< draft-ietf-sip-events-04.txt		draft-ietf-sip-events-05.txt >
	Internet Engineering Task Force Adam Roach		A new Request for Comments is now available in online RFC libraries.
	Internet Draft dynamicsoft
	Category: Standards Track February 2002
	Expires August 2002
	<draft-ietf-sip-events-04.txt>
	SIP-Specific Event Notification
	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 cite them other than as "work in
	progress".
	The list of current Internet-Drafts can be accessed at
	http://www.ietf.org/ietf/lid-abstracts.txt
	The list of Internet-Draft Shadow Directories can be accessed at
	http://www.ietf.org/shadow.html
	This document is an individual submission to the IETF. Comments
	should be directed to the authors.
	Abstract
	This document describes an extension to the Session Initiation
	Protocol (SIP). The purpose of this extension is to provide an
	extensible framework by which SIP nodes can request notification
	from remote nodes indicating that certain events have occurred.
	Concrete uses of the mechanism described in this document may be
	standardized in the future.
	Note that the event notification mechanisms defined herein are
	NOT intended to be a general-purpose infrastructure for all
	classes of event subscription and notification.
	1. Table of Contents
	1. Table of Contents...................................... 1
	2. Introduction........................................... 3
	2.1. Overview of Operation.................................. 4
	2.2. Documentation Conventions.............................. 4
	3. Definitions............................................ 4
	4. Node Behavior.......................................... 5
	4.1. Description of SUBSCRIBE Behavior...................... 5
	4.1.1. Subscription Duration.................................. 6
	4.1.2. Identification of Subscribed Events and Event Classes.. 6
	4.1.3. Additional SUBSCRIBE Header Values..................... 7
	4.1.4. Subscriber SUBSCRIBE Behavior.......................... 7
	4.1.5. Proxy SUBSCRIBE Behavior............................... 9
	4.1.6. Notifier SUBSCRIBE Behavior............................ 9
	4.2. Description of NOTIFY Behavior......................... 12
	4.2.1. Identification of Reported Events, Event Classes, and C 13
	4.2.2. Notifier NOTIFY Behavior............................... 13
	4.2.3. Proxy NOTIFY Behavior.................................. 15
	4.2.4. Subscriber NOTIFY Behavior............................. 15
	4.3. General................................................ 17
	4.3.1. Detecting support for SUBSCRIBE and NOTIFY............. 17
	4.3.2. CANCEL requests........................................ 18
	4.3.3. Forking................................................ 18
	4.3.4. Dialog creation and termination........................ 18
	4.3.5. State Agents and Notifier Migration.................... 19
	4.3.6. Polling Resource State................................. 20
	4.3.7. Allow-Events header usage.............................. 20
	4.3.8. PINT Compatibility..................................... 21
	5. Event Packages......................................... 21
	5.1. Appropriateness of Usage............................... 21
	5.2. Event Template-packages................................ 22
	5.3. Amount of State to be Conveyed......................... 22
	5.3.1. Complete State Information............................. 22
	5.3.2. State Deltas........................................... 23
	5.4. Event Package Responsibilities......................... 23
	5.4.1. Event Package Name..................................... 24
	5.4.2. Event Package Parameters............................... 24
	5.4.3. SUBSCRIBE Bodies....................................... 24
	5.4.4. Subscription Duration.................................. 24
	5.4.5. NOTIFY Bodies.......................................... 24
	5.4.6. Notifier processing of SUBSCRIBE requests.............. 25
	5.4.7. Notifier generation of NOTIFY requests................. 25
	5.4.8. Subscriber processing of NOTIFY requests............... 25
	5.4.9. Handling of forked requests............................ 25
	5.4.10. Rate of notifications.................................. 26
	5.4.11. State Agents........................................... 26
	5.4.12. Examples............................................... 26
	5.4.13. Use of URIs to Retrieve State.......................... 27
	6. Security Considerations................................ 27
	6.1. Access Control......................................... 27
	6.2. Notifier Privacy Mechanism............................. 27
	6.3. Denial-of-Service attacks.............................. 27
	6.4. Replay Attacks......................................... 28
	6.5. Man-in-the middle attacks.............................. 28
	6.6. Confidentiality........................................ 28
	7. IANA Considerations.................................... 29
	7.1. Registration Information............................... 29
	7.2. Registration Template.................................. 30
	7.3. Syntax................................................. 31
	7.4. New Methods............................................ 31
	7.4.1. SUBSCRIBE method....................................... 32
	7.4.2. NOTIFY method.......................................... 33
	7.5. New Headers............................................ 33
	7.5.1. "Event" header......................................... 33
	7.5.2. "Allow-Events" Header.................................. 34
	7.5.3. "Subscription-State" Header............................ 34
	7.6. New Response Codes..................................... 34
	7.6.1. "202 Accepted" Response Code........................... 34
	7.6.2. "489 Bad Event" Response Code.......................... 34
	7.7. Augmented BNF Definitions.............................. 34
	8. References............................................. 35
	9. Acknowledgements....................................... 36
	10. Author's Address....................................... 36
	11. Notice Regarding Intellectual Property Rights.......... 36
	12. Full Copyright Statement............................... 36
	2. Introduction
	The ability to request asynchronous notification of events proves
	useful in many types of SIP services for which cooperation
	between end-nodes is required. Examples of such services include
	automatic callback services (based on terminal state events),
	buddy lists (based on user presence events), message waiting
	indications (based on mailbox state change events), and PSTN and
	Internet Internetworking (PINT) [3] status (based on call state
	events).
	The methods described in this document provide a framework by
	which notification of these events can be ordered.
	The event notification mechanisms defined herein are NOT intended
	to be a general-purpose infrastructure for all classes of event
	subscription and notification. Meeting requirements for the
	general problem set of subscription and notification is far too
	complex for a single protocol. Our goal is to provide a
	SIP-specific framework for event notification which is not so
	complex as to be unusable for simple features, but which is still
	flexible enough to provide powerful services. Note, however, that
	event packages based on this framework may define arbitrarily
	elaborate rules which govern the subscription and notification
	for the events or classes of events they describe.
	This draft does not describe an extension which may be used
	directly; it must be extended by other drafts (herein referred to
	as "event packages".) In object-oriented design terminology, it
	may be thought of as an abstract base class which must be derived
	into an instantiatable class by further extensions. Guidelines
	for creating these extensions are described in section 5.
	2.1. Overview of Operation
	The general concept is that entities in the network can subscribe
	to resource or call state for various resources or calls in the
	network, and those entities (or entities acting on their behalf)
	can send notifications when those states change.
	A typical flow of messages would be:
	Subscriber Notifier
	|-----SUBSCRIBE---->| Request state subscription
	|<-------200--------| Acknowledge subscription
	|<------NOTIFY----- | Return current state information
	|--------200------->|
	|<------NOTIFY----- | Return current state information
	|--------200------->|
	Subscriptions are expired and must be refreshed by subsequent
	SUBSCRIBE messages.
	2.2. Documentation Conventions
	There are several paragraphs throughout the document which
	provide motivational or clarifying text. Such passages are
	non-normative, and are provided only to assist with reader
	comprehension. These passages are set off from the remainder of
	the text by being indented thus:
	This is an example of non-normative explanatory text. It does
	not form part of the specification, and is used only for
	clarification.
	Numbers in square brackets (e.g. [1]) denote a reference to one
	of the entries in the References section; see section 8.
	The all-capital terms "MUST", "SHOULD", "MAY", "SHOULD NOT", and
	"MUST NOT" are used as defined in RFC 2119 [7].
	The use of quotation marks next to periods and commas follows the
	convention used by the American Mathematical Society; although
	contrary to traditional American English convention, this usage
	lends clarity to certain passages.
	3. Definitions
	Event Package: An event package is an additional specification
	which defines a set of state information to be reported by a
	notifier to a subscriber. Event packages also define further
	syntax and semantics based on the framework defined by this
	document required to convey such state information.
	Event Template-Package: An event template-package is a special
	kind of event package which defines a set of state which may
	be applied to all possible event packages, including itself.
	Notification: Notification is the act of a notifier sending a
	NOTIFY message to a subscriber to inform the subscriber of
	the state of a resource.
	Notifier: A notifier is a user agent which generates NOTIFY
	requests for the purpose of notifying subscribers of the
	state of a resource. Notifiers typically also accept
	SUBSCRIBE requests to create subscriptions.
	State Agent: A state agent is a notifier which publishes state
	information on behalf of a resource; in order to do so, it
	may need to gather such state information from multiple
	sources. State agents always have complete state information
	for the resource for which they are creating notifications.
