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rfc:rfc3265

Network Working Group A. B. Roach Request for Comments: 3265 dynamicsoft Updates: 2543 June 2002 Category: Standards Track

   Session Initiation Protocol (SIP)-Specific Event Notification

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2002).  All Rights Reserved.

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.

Table of Contents

 1.       Introduction...........................................  3
 1.1.     Overview of Operation..................................  4
 1.2.     Documentation Conventions..............................  4
 2.       Definitions............................................  5
 3.       Node Behavior..........................................  6
 3.1.     Description of SUBSCRIBE Behavior......................  6
 3.1.1.   Subscription Duration..................................  6
 3.1.2.   Identification of Subscribed Events and Event Classes..  6
 3.1.3.   Additional SUBSCRIBE Header Values.....................  7
 3.1.4.   Subscriber SUBSCRIBE Behavior..........................  7
 3.1.5.   Proxy SUBSCRIBE Behavior...............................  9
 3.1.6.   Notifier SUBSCRIBE Behavior............................ 10

Roach Standards Track [Page 1] RFC 3265 SIP-Specific Event Notification June 2002

 3.2.     Description of NOTIFY Behavior......................... 13
 3.2.1.   Identification of Reported Events, Event Classes, and
          Current State.......................................... 13
 3.2.2.   Notifier NOTIFY Behavior............................... 14
 3.2.3.   Proxy NOTIFY Behavior.................................. 15
 3.2.4.   Subscriber NOTIFY Behavior............................. 16
 3.3.     General................................................ 18
 3.3.1.   Detecting support for SUBSCRIBE and NOTIFY............. 18
 3.3.2.   CANCEL requests........................................ 18
 3.3.3.   Forking................................................ 18
 3.3.4.   Dialog creation and termination........................ 18
 3.3.5.   State Agents and Notifier Migration.................... 19
 3.3.6.   Polling Resource State................................. 20
 3.3.7.   Allow-Events header usage.............................. 21
 3.3.8.   PINT Compatibility..................................... 21
 4.       Event Packages......................................... 21
 4.1.     Appropriateness of Usage............................... 21
 4.2.     Event Template-packages................................ 22
 4.3.     Amount of State to be Conveyed......................... 22
 4.3.1.   Complete State Information............................. 23
 4.3.2.   State Deltas........................................... 23
 4.4.     Event Package Responsibilities......................... 24
 4.4.1.   Event Package Name..................................... 24
 4.4.2.   Event Package Parameters............................... 24
 4.4.3.   SUBSCRIBE Bodies....................................... 24
 4.4.4.   Subscription Duration.................................. 25
 4.4.5.   NOTIFY Bodies.......................................... 25
 4.4.6.   Notifier processing of SUBSCRIBE requests.............. 25
 4.4.7.   Notifier generation of NOTIFY requests................. 25
 4.4.8.   Subscriber processing of NOTIFY requests............... 26
 4.4.9.   Handling of forked requests............................ 26
 4.4.10.  Rate of notifications.................................. 26
 4.4.11.  State Agents........................................... 27
 4.4.12.  Examples............................................... 27
 4.4.13.  Use of URIs to Retrieve State.......................... 27
 5.       Security Considerations................................ 28
 5.1.     Access Control......................................... 28
 5.2.     Notifier Privacy Mechanism............................. 28
 5.3.     Denial-of-Service attacks.............................. 28
 5.4.     Replay Attacks......................................... 29
 5.5.     Man-in-the middle attacks.............................. 29
 5.6.     Confidentiality........................................ 29
 6.       IANA Considerations.................................... 30
 6.1.     Registration Information............................... 30
 6.2.     Registration Template.................................. 31
 6.3.     Header Field Names..................................... 31
 6.4.     Response Codes......................................... 32
 7.       Syntax................................................. 32

Roach Standards Track [Page 2] RFC 3265 SIP-Specific Event Notification June 2002

 7.1.     New Methods............................................ 32
 7.1.1.   SUBSCRIBE method....................................... 34
 7.1.2.   NOTIFY method.......................................... 34
 7.2.     New Headers............................................ 34
 7.2.1.   "Event" header......................................... 34
 7.2.2.   "Allow-Events" Header.................................. 35
 7.2.3.   "Subscription-State" Header............................ 35
 7.3.     New Response Codes..................................... 35
 7.3.1.   "202 Accepted" Response Code........................... 35
 7.3.2.   "489 Bad Event" Response Code.......................... 35
 7.4.     Augmented BNF Definitions.............................. 35
 8.       Normative References................................... 36
 9.       Informative References................................. 37
 10.      Acknowledgements....................................... 37
 11.      Notice Regarding Intellectual Property Rights.......... 37
 12.      Author's Address....................................... 37
 13.      Full Copyright Statement............................... 38

1. 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) [2] 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 document does not describe an extension which may be used
 directly; it must be extended by other documents (herein referred to
 as "event packages").  In object-oriented design terminology, it may

Roach Standards Track [Page 3] RFC 3265 SIP-Specific Event Notification June 2002

 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 4.

1.1. 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.

