i.e.,
increase or decrease) appropriately the subscriber-requested rate
control values. The notifier MUST reflect back the possibly adjusted
rate control values in a "max-rate", "min-rate", or "adaptive-min-
rate" Subscription-State header field parameter of the subsequent
NOTIFY requests.
5. Operation of the Maximum Rate Mechanism
5.1. Subscriber Behavior
A subscriber that wishes to apply a maximum rate to notifications in
a subscription MUST construct a SUBSCRIBE request that includes the
"max-rate" Event header field parameter. This parameter specifies
the requested maximum number of notifications per second. The value
of this parameter is a positive real number given by a finite decimal
representation.
Note that the grammar in section 9.2 constrains this value to be
between 0.0000000001 and 99.9999999999. Zero is not an allowed
value.
Note that the witnessed notification rate may not conform to the
"max-rate" value for a number of reasons. For example, network
jitter and retransmissions may result in the subscriber receiving
the notifications more frequently than the "max-rate" value
recommends.
A subscriber that wishes to update the previously agreed maximum rate
of notifications MUST include the updated "max-rate" Event header
field parameter in a subsequent SUBSCRIBE request or a 2xx response
to the NOTIFY request.
A subscriber that wishes to remove the maximum rate control from
notifications MUST indicate so by not including a "max-rate" Event
header field parameter in a subsequent SUBSCRIBE request or a 2xx
response to the NOTIFY request.
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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 utilizes the event
notifications, developers are instructed not to use the mechanism.
5.2. Notifier Behavior
A notifier that supports the maximum rate mechanism MUST extract the
value of the "max-rate" Event header parameter from a SUBSCRIBE
request or a 2xx response to the NOTIFY request and use it as the
suggested maximum number of notifications per second. This value can
be adjusted by the notifier, as defined in Section 5.3.
A compliant notifier MUST reflect back the possibly adjusted maximum
rate of notifications in a "max-rate" Subscription-State header field
parameter of the subsequent NOTIFY requests. The indicated "max-
rate" 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-rate" Subscription-State header field parameter in the
NOTIFY requests; the absence of this parameter indicates to the
subscriber that no rate control is supported by the notifier.
A compliant notifier MUST NOT generate a notification if the interval
since the most recent notification is less than the reciprocal value
of the "max-rate" parameter, except when generating the notification
either upon receipt of a SUBSCRIBE request, when the subscription
state is changing from "pending" to "active" state, or upon
termination of the subscription (the last notification).
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 a local policy maximum rate using the "max-rate" Subscription-
State header field parameter included in subsequent NOTIFY requests.
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 specified in RFC 3261 [RFC3261].
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5.3. Selecting the Maximum Rate
Special care needs to be taken when selecting the maximum rate. 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 being generated. If the subscriber
requests a maximum rate that would result in no notification before
the subscription expiration, the notifier MUST increase the maximum
rate and set it to the reciprocal value of the remaining subscription
expiration time. According to RFC 3265 [RFC3265], the notifier may
also shorten the subscription expiry anytime during an active
subscription. If the subscription expiry is shortened during an
active subscription, the notifier MUST also increase the "max-rate"
value and set it to the reciprocal value of the reduced subscription
expiration time.
In some cases, it makes sense to temporarily pause the notification
stream on an existing subscription dialog without terminating the
subscription, e.g., due to inactivity on the application user
interface. Whenever a subscriber discovers the need to perform the
notification pause operation, it SHOULD set the maximum rate to the
reciprocal value of the remaining subscription expiration value.
This results in receiving no further notifications until the
subscription expires or the subscriber sends a SUBSCRIBE request
resuming notifications.
The notifier MAY decide to increase or decrease the proposed "max-
rate" value by the subscriber based on its local policy, static
configuration, or other implementation-determined constraints. In
addition, different event packages MAY define other constraints for
the allowed maximum rate ranges. Such constraints are out of the
scope of this specification.
5.4. The Maximum Rate Mechanism for the Resource List Server
When applied to a list subscription [RFC4662], 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 RLS can use the maximum rate mechanism on its own to control the
rate of the back-end subscriptions to avoid overflowing its buffer.