	Subscriber: A subscriber is a user agent which receives NOTIFY
	requests from notifiers; these NOTIFY requests contain
	information about the state of a resource in which the
	subscriber is interested. Subscribers typically also generate
	SUBSCRIBE requests and send them to notifiers to create
	subscriptions.
	Subscription: A subscription is a set of application state
	associated with a dialog. This application state includes a
	pointer to the associated dialog, the event package name, and
	possibly an identification token. Event packages will define
	additional subscription state information. By definition,
	subscriptions exist in both a subscriber and a notifier.
	Subscription Migration: Subscription migration is the act of
	moving a subscription from one notifier to another notifier.
	4. Node Behavior
	4.1. Description of SUBSCRIBE Behavior
	The SUBSCRIBE method is used to request current state and state
	updates from a remote node.
	4.1.1. Subscription Duration
	SUBSCRIBE requests SHOULD contain an "Expires" header (defined in
	SIP [1]). This expires value indicates the duration of the
	subscription. In order to keep subscriptions effective beyond the
	duration communicated in the "Expires" header, subscribers need
	to refresh subscriptions on a periodic basis using a new
	SUBSCRIBE message on the same dialog as defined in SIP [1].
	If no "Expires" header is present in a SUBSCRIBE request, the
	implied default is defined by the event package being used.
	200-class responses to SUBSCRIBE requests also MUST contain an
	"Expires" header. The period of time in the response MAY be
	shorter but MUST NOT be longer than specified in the request. The
	period of time in the response is the one which defines the
	duration of the subscription.
	An "expires" parameter on the "Contact" header has no semantics
	for SUBSCRIBE and is explicitly not equivalent to an "Expires"
	header in a SUBSCRIBE request or response.
	A natural consequence of this scheme is that a SUBSCRIBE with an
	"Expires" of 0 constitutes a request to unsubscribe from an
	event.
	In addition to being a request to unsubscribe, a SUBSCRIBE
	message with "Expires" of 0 also causes a fetch of state; see
	section 4.3.6.
	Notifiers may also wish to cancel subscriptions to events; this
	is useful, for example, when the resource to which a subscription
	refers is no longer available. Further details on this mechanism
	are discussed in section 4.2.2.
	4.1.2. Identification of Subscribed Events and Event Classes
	Identification of events is provided by three pieces of
	information: Request URI, Event Type, and (optionally) message
	body.
	The Request URI of a SUBSCRIBE request, most importantly,
	contains enough information to route the request to the
	appropriate entity per the request routing procedures outlined in
	SIP [1]. It also contains enough information to identify the
	resource for which event notification is desired, but not
	necessarily enough information to uniquely identify the nature of
	the event (e.g. "sip:adam@dynamicsoft.com" would be an
	appropriate URI to subscribe to for my presence state; it would
	also be an appropriate URI to subscribe to the state of my voice
	mailbox).
	Subscribers MUST include exactly one "Event" header in SUBSCRIBE
	requests, indicating to which event or class of events they are
	subscribing. The "Event" header will contain a token which
	indicates the type of state for which a subscription is being
	requested. This token will be registered with the IANA and will
	correspond to an event package which further describes the
	semantics of the event or event class. The "Event" header MAY
	also contain an "id" parameter. This "id" parameter, if present,
	contains an opaque token which identifies the specific
	subscription within a dialog. An "id" parameter is only valid
	within the scope of a single dialog.
	If the event package to which the event token corresponds defines
	behavior associated with the body of its SUBSCRIBE requests,
	those semantics apply.
	Event packages may also define parameters for the Event header;
	if they do so, they must define the semantics for such
	parameters.
	4.1.3. Additional SUBSCRIBE Header Values
	Because SUBSCRIBE requests create a dialog as defined in SIP [1],
	they MAY contain an "Accept" header. This header, if present,
	indicates the body formats allowed in subsequent NOTIFY requests.
	Event packages MUST define the behavior for SUBSCRIBE requests
	without "Accept" headers; usually, this will connote a single,
	default body type.
	Header values not described in this document are to be
	interpreted as described in SIP [1].
	4.1.4. Subscriber SUBSCRIBE Behavior
	4.1.4.1. Requesting a Subscription
	SUBSCRIBE is a dialog-creating method, as described in SIP [1].
	When a subscriber wishes to subscribe to a particular state for a
	resource, it forms a SUBSCRIBE message. If the initial SUBSCRIBE
	represents a request outside of a dialog (as it typically will),
	its construction follows the procedures outlined in SIP [1] for
	UAC request generation outside of a dialog.
	This SUBSCRIBE request will be confirmed with a final response.
	200-class responses indicate that the subscription has been
	accepted, and that a NOTIFY will be sent immediately. A 200
	response indicates that the subscription has been accepted and
	that the user is authorized to subscribe to the requested
	resource. A 202 response merely indicates that the subscription
	has been understood, and that authorization may or may not have
	been granted.
	The "Expires" header in a 200-class response to SUBSCRIBE
	indicates the actual duration for which the subscription will
	remain active (unless refreshed).
	Non-200 class final responses indicate that no subscription or
	dialog has been created, and no subsequent NOTIFY message will be
	sent. All non-200 class responses (with the exception of "489",
	described herein) have the same meanings and handling as
	described in SIP [1].
	A SUBSCRIBE request MAY include an "id" parameter in its "Event"
	header to allow differentiation between multiple subscriptions in
	the same dialog.
	4.1.4.2. Refreshing of Subscriptions
	At any time before a subscription expires, the subscriber may
	refresh the timer on such a subscription by sending another
	SUBSCRIBE request on the same dialog as the existing
	subscription, and with the same "Event" header "id" parameter (if
	one was present in the initial subscription). The handling for
	such a request is the same as for the initial creation of a
	subscription except as described below.
	If the initial SUBSCRIBE message contained an "id" parameter
	on the "Event" header, then refreshes of the subscription
	must also contain an identical "id" parameter; they will
	otherwise be considered new subscriptions in an existing
	dialog.
	If a SUBSCRIBE request to refresh a subscription receives a "481"
	response, this indicates that the subscription has been
	terminated and that the subscriber did not receive notification
	of this fact. In this case, the subscriber should consider the
	subscription invalid. If the subscriber wishes to re-subscribe to
	the state, he does so by composing an unrelated initial SUBSCRIBE
	request with a freshly-generated Call-ID and a new, unique "From"
	tag (see section 4.1.4.1.)
	If a SUBSCRIBE request to refresh a subscription fails with a
	non-481 response, the original subscription is still considered
	valid for the duration of the most recently known "Expires" value
	as negotiated by SUBSCRIBE and its response, or as communicated
	by NOTIFY in the "Subscription-State" header "expires" parameter.
	Note that many such errors indicate that there may be a
	problem with the network or the notifier such that no further
	NOTIFY messages will be received.
	4.1.4.3. Unsubscribing
	Unsubscribing is handled in the same way as refreshing of a
	subscription, with the "Expires" header set to "0". Note that a
	successful unsubscription will also trigger a final NOTIFY
	message.
	4.1.4.4. Confirmation of Subscription Creation
	The subscriber can expect to receive a NOTIFY message from each
	node which has processed a successful subscription or
	subscription refresh. Until the first NOTIFY message arrives, the
	subscriber should consider the state of the subscribed resource
	to be in a neutral state. Event packages which define new event
	packages MUST define this "neutral state" in such a way that
	makes sense for their application (see section 5.4.7.).
	Due to the potential for both out-of-order messages and forking,
	the subscriber MUST be prepared to receive NOTIFY messages before
	the SUBSCRIBE transaction has completed.
	Except as noted above, processing of this NOTIFY is the same as
	in section 4.2.4.
	4.1.5. Proxy SUBSCRIBE Behavior
	Proxies need no additional behavior beyond that described in SIP
	[1] to support SUBSCRIBE. If a proxy wishes to see all of the
	SUBSCRIBE and NOTIFY requests for a given dialog, it MUST
	record-route the initial SUBSCRIBE and any dialog-establishing
	NOTIFY requests. Such proxies SHOULD also record-route all other
	SUBSCRIBE and NOTIFY requests.
	Note that subscribers and notifiers may elect to use S/MIME
	encryption of SUBSCRIBE and NOTIFY requests; consequently,
	proxies cannot rely on being able to access any information
	that is not explicitly required to be proxy-readable by SIP
	[1].
	4.1.6. Notifier SUBSCRIBE Behavior
	4.1.6.1. Initial SUBSCRIBE Transaction Processing
	In no case should a SUBSCRIBE transaction extend for any longer
	than the time necessary for automated processing. In particular,
	notifiers MUST NOT wait for a user response before returning a
	final response to a SUBSCRIBE request.