1.2. Documentation Conventions

 There are several paragraphs throughout this 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 reference sections; see sections 8 and 9.
 The all-capital terms "MUST", "SHOULD", "MAY", "SHOULD NOT", "MUST
 NOT", and "RECOMMENDED" are used as defined in RFC 2119 [5].
 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.

Roach Standards Track [Page 4] RFC 3265 SIP-Specific Event Notification June 2002

2. 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 states 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.

Roach Standards Track [Page 5] RFC 3265 SIP-Specific Event Notification June 2002

3. Node Behavior

3.1. Description of SUBSCRIBE Behavior

 The SUBSCRIBE method is used to request current state and state
 updates from a remote node.

3.1.1. 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
    3.3.6.
 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 3.2.2.

3.1.2. 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.

Roach Standards Track [Page 6] RFC 3265 SIP-Specific Event Notification June 2002

 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.

3.1.3. 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].

3.1.4. Subscriber SUBSCRIBE Behavior

3.1.4.1. 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

Roach Standards Track [Page 7] RFC 3265 SIP-Specific Event Notification June 2002

 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.

3.1.4.2. 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
 3.1.4.1.)

Roach Standards Track [Page 8] RFC 3265 SIP-Specific Event Notification June 2002

 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.

3.1.4.3. 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.

3.1.4.4. 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.  Documents which define new event packages MUST define
 this "neutral state" in such a way that makes sense for their
 application (see section 4.4.7.).
 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 3.2.4.

3.1.5. 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].

Roach Standards Track [Page 9] RFC 3265 SIP-Specific Event Notification June 2002

3.1.6. Notifier SUBSCRIBE Behavior

3.1.6.1. 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 3.1.6.3.
 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
 5.2.).
 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.

Roach Standards Track [Page 10] RFC 3265 SIP-Specific Event Notification June 2002

 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.

3.1.6.2. 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 3.2.2. 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.

3.1.6.3. 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.

Roach Standards Track [Page 11] RFC 3265 SIP-Specific Event Notification June 2002

    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 5.2.
 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".

3.1.6.4. Refreshing of Subscriptions

 When a notifier receives a subscription refresh, assuming that the
 subscriber is still authorized, the notifier updates the expiration
 time for the 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.

Roach Standards Track [Page 12] RFC 3265 SIP-Specific Event Notification June 2002

3.2. 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 3.2.4.  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.

3.2.1. 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
 3.1.2.).
 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 (see section 4.4.13).

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3.2.2. 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).

Roach Standards Track [Page 14] RFC 3265 SIP-Specific Event Notification June 2002

    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 5.2.).  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 3.2.4.

3.2.3. 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.

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    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].

3.2.4. 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 3.3.4.  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 7.2.1.)
 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-

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 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 subscriber
    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.

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3.3. General

3.3.1. 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 [8] for details on the "methods"
    parameter).

3.3.2. CANCEL requests

 No semantics are associated with cancelling SUBSCRIBE or NOTIFY.

3.3.3. 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 different dialogs in
 response to a single SUBSCRIBE message, each dialog represents a
 different destination to which the SUBSCRIBE request was forked.  For
 information on subscriber handling in such situations, see section
 4.4.9.

3.3.4. 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).

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 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 7.2.1.  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.

3.3.5. State Agents and Notifier Migration

 When state agents (see section 4.4.11.) 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

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 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 3.2.2.
 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.

3.3.6. 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.

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 When polling is used, subscribers SHOULD attempt to cache
 authentication credentials between polls so as to reduce the number
 of messages sent.

3.3.7. 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.

3.3.8. PINT Compatibility

 The "Event" header is considered mandatory for the purposes of this
 document.  However, to maintain compatibility with PINT (see [2]),
 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.

4. Event Packages

 This section covers several issues which should be taken into
 consideration when event packages based on SUBSCRIBE and NOTIFY are
 proposed.

4.1. Appropriateness of Usage

 When designing an event package using the methods described in this
 document 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

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 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" [7] 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 hundredth of a second could easily overload such a network.

4.2. 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.

4.3. 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.

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 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.

4.3.1. 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".

4.3.2. 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
 transaction 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

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 (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.

4.4. Event Package Responsibilities

  Event packages are not required to reiterate 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.

4.4.1. 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 6.

4.4.2. 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.

4.4.3. 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.

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 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.

4.4.4. 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.

4.4.5. 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.

4.4.6. 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 5.2.).

4.4.7. 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 4.3.  Such a section is
 required.
 This section may optionally describe the behavior used to process the
 subsequent response.

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4.4.8. 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).

4.4.9. 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.

4.4.10. 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.

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4.4.11. 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.

4.4.12. 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 document
 for exact protocol details.

4.4.13. 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.

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5. Security Considerations

5.1. 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 document.

5.2. 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 documents 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 [6] for user presence information.

5.3. 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].

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5.4. 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].

5.5. 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].

5.6. 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.

Roach Standards Track [Page 29] RFC 3265 SIP-Specific Event Notification June 2002

 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].

6. IANA Considerations

 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 4.2.  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" [4], 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.

