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 This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 19, 2004. 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. Niemi Expires April 19, 2004 [Page 1] Internet-Draft Event Throttle October 2003 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 Niemi Expires April 19, 2004 [Page 2] Internet-Draft Event Throttle October 2003 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 Niemi Expires April 19, 2004 [Page 3] Internet-Draft Event Throttle October 2003 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. Niemi Expires April 19, 2004 [Page 4] Internet-Draft Event Throttle October 2003 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. Niemi Expires April 19, 2004 [Page 5] Internet-Draft Event Throttle October 2003 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. Niemi Expires April 19, 2004 [Page 6] Internet-Draft Event Throttle October 2003 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. Niemi Expires April 19, 2004 [Page 7] Internet-Draft Event Throttle October 2003 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. Niemi Expires April 19, 2004 [Page 8] Internet-Draft Event Throttle October 2003 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 Niemi Expires April 19, 2004 [Page 9] Internet-Draft Event Throttle October 2003 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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