	This requirement is imposed primarily to prevent the
	non-INVITE transaction timeout timer F (see [1]) from firing
	during the SUBSCRIBE transaction, since interaction with a
	user would often exceed 64*T1 seconds.
	The notifier SHOULD check that the event package specified in the
	"Event" header is understood. If not, the notifier SHOULD return
	a "489 Bad Event" response to indicate that the specified
	event/event class is not understood.
	The notifier SHOULD also perform any necessary authentication and
	authorization per its local policy. See section 4.1.6.3.
	The notifier MAY also check that the duration in the "Expires"
	header is not too small. If and only if the expiration interval
	is greater than zero AND smaller than one hour AND less than a
	notifier-configured minimum, the notifier MAY return a "423
	Interval too small" error which contains a "Min-Expires" header
	field. The "Min-Expires" header field is described in SIP [1].
	If the notifier is able to immediately determine that it
	understands the event package, that the authenticated subscriber
	is authorized to subscribe, and that there are no other barriers
	to creating the subscription, it creates the subscription and a
	dialog (if necessary), and returns a "200 OK" response (unless
	doing so would reveal authorization policy in an undesirable
	fashion; see section 6.2.).
	If the notifier cannot immediately create the subscription (e.g.
	it needs to wait for user input for authorization, or is acting
	for another node which is not currently reachable), or wishes to
	mask authorization policy, it will return a "202 Accepted"
	response. This response indicates that the request has been
	received and understood, but does not necessarily imply that the
	subscription has been authorized yet.
	When a subscription is created in the notifier, it stores the
	event package name and the "Event" header "id" parameter (if
	present) as part of the subscription information.
	The "Expires" values present in SUBSCRIBE 200-class responses
	behave in the same way as they do in REGISTER responses: the
	server MAY shorten the interval, but MUST NOT lengthen it.
	If the duration specified in a SUBSCRIBE message is
	unacceptably short, the notifier may be able to send a 423
	response, as described earlier in this section.
	200-class responses to SUBSCRIBE requests will not generally
	contain any useful information beyond subscription duration;
	their primary purpose is to serve as a reliability mechanism.
	State information will be communicated via a subsequent NOTIFY
	request from the notifier.
	The other response codes defined in SIP [1] may be used in
	response to SUBSCRIBE requests, as appropriate.
	4.1.6.2. Confirmation of Subscription Creation/Refreshing
	Upon successfully accepting or refreshing a subscription,
	notifiers MUST send a NOTIFY message immediately to communicate
	the current resource state to the subscriber. This NOTIFY message
	is sent on the same dialog as created by the SUBSCRIBE response.
	If the resource has no meaningful state at the time that the
	SUBSCRIBE message is processed, this NOTIFY message MAY contain
	an empty or neutral body. See section 4.2.2. for further details
	on NOTIFY message generation.
	Note that a NOTIFY message is always sent immediately after any
	200-class response to a SUBSCRIBE request, regardless of whether
	the subscription has already been authorized.
	4.1.6.3. Authentication/Authorization of SUBSCRIBE requests
	Privacy concerns may require that notifiers apply policy to
	determine whether a particular subscriber is authorized to
	subscribe to a certain set of events. Such policy may be defined
	by mechanisms such as access control lists or real-time
	interaction with a user. In general, authorization of subscribers
	prior to authentication is not particularly useful.
	SIP authentication mechanisms are discussed in SIP [1]. Note
	that, even if the notifier node typically acts as a proxy,
	authentication for SUBSCRIBE requests will always be performed
	via a "401" response, not a "407;" notifiers always act as a user
	agents when accepting subscriptions and sending notifications.
	Of course, when acting as a proxy, a node will perform normal
	proxy authentication (using 407). The foregoing explanation
	is a reminder that notifiers are always UAs, and as such
	perform UA authentication.
	If authorization fails based on an access list or some other
	automated mechanism (i.e. it can be automatically authoritatively
	determined that the subscriber is not authorized to subscribe),
	the notifier SHOULD reply to the request with a "403 Forbidden"
	or "603 Decline" response, unless doing so might reveal
	information that should stay private; see section 6.2.
	If the notifier owner is interactively queried to determine
	whether a subscription is allowed, a "202 Accept" response is
	returned immediately. Note that a NOTIFY message is still formed
	and sent under these circumstances, as described in the previous
	section.
	If subscription authorization was delayed and the notifier wishes
	to convey that such authorization has been declined, it may do so
	by sending a NOTIFY message containing a "Subscription-State"
	header with a value of "terminated" and a reason parameter of
	"rejected".
	4.1.6.4. Refreshing of Subscriptions
	When a notifier receives a subscription refresh, assuming that
	the subscriber is still authorized, the notifier updates the
	expiration time for subscription. As with the initial
	subscription, the server MAY shorten the amount of time until
	expiration, but MUST NOT increase it. The final expiration time
	is placed in the "Expires" header in the response. If the
	duration specified in a SUBSCRIBE message is unacceptably short,
	the notifier SHOULD respond with a "423 Subscription Too Brief"
	message.
	If no refresh for a notification address is received before its
	expiration time, the subscription is removed. When removing a
	subscription, the notifier SHOULD send a NOTIFY message with a
	"Subscription-State" value of "terminated" to inform it that the
	subscription is being removed. If such a message is sent, the
	"Subscription-State" header SHOULD contain a "reason=timeout"
	parameter.
	The sending of a NOTIFY when a subscription expires allows
	the corresponding dialog to be terminated, if appropriate.
	4.2. Description of NOTIFY Behavior
	NOTIFY messages are sent to inform subscribers of changes in
	state to which the subscriber has a subscription. Subscriptions
	are typically put in place using the SUBSCRIBE method; however,
	it is possible that other means have been used.
	If any non-SUBSCRIBE mechanisms are defined to create
	subscriptions, it is the responsibility of the parties defining
	those mechanisms to ensure that correlation of a NOTIFY message
	to the corresponding subscription is possible. Designers of such
	mechanisms are also warned to make a distinction between sending
	a NOTIFY message to a subscriber who is aware of the
	subscription, and sending a NOTIFY message to an unsuspecting
	node. The latter behavior is invalid, and MUST receive a "481
	Subscription does not exist" response (unless some other 400- or
	500-class error code is more applicable), as described in section
	4.2.4. In other words, knowledge of a subscription must exist in
	both the subscriber and the notifier to be valid, even if
	installed via a non-SUBSCRIBE mechanism.
	A NOTIFY does not terminate its corresponding subscription; in
	other words, a single SUBSCRIBE request may trigger several
	NOTIFY requests.
	4.2.1. Identification of Reported Events, Event Classes, and Current
	State
	Identification of events being reported in a notification is very
	similar to that described for subscription to events (see section
	4.1.2.).
	As in SUBSCRIBE requests, NOTIFY "Event" headers will contain a
	single event package name for which a notification is being
	generated. The package name in the "Event" header MUST match the
	"Event" header in the corresponding SUBSCRIBE message. If an "id"
	parameter was present in the SUBSCRIBE message, that "id"
	parameter MUST also be present in the corresponding NOTIFY
	messages.
	Event packages may define semantics associated with the body of
	their NOTIFY requests; if they do so, those semantics apply.
	NOTIFY bodies are expected to provide additional details about
	the nature of the event which has occurred and the resultant
	resource state.
	When present, the body of the NOTIFY request MUST be formatted
	into one of the body formats specified in the "Accept" header of
	the corresponding SUBSCRIBE request. This body will contain
	either the state of the subscribed resource or a pointer to such
	state in the form of a URI.
	4.2.2. Notifier NOTIFY Behavior
	When a SUBSCRIBE request is answered with a 200-class response,
	the notifier MUST immediately construct and send a NOTIFY request
	to the subscriber. When a change in the subscribed state occurs,
	the notifier SHOULD immediately construct and send a NOTIFY
	request, subject to authorization, local policy, and throttling
	considerations.
	A NOTIFY request is considered failed if the response times out,
	or a non-200 class response code is received which has no
	"Retry-After" header and no implied further action which can be
	taken to retry the request (e.g. "401 Authorization Required".)
	If the NOTIFY request fails (as defined above) due to a timeout
	condition, and the subscription was installed using a soft-state
	mechanism (such as SUBSCRIBE), the notifier SHOULD remove the
	subscription.
