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rfc:rfc6665

Internet Engineering Task Force (IETF) A.B. Roach Request for Comments: 6665 Tekelec Obsoletes: 3265 July 2012 Updates: 3261, 4660 Category: Standards Track ISSN: 2070-1721

                  SIP-Specific Event Notification

Abstract

 This document describes an extension to the Session Initiation
 Protocol (SIP) defined by RFC 3261.  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.
 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.
 This document represents a backwards-compatible improvement on the
 original mechanism described by RFC 3265, taking into account several
 years of implementation experience.  Accordingly, this document
 obsoletes RFC 3265.  This document also updates RFC 4660 slightly to
 accommodate some small changes to the mechanism that were discussed
 in that document.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6665.

Roach Standards Track [Page 1] RFC 6665 SIP-Specific Event Notification July 2012

Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   1.1.  Overview of Operation  . . . . . . . . . . . . . . . . . .  5
   1.2.  Documentation Conventions  . . . . . . . . . . . . . . . .  6
 2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  6
 3.  SIP Methods for Event Notification . . . . . . . . . . . . . .  7
   3.1.  SUBSCRIBE  . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.1.  Subscription Duration  . . . . . . . . . . . . . . . .  7
     3.1.2.  Identification of Subscribed Events and Event
             Classes  . . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.3.  Additional SUBSCRIBE Header Field Values . . . . . . .  9
   3.2.  NOTIFY . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.2.1.  Identification of Reported Events, Event Classes,
             and Current State  . . . . . . . . . . . . . . . . . .  9
 4.  Node Behavior  . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.1.  Subscriber Behavior  . . . . . . . . . . . . . . . . . . . 10
     4.1.1.  Detecting Support for SIP Events . . . . . . . . . . . 10
     4.1.2.  Creating and Maintaining Subscriptions . . . . . . . . 10
     4.1.3.  Receiving and Processing State Information . . . . . . 14
     4.1.4.  Forking of SUBSCRIBE Requests  . . . . . . . . . . . . 16
   4.2.  Notifier Behavior  . . . . . . . . . . . . . . . . . . . . 17
     4.2.1.  Subscription Establishment and Maintenance . . . . . . 17
     4.2.2.  Sending State Information to Subscribers . . . . . . . 20
     4.2.3.  PSTN/Internet Interworking (PINT) Compatibility  . . . 23
   4.3.  Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 23
   4.4.  Common Behavior  . . . . . . . . . . . . . . . . . . . . . 24
     4.4.1.  Dialog Creation and Termination  . . . . . . . . . . . 24
     4.4.2.  Notifier Migration . . . . . . . . . . . . . . . . . . 24
     4.4.3.  Polling Resource State . . . . . . . . . . . . . . . . 25
     4.4.4.  "Allow-Events" Header Field Usage  . . . . . . . . . . 26
   4.5.  Targeting Subscriptions at Devices . . . . . . . . . . . . 26
     4.5.1.  Using GRUUs to Route to Devices  . . . . . . . . . . . 27

Roach Standards Track [Page 2] RFC 6665 SIP-Specific Event Notification July 2012