The notifier is responsible for sending event notifications upon
state changes of the subscribed resource. We can model the notifier
as consisting of four components: the event state resource(s), the
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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|<======'max-rate|
| | `--------'
)--.---(
|
|
.---+---.
| Event |
|Watcher|
`-------'
Figure 1: Model for the RLS Supporting Event Rate Control
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 specification. 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.
5.5. Buffer Policy Description
5.5.1. Partial-State Notifications
With partial notifications, the notifier needs to maintain a separate
buffer for each subscriber since each subscriber may have a different
value for the maximum rate of 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' be replaced with F only when the difference of F
and F' is 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.
5.5.2. 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 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.
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5.6. 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 "max-rate" 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
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.
6. Operation of the Minimum Rate Mechanism
6.1. Subscriber Behavior
A subscriber that wishes to apply a minimum rate to notifications in
a subscription MUST construct a SUBSCRIBE request that includes the
"min-rate" Event header field parameter. This parameter specifies
the requested minimum number of notifications per second. The value
of this parameter is a positive real number given by a finite decimal
representation.
Note that the grammar in section 9.2 constrains this value to be
between 0.0000000001 and 99.9999999999. Zero is not an allowed
value.
A subscriber that wishes to update the previously agreed minimum rate
of notifications MUST include the updated "min-rate" Event header
field parameter in a subsequent SUBSCRIBE request or a 2xx response
to the NOTIFY request.
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A subscriber that wishes to remove the minimum rate control from
notifications MUST indicate so by not including a "min-rate" Event
header field parameter in a subsequent SUBSCRIBE request or a 2xx
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. However, RFC 5839
[RFC5839] defines a mechanism that allows suppression of a NOTIFY
request or a NOTIFY request body if the state has not changed.
6.2. Notifier Behavior
A notifier that supports the minimum rate mechanism MUST extract the
value of the "min-rate" Event header field parameter from a SUBSCRIBE
request or a 2xx response to the NOTIFY request and use it as the
suggested minimum number of notifications per second. This value can
be adjusted by the notifier, as defined in Section 6.3.
A compliant notifier MUST reflect back the possibly adjusted minimum
rate of notifications in a "min-rate" Subscription-State header field
parameter of the subsequent NOTIFY requests. The indicated "min-
rate" 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-rate" Subscription-State header field parameter in the
NOTIFY requests; the absence of this parameter indicates to the
subscriber that no rate control is supported by the notifier.
A compliant notifier MUST generate notifications when state changes
occur or when the time since the most recent notification exceeds the
reciprocal value of the "min-rate" parameter. Depending on the event
package and subscriber preferences indicated in the SUBSCRIBE
request, the NOTIFY request sent as a result of a minimum rate
mechanism MUST contain either the current full state or the partial
state showing the difference between the current state and the last
successfully communicated state. If the subscriber and the notifier
support the procedures in RFC 5839 [RFC5839], the complete NOTIFY
request or the NOTIFY request body can be suppressed if the state has
not changed from the previous notification.
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.
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6.3. Selecting the Minimum Rate
The minimum rate mechanism can be used to generate a lot of
notifications, creating additional processing load for the notifier.
Some of the notifications may also be unnecessary possibly repeating
already known state information to the subscriber. It is difficult
to provide generic guidelines for the acceptable minimum rate value
ranges; however, the subscriber SHOULD request the lowest possible
minimum rate. Different event packages MAY define other constraints
for the allowed minimum rate values. Such constraints are out of the
scope of this specification.
The notifier MAY decide to increase or decrease the proposed "min-
rate" value by the subscriber based on its local policy, static
configuration, or other implementation-determined constraints.
7. Operation of the Adaptive Minimum Rate Mechanism
7.1. Subscriber Behavior
A subscriber that wishes to apply an adaptive minimum rate to
notifications in a subscription MUST construct a SUBSCRIBE request
that includes the "adaptive-min-rate" Event header field parameter.
This parameter specifies an adaptive minimum number of notifications
per second. The value of this parameter is a positive real number
given by a finite decimal representation.
Note that the grammar in section 9.2 constrains this value to be
between 0.0000000001 and 99.9999999999. Zero is not an allowed
value.