	This behavior prevents unnecessary transmission of state
	information for subscribers who have crashed or disappeared
	from the network. Because such transmissions will be sent
	multiple times, per the retransmission algorithm defined in
	SIP [1] (instead of the typical single transmission for
	functioning clients), continuing to service them when no
	client is available to acknowledge them could place undue
	strain on a network. Upon client restart or reestablishment
	of a network connection, it is expected that clients will
	send SUBSCRIBE messages to refresh potentially stale state
	information; such messages will re-install subscriptions in
	all relevant nodes.
	If the NOTIFY request fails (as defined above) due to an error
	response, and the subscription was installed using a soft-state
	mechanism, the notifier MUST remove the corresponding
	subscription.
	A notify error response would generally indicate that
	something has gone wrong with the subscriber or with some
	proxy on the way to the subscriber. If the subscriber is in
	error, it makes the most sense to allow the subscriber to
	rectify the situation (by re-subscribing) once the error
	condition has been handled. If a proxy is in error, the
	periodic SUBSCRIBE refreshes will re-install subscription
	state once the network problem has been resolved.
	If a NOTIFY request receives a 481 response, the notifier MUST
	remove the corresponding subscription even if such subscription
	was installed by non-SUBSCRIBE means (such as an administrative
	interface).
	If the above behavior were not required, subscribers
	receiving a notify for an unknown subscription would need to
	send an error status code in response to the NOTIFY and also
	send a SUBSCRIBE request to remove the subscription. Since
	this behavior would make subscribers available for use as
	amplifiers in denial of service attacks, we have instead
	elected to give the 481 response special meaning: it is used
	to indicate that a subscription must be cancelled under all
	circumstances.
	NOTIFY requests MUST contain a "Subscription-State" header with a
	value of "active", "pending", or "terminated". The "active" value
	indicates that the subscription has been accepted and has been
	authorized (in most cases; see section 6.2.). The "pending" value
	indicates that the subscription has been received, but that
	policy information is insufficient to accept or deny the
	subscription at this time. The "terminated" value indicates that
	the subscription is not active.
	If the value of the "Subscription-State" header is "active" or
	"pending", the notifier SHOULD also include in the
	"Subscription-State" header an "expires" parameter which
	indicates the time remaining on the subscription. The notifier
	MAY use this mechanism to shorten a subscription; however, this
	mechanism MUST NOT be used to lengthen a subscription.
	Including expiration information for active and pending
	subscriptions is useful in case the SUBSCRIBE request forks,
	since the response to a forked SUBSCRIBE may not be received
	by the subscriber. Note well that this "expires" value is a
	parameter on the "Subscription-State" header, NOT an
	"Expires" header.
	If the value of the "Subscription-State" header is "terminated",
	the notifier SHOULD also include a "reason" parameter. The
	notifier MAY also include a "retry-after" parameter, where
	appropriate. For details on the value and semantics of the
	"reason" and "retry-after" parameters, see section 4.2.4.
	4.2.3. Proxy NOTIFY Behavior
	Proxies need no additional behavior beyond that described in SIP
	[1] to support NOTIFY. If a proxy wishes to see all of the
	SUBSCRIBE and NOTIFY requests for a given dialog, it MUST
	record-route the initial SUBSCRIBE and any dialog-establishing
	NOTIFY requests. Such proxies SHOULD also record-route all other
	SUBSCRIBE and NOTIFY requests.
	Note that subscribers and notifiers may elect to use S/MIME
	encryption of SUBSCRIBE and NOTIFY requests; consequently,
	proxies cannot rely on being able to access any information
	that is not explicitly required to be proxy-readable by SIP
	[1].
	4.2.4. Subscriber NOTIFY Behavior
	Upon receiving a NOTIFY request, the subscriber should check that
	it matches at least one of its outstanding subscriptions; if not,
	it MUST return a "481 Subscription does not exist" response
	unless another 400- or 500-class response is more appropriate.
	The rules for matching NOTIFY requests with subscriptions that
	create a new dialog are described in section 4.3.4. Notifications
	for subscriptions which were created inside an existing dialog
	match if they are in the same dialog and the "Event" headers
	match (as described in section 7.5.1.)
	If, for some reason, the event package designated in the "Event"
	header of the NOTIFY request is not supported, the subscriber
	will respond with a "489 Bad Event" response.
	To prevent spoofing of events, NOTIFY requests SHOULD be
	authenticated, using any defined SIP authentication mechanism.
	NOTIFY requests MUST contain "Subscription-State" headers which
	indicate the status of the subscription.
	If the "Subscription-State" header value is "active", it means
	that the subscription has been accepted and (in general) has been
	authorized. If the header also contains an "expires" parameter,
	the subscriber SHOULD take it as the authoritative subscription
	duration and adjust accordingly. The "retry-after" and "reason"
	parameters have no semantics for "active".
	If the "Subscription-State" value is "pending", the subscription
	has been received by the notifier, but there is insufficient
	policy information to grant or deny the subscription yet. If the
	header also contains an "expires" parameter, the subscriber
	SHOULD take it as the authoritative subscription duration and
	adjust accordingly. No further action is necessary on the part of
	the subscriber. The "retry-after" and "reason" parameters have no
	semantics for "pending".
	If the "Subscription-State" value is "terminated", the subscriber
	should consider the subscription terminated. The "expires"
	parameter has no semantics for "terminated". If a reason code is
	present, the client should behave as described below. If no
	reason code or an unknown reason code is present, the client MAY
	attempt to re-subscribe at any time (unless a "retry-after"
	parameter is present, in which case the client SHOULD NOT attempt
	re-subscription until after the number of seconds specified by
	the "retry-after" parameter). The defined reason codes are:
	deactivated: The subscription has been terminated, but the client
	SHOULD retry immediately with a new subscription. One primary
	use of such a status code is to allow migration of
	subscriptions between nodes. The "retry-after" parameter has
	no semantics for "deactivated".
	probation: The subscription has been terminated, but the client
	SHOULD retry at some later time. If a "retry-after" parameter
	is also present, the client SHOULD wait at least the number
	of seconds specified by that parameter before attempting to
	re-subscribe.
	rejected: The subscription has been terminated due to change in
	authorization policy. Clients SHOULD NOT attempt to
	re-subscribe. The "retry-after" parameter has no semantics
	for "rejected".
	timeout: The subscription has been terminated because it was not
	refreshed before it expired. Clients MAY re-subscribe
	immediately. The "retry-after" parameter has no semantics for
	"timeout".
	giveup: The subscription has been terminated because the notifier
	could not obtain authorization in a timely fashion. If a
	"retry-after" parameter is also present, the client SHOULD
	wait at least the number of seconds specified by that
	parameter before attempting to re-subscribe; otherwise, the
	client MAY retry immediately, but will likely get put back
	into pending state.
	noresource: The subscription has been terminated because the
	resource state which was being monitored no longer exists.
	Clients SHOULD NOT attempt to re-subscribe. The "retry-after"
	parameter has no semantics for "noresource".
	Once the notification is deemed acceptable to the subscriber, the
	subscriber SHOULD return a 200 response. In general, it is not
	expected that NOTIFY responses will contain bodies; however, they
	MAY, if the NOTIFY request contained an "Accept" header.
	Other responses defined in SIP [1] may also be returned, as
	appropriate. In no case should a NOTIFY transaction extend for
	any longer than the time necessary for automated processing. In
	particular, subscribers MUST NOT wait for a user response before
	returning a final response to a NOTIFY request.
	4.3. General
	4.3.1. Detecting support for SUBSCRIBE and NOTIFY
	Neither SUBSCRIBE nor NOTIFY necessitate the use of "Require" or
	"Proxy-Require" headers; similarly, there is no token defined for
	"Supported" headers. If necessary, clients may probe for the
	support of SUBSCRIBE and NOTIFY using the OPTIONS request defined
	in SIP[1].
	The presence of the "Allow-Events" header in a message is
	sufficient to indicate support for SUBSCRIBE and NOTIFY.
	The "methods" parameter for Contact may also be used to
	specifically announce support for SUBSCRIBE and NOTIFY
	messages when registering. (See reference [6] for details on
	the "methods" parameter).
	4.3.2. CANCEL requests
	No semantics are associated with cancelling SUBSCRIBE or NOTIFY.
	4.3.3. Forking
	In accordance with the rules for proxying non-INVITE requests as
	defined in SIP [1], successful SUBSCRIBE requests will receive
	only one 200-class response; however, due to forking, the
	subscription may have been accepted by multiple nodes. The
	subscriber MUST therefore be prepared to receive NOTIFY requests
	with "From:" tags which differ from the "To:" tag received in the
	SUBSCRIBE 200-class response.
	If multiple NOTIFY messages are received in response to a single
	SUBSCRIBE message, they represent different destinations to which
	the SUBSCRIBE request was forked. For information on subscriber
	handling in such situations, see section 5.4.9.
