One document matched: draft-niemi-sipping-event-throttle-05.txt
Differences from draft-niemi-sipping-event-throttle-04.txt
Network Working Group A. Niemi
Internet-Draft Nokia
Intended status: Standards Track March 4, 2007
Expires: September 5, 2007
Session Initiation Protocol (SIP) Event Notification Extension for
Notification Throttling
draft-niemi-sipping-event-throttle-05
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This memo specifies a throttle mechanism for limiting the rate of
Session Initiation Protocol (SIP) event notifications. This
mechanism can be applied in subscriptions to all SIP event packages,
but the mechanism is especially designed to be used in combination
with a subscription to a Resource List Server (RLS).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions and Document Conventions . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Use Case . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Requirements . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Event Throttle Model . . . . . . . . . . . . . . . . . . . 6
3.4. Basic Operation . . . . . . . . . . . . . . . . . . . . . 7
4. Operation of Event Throttles . . . . . . . . . . . . . . . . . 7
4.1. Negotiating the Use of Throttle . . . . . . . . . . . . . 8
4.2. Setting the Throttle . . . . . . . . . . . . . . . . . . . 8
4.2.1. Subscriber Behavior . . . . . . . . . . . . . . . . . 8
4.2.2. Notifier Behavior . . . . . . . . . . . . . . . . . . 8
4.3. Selecting the Throttle Interval . . . . . . . . . . . . . 9
4.4. Buffer Policy Description . . . . . . . . . . . . . . . . 9
4.4.1. Partial State Notifications . . . . . . . . . . . . . 9
4.4.2. Full State Notifications . . . . . . . . . . . . . . . 10
4.5. Estimated Bandwidth Savings . . . . . . . . . . . . . . . 10
5. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. "throttle" Header Field Parameter . . . . . . . . . . . . 11
5.2. Augmented BNF Definitions . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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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 [I-D.ietf-simple-event-list] to the SIP
events framework also deals with rate limiting of event
notifications. The extension allows a subscriber to subscribe to a
heterogenous list of resources with a single SUBSCRIBE request,
rather than having to install a subscription for each resource
separately. The event list subscription also allows rate limiting,
or throttling of notifications, by means of the Resource List Server
(RLS) buffering notifications of resource state changes, and sending
them in batches. However, the event list mechanism provides no means
for the subscriber to set the interval for the throttling; it is
strictly an implementation decision whether batching of notifications
is supported, and by what means.
This document defines an extension to the SIP events framework that
allows a subscriber to set a throttle to event notifications
generated by the notifier. The requirements and model for generic
event throttles are further discussed in Section 3. A throttle is
simply a timer value that indicates the minimum time period allowed
between two notifications. As a result of this throttle, a compliant
notifier will limit the rate at which it generates notifications.
This mechanism is applicable to any event subscription, but it is
mainly intended for use with an event list subscription.
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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
There are many applications that potentially would make use of a
throttle mechanism. This chapter only illustrates one, albeit the
main use case, in which a mobile device uses the event list
subscription in combination with the event throttling mechanism to
limit the amount of traffic it may expect to receive.
3.1. Use Case
A presence application in a mobile device contains a list of 100
buddies or presentities. In order to decrease the processing and
network load of watching 100 presentities, the presence application
has employed a Resource List Server (RLS) with the list of buddies,
and therefore only needs a single subscription to the RLS in order to
receive notification of the presence state of the resource list.
In order to control the buffer policy of the RLS, the presence
application sets a throttle interval via the event throttle
extension. Alternatively, the presence application could set a
default throttle for the resource list, via a list manipulation
interface, e.g., using the XML Configuration Access Protocol (XCAP)
[I-D.ietf-simple-xcap].
The RLS will buffer notifications that do not comply with the
throttle interval, and batch all of the buffered state changes
together in a single notification when allowed by the throttle. The
throttle applies to the overall resource list, which means that there
is a hard cap imposed by the throttle to the amount of traffic the
presence application can expect to receive.