A subscriber that wishes to update the previously agreed adaptive
minimum rate of notifications MUST include the updated "adaptive-min-
rate" Event header field parameter in a subsequent SUBSCRIBE request
or a 2xx response to the NOTIFY request.
A subscriber that wishes to remove the adaptive minimum rate control
from notifications MUST indicate so by not including an "adaptive-
min-rate" Event header field parameter in a subsequent SUBSCRIBE
request or a 2xx response to the NOTIFY request.
The main consequence for the subscriber when applying the adaptive
minimum rate mechanism is that it can receive a notification, even if
nothing has changed in the current state of the notifier. However,
RFC 5839 [RFC5839] defines a mechanism that allows suppression of a
NOTIFY request or a NOTIFY request body if the state has not changed.
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7.2. Notifier Behavior
A notifier that supports the adaptive minimum rate mechanism MUST
extract the value of the "adaptive-min-rate" Event header parameter
from a SUBSCRIBE request or a 2xx response to the NOTIFY request and
use it to calculate the actual maximum time between two
notifications, as defined in Section 7.4.
The "adaptive-min-rate" value can be adjusted by the notifier, as
defined in Section 7.3.
A compliant notifier MUST reflect back the possibly adjusted adaptive
minimum rate of notifications in an "adaptive-min-rate" Subscription-
State header field parameter of the subsequent NOTIFY requests. The
indicated "adaptive-min-rate" 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 "adaptive-min-rate" Subscription-State header parameter in the
NOTIFY requests; the absence of this parameter indicates to the
subscriber that no rate control is supported by the notifier.
A compliant notifier MUST generate notifications when state changes
occur or when the time since the most recent notification exceeds the
value calculated using the formula defined in Section 7.4. Depending
on the event package and subscriber preferences indicated in the
SUBSCRIBE request, the NOTIFY request sent as a result of a minimum
rate mechanism MUST contain either the current full state or the
partial state showing the difference between the current state and
the last successfully communicated state. If the subscriber and the
notifier support the procedures in RFC 5839 [RFC5839], the complete
NOTIFY request or the NOTIFY request body can be suppressed if the
state has not changed from the previous notification.
The adaptive minimum rate mechanism is implemented as follows:
1) When a subscription is first created, the notifier creates a
record ("count" parameter) that keeps track of the number of
notifications that have been sent in the "period". The "count"
parameter is initialized to contain a history of having sent a
"period * adaptive-min-rate" number of notifications for the
"period".
2) The "timeout" value is calculated according to the equation given
in Section 7.4.
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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" event or a state change), the notifier updates the
notification history record stored in the "count" parameter,
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 specified in RFC 3261 [RFC3261].
7.3. Selecting the Adaptive Minimum Rate
The adaptive minimum rate mechanism can be used to generate a lot of
notifications, creating additional processing load for the notifier.
Some of the notifications may also be unnecessary, possibly repeating
already known state information to the subscriber. It is difficult
to provide generic guidelines for the acceptable adaptive minimum
rate value ranges; however, the subscriber SHOULD request the lowest
possible adaptive minimum rate value. Different event packages MAY
define other constraints for the allowed adaptive minimum rate
values. Such constraints are out of the scope of this specification.
The notifier MAY decide to increase or decrease the proposed
"adaptive-min-rate" value based on its local policy, static
configuration, or other implementation-determined constraints.
7.4. Calculating the Timeout
The formula used to vary the absolute pacing in a way that will meet
the adaptive minimum rate requested over the period is given in
equation (1):
timeout = count / ((adaptive-min-rate ^ 2) * period) (1)
The output of the formula, "timeout", is the time to the next
notification, expressed in seconds. The formula has three inputs:
adaptive-min-rate: The value of the "adaptive-min-rate" parameter
conveyed in the Subscription-State header field.
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period: The rolling average period, in seconds. The granularity of
the values for the "period" parameter is set by local policy at
the notifier; however, the notifier MUST choose a value greater
than the reciprocal value of the "adaptive-min-rate" parameter.