     4.5.2.  Sharing Dialogs  . . . . . . . . . . . . . . . . . . . 27
   4.6.  CANCEL Requests for SUBSCRIBE and NOTIFY Transactions  . . 29
 5.  Event Packages . . . . . . . . . . . . . . . . . . . . . . . . 29
   5.1.  Appropriateness of Usage . . . . . . . . . . . . . . . . . 29
   5.2.  Event Template-Packages  . . . . . . . . . . . . . . . . . 30
   5.3.  Amount of State to Be Conveyed . . . . . . . . . . . . . . 31
     5.3.1.  Complete State Information . . . . . . . . . . . . . . 31
     5.3.2.  State Deltas . . . . . . . . . . . . . . . . . . . . . 32
   5.4.  Event Package Responsibilities . . . . . . . . . . . . . . 32
     5.4.1.  Event Package Name . . . . . . . . . . . . . . . . . . 33
     5.4.2.  Event Package Parameters . . . . . . . . . . . . . . . 33
     5.4.3.  SUBSCRIBE Request Bodies . . . . . . . . . . . . . . . 33
     5.4.4.  Subscription Duration  . . . . . . . . . . . . . . . . 33
     5.4.5.  NOTIFY Request Bodies  . . . . . . . . . . . . . . . . 34
     5.4.6.  Notifier Processing of SUBSCRIBE Requests  . . . . . . 34
     5.4.7.  Notifier generation of NOTIFY requests . . . . . . . . 34
     5.4.8.  Subscriber Processing of NOTIFY Requests . . . . . . . 34
     5.4.9.  Handling of Forked Requests  . . . . . . . . . . . . . 34
     5.4.10. Rate of Notifications  . . . . . . . . . . . . . . . . 35
     5.4.11. State Aggregation  . . . . . . . . . . . . . . . . . . 35
     5.4.12. Examples . . . . . . . . . . . . . . . . . . . . . . . 36
     5.4.13. Use of URIs to Retrieve State  . . . . . . . . . . . . 36
 6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 36
   6.1.  Access Control . . . . . . . . . . . . . . . . . . . . . . 36
   6.2.  Notifier Privacy Mechanism . . . . . . . . . . . . . . . . 36
   6.3.  Denial-of-Service Attacks  . . . . . . . . . . . . . . . . 37
   6.4.  Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 37
   6.5.  Man-in-the-Middle Attacks  . . . . . . . . . . . . . . . . 37
   6.6.  Confidentiality  . . . . . . . . . . . . . . . . . . . . . 38
 7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
   7.1.  Event Packages . . . . . . . . . . . . . . . . . . . . . . 38
     7.1.1.  Registration Information . . . . . . . . . . . . . . . 39
     7.1.2.  Registration Template  . . . . . . . . . . . . . . . . 40
   7.2.  Reason Codes . . . . . . . . . . . . . . . . . . . . . . . 40
   7.3.  Header Field Names . . . . . . . . . . . . . . . . . . . . 41
   7.4.  Response Codes . . . . . . . . . . . . . . . . . . . . . . 41
 8.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
   8.1.  New Methods  . . . . . . . . . . . . . . . . . . . . . . . 42
     8.1.1.  SUBSCRIBE Method . . . . . . . . . . . . . . . . . . . 42
     8.1.2.  NOTIFY Method  . . . . . . . . . . . . . . . . . . . . 42
   8.2.  New Header Fields  . . . . . . . . . . . . . . . . . . . . 42
     8.2.1.  "Event" Header Field . . . . . . . . . . . . . . . . . 42
     8.2.2.  "Allow-Events" Header Field  . . . . . . . . . . . . . 43
     8.2.3.  "Subscription-State" Header Field  . . . . . . . . . . 43
   8.3.  New Response Codes . . . . . . . . . . . . . . . . . . . . 43
     8.3.1.  202 (Accepted) Response Code . . . . . . . . . . . . . 43
     8.3.2.  489 (Bad Event) Response Code  . . . . . . . . . . . . 44
   8.4.  Augmented BNF Definitions  . . . . . . . . . . . . . . . . 44

Roach Standards Track [Page 3] RFC 6665 SIP-Specific Event Notification July 2012