For example, with a throttle of 20 seconds, the presence application
can expect to receive a notification every 20 seconds at a maximum.
The presence application can also modify the throttle during the
lifetime of the subscription. For example, if the User Interface
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(UI) of the application shows inactivity for a period of time, it can
throttle the event list subscription little by little until it is
completely squelched. After all, if the user isn't actively access
the buddy list, there is no reason to keep it absolutely current
either.
Currently, a subscription refresh is needed in order to update the
throttle interval. However, this is highly inefficient, since
each refresh automatically generates a (full-state) notification
carrying the latest resource state. There is work
[I-D.niemi-sip-subnot-etags] ongoing to solve these
inefficiencies.
3.2. Requirements
REQ1: The subscriber must be able to set using a throttle mechanism
the minimum time period between two notifications in a
specific subscription.
REQ2: It must be possible to use of the throttle mechanism in
subscriptions to all events.
REQ3: It must be possible to use the throttle mechanism together
with any event filtering mechanism.
REQ4: The notifier must be allowed to use a throttling policy in
which the minimum time period between two notifications is
longer than the one given by the subscriber.
For example, due to congestion reasons, local policy at the
notifier could temporarily dictate a throttling policy that
in effect increases the subscriber-configured minimum time
period between two notifications.
REQ5: The throttle mechanism must provide a reasonable resolution
for setting the minimum period between two notifications. At
a minimum, the throttling mechanism must include discussion of
the situation resulting from a minimum time period which
exceeds the subscription duration, and should provide
mechanisms for avoiding this situation.
REQ6: A throttle must be possible to be installed, adapted, or
removed in the course of an active subscription.
REQ7: A throttle mechanism must allow for the application of
authentication and integrity protection mechanisms to
subscriptions invoking that mechanism.
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Note that Section 7 contains further discussion on the security
implications of the throttle mechanism.
3.3. Event Throttle Model
The notifier is responsible for sending out event notifications upon
state changes of the subscribed resource. We can model the notifier
as consisting of three components: the event state resource(s), the
Resource List Server (RLS) (or any other notifier), a notification
buffer, and finally the subcriber, or watcher of the event state, as
shown in Figure 1.
+--------+
| Event |
+--------+ |Resource| +--------+
| Event | +--------+ | Event |
|Resource| | |Resource|
+---.=---+ | +---=----+
`-.. | _.--'
``-._ | _.--'
+'--'--'-+
|Resource|
| List |
| Server |
+---.----+
|
|
)--+---(
| | .--------.
|Buffer|<======'Throttle|
| | `--------'
)--.---(
|
|
.---+---.
| Event |
|Watcher|
`-------'
Figure 1: Model for the Resource List Server (RLS) Supporting
Throttling
In short, the RLS reads event state changes from the event state
resource, either by creating a backend subscription, or by other
means; it packages them into event notifications, and submits them
into the output buffer. The rate at which this output buffer drains
is controlled by the subscriber via the event throttle mechanism.
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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.
3.4. Basic Operation
A subscriber that wants to limit the rate of event notification in a
specific event list subscription does so by suggesting a throttle as
part of the SUBSCRIBE message. The throttle indicating the minimum
time allowed between transmission of two consecutive notifications in
a subscription is given as an Event header parameter in the SUBSCRIBE
request.
Note that the witnessed time between two consecutive received
notifications may not conform to the set throttle for a number of
reasons. For example, network jitter and retransmissions may
result in the subscriber receiving the notifications in lesser
intervals than what the throttle recommends.
A notifier that supports the throttle mechanism will comply with
value given in the throttle, and adjust its rate of notification
accordingly.
Throttled notifications will have exactly the same properties as the
un-throttled ones, with the exception that they will not be generated
more frequent than what the throttle allows.
4. Operation of Event Throttles
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4.1. Negotiating the Use of Throttle
A subscriber that wishes to apply a throttle to notifications in a
subscription constructs a SUBSCRIBE request that includes a proposed
throttle interval in a "throttle" Event header field parameter.
