One document matched: draft-niemi-sipping-event-throttle-00.txt
Network Working Group A. Niemi
Internet-Draft Nokia
Expires: April 19, 2004 October 20, 2003
Session Initiation Protocol (SIP) Event Notification Throttles
draft-niemi-sipping-event-throttle-00
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo specifies a throttle mechanism for limiting the rate of
Session Initiation Protocol (SIP) event notifications. This mechanism
can be applied in subscriptions to all SIP event packages.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions and Document Conventions . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1 Event Throttle Model . . . . . . . . . . . . . . . . . . . . 4
3.2 Basic Operation . . . . . . . . . . . . . . . . . . . . . . 4
4. Operation of Event Throttles . . . . . . . . . . . . . . . . 5
4.1 Negotiating the Use of Throttle . . . . . . . . . . . . . . 5
4.2 Setting the Throttle . . . . . . . . . . . . . . . . . . . . 5
4.2.1 Subscriber Behavior . . . . . . . . . . . . . . . . . . . . 5
4.2.2 Notifier Behavior . . . . . . . . . . . . . . . . . . . . . 6
4.3 Selecting the Throttle interval . . . . . . . . . . . . . . 6
5. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1 "event-throttle" SIP Option Tag . . . . . . . . . . . . . . 7
5.2 "throttle" Header Parameter . . . . . . . . . . . . . . . . 7
5.3 Augmented BNF Definitions . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
Normative References . . . . . . . . . . . . . . . . . . . . 8
Informative References . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . 10
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1. Introduction
The SIP events framework [2] 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 [4], [5], [6] to once per second [7].
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. Requirements and model for generic event throttles are
discussed in [8].
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. A throttle is simply a timer value that
indicates the minimum time period allowed between two notifications.
As a result of this throttle, a compliant notifier will limit the
rate at which it generates notifications.
2. Definitions and Document Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1] and
indicate requirement levels for compliant implementations.
Indented passages such as this one are used in this document to
provide additional information and clarifying text. They do not
contain normative protocol behavior.
3. Overview
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3.1 Event Throttle Model
Using notations from traffic theory, we can model the notifier as a
statistical multiplexer with an input rate of Ci (i = 1,...,n), and
an output rate of C <= C1 + ... + Cn. Typically, the statistical
multiplexer is lossy, with a finite buffer size. The loss probability
of the statistical multiplexer can be decreased by enlarging this
buffer. Figure 1 illustrates the model.
C1 |\
1 ---------| \
---------| \ C
---------| ||||O---------- 1
---------| /
n ---------| /
Cn |/
Figure 1: Notifier modeled as a statistical multiplexer
The output connection has a default rate that is generally dictated
by each individual event package. The rate can also be set using the
throttle mechanism described in this document. A notifer that accepts
a subscriber-defined throttle, adjusts its output rate accordingly.
There is typically only a single input connection, characterized by
the event package, and consisting of a stream of event notification
packets. Properties of the buffer, such as buffer size, policy (e.g.,
FIFO, LIFO), and packet treatment in lossy conditions, are all
implementation and event package specific.
In general, applying a throttle MUST NOT alter the properties of the
buffer. I.e., the event notifications are expected to deliver
consistent information even when throttled.
3.2 Basic Operation
A subscriber that wants to limit the rate of event notification in a
specific subscription does so by suggesting a throttle as part of the
SUBSCRIBE message. The throttle indicating the minimum time allowed
between two notifications in a subscription is given as an Event
header parameter in the SUBSCRIBE request.
The subscriber also indicates that it requires the throttle to be
applied to the subscription. This is done using the SIP option-tag
mechanism, by insisting that the notifier applies the event throttle
extension when processing the request. A notifier that does not
support the event throttle extension will reject the subscription.
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A notifier that supports the throttle mechanism will comply with
value given in the throttle, and adjust its rate of notification
accordingly.
Throttled notifications will have exactly the same properties as the
un-throttled ones, with the exception that they will not be generated
more frequent than what the throttle allows.
4. Operation of Event Throttles
4.1 Negotiating the Use of Throttle
This specification uses the SIP option-tag mechanism for negotiating
use of the throttle mechanism. Use of the "Require" header field and
the 420 (Bad Extension) are according to SIP [3].
A subscriber that wishes to apply a throttle to notifications in a
subscription insists that the notifier applies this throttle by
including an "event-throttle" option-tag to the Require header field
of the SUBSCRIBE request.
A notifier that does not understand the event-throttle extension,
will respond with a 420 (Bad Extension) response. Otherwise, the
throttle is processed by the notifier, and the notification rate is
adjusted accordingly.
4.2 Setting the Throttle
4.2.1 Subscriber Behavior
In general, the way in which a subscriber generates SUBSCRIBE
requests and processes NOTIFY requests is according to RFC 3265 [2].