 9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 45
   9.1.  Normative References . . . . . . . . . . . . . . . . . . . 45
   9.2.  Informative References . . . . . . . . . . . . . . . . . . 46
 Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 48
 Appendix B.  Changes from RFC 3265 . . . . . . . . . . . . . . . . 48
   B.1.  Bug 666: Clarify use of "expires=xxx" with "terminated"  . 48
   B.2.  Bug 667: Reason code for unsub/poll not clearly
         spelled out  . . . . . . . . . . . . . . . . . . . . . . . 48
   B.3.  Bug 669: Clarify: SUBSCRIBE for a duration might be
         answered with a NOTIFY/expires=0 . . . . . . . . . . . . . 48
   B.4.  Bug 670: Dialog State Machine needs clarification  . . . . 49
   B.5.  Bug 671: Clarify timeout-based removal of subscriptions  . 49
   B.6.  Bug 672: Mandate "expires" in NOTIFY . . . . . . . . . . . 49
   B.7.  Bug 673: INVITE 481 response effect clarification  . . . . 49
   B.8.  Bug 677: SUBSCRIBE response matching text in error . . . . 49
   B.9.  Bug 695: Document is not explicit about response to
         NOTIFY at subscription termination . . . . . . . . . . . . 49
   B.10. Bug 696: Subscription state machine needs clarification  . 49
   B.11. Bug 697: Unsubscription behavior could be clarified  . . . 49
   B.12. Bug 699: NOTIFY and SUBSCRIBE are target refresh
         requests . . . . . . . . . . . . . . . . . . . . . . . . . 50
   B.13. Bug 722: Inconsistent 423 reason phrase text . . . . . . . 50
   B.14. Bug 741: Guidance needed on when to not include
         "Allow-Events" . . . . . . . . . . . . . . . . . . . . . . 50
   B.15. Bug 744: 5xx to NOTIFY terminates a subscription, but
         should not . . . . . . . . . . . . . . . . . . . . . . . . 50
   B.16. Bug 752: Detection of forked requests is incorrect . . . . 50
   B.17. Bug 773: Reason code needs IANA registry . . . . . . . . . 50
   B.18. Bug 774: Need new reason for terminating subscriptions
         to resources that never change . . . . . . . . . . . . . . 50
   B.19. Clarify Handling of "Route"/"Record-Route" in NOTIFY . . . 50
   B.20. Eliminate Implicit Subscriptions . . . . . . . . . . . . . 51
   B.21. Deprecate Dialog Reuse . . . . . . . . . . . . . . . . . . 51
   B.22. Rationalize Dialog Creation  . . . . . . . . . . . . . . . 51
   B.23. Refactor Behavior Sections . . . . . . . . . . . . . . . . 51
   B.24. Clarify Sections That Need to Be Present in Event
         Packages . . . . . . . . . . . . . . . . . . . . . . . . . 51
   B.25. Make CANCEL Handling More Explicit . . . . . . . . . . . . 51
   B.26. Remove "State Agent" Terminology . . . . . . . . . . . . . 51
   B.27. Miscellaneous Changes  . . . . . . . . . . . . . . . . . . 52

Roach Standards Track [Page 4] RFC 6665 SIP-Specific Event Notification July 2012

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) [RFC2848] 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 that is not so complex as to be unusable for
 simple features, but that is still flexible enough to provide
 powerful services.  Note, however, that event packages based on this
 framework may define arbitrarily elaborate rules that govern the
 subscription and notification for the events or classes of events
 they describe.
 This document does not describe an extension that may be used
 directly; it must be extended by other documents (herein referred to
 as "event packages").  In object-oriented design terminology, it may
 be thought of as an abstract base class that must be derived into an
 instantiable class by further extensions.  Guidelines for creating
 these extensions are described in Section 5.

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------->|

Roach Standards Track [Page 5] RFC 6665 SIP-Specific Event Notification July 2012

 Subscriptions are expired and must be refreshed by subsequent
 SUBSCRIBE requests.

1.2. Documentation Conventions

 There are several paragraphs throughout this document that 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.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].
 In particular, implementors need to take careful note of the meaning
 of "SHOULD" defined in RFC 2119.  To rephrase: violation of "SHOULD"-
 strength requirements requires careful analysis and clearly
 enumerable reasons.  It is a protocol violation to fail to comply
 with "SHOULD"-strength requirements whimsically or for ease of
 implementation.

2. Definitions

 Event Package:  An event package is an additional specification that
    defines a set of state information to be reported by a notifier to
    a subscriber.  Event packages also define further syntax and
    semantics that are based on the framework defined by this document
    and are required to convey such state information.
 Event Template-Package:  An event template-package is a special kind
    of event package that defines a set of states that may be applied
    to all possible event packages, including itself.
 Notification:  Notification is the act of a notifier sending a NOTIFY
    request to a subscriber to inform the subscriber of the state of a
    resource.
 Notifier:  A notifier is a user agent that generates NOTIFY requests
    for the purpose of notifying subscribers of the state of a
    resource.  Notifiers typically also accept SUBSCRIBE requests to
    create subscriptions.