	4.3.4. Dialog creation and termination
	If an initial SUBSCRIBE request is not sent on a pre-existing
	dialog, the subscriber will wait for a response to the SUBSCRIBE
	request or a matching NOTIFY.
	Responses are matched to such SUBSCRIBE requests if they contain
	the same the same "Call-ID", the same "From" header "tag", and
	the same "CSeq". Rules for the comparison of these headers are
	described in SIP [1]. If a 200-class response matches such a
	SUBSCRIBE request, it creates a new subscription and a new dialog
	(unless they have already been created by a matching NOTIFY
	request; see below).
	NOTIFY requests are matched to such SUBSCRIBE requests if they
	contain the same "Call-ID", a "To" header "tag" parameter which
	matches the "From" header "tag" parameter of the SUBSCRIBE, and
	the same "Event" header field. Rules for comparisons of the
	"Event" headers are described in section 7.5.1. If a matching
	NOTIFY request contains a "Subscription-State" of "active" or
	"pending", it creates a new subscription and a new dialog (unless
	they have already been created by a matching response, as
	described above).
	If an initial SUBSCRIBE is sent on a pre-existing dialog, a
	matching 200-class response or successful NOTIFY request merely
	creates a new subscription associated with that dialog.
	Multiple subscriptions can be associated with a single dialog.
	Subscriptions may also exist in dialogs associated with
	INVITE-created application state and other application state
	created by mechanisms defined in other specifications. These sets
	of application state do not interact beyond the behavior
	described for a dialog (e.g. route set handling).
	A subscription is destroyed when a notifier sends a NOTIFY
	request with a "Subscription-State" of "terminated".
	A subscriber may send a SUBSCRIBE request with an "Expires"
	header of 0 in order to trigger the sending of such a NOTIFY
	request; however, for the purposes of subscription and dialog
	lifetime, the subscription is not considered terminated until
	the NOTIFY with a "Subscription-State" of "terminated" is
	sent.
	If a subscription's destruction leaves no other application state
	associated with the dialog, the dialog terminates. The
	destruction of other application state (such as that created by
	an INVITE) will not terminate the dialog if a subscription is
	still associated with that dialog.
	Note that the above behavior means that a dialog created with
	an INVITE does not necessarily terminate upon receipt of a
	BYE. Similarly, in the case that several subscriptions are
	associated with a single dialog, the dialog does not
	terminate until all the subscriptions in it are destroyed.
	4.3.5. State Agents and Notifier Migration
	When state agents (see section 5.4.11.) are used, it is often
	useful to allow migration of subscriptions between state agents
	and the nodes for which they are providing state aggregation (or
	even among various state agents). Such migration may be effected
	by sending a NOTIFY message with a "Subscription-State" header of
	"terminated", and a reason parameter of "deactivated". This
	NOTIFY request is otherwise normal, and is formed as described in
	section 4.2.2.
	Upon receipt of this NOTIFY message, the subscriber SHOULD
	attempt to re-subscribe (as described in the preceding sections).
	Note that this subscription is established on a new dialog, and
	does not re-use the route set from the previous subscription
	dialog.
	The actual migration is effected by making a change to the policy
	(such as routing decisions) of one or more servers to which the
	SUBSCRIBE request will be sent in such a way that a different
	node ends up responding to the SUBSCRIBE request. This may be as
	simple as a change in the local policy in the notifier from which
	the subscription is migrating so that it serves as a proxy or
	redirect server instead of a notifier.
	Whether, when, and why to perform notifier migrations may be
	described in individual event packages; otherwise, such decisions
	are a matter of local notifier policy, and are left up to
	individual implementations.
	4.3.6. Polling Resource State
	A natural consequence of the behavior described in the preceding
	sections is that an immediate fetch without a persistent
	subscription may be effected by sending a SUBSCRIBE with an
	"Expires" of 0.
	Of course, an immediate fetch while a subscription is active may
	be effected by sending a SUBSCRIBE with an "Expires" equal to the
	number of seconds remaining in the subscription.
	Upon receipt of this SUBSCRIBE request, the notifier (or
	notifiers, if the SUBSCRIBE request was forked) will send a
	NOTIFY request containing resource state in the same dialog.
	Note that the NOTIFY messages triggered by SUBSCRIBE messages
	with "Expires" headers of 0 will contain a "Subscription-State"
	value of "terminated", and a "reason" parameter of "timeout".
	Polling of event state can cause significant increases in load on
	the network and notifiers; as such, it should be used only
	sparingly. In particular, polling SHOULD NOT be used in
	circumstances in which it will typically result in more network
	messages than long-running subscriptions.
	When polling is used, subscribers SHOULD attempt to cache
	authentication credentials between polls so as to reduce the
	number of messages sent.
	4.3.7. Allow-Events header usage
	The "Allow-Events" header, if present, includes a list of tokens
	which indicates the event packages supported by the client (if
	sent in a request) or server (if sent in a response). In other
	words, a node sending an "Allow-Events" header is advertising
	that it can process SUBSCRIBE requests and generate NOTIFY
	requests for all of the event packages listed in that header.
	Any node implementing one or more event packages SHOULD include
	an appropriate "Allow-Events" header indicating all supported
	events in all methods which initiate dialogs and their responses
	(such as INVITE) and OPTIONS responses.
	This information is very useful, for example, in allowing user
	agents to render particular interface elements appropriately
	according to whether the events required to implement the
	features they represent are supported by the appropriate nodes.
	Note that "Allow-Events" headers MUST NOT be inserted by proxies.
	4.3.8. PINT Compatibility
	The "Event" header is considered mandatory for the purposes of
	this document. However, to maintain compatibility with PINT (see
	[3]), servers MAY interpret a SUBSCRIBE request with no "Event"
	header as requesting a subscription to PINT events. If a server
	does not support PINT, it SHOULD return "489 Bad Event" to any
	SUBSCRIBE messages without an "Event" header.
	5. Event Packages
	This section covers several issues which should be taken into
	consideration when event packages based on SUBSCRIBE and NOTIFY
	are proposed.
	5.1. Appropriateness of Usage
	When designing an event package using the methods described in
	this draft for event notification, it is important to consider:
	is SIP an appropriate mechanism for the problem set? Is SIP being
	selected because of some unique feature provided by the protocol
	(e.g. user mobility), or merely because "it can be done?" If you
	find yourself defining event packages for notifications related
	to, for example, network management or the temperature inside
	your car's engine, you may want to reconsider your selection of
	protocols.
	Those interested in extending the mechanism defined in this
	document are urged to follow the development of "Guidelines
	for Authors of SIP Extensions"[2] for further guidance
	regarding appropriate uses of SIP.
	Further, it is expected that this mechanism is not to be used in
	applications where the frequency of reportable events is
	excessively rapid (e.g. more than about once per second). A SIP
	network is generally going to be provisioned for a reasonable
	signalling volume; sending a notification every time a user's GPS
	position changes by one hundreth of a second could easily
	overload such a network.
	5.2. Event Template-packages
	Normal event packages define a set of state applied to a specific
	type of resource, such as user presence, call state, and
	messaging mailbox state.
	Event template-packages are a special type of package which
	define a set of state applied to other packages, such as
	statistics, access policy, and subscriber lists. Event
	template-packages may even be applied to other event
	template-packages.
	To extend the object-oriented analogy made earlier, event
	template-packages can be thought of as templatized C++ packages
	which must be applied to other packages to be useful.
	The name of an event template-package as applied to a package is
	formed by appending a period followed by the event
	template-package name to the end of the package. For example, if
	a template-package called "winfo" were being applied to a package
	called "presence", the event token used in "Event" and
	"Allow-Events" would be "presence.winfo".
	Event template-packages must be defined so that they can be
	applied to any arbitrary package. In other words, event
	template-packages cannot be specifically tied to one or a few
	"parent" packages in such a way that they will not work with
	other packages.
	5.3. Amount of State to be Conveyed
	When designing event packages, it is important to consider the
	type of information which will be conveyed during a notification.
	A natural temptation is to convey merely the event (e.g. "a new
	voice message just arrived") without accompanying state (e.g. "7
	total voice messages"). This complicates implementation of
	subscribing entities (since they have to maintain complete state
	for the entity to which they have subscribed), and also is
	particularly susceptible to synchronization problems.
	There are two possible solutions to this problem that event
	packages may choose to implement.
	5.3.1. Complete State Information
	For packages which typically convey state information that is
	reasonably small (on the order of 1 kb or so), it is suggested
	that event packages are designed so as to send complete state
	information when an event occurs.