6.1. 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" [RFC3265].  Each name is
 designated as a package or a template-package under "Type".

Roach Standards Track [Page 30] RFC 3265 SIP-Specific Event Notification June 2002

 Package Name      Type         Contact      Reference
 ------------      ----         -------      ---------
 example1          package      [Roach]
 example2          package      [Roach]      [RFC3265]
 example3          package                   [RFC3265]
 example4          template     [Roach]      [RFC3265]
 example5          template                  [RFC3265]
 PEOPLE
 ------
 [Roach] Adam Roach <adam@dynamicsoft.com>
 REFERENCES
 ----------
 [RFC3265] Roach, A., "SIP-Specific Event Notification", RFC 3265,
           June 2002.

6.2. 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 7.2.1.)
 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:

6.3. Header Field Names

 This document registers three new header field names, described
 elsewhere in this document. These headers are defined by the
 following information, which is to be added to the header sub-
 registry under http://www.iana.org/assignments/sip-parameters.
 Header Name:   Allow-Events
 Compact Form:  u

Roach Standards Track [Page 31] RFC 3265 SIP-Specific Event Notification June 2002

 Header Name:   Subscription-State
 Compact Form:  (none)
 Header Name:   Event
 Compact Form:  o

6.4. Response Codes

 This document registers two new response codes. These response codes
 are defined by the following information, which is to be added to the
 method and response-code sub-registry under
 http://www.iana.org/assignments/sip-parameters.
 Response Code Number:   202
 Default Reason Phrase:  Accepted
 Response Code Number:   489
 Default Reason Phrase:  Bad Event

7. Syntax

 This section describes the syntax extensions required for event
 notification in SIP.  Semantics are described in section 3.  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.

7.1. 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

Roach Standards Track [Page 32] RFC 3265 SIP-Specific Event Notification June 2002

 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

Roach Standards Track [Page 33] RFC 3265 SIP-Specific Event Notification June 2002

 User-Agent                          o   o
 Via                         c       m   m
 Warning                     R       -   o
 Warning                     r       o   o
 WWW-Authenticate           401      m   m

7.1.1. 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.

7.1.2. 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.

7.2. New Headers

 This table expands on tables 2 and 3 in SIP [1], as amended by the
 changes described in section 7.1.
 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

7.2.1. "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.

Roach Standards Track [Page 34] RFC 3265 SIP-Specific Event Notification June 2002

    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.

7.2.2. "Allow-Events" Header

 Allow-Events is added to the definition of the element "general-
 header" in the SIP message grammar.  Its usage is described in
 section 3.3.7.

7.2.3. "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 3.2.4.

7.3. New Response Codes

7.3.1. "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 [3].

7.3.2. "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.

7.4. 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

Roach Standards Track [Page 35] RFC 3265 SIP-Specific Event Notification June 2002

 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 event-reason-value)
                        / ("expires" EQUAL delta-seconds)
                        / ("retry-after" EQUAL delta-seconds)
                        / generic-param
 event-reason-value   =   "deactivated"
                        / "probation"
                        / "rejected"
                        / "timeout"
                        / "giveup"
                        / "noresource"
                        / event-reason-extension
 event-reason-extension = token

8. Normative References

 [1]   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.
 [2]   Petrack, S. and L. Conroy, "The PINT Service Protocol", RFC
       2848, June 2000.
 [3]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
       Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
       HTTP/1.1", RFC 2616, June 1999.
 [4]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
       Considerations Section in RFCs", BCP 26, RFC 2434, October
       1998.

Roach Standards Track [Page 36] RFC 3265 SIP-Specific Event Notification June 2002

 [5]   Bradner, S., "Key Words for Use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
 [6]   Day, M., Aggarwal, S., Mohr, G. and J. Vincent, "Instant
       Messaging/Presence Protocol Requirements", RFC 2779, February
       2000.

9. Informative References

 [7]   Rosenberg, J. and H. Schulzrinne, "Guidelines for Authors of
       SIP Extensions", Work in Progress.
 [8]   Schulzrinne, H. and J. Rosenberg, "SIP Caller Preferences and
       Callee Capabilities", Work in Progress.

10. Acknowledgements

 Thanks to the participants in the Events BOF at the 48th IETF meeting
 in Pittsburgh, as well as those who gave ideas and 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" document for their input to SUBSCRIBE and NOTIFY request
 semantics.

11. Notice Regarding Intellectual Property Rights

 The IETF has been notified of intellectual property rights claimed in
 regard to some or all of the specification contained in this
 document.  For more information, consult the online list of claimed
 rights at http://www.ietf.org/ipr.html

12. Author's Address

 Adam Roach
 dynamicsoft
 5100 Tennyson Parkway
 Suite 1200
 Plano, TX 75024
 USA
 EMail: adam@dynamicsoft.com
 Voice: sip:adam@dynamicsoft.com

Roach Standards Track [Page 37] RFC 3265 SIP-Specific Event Notification June 2002

13. Full Copyright Statement

 Copyright (C) The Internet Society (2002).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise 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 be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Roach Standards Track [Page 38]

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