Roach Standards Track [Page 6] RFC 6665 SIP-Specific Event Notification July 2012

 Subscriber:  A subscriber is a user agent that 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.

3. SIP Methods for Event Notification

3.1. SUBSCRIBE

 The SUBSCRIBE method is used to request current state and state
 updates from a remote node.  SUBSCRIBE requests are target refresh
 requests, as that term is defined in [RFC3261].

3.1.1. Subscription Duration

 SUBSCRIBE requests SHOULD contain an "Expires" header field (defined
 in [RFC3261]).  This expires value indicates the duration of the
 subscription.  In order to keep subscriptions effective beyond the
 duration communicated in the "Expires" header field, subscribers need
 to refresh subscriptions on a periodic basis using a new SUBSCRIBE
 request on the same dialog as defined in [RFC3261].
 If no "Expires" header field is present in a SUBSCRIBE request, the
 implied default MUST be defined by the event package being used.
 200-class responses to SUBSCRIBE requests also MUST contain an
 "Expires" header field.  The period of time in the response MAY be
 shorter but MUST NOT be longer than specified in the request.  The
 notifier is explicitly allowed to shorten the duration to zero.  The
 period of time in the response is the one that defines the duration
 of the subscription.
 An "expires" parameter on the "Contact" header field has no semantics
 for the SUBSCRIBE method and is explicitly not equivalent to an
 "Expires" header field in a SUBSCRIBE request or response.

Roach Standards Track [Page 7] RFC 6665 SIP-Specific Event Notification July 2012

 A natural consequence of this scheme is that a SUBSCRIBE request with
 an "Expires" of 0 constitutes a request to unsubscribe from the
 matching subscription.
    In addition to being a request to unsubscribe, a SUBSCRIBE request
    with "Expires" of 0 also causes a fetch of state; see
    Section 4.4.3.
 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.

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.
 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 [RFC3261].  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@example.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 field in
 SUBSCRIBE requests, indicating to which event or class of events they
 are subscribing.  The "Event" header field will contain a token that
 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 that further describes the semantics
 of the event or event class.
 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
 field; if they do so, they must define the semantics for such
 parameters.

Roach Standards Track [Page 8] RFC 6665 SIP-Specific Event Notification July 2012

3.1.3. Additional SUBSCRIBE Header Field Values

 Because SUBSCRIBE requests create a dialog usage as defined in
 [RFC3261], they MAY contain an "Accept" header field.  This header
 field, if present, indicates the body formats allowed in subsequent
 NOTIFY requests.  Event packages MUST define the behavior for
 SUBSCRIBE requests without "Accept" header fields; usually, this will
 connote a single, default body type.
 Header values not described in this document are to be interpreted as
 described in [RFC3261].

3.2. NOTIFY

 NOTIFY requests are sent to inform subscribers of changes in state to
 which the subscriber has a subscription.  Subscriptions are created
 using the SUBSCRIBE method.  In legacy implementations, it is
 possible that other means of subscription creation have been used.
 However, this specification does not allow the creation of
 subscriptions except through SUBSCRIBE requests and (for backwards-
 compatibility) REFER requests [RFC3515].
 NOTIFY is a target refresh request, as that term is defined in
 [RFC3261].
 A NOTIFY request 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 request "Event" header fields MUST
 contain a single event package name for which a notification is being
 generated.  The package name in the "Event" header field MUST match
 the "Event" header field in the corresponding SUBSCRIBE request.
 Event packages may define semantics associated with the body of their
 NOTIFY requests; if they do so, those semantics apply.  NOTIFY
 request bodies are expected to provide additional details about the
 nature of the event that has occurred and the resultant resource
 state.

Roach Standards Track [Page 9] RFC 6665 SIP-Specific Event Notification July 2012

 When present, the body of the NOTIFY request MUST be formatted into
 one of the body formats specified in the "Accept" header field of the
 corresponding SUBSCRIBE request (or the default type according to the
 event package description, if no "Accept" header field was
 specified).  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 5.4.13).