It is also RECOMMENDED that the notifier choose a "period"
parameter several times larger than reciprocal value of the
"adaptive-min-rate" parameter in order to maximize the
effectiveness of equation (1). It is an implementation decision
whether the notifier uses the same value of the "period" parameter
for all subscriptions or individual values for each subscription.
count: The number of notifications that have been sent during the
last "period" of seconds, not including any retransmissions of
requests.
In case both the maximum rate and the adaptive minimum rate
mechanisms are used in the same subscription, the formula used to
dynamically calculate the timeout is given in equation (2):
timeout = MAX[(1/max-rate), count/((adaptive-min-rate ^ 2)*period)] (2)
max-rate: The value of the "max-rate" parameter conveyed in the
Subscription-State header field.
The formula in (2) makes sure that for all the possible values of the
"max-rate" and "adaptive-min-rate" parameters, with "adaptive-min-
rate" < "max-rate", the timeout never results in a lower value than
the reciprocal value of the "max-rate" parameter.
In some situations, it may be beneficial for the notifier to achieve
an adaptive minimum rate in a different way than the algorithm
detailed in this document allows. However, the notifier MUST comply
with any "max-rate" or "min-rate" parameters that have been
negotiated.
8. Usage of the Maximum Rate, Minimum Rate, and Adaptive Minimum Rate
Mechanisms 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
ability to use each combination in a subscription is also analyzed.
Niemi, et al. Standards Track [Page 19]
RFC 6446 Event Rate Control January 2012
maximum rate and minimum rate: This combination allows a reduced
notification rate, but at the same time assures the reception of
periodic notifications.
A subscriber SHOULD choose a "min-rate" value lower than the "max-
rate" value, otherwise, the notifier MUST adjust the subscriber
provided "min-rate" value to a value equal to or lower than the
"max-rate" value.
maximum rate and adaptive minimum rate: 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 "adaptive-min-rate" value lower than
the "max-rate" value, otherwise, the notifier MUST adjust the
subscriber provided "adaptive-min-rate" value to a value equal to
or lower than the "max-rate" value.
minimum rate and adaptive minimum rate: When using the adaptive
minimum rate mechanism, frequent state changes in a short period
can result in no notifications for a longer period following the
short period. The addition of the minimum rate mechanism ensures
that the subscriber always receives notifications after a
specified interval.
A subscriber SHOULD choose a "min-rate" value lower than the
"adaptive-min-rate" value, otherwise, the notifier MUST NOT
consider the "min-rate" value.
maximum rate, minimum rate, and adaptive minimum rate: This
combination makes little sense to be used, although it is not
forbidden.
A subscriber SHOULD choose a "min-rate" and "adaptive-min-rate"
values lower than the "max-rate" value, otherwise, the notifier
MUST adjust the subscriber provided "min-rate" and "adaptive-min-
rate" values to a value equal to or lower than the "max-rate"
value.
A subscriber SHOULD choose a "min-rate" value lower than the
"adaptive-min-rate" value, otherwise, the notifier MUST NOT
consider the "min-rate" value.
9. Protocol Element Definitions
This section describes the protocol extensions required for the
different rate control mechanisms.
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9.1. "max-rate", "min-rate", and "adaptive-min-rate" Header Field
Parameters
The "max-rate", "min-rate", and "adaptive-min-rate" parameters are
added to the rule definitions of the Event header field and the
Subscription-State header field in RFC 3265 [RFC3265] grammar. Usage
of this parameter is described in Sections 5, 6, and 7.
9.2. Grammar
This section describes the Augmented BNF [RFC5234] definitions for
the new header field parameters. Note that we derive here from the
ruleset present in RFC 3265 [RFC3265], adding additional alternatives
to the alternative sets of "event-param" and "subexp-params" defined
therein.
event-param = max-rate-param
/ min-rate-param
/ amin-rate-param
subexp-params = max-rate-param
/ min-rate-param
/ amin-rate-param
max-rate-param = "max-rate" EQUAL
(1*2DIGIT ["." 1*10DIGIT])
min-rate-param = "min-rate" EQUAL
(1*2DIGIT ["." 1*10DIGIT])
amin-rate-param = "adaptive-min-rate" EQUAL
(1*2DIGIT ["." 1*10DIGIT])
9.3. Event Header Field Usage in Responses to the NOTIFY Request
This table expands the table described in Section 7.2 of RFC 3265
[RFC3265], allowing the Event header field to appear in a 2xx
response to a NOTIFY request. The use of the Event header field in
responses other than 2xx to NOTIFY requests is undefined and out of
scope of this specification.