	In some circumstances, conveying the current state alone may be
	insufficient for a particular class of events. In these cases,
	the event packages should include complete state information
	along with the event that occurred. For example, conveying "no
	customer service representatives available" may not be as useful
	as conveying "no customer service representatives available;
	representative sip:46@cs.xyz.int just logged off".
	5.3.2. State Deltas
	In the case that the state information to be conveyed is large,
	the event package may choose to detail a scheme by which NOTIFY
	messages contain state deltas instead of complete state.
	Such a scheme would work as follows: any NOTIFY sent in immediate
	response to a SUBSCRIBE contains full state information. NOTIFY
	messages sent because of a state change will contain only the
	state information that has changed; the subscriber will then
	merge this information into its current knowledge about the state
	of the resource.
	Any event package that supports delta changes to states MUST
	include a version number that increases by exactly one for each
	NOTIFY message in a subscription. Note that the state version
	number appears in the body of the message, not in a SIP header.
	If a NOTIFY arrives that has a version number that is incremented
	by more than one, the subscriber knows that a state delta has
	been missed; it ignores the NOTIFY message containing the state
	delta (except for the version number, which it retains to detect
	message loss), and re-sends a SUBSCRIBE to force a NOTIFY
	containing a complete state snapshot.
	5.4. Event Package Responsibilities
	Event packages are not required to re-iterate any of the behavior
	described in this document, although they may choose to do so for
	clarity or emphasis. In general, though, such packages are
	expected to describe only the behavior that extends or modifies
	the behavior described in this document.
	Note that any behavior designated with "SHOULD" or "MUST" in this
	document is not allowed to be weakened by extension documents;
	however, such documents may elect to strengthen "SHOULD"
	requirements to "MUST" strength if required by their application.
	In addition to the normal sections expected in
	standards-track RFCs and SIP extension documents, authors of
	event packages need to address each of the issues detailed in
	the following subsections, whenever applicable.
	5.4.1. Event Package Name
	This section, which MUST be present, defines the token name to be
	used to designate the event package. It MUST include the
	information which appears in the IANA registration of the token.
	For information on registering such types, see section 7.
	5.4.2. Event Package Parameters
	If parameters are to be used on the "Event" header to modify the
	behavior of the event package, the syntax and semantics of such
	headers MUST be clearly defined.
	5.4.3. SUBSCRIBE Bodies
	It is expected that most, but not all, event packages will define
	syntax and semantics for SUBSCRIBE method bodies; these bodies
	will typically modify, expand, filter, throttle, and/or set
	thresholds for the class of events being requested. Designers of
	event packages are strongly encouraged to re-use existing MIME
	types for message bodies where practical.
	This mandatory section of an event package defines what type or
	types of event bodies are expected in SUBSCRIBE requests (or
	specify that no event bodies are expected). It should point to
	detailed definitions of syntax and semantics for all referenced
	body types.
	5.4.4. Subscription Duration
	It is RECOMMENDED that event packages give a suggested range of
	times considered reasonable for the duration of a subscription.
	Such packages MUST also define a default "Expires" value to be
	used if none is specified.
	5.4.5. NOTIFY Bodies
	The NOTIFY body is used to report state on the resource being
	monitored. Each package MUST define what type or types of event
	bodies are expected in NOTIFY requests. Such packages MUST
	specify or cite detailed specifications for the syntax and
	semantics associated with such event body.
	Event packages also MUST define which MIME type is to be assumed
	if none are specified in the "Accept" header of the SUBSCRIBE
	request.
	5.4.6. Notifier processing of SUBSCRIBE requests
	This section describes the processing to be performed by the
	notifier upon receipt of a SUBSCRIBE request. Such a section is
	required.
	Information in this section includes details of how to
	authenticate subscribers and authorization issues for the
	package. Such authorization issues may include, for example,
	whether all SUBSCRIBE requests for this package are answered with
	202 responses (see section 6.2.).
	5.4.7. Notifier generation of NOTIFY requests
	This section of an event package describes the process by which
	the notifier generates and sends a NOTIFY request. This includes
	detailed information about what events cause a NOTIFY to be sent,
	how to compute the state information in the NOTIFY, how to
	generate neutral or fake state information to hide authorization
	delays and decisions from users, and whether state information is
	complete or deltas for notifications; see section 5.3. Such a
	section is required.
	This section may optionally describe the behavior used to process
	the subsequent response.
	5.4.8. Subscriber processing of NOTIFY requests
	This section of an event package describes the process followed
	by the subscriber upon receipt of a NOTIFY request, including any
	logic required to form a coherent resource state (if applicable).
	5.4.9. Handling of forked requests
	Each event package MUST specify whether forked SUBSCRIBE requests
	are allowed to install multiple subscriptions.
	If such behavior is not allowed, the first potential
	dialog-establishing message will create a dialog. All subsequent
	NOTIFY messages which correspond to the SUBSCRIBE message (i.e.
	match "To", "From", "From" header "tag" parameter, "Call-ID",
	"CSeq", "Event", and "Event" header "id" parameter) but which do
	not match the dialog would be rejected with a 481 response. Note
	that the 200-class response to the SUBSCRIBE can arrive after a
	matching NOTIFY has been received; such responses might not
	correlate to the same dialog established by the NOTIFY. Except as
	required to complete the SUBSCRIBE transaction, such non-matching
	200-class responses are ignored.
	If installing of multiple subscriptions by way of a single forked
	SUBSCRIBE is allowed, the subscriber establishes a new dialog
	towards each notifier by returning a 200-class response to each
	NOTIFY. Each dialog is then handled as its own entity, and is
	refreshed independent of the other dialogs.
	In the case that multiple subscriptions are allowed, the event
	package MUST specify whether merging of the notifications to form
	a single state is required, and how such merging is to be
	performed. Note that it is possible that some event packages may
	be defined in such a way that each dialog is tied to a mutually
	exclusive state which is unaffected by the other dialogs; this
	MUST be clearly stated if it is the case.
	5.4.10. Rate of notifications
	Each event package is expected to define a requirement (SHOULD or
	MUST strength) which defines an absolute maximum on the rate at
	which notifications are allowed to be generated by a single
	notifier.
	Each package MAY further define a throttle mechanism which allows
	subscribers to further limit the rate of notification.
	5.4.11. State Agents
	Designers of event packages should consider whether their package
	can benefit from network aggregation points (state agents) and/or
	nodes which act on behalf of other nodes. (For example, nodes
	which provide state information about a resource when such a
	resource is unable or unwilling to provide such state information
	itself). An example of such an application is a node which tracks
	the presence and availability of a user in the network.
	If state agents are to be used by the package, the package MUST
	specify how such state agents aggregate information and how they
	provide authentication and authorization.
	Event packages MAY also outline specific scenarios under which
	notifier migrations take place.
	5.4.12. Examples
	Event packages SHOULD include several demonstrative message flow
	diagrams paired with several typical, syntactically correct and
	complete messages.
	It is RECOMMENDED that documents describing event packages
	clearly indicate that such examples are informative and not
	normative, with instructions that implementors refer to the main
	text of the draft for exact protocol details.
	5.4.13. Use of URIs to Retrieve State
	Some types of event packages may define state information which
	is potentially too large to reasonably send in a SIP message. To
	alleviate this problem, event packages may include the ability to
	convey a URI instead of state information; this URI will then be
	used to retrieve the actual state information.
	The precise mechanisms for conveying such URIs are out of the
	scope of this document.
	6. Security Considerations
	6.1. Access Control
	The ability to accept subscriptions should be under the direct
	control of the notifier's user, since many types of events may be
	considered sensitive for the purposes of privacy. Similarly, the
	notifier should have the ability to selectively reject
	subscriptions based on the subscriber identity (based on access
	control lists), using standard SIP authentication mechanisms. The
	methods for creation and distribution of such access control
	lists is outside the scope of this draft.
	6.2. Notifier Privacy Mechanism
	The mere act of returning a 200 or certain 4xx and 6xx responses
	to SUBSCRIBE requests may, under certain circumstances, create
	privacy concerns by revealing sensitive policy information. In
	these cases, the notifier SHOULD always return a 202 response.
	While the subsequent NOTIFY message may not convey true state, it
	MUST appear to contain a potentially correct piece of data from
	the point of view of the subscriber, indistinguishable from a
	valid response. Information about whether a user is authorized to
	subscribe to the requested state is never conveyed back to the
	original user under these circumstances.
	Individual packages and their related drafts for which such a
	mode of operation makes sense can further describe how and why to
	generate such potentially correct data. For example, such a mode
	of operation is mandated by RFC 2779 [8] for user presence
	information.