4. Node Behavior

4.1. Subscriber Behavior

4.1.1. Detecting Support for SIP Events

 The extension described in this document does not make use of the
 "Require" or "Proxy-Require" header fields; similarly, there is no
 token defined for "Supported" header fields.  Potential subscribers
 may probe for the support of SIP events using the OPTIONS request
 defined in [RFC3261].
 The presence of "SUBSCRIBE" in the "Allow" header field of any
 request or response indicates support for SIP events; further, in the
 absence of an "Allow" header field, the simple presence of an "Allow-
 Events" header field is sufficient to indicate that the node that
 sent the message is capable of acting as a notifier (see
 Section 4.4.4).
    The "methods" parameter for Contact may also be used to
    specifically announce support for SUBSCRIBE and NOTIFY requests
    when registering.  (See [RFC3840] for details on the "methods"
    parameter.)

4.1.2. Creating and Maintaining Subscriptions

 From the subscriber's perspective, a subscription proceeds according
 to the following state diagram.  Events that result in a transition
 back to the same state are not represented in this diagram.

Roach Standards Track [Page 10] RFC 6665 SIP-Specific Event Notification July 2012

                        +-------------+
                        |    init     |<-----------------------+
                        +-------------+                        |
                               |                           Retry-after
                         Send SUBSCRIBE                    expires
                               |                               |
                               V          Timer N Fires;       |
                        +-------------+   SUBSCRIBE failure    |
           +------------| notify_wait |-- response; --------+  |
           |            +-------------+   or NOTIFY,        |  |
           |                   |          state=terminated  |  |
           |                   |                            |  |
 ++========|===================|============================|==|====++
 ||        |                   |                            V  |    ||
 ||  Receive NOTIFY,    Receive NOTIFY,             +-------------+ ||
 ||  state=active       state=pending               | terminated  | ||
 ||        |                   |                    +-------------+ ||
 ||        |                   |          Re-subscription     A  A  ||
 ||        |                   V          times out;          |  |  ||
 ||        |            +-------------+   Receive NOTIFY,     |  |  ||
 ||        |            |   pending   |-- state=terminated; --+  |  ||
 ||        |            +-------------+   or 481 response        |  ||
 ||        |                   |          to SUBSCRIBE           |  ||
 ||        |            Receive NOTIFY,   refresh                |  ||
 ||        |            state=active                             |  ||
 ||        |                   |          Re-subscription        |  ||
 ||        |                   V          times out;             |  ||
 ||        |            +-------------+   Receive NOTIFY,        |  ||
 ||        +----------->|   active    |-- state=terminated; -----+  ||
 ||                     +-------------+   or 481 response           ||
 ||                                       to SUBSCRIBE              ||
 || Subscription                          refresh                   ||
 ++=================================================================++
 In the state diagram, "Re-subscription times out" means that an
 attempt to refresh or update the subscription using a new SUBSCRIBE
 request does not result in a NOTIFY request before the corresponding
 Timer N expires.
 Any transition from "notify_wait" into a "pending" or "active" state
 results in a new subscription.  Note that multiple subscriptions can
 be generated as the result of a single SUBSCRIBE request (see
 Section 4.4.1).  Each of these new subscriptions exists in its own
 independent state machine and runs its own set of timers.

Roach Standards Track [Page 11] RFC 6665 SIP-Specific Event Notification July 2012

4.1.2.1. Requesting a Subscription

 SUBSCRIBE is a dialog-creating method, as described in [RFC3261].
 When a subscriber wishes to subscribe to a particular state for a
 resource, it forms a SUBSCRIBE request.  If the initial SUBSCRIBE
 request represents a request outside of a dialog (as it typically
 will), its construction follows the procedures outlined in [RFC3261]
 for User Agent Client (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 request will be sent immediately.
 The "Expires" header field in a 200-class response to SUBSCRIBE
 request indicates the actual duration for which the subscription will
 remain active (unless refreshed).  The received value might be
 smaller than the value indicated in the SUBSCRIBE request but cannot
 be larger; see Section 4.2.1 for details.
 Non-200-class final responses indicate that no subscription or new
 dialog usage has been created, and no subsequent NOTIFY request will
 be sent.  All non-200-class responses (with the exception of 489 (Bad
 Event), described herein) have the same meanings and handling as
 described in [RFC3261].  For the sake of clarity: if a SUBSCRIBE
 request contains an "Accept" header field, but that field does not
 indicate a media type that the notifier is capable of generating in
 its NOTIFY requests, then the proper error response is 406 (Not
 Acceptable).