A compliant notifier will reflect back the possibly adjusted throttle
interval in a "throttle" Subscription-State header field parameter of
the subsequent NOTIFY requests.
A notifier that does not understand the event-throttle extension,
will not reflect the "throttle" parameter in the NOTIFY requests; the
absence of this parameter serves as a hint to the subscriber that no
throttling is supported by the notifier.
Otherwise, the indicated throttle value is adopted by the notifier,
and the notification rate is adjusted accordingly.
4.2. Setting the Throttle
4.2.1. Subscriber Behavior
In general, the way in which a subscriber generates SUBSCRIBE
requests and processes NOTIFY requests is according to RFC 3265
[RFC3265].
A subscriber that wishes to throttle the notifications in a
subscription includes a "throttle" Event header parameter in the
SUBSCRIBE request, indicating in seconds the desired throttle value.
The value of this parameter is an integral number of seconds in
decimal.
The notifier is allowed to lower the suggested throttle interval.
The adjusted throttle value will be reflected back in the
Subscription-State header field of the subsequent NOTIFY requests,
which the subscriber MUST take as the current, possibly adjusted
throttle interval for the subscription.
There are two main consequencies for the subscriber when applying the
throttle mechanism: state transitions may be lost, and event
notifications may be delayed. If either of these side effects
constitute a problem to the application that is to utilize event
throttles, developers are instructed not to use the mechanism.
4.2.2. Notifier Behavior
In general, the way in which a notifier processes SUBSCRIBE requests
and generates NOTIFY requests is according to RFC 3265 [RFC3265].
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A notifier that supports the event-throttle extension extracts the
value of the "throttle" Event header parameter, and uses it as the
suggested minimum time allowed between two notifications. This value
can be adjusted by the notifier, as defined in Section 4.3.
A compliant notifier MUST NOT generate notifications more frequent
than what the throttle allows for, except when generating the
notification either upon receipt of a SUBSCRIBE request (the first
notification) or upon termination of the subscription (the last
notification). Such notifications reset the throttle timer, even
though they do not need to abide by it.
Retransmissions of NOTIFY requests are not affected by the throttle,
i.e., the throttle only applies to the generation of new
transactions. In other words, the throttle is reset only after the
previous transaction has completed.
4.3. Selecting the Throttle Interval
Special care needs to be taken when selecting the throttle value.
Using the throttle syntax it is possible to insist both very short
and very long throttles to be applied to the subscription. For
example, a throttle could potentially set a minimum time value
between notifications that exceeds the subscription expiration value.
Such a configuration would effectively quench the notifier, resulting
in exactly two notifications to be generated.
The notifier is responsible for adjusting the proposed throttle value
based on its local policy. The notifier MAY lower the throttle
value, e.g., because of lowering the subscription expiration. The
notifier MUST include the adjusted throttle value in the
Subscription-State header field's "throttle" parameter in each of the
NOTIFY requests. In addition, different event packages MAY define
additional constraints to the allowed throttle intervals. Such
constraints are out of the scope of this specification.
4.4. Buffer Policy Description
4.4.1. Partial State Notifications
With partial notifications, the notifier will always need to keep
both a copy of the current full state of the resource F, as well as
the last successfully communicated full state view F' of the resource
in a specific subscription. The construction of a partial
notification then involves creating a diff of the two states, and
generating a notification that contains that diff.
When a throttle is applied to the subscription, it is important that
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F' is replaced with F only when the throttle is reset. Additionally,
the notifier implementation SHOULD check to see that the size of an
accumulated partial state notification is smaller than the full
state, and if not, the notifier SHOULD send the full state
notification instead.
4.4.2. Full State Notifications
With full state notifications, the notifier only needs to keep the
full state of the resource, and when that changes, send the resulting
notification over to the subscriber.