	6.3. Denial-of-Service attacks
	The current model (one SUBSCRIBE request triggers a SUBSCRIBE
	response and one or more NOTIFY requests) is a classic setup for
	an amplifier node to be used in a smurf attack.
	Also, the creation of state upon receipt of a SUBSCRIBE request
	can be used by attackers to consume resources on a victim's
	machine, rendering it unusable.
	To reduce the chances of such an attack, implementations of
	notifiers SHOULD require authentication. Authentication issues
	are discussed in SIP [1].
	6.4. Replay Attacks
	Replaying of either SUBSCRIBE or NOTIFY can have detrimental
	effects.
	In the case of SUBSCRIBE messages, attackers may be able to
	install any arbitrary subscription which it witnessed being
	installed at some point in the past. Replaying of NOTIFY messages
	may be used to spoof old state information (although a good
	versioning mechanism in the body of the NOTIFY messages may help
	mitigate such an attack). Note that the prohibition on sending
	NOTIFY messages to nodes which have not subscribed to an event
	also aids in mitigating the effects of such an attack.
	To prevent such attacks, implementations SHOULD require
	authentication with anti-replay protection. Authentication issues
	are discussed in SIP [1].
	6.5. Man-in-the middle attacks
	Even with authentication, man-in-the-middle attacks using
	SUBSCRIBE may be used to install arbitrary subscriptions, hijack
	existing subscriptions, terminate outstanding subscriptions, or
	modify the resource to which a subscription is being made. To
	prevent such attacks, implementations SHOULD provide integrity
	protection across "Contact", "Route", "Expires", "Event", and
	"To" headers of SUBSCRIBE messages, at a minimum. If SUBSCRIBE
	bodies are used to define further information about the state of
	the call, they SHOULD be included in the integrity protection
	scheme.
	Man-in-the-middle attacks may also attempt to use NOTIFY messages
	to spoof arbitrary state information and/or terminate outstanding
	subscriptions. To prevent such attacks, implementations SHOULD
	provide integrity protection across the "Call-ID", "CSeq", and
	"Subscription-State" headers and the bodies of NOTIFY messages.
	Integrity protection of message headers and bodies is discussed
	in SIP [1].
	6.6. Confidentiality
	The state information contained in a NOTIFY message has the
	potential to contain sensitive information. Implementations MAY
	encrypt such information to ensure confidentiality.
	While less likely, it is also possible that the information
	contained in a SUBSCRIBE message contains information that users
	might not want to have revealed. Implementations MAY encrypt such
	information to ensure confidentiality.
	To allow the remote party to hide information it considers
	sensitive, all implementations SHOULD be able to handle encrypted
	SUBSCRIBE and NOTIFY messages.
	The mechanisms for providing confidentiality are detailed in SIP
	[1].
	7. IANA Considerations
	(This section is not applicable until this document is published
	as an RFC.)
	This document defines an event-type namespace which requires a
	central coordinating body. The body chosen for this coordination
	is the Internet Assigned Numbers Authority (IANA).
	There are two different types of event-types: normal event
	packages, and event template-packages; see section 5.2. To avoid
	confusion, template-package names and package names share the
	same namespace; in other words, an event template-package MUST
	NOT share a name with a package.
	Following the policies outlined in "Guidelines for Writing an
	IANA Considerations Section in RFCs"[5], normal event package
	identification tokens are allocated as First Come First Served,
	and event template-package identification tokens are allocated on
	a IETF Consensus basis.
	Registrations with the IANA MUST include the token being
	registered and whether the token is a package or a
	template-package. Further, packages MUST include contact
	information for the party responsible for the registration and/or
	a published document which describes the event package. Event
	template-package token registrations MUST include a pointer to
	the published RFC which defines the event template-package.
	Registered tokens to designate packages and template-packages
	MUST NOT contain the character ".", which is used to separate
	template-packages from packages.
	7.1. Registration Information
	As this document specifies no package or template-package names,
	the initial IANA registration for event types will be empty. The
	remainder of the text in this section gives an example of the
	type of information to be maintained by the IANA; it also
	demonstrates all five possible permutations of package type,
	contact, and reference.
	The table below lists the event packages and template-packages
	defined in "SIP-Specific Event Notification" [RFC xxxx]. Each
	name is designated as a package or a template-package under
	"Type".
	Package Name Type Contact Reference
	------------ ---- ------- ---------
	example1 package [Roach]
	example2 package [Roach] [RFC xxxx]
	example3 package [RFC xxxx]
	example4 template [Roach] [RFC xxxx]
	example5 template [RFC xxxx]
	PEOPLE
	------
	[Roach] Adam Roach <adam@dynamicsoft.com>
	REFERENCES
	----------
	[RFC xxxx] A. Roach "SIP-Specific Event Notification", RFC XXXX,
	August 2002.
	7.2. Registration Template
	To: ietf-sip-events@iana.org
	Subject: Registration of new SIP event package
	Package Name:
	(Package names must conform to the syntax described in
	section 7.5.1.)
	Is this registration for a Template Package:
	(indicate yes or no)
	Published Specification(s):
	(Template packages require a published RFC. Other packages
	may reference a specification when appropriate).
	Person & email address to contact for further information:
	7.3. Syntax
	This section describes the syntax extensions required for event
	notification in SIP. Semantics are described in section 4. Note
	that the formal syntax definitions described in this document are
	expressed in the ABNF format used in SIP [1], and contain
	references to elements defined therein.
	7.4. New Methods
	This document describes two new SIP methods: SUBSCRIBE and
	NOTIFY.
	This table expands on tables 2 and 3 in SIP [1].
	Header Where SUB NOT
	------ ----- --- ---
	Accept R o o
	Accept 2xx - -
	Accept 415 o o
	Accept-Encoding R o o
	Accept-Encoding 2xx - -
	Accept-Encoding 415 o o
	Accept-Language R o o
	Accept-Language 2xx - -
	Accept-Language 415 o o
	Alert-Info R - -
	Alert-Info 180 - -
	Allow R o o
	Allow 2xx o o
	Allow r o o
	Allow 405 m m
	Authentication-Info 2xx o o
	Authorization R o o
	Call-ID c m m
	Contact R m m
	Contact 1xx o o
	Contact 2xx m o
	Contact 3xx m m
	Contact 485 o o
	Content-Disposition o o
	Content-Encoding o o
	Content-Language o o
	Content-Length t t
	Content-Type * *
	CSeq c m m
	Date o o
	Error-Info 300-699 o o
	Expires o -
	Expires 2xx m -
	From c m m
	In-Reply-To R - -
	Max-Forwards R m m
	Min-Expires 423 m -
	MIME-Version o o
	Organization o -
	Priority R o -
	Proxy-Authenticate 407 m m
	Proxy-Authorization R o o
	Proxy-Require R o o
	RAck R - -
	Record-Route R o o
	Record-Route 2xx,401,484 o o
	Reply-To - -
	Require o o
	Retry-After 404,413,480,486 o o
	Retry-After 500,503 o o
	Retry-After 600,603 o o
	Route R c c
	RSeq 1xx o o
	Server r o o
	Subject R - -
	Supported R o o
	Supported 2xx o o
	Timestamp o o
	To c(1) m m
	Unsupported 420 o o
	User-Agent o o
	Via c m m
	Warning R - o
	Warning r o o
	WWW-Authenticate 401 m m
	7.4.1. SUBSCRIBE method
	"SUBSCRIBE" is added to the definition of the element "Method" in
	the SIP message grammar.
	Like all SIP method names, the SUBSCRIBE method name is case
	sensitive. The SUBSCRIBE method is used to request asynchronous
	notification of an event or set of events at a later time.
	7.4.2. NOTIFY method
	"NOTIFY" is added to the definition of the element "Method" in
	the SIP message grammar.
	The NOTIFY method is used to notify a SIP node that an event
	which has been requested by an earlier SUBSCRIBE method has
	occurred. It may also provide further details about the event.
	7.5. New Headers
	This table expands on tables 2 and 3 in SIP [1], as amended by
	the changes described in section 7.4.
	Header field where proxy ACK BYE CAN INV OPT REG PRA SUB NOT
	-----------------------------------------------------------------
	Allow-Events R o o - o o o o o o
	Allow-Events 2xx - o - o o o o o o
	Allow-Events 489 - - - - - - - m m
	Event R - - - - - - - m m
	Subscription-State R - - - - - - - - m
	7.5.1. "Event" header
	Event is added to the definition of the element "message-header"
	in the SIP message grammar.