4.1.2.2. 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.  The handling for
 such a request is the same as for the initial creation of a
 subscription except as described below.
 If a SUBSCRIBE request to refresh a subscription receives a 404, 405,
 410, 416, 480-485, 489, 501, or 604 response, the subscriber MUST
 consider the subscription terminated.  (See [RFC5057] for further
 details and notes about the effect of error codes on dialogs and
 usages within dialog, such as subscriptions).  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 4.1.2.1).

Roach Standards Track [Page 12] RFC 6665 SIP-Specific Event Notification July 2012

 If a SUBSCRIBE request to refresh a subscription fails with any error
 code other than those listed above, the original subscription is
 still considered valid for the duration of the most recently known
 "Expires" value as negotiated by the most recent successful SUBSCRIBE
 transaction, or as communicated by a NOTIFY request in its
 "Subscription-State" header field "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
    requests will be received.
 When refreshing a subscription, a subscriber starts Timer N, set to
 64*T1, when it sends the SUBSCRIBE request.  If this Timer N expires
 prior to the receipt of a NOTIFY request, the subscriber considers
 the subscription terminated.  If the subscriber receives a success
 response to the SUBSCRIBE request that indicates that no NOTIFY
 request will be generated -- such as the 204 response defined for use
 with the optional extension described in [RFC5839] -- then it MUST
 cancel Timer N.

4.1.2.3. Unsubscribing

 Unsubscribing is handled in the same way as refreshing of a
 subscription, with the "Expires" header field set to "0".  Note that
 a successful unsubscription will also trigger a final NOTIFY request.
 The final NOTIFY request may or may not contain information about the
 state of the resource; subscribers need to be prepared to receive
 final NOTIFY requests both with and without state.

4.1.2.4. Confirmation of Subscription Creation

 The subscriber can expect to receive a NOTIFY request from each node
 which has processed a successful subscription or subscription
 refresh.  To ensure that subscribers do not wait indefinitely for a
 subscription to be established, a subscriber starts a Timer N, set to
 64*T1, when it sends a SUBSCRIBE request.  If this Timer N expires
 prior to the receipt of a NOTIFY request, the subscriber considers
 the subscription failed, and cleans up any state associated with the
 subscription attempt.
 Until Timer N expires, several NOTIFY requests may arrive from
 different destinations (see Section 4.4.1).  Each of these requests
 establishes a new dialog usage and a new subscription.  After the
 expiration of Timer N, the subscriber SHOULD reject any such NOTIFY
 requests that would otherwise establish a new dialog usage with a 481
 (Subscription does not exist) response code.

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 Until the first NOTIFY request arrives, the subscriber should
 consider the state of the subscribed resource to be in a neutral
 state.  Event package specifications 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 out-of-order messages, packet loss, and
 forking, the subscriber MUST be prepared to receive NOTIFY requests
 before the SUBSCRIBE transaction has completed.
 Except as noted above, processing of this NOTIFY request is the same
 as in Section 4.1.3.

4.1.3. Receiving and Processing State Information

 Subscribers receive information about the state of a resource to
 which they have subscribed in the form of NOTIFY requests.
 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 usage are described in Section 4.4.1.  Notifications for
 subscriptions that were created inside an existing dialog match if
 they are in the same dialog and the "Event" header fields match (as
 described in Section 8.2.1).
 If, for some reason, the event package designated in the "Event"
 header field 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, such as
 those described in Sections 22.2 and 23 of [RFC3261].
 NOTIFY requests MUST contain "Subscription-State" header fields that
 indicate the status of the subscription.
 If the "Subscription-State" header field value is "active", it means
 that the subscription has been accepted and (in general) has been
 authorized.  If the header field 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".