When a throttle is applied in the subscription, the notifier receives
the state changes of the resource, and generates a notification. If
there is a pending notification, the notifier simply replaces that
notification with the new notification, discarding the older state.
4.5. Estimated Bandwidth Savings
It is difficult to estimate the total bandwidth savings accrued by
using the throttle mechanism over a subscription, since such
estimates will vary depending on the useage scenarios. However, it
is easy to see that given a subscription where several full state
notification would have normally been sent in any given throttle
interval, a throttled subscription would only send a single
notification during the same interval, yielding bandwidth savings of
several times the notification size.
With partial-state notifications, drawing estimates is further
complicated by the fact that the states of consequtive updates may or
may not overlap. However, even in the worst case scenario, where
each partial update is to a different part of the full state, a
throttled notification merging all of these n partial states together
should at a maximum be the size of a full-state update. In this
case, the bandwidth savings are approximately n times the size of the
NOTIFY header.
It is also true that there are several compression schemes available
that have been designed to save bandwidth in SIP, e.g., SigComp
[RFC3320] and TLS compression [RFC3943]. However, such comression
schemes are complementary rather than competing mechanisms to the
throttle mechanism. After all, they can both be applied
simultaniously, and in such a way that the compound savings are as
good as the sum of applying each one alone.
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5. Syntax
This section describes the syntax extensions required for the
throttle mechanism.
5.1. "throttle" Header Field Parameter
The "throttle" parameter is added to the rule definitions of the
Event header field and the Subscription-State header field in the SIP
Events [RFC3265] grammar. Usage of this parameter is described in
section Section 4.2.
5.2. Augmented BNF Definitions
This section describes the Augmented BNF [RFC2234] definitions for
the new syntax elements. Note that we derive here from the ruleset
present in both SIP Events [RFC3265] and SIP [RFC3261], adding
additional alternatives to the alternative sets of "event-param" and
"subexp-params" defined therein.
event-param =/ throttle-param
subexp-params =/ throttle-param
throttle-param = "throttle" EQUAL delta-seconds
6. IANA Considerations
This specification registers a new SIP header field parameter,
defined by the following information which is to be added to the
Header Field Parameters and Parameter Values sub-registry under
http://www.iana.org/assignments/sip-parameters.
Predefined
Header Field Parameter Name Values Reference
-------------------- --------------- ---------- ---------
Event throttle No [RFCxxxx]
Subscription-State throttle No [RFCxxxx]
(Note to the RFC Editor: please replace "xxxx" with the RFC number of
this specification, when assigned.)
7. Security Considerations
Naturally, the security considerations listed in SIP events
[RFC3265], which the throttle mechanism extends, apply in entirety.
In particular, authentication and message integrity SHOULD be applied
to subscriptions with the event-throttle extension.
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8. Acknowledgements
Thanks to Pekka Pessi, Dean Willis, Eric Burger, Alex Audu, Alexander
Milinski, Jonathan Rosenberg, Cullen Jennings and Adam Roach for
support and/or review of this work.
9. References
9.1. Normative References
[I-D.ietf-simple-event-list]
Roach, A., Rosenberg, J., and B. Campbell, "A Session
Initiation Protocol (SIP) Event Notification Extension for
Resource Lists", draft-ietf-simple-event-list-07 (work in
progress), January 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 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.
9.2. Informative References
[I-D.ietf-simple-xcap]
Rosenberg, J., "The Extensible Markup Language (XML)
Configuration Access Protocol (XCAP)",
draft-ietf-simple-xcap-12 (work in progress),
October 2006.
[I-D.niemi-sip-subnot-etags]
Niemi, A., "An Extension to Session Initiation Protocol
(SIP) Events for Issuing Conditional Subscriptions",
draft-niemi-sip-subnot-etags-01 (work in progress),
June 2006.
[RFC3320] Price, R., Bormann, C., Christoffersson, J., Hannu, H.,
Liu, Z., and J. Rosenberg, "Signaling Compression
(SigComp)", RFC 3320, January 2003.
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[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.
Author's Address
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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