	For the purposes of matching responses and NOTIFY messages with
	SUBSCRIBE messages, the event-type portion of the "Event" header
	is compared byte-by-byte, and the "id" parameter token (if
	present) is compared byte-by-byte. An "Event" header containing
	an "id" parameter never matches an "Event" header without an "id"
	parameter. No other parameters are considered when performing a
	comparison.
	Note that the forgoing text means that "Event: foo; id=1234"
	would match "Event: foo; param=abcd; id=1234", but not
	"Event: foo" (id does not match) or "Event: Foo; id=1234"
	(event portion does not match).
	This document does not define values for event-types. These
	values will be defined by individual event packages, and MUST be
	registered with the IANA.
	There MUST be exactly one event type listed per event header.
	Multiple events per message are disallowed.
	7.5.2. "Allow-Events" Header
	Allow-Events is added to the definition of the element
	"general-header" in the SIP message grammar. Its usage is
	describe in section 4.3.7.
	7.5.3. "Subscription-State" Header
	Subscription-State is added to the definition of the element
	"request-header" in the SIP message grammar. Its usage is
	described in section 4.2.4.
	7.6. New Response Codes
	7.6.1. "202 Accepted" Response Code
	The 202 response is added to the "Success" header field
	definition. "202 Accepted" has the same meaning as that defined
	in HTTP/1.1 [4].
	7.6.2. "489 Bad Event" Response Code
	The 489 event response is added to the "Client-Error" header
	field definition. "489 Bad Event" is used to indicate that the
	server did not understand the event package specified in a
	"Event" header field.
	7.7. Augmented BNF Definitions
	The Augmented BNF definitions for the various new and modified
	syntax elements follows. The notation is as used in SIP [1], and
	any elements not defined in this section are as defined in SIP
	and the documents to which it refers.
	SUBSCRIBEm = %x53.55.42.53.43.52.49.42.45 ; SUBSCRIBE in caps
	NOTIFYm = %x4E.4F.54.49.46.59 ; NOTIFY in caps
	extension-method = SUBSCRIBEm / NOTIFYm / token
	Event = ( "Event" / "o" ) HCOLON event-type
	*( SEMI event-param )
	event-type = event-package *( "." event-template )
	event-package = token-nodot
	event-template = token-nodot
	token-nodot = 1*( alphanum / "-" / "!" / "%" / "*"
	/ "_" / "+" / "`" / "'" / "~" )
	event-param = generic-param / ( "id" EQUAL token )
	Allow-Events = ( "Allow-Events" / "u" ) HCOLON event-type
	*(COMMA event-type)
	Subscription-State = "Subscription-State" HCOLON substate-value
	*( SEMI subexp-params )
	substate-value = "active" / "pending" / "terminated"
	/ extension-substate
	extension-substate = token
	subexp-params = ("reason" EQUAL reason-value)
	/ ("expires" EQUAL delta-seconds)
	/ ("retry-after" EQUAL delta-seconds)
	/ generic-param
	reason-value = "deactivated"
	/ "probation"
	/ "rejected"
	/ "timeout"
	/ "giveup"
	/ "noresource"
	/ reason-extension
	reason-extension = token
	8. References
	NOTE: Non-normative references are so labeled.
	[1] J. Rosenberg et. al., "SIP: Session Initiation Protocol",
	<draft-ietf-sip-rfc2543bis-07>, IETF; February 2002. Work in
	progress.
	[2] J. Rosenberg, H. Schulzrinne, "Guidelines for Authors of SIP
	Extensions", <draft-ietf-sip-guidelines-03.txt>, IETF;
	November 2001. Work in progress. Non-normative.
	[3] S. Petrack, L. Conroy, "The PINT Service Protocol", RFC 2848,
	IETF; June 2000.
	[4] R. Fielding et. al., "Hypertext Transfer Protocol --
	HTTP/1.1", RFC 2616, IETF, June 1999.
	[5] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA		RFC 3265
	Considerations Section in RFCs", BCP 26, IETF, October 1998.
	[6] Schulzrinne/Rosenberg, "SIP Caller Preferences and Callee		Title: Session Initiation Protocol (SIP)-Specific Event
	Capabilities", <draft-ietf-sip-callerprefs-05.txt>, IETF;		Notification
	November 2001. Work in progress. Non-normative.		Author(s): A. B. Roach
			Status: Standards Track
			Date: June 2002
			Mailbox: adam@dynamicsoft.com
			Pages: 38
			Characters: 89005
			Obsoletes: 2543
	[7] S. Bradner, "Key words for use in RFCs to indicate		I-D Tag: draft-ietf-sip-events-05.txt
	requirement levels", RFC 2119, IETF, March 1997
	[8] M. Day et. al., "Instant Messaging/Presence Protocol		URL: ftp://ftp.rfc-editor.org/in-notes/rfc3265.txt
	Requirements", RFC 2779, IETF, February 2000
	9. Acknowledgements		This document describes an extension to the Session Initiation
			Protocol (SIP). The purpose of this extension is to provide an
			extensible framework by which SIP nodes can request notification
			from remote nodes indicating that certain events have occurred.
	Thanks to the participants in the Events BOF at the 48th IETF		Concrete uses of the mechanism described in this document may be
	meeting in Pittsburgh, as well as those who gave ideas and		standardized in the future.
	suggestions on the SIP Events mailing list. In particular, I wish
	to thank Henning Schulzrinne of Columbia University for coming up
	with the final three-tiered event identification scheme, Sean
	Olson for miscellaneous guidance, Jonathan Rosenberg for a
	thorough scrubbing of the -00 draft, and the authors of the "SIP
	Extensions for Presence" draft for their input to SUBSCRIBE and
	NOTIFY request semantics.
	10. Author's Address		Note that the event notification mechanisms defined herein are
			NOT intended to be a general-purpose infrastructure for all
			classes of event subscription and notification.
	Adam Roach		This document is a product of the Session Initiation Protocol Working
	dynamicsoft		Group of the IETF.
	5100 Tennyson Parkway
	Suite 1200
	Plano, TX 75024
	USA
	E-Mail: <adam@dynamicsoft.com>
	Voice: <sip:adam@dynamicsoft.com>
	11. Notice Regarding Intellectual Property Rights		This is now a Proposed Standard Protocol.
	The IETF has been notified of intellectual property rights		This document specifies an Internet standards track protocol for
	claimed in regard to some or all of the specification contained		the Internet community, and requests discussion and suggestions
	in this document. For more information, consult the online list		for improvements. Please refer to the current edition of the
	of claimed rights at http://www.ietf.org/ipr.html		"Internet Official Protocol Standards" (STD 1) for the
			standardization state and status of this protocol. Distribution
			of this memo is unlimited.
	12. Full Copyright Statement		This announcement is sent to the IETF list and the RFC-DIST list.
			Requests to be added to or deleted from the IETF distribution list
			should be sent to IETF-REQUEST@IETF.ORG. Requests to be
			added to or deleted from the RFC-DIST distribution list should
			be sent to RFC-DIST-REQUEST@RFC-EDITOR.ORG.
	Copyright (c) The Internet Society (2002). All Rights Reserved.		Details on obtaining RFCs via FTP or EMAIL may be obtained by sending
			an EMAIL message to rfc-info@RFC-EDITOR.ORG with the message body
			help: ways_to_get_rfcs. For example:
	This document and translations of it may be copied and furnished		To: rfc-info@RFC-EDITOR.ORG
	to others, and derivative works that comment on or otherwise		Subject: getting rfcs
	explain it or assist in its implementation may be prepared,
	copied, published and distributed, in whole or in part, without
	restriction of any kind, provided that the above copyright notice
	and this paragraph are included on all such copies and derivative
	works. However, this document itself may not be modified in any
	way, such as by removing the copyright notice or references to
	the Internet Society or other Internet organizations, except as
	needed for the purpose of developing Internet standards in which
	case the procedures for copyrights defined in the Internet
	Standards process must be followed, or as required to translate
	it into languages other than English.
	The limited permissions granted above are perpetual and will not		help: ways_to_get_rfcs
	be revoked by the Internet Society or its successors or assigns.
	This document and the information contained herein is provided on		Requests for special distribution should be addressed to either the
	an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET		author of the RFC in question, or to RFC-Manager@RFC-EDITOR.ORG. Unless
	ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR		specifically noted otherwise on the RFC itself, all RFCs are for
	IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE		unlimited distribution.echo
	OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY		Submissions for Requests for Comments should be sent to
	IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR		RFC-EDITOR@RFC-EDITOR.ORG. Please consult RFC 2223, Instructions to RFC
	PURPOSE.		Authors, for further information.
End of changes. 16 change blocks.
	1772 lines changed or deleted		42 lines changed or added
This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/