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 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
 field 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
 MUST consider the subscription terminated.  The "expires" parameter
 has no semantics for "terminated" -- notifiers SHOULD NOT include an
 "expires" parameter on a "Subscription-State" header field with a
 value of "terminated", and subscribers MUST ignore any such
 parameter, if present.  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 reason codes
 defined by this document 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 (as long as the resource's state
    is still relevant to the client at that 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".  This reason code is also associated with polling of
    resource state, as detailed in Section 4.4.3.
 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

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    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 that was being monitored no longer exists.  Clients
    SHOULD NOT attempt to re-subscribe.  The "retry-after" parameter
    has no semantics for "noresource".
 invariant:  The subscription has been terminated because the resource
    state is guaranteed not to change for the foreseeable future.
    This may be the case, for example, when subscribing to the
    location information of a fixed-location land-line telephone.
    When using this reason code, notifiers are advised to include a
    "retry-after" parameter with a large value (for example, 31536000
    -- or one year) to prevent older clients that are RFC 3265
    compliant from periodically re-subscribing.  Clients SHOULD NOT
    attempt to re-subscribe after receiving a reason code of
    "invariant", regardless of the presence of or value of a "retry-
    after" parameter.
 Other specifications may define new reason codes for use with the
 "Subscription-State" header field.
 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 field.
 Other responses defined in [RFC3261] 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.

4.1.4. Forking of SUBSCRIBE Requests

 In accordance with the rules for proxying non-INVITE requests as
 defined in [RFC3261], 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 that differ
 from the "To:" tag received in the SUBSCRIBE 200-class response.

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 If multiple NOTIFY requests are received in different dialogs in
 response to a single SUBSCRIBE request, each dialog represents a
 different destination to which the SUBSCRIBE request was forked.
 Subscriber handling in such situations varies by event package; see
 Section 5.4.9 for details.

4.2. Notifier Behavior

4.2.1. Subscription Establishment and Maintenance

 Notifiers learn about subscription requests by receiving SUBSCRIBE
 requests from interested parties.  Notifiers MUST NOT create
 subscriptions except upon receipt of a SUBSCRIBE request.  However,
 for historical reasons, the implicit creation of subscriptions as
 defined in [RFC3515] is still permitted.
    [RFC3265] allowed the creation of subscriptions using means other
    than the SUBSCRIBE method.  The only standardized use of this
    mechanism is the REFER method [RFC3515].  Implementation
    experience with REFER has shown that the implicit creation of a
    subscription has a number of undesirable effects, such as the
    inability to signal the success of a REFER request while signaling
    a problem with the subscription, and difficulty performing one
    action without the other.  Additionally, the proper exchange of
    dialog identifiers is difficult without dialog reuse (which has
    its own set of problems; see Section 4.5).

4.2.1.1. 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 [RFC3261]) 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 field 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 4.2.1.3.

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 The notifier MAY also check that the duration in the "Expires" header
 field 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
 Brief) error that contains a "Min-Expires" header field.  The
 "Min-Expires" header field is described in [RFC3261].
 Once the notifier determines that it has enough information to create
 the subscription (i.e., it understands the event package, the
 subscription pertains to a known resource, and there are no other
 barriers to creating the subscription), it creates the subscription
 and a dialog usage, and returns a 200 (OK) response.
 When a subscription is created in the notifier, it stores the event
 package name 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 request 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 [RFC3261] may be used in response
 to SUBSCRIBE requests, as appropriate.

4.2.1.2. Confirmation of Subscription Creation/Refreshing

 Upon successfully accepting or refreshing a subscription, notifiers
 MUST send a NOTIFY request immediately to communicate the current
 resource state to the subscriber.  This NOTIFY request 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 request is
 processed, this NOTIFY request MAY contain an empty or neutral body.
 See Section 4.2.2 for further details on NOTIFY request generation.
 Note that a NOTIFY request is always sent immediately after any
 200-class response to a SUBSCRIBE request, regardless of whether the
 subscription has already been authorized.

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4.2.1.3. 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 [RFC3261].  Note that,
 even if the notifier node typically acts as a proxy, authentication
 for SUBSCRIBE requests will always be performed via a 401
 (Unauthorized) response, not a 407 (Proxy Authentication Required).
 Notifiers always act as 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 user agents and, as such,
    perform user agent 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 200 (OK) response is returned immediately.
 Note that a NOTIFY request 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 request containing a "Subscription-State" header
 field with a value of "terminated" and a reason parameter of
 "rejected".