A subscriber that wishes to throttle the notifications in a
subscription, MUST include a "throttle" Event header parameter in the
SUBSCRIBE request, indicating in seconds the throttle value. The
value of this parameter is an integral number of seconds in decimal.
In case the notifier does not support the "event-throttle" extension,
the subscriber SHOULD retry the subscription without that extension,
i.e., without the "throttle" parameter.
In this case the subscriber can resort to other means of limiting
the notification rate. For example, instead of a subscription, it
can fetch or poll the event state.
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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 [2].
A notifier that supports the "event-throttle" extension extracts the
value of the "throttle" Event header parameter, and uses it as the
minimum time allowed between two notifications. However, a notifier
MUST set the throttle to a value that is within the general throttle
resoulution, and is less than the subscription expiration. This
ensures that the throttle does not end up quenching the subscription.
OPEN ISSUE: Is this appropriate? Should we also handle somehow the
scenario where a notification is generated at such time that the
remaining subscription lifetime is less than the throttle value?
A notifier MUST include the selected throttle value in a "throttle"
parameter to the Subscription-State header field of the NOTIFY
requests sent to the subscriber.
A compliant notifier MUST NOT generate notifications more frequent
than what the throttle allows for, except when generating the
notification upon termination of the subscription. A notifier also
MUST NOT alter its notification buffer properties when applying a
throttle to it.
As specified in RFC 3261 [3] a notifier that supports event throttles
SHOULD advertise its support by including the "event-throttle"
option-tag in the Supported header field of a response to an OPTIONS
request.
4.3 Selecting the Throttle interval
Special care needs to be taken when selecting the throttle value.
Using the throttle syntax it is potentially 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 a useless subscription.
To avoid such complications, certain limits for the throttle value
are used. A throttle MUST be greater than the default minimum time
between two notifications, for a given event package. This ensures
that the notifications always follow the default notification rate
given by each event package. The throttle also MUST NOT exceed 600
seconds.
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OPEN ISSUE: This hard limit is open to discussion, but
intuitively, this seems as good a value as any. This resolution
seems to give enough flexibility so that the throttle mechanism is
usable, with being low enough to keep clear of most default
subscription expiration values.
Implementations MUST consider values greater than 600 seconds as
equal to 600, and values lower than the event package default as
equal to the event package default.
5. Syntax
This section describes the syntax extensions required for watcherinfo
history. Note that the formal syntax definitions described in this
section are expressed in the Augmented BNF format used in SIP [3],
and contain references to elements defined therein.
5.1 "event-throttle" SIP Option Tag
The "event-throttle" SIP option-tag is added to the "option-tag"
definition in the SIP grammar. Usage of this option-tag is defined in
Section 4.1.
5.2 "throttle" Header Parameter
The "throttle" header parameter is added to the "generic-param"
definition in the SIP grammar. Usage of this Event header parameter
is described in section Section 4.2.
The maximum value of this parameter is 600.
5.3 Augmented BNF Definitions
This section describes the Augmented BNF definitions for the new
syntax element. The notation is as used in SIP [3] and the documents
to which is refers.
generic-param = throttle-param / token [ EQUAL gen-value ]
throttle-param = "throttle" EQUAL delta-seconds ; value less than 600
option-tag = throttle-tag / token
throttle-tag = "event-throttle"
6. IANA Considerations
TBD: New SIP option tag (event-throttle), and possibly new header
parameter (throttle) need to be registered with IANA.
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7. Security Considerations
Naturally, the security considerations listed in SIP events [2],
which the throttle mechanism extends, apply in entirety. In
particular, authentication and message integrity SHOULD be applied to
subscriptions with the event-throttle extension.
8. Acknowledgements
Thanks for Pekka Pessi for comments and suggestions on the draft.
Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
Informative References
[4] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", draft-ietf-simple-presence-10 (work
in progress), January 2003.
[5] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Registrations", draft-ietf-sipping-reg-event-00
(work in progress), October 2002.
[6] Rosenberg, J., "A Watcher Information Event Template-Package for
the Session Initiation Protocol (SIP)",
draft-ietf-simple-winfo-package-05 (work in progress), January
2003.
[7] Mahy, R., "A Message Summary and Message Waiting Indication
Event Package for the Session Initiation Protocol (SIP)",
draft-ietf-sipping-mwi-03 (work in progress), July 2003.
[8] Niemi, A., "Requirements for Limiting the Rate of Event
Notifications", draft-ietf-sipping-event-throttle-reqs-00 (work
in progress), September 2003.
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Author's Address
Aki Niemi
Nokia
P.O. Box 321
NOKIA GROUP, FIN 00045
Finland
Phone: +358 50 389 1644
EMail: aki.niemi@nokia.com
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