Header field where proxy ACK BYE CAN INV OPT REG PRA SUB NOT
-----------------------------------------------------------------
Event 2xx - - - - - - - - o
A subscriber that wishes to update the previously agreed value for
maximum, minimum, or adaptive minimum rate of notifications MUST
include all desired values for the "max-rate", "min-rate", and
"adaptive-min-rate" parameters in an Event header field of the 2xx
response to a NOTIFY request. Any of the other header field
Niemi, et al. Standards Track [Page 21]
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parameters currently defined for the Event header field by other
specifications do not have a meaning if the Event header field is
included in the 2xx response to the NOTIFY request. These header
field parameters MUST be ignored by the notifier, if present.
The event type listed in the Event header field of the 2xx response
to the NOTIFY request MUST match the event type of the Event header
field in the corresponding NOTIFY request.
10. IANA Considerations
This specification registers three new SIP header field parameters in
the "Header Field Parameters and Parameter Values" sub-registry of
the "Session Initiation Protocol (SIP) Parameters" registry.
Predefined
Header Field Parameter Name Values Reference
-------------------- --------------- ---------- ---------
Event max-rate No [RFC6446]
Subscription-State max-rate No [RFC6446]
Event min-rate No [RFC6446]
Subscription-State min-rate No [RFC6446]
Event adaptive-min-rate No [RFC6446]
Subscription-State adaptive-min-rate No [RFC6446]
This specification also updates the reference defining the Event
header field in the "Header Fields" sub-registry of the "Session
Initiation Protocol (SIP) Parameters" registry.
Header Name compact Reference
----------- ------- ------------------
Event o [RFC3265][RFC6446]
11. Security Considerations
Naturally, the security considerations listed in RFC 3265 [RFC3265],
which the rate control mechanisms described in this document extends,
apply in their entirety. In particular, authentication and message
integrity SHOULD be applied to subscriptions with this extension.
RFC 3265 [RFC3265] recommends the integrity protection of the Event
header field of SUBSCRIBE requests. Implementations of this
extension SHOULD also provide integrity protection for the Event
header field included in the 2xx response to the NOTIFY request.
Without integrity protection, an eavesdropper could see and modify
the Event header field, or it could manipulate the transmission of a
200 (OK) response to the NOTIFY request to suppress or flood
notifications without the subscriber seeing what caused the problem.
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When the maximum rate mechanism involves partial-state notifications,
the security considerations listed in RFC 5263 [RFC5263] apply in
their entirety.
12. 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, Byron Campen, Alan Johnston,
Michael Procter, Janet Gunn, and Ari Keranen for support and/or
review of this work.
Thanks to Brian Rosen for the idea of the minimum and adaptive
minimum rate mechanisms, and to Adam Roach for the work on the
algorithm for the adaptive minimum rate mechanism and other feedback.
13. References
13.1. 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., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5263] Lonnfors, M., Costa-Requena, J., Leppanen, E., and H.
Khartabil, "Session Initiation Protocol (SIP) Extension
for Partial Notification of Presence Information",
RFC 5263, September 2008.
Niemi, et al. Standards Track [Page 23]
RFC 6446 Event Rate Control January 2012
13.2. Informative References
[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.
[RFC5839] Niemi, A. and D. Willis, Ed., "An Extension to Session
Initiation Protocol (SIP) Events for Conditional Event
Notification", RFC 5839, May 2010.
[RFC6447] Mahy, R., Rosen, B., and H. Tschofenig, "Filtering
Location Notifications in the Session Initiation Protocol
(SIP)", RFC 6447, January 2012.
Niemi, et al. Standards Track [Page 24]
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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
200 South Mathilda Ave
Sunnyvale, CA 94086
US
Phone: +1 650 391 5969
EMail: krisztian.kiss@nokia.com
Salvatore Loreto
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: salvatore.loreto@ericsson.com
Niemi, et al. Standards Track [Page 25]