4.2.1.4. 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

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 field in the response.  If the duration specified in a SUBSCRIBE
 request is unacceptably short, the notifier SHOULD respond with a 423
 (Interval Too Brief) response.
 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 request with a
 "Subscription-State" value of "terminated" to inform it that the
 subscription is being removed.  If such a request is sent, the
 "Subscription-State" header field SHOULD contain a "reason=timeout"
 parameter.
 Clients can cause a subscription to be terminated immediately by
 sending a SUBSCRIBE request with an "Expires" header field set to
 '0'.  Notifiers largely treat this the same way as any other
 subscription expiration: they send a NOTIFY request containing a
 "Subscription-State" of "terminated", with a reason code of
 "timeout."  For consistency with state polling (see Section 4.4.3)
 and subscription refreshes, the notifier may choose to include
 resource state in this final NOTIFY request.  However, in some cases,
 including such state makes no sense.  Under such circumstances, the
 notifier may choose to omit state information from the terminal
 NOTIFY request.
    The sending of a NOTIFY request when a subscription expires allows
    the corresponding dialog usage to be terminated, if appropriate.

4.2.2. Sending State Information to Subscribers

 Notifiers use the NOTIFY method to send information about the state
 of a resource to subscribers.  The notifier's view of a subscription
 is shown in the following state diagram.  Events that result in a
 transition back to the same state are not represented in this
 diagram.

Roach Standards Track [Page 20] RFC 6665 SIP-Specific Event Notification July 2012

                       +-------------+
                       |    init     |
                       +-------------+
                              |
                        Receive SUBSCRIBE,
                        Send NOTIFY
                              |
                              V          NOTIFY failure,
                       +-------------+   subscription expires,
          +------------|  resp_wait  |-- or terminated ----+
          |            +-------------+   per local policy  |
          |                   |                            |
          |                   |                            |
          |                   |                            V
    Policy grants       Policy needed              +-------------+
    permission                |                    | terminated  |
          |                   |                    +-------------+
          |                   |                               A A
          |                   V          NOTIFY failure,      | |
          |            +-------------+   subscription expires,| |
          |            |   pending   |-- or terminated -------+ |
          |            +-------------+   per local policy       |
          |                   |                                 |
          |            Policy changed to                        |
          |            grant permission                         |
          |                   |                                 |
          |                   V          NOTIFY failure,        |
          |            +-------------+   subscription expires,  |
          +----------->|   active    |-- or terminated ---------+
                       +-------------+   per local policy
 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,
 unless the state transition is caused by a NOTIFY transaction
 failure.  The sending of this NOTIFY message is also subject to
 authorization, local policy, and throttling considerations.
 If the NOTIFY request fails due to expiration of SIP Timer F
 (transaction timeout), 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 [RFC3261]
    (instead of the typical single transmission for functioning
    clients), continuing to service them when no client is available

Roach Standards Track [Page 21] RFC 6665 SIP-Specific Event Notification July 2012

    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 requests to refresh
    potentially stale state information; such requests will reinstall
    subscriptions in all relevant nodes.
 If the NOTIFY transaction fails due to the receipt of a 404, 405,
 410, 416, 480-485, 489, 501, or 604 response to the NOTIFY request,
 the notifier MUST remove the corresponding subscription.  See
 [RFC5057] for further details and notes about the effect of error
 codes on dialogs and usages within dialog (such as subscriptions).
    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 sending of SUBSCRIBE requests to
    refresh the expiration timer will reinstall subscription state
    once the network problem has been resolved.
 NOTIFY requests MUST contain a "Subscription-State" header field 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 field is "active" or
 "pending", the notifier MUST also include in the "Subscription-State"
 header field an "expires" parameter that 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 necessary in case the SUBSCRIBE request forks,
    since the response to a forked SUBSCRIBE request may not be
    received by the subscriber.  [RFC3265] allowed the notifier some
    discretion in the inclusion of this parameter, so subscriber
    implementations are warned to handle the lack of an "expires"
    parameter gracefully.  Note well that this "expires" value is a
    parameter on the "Subscription-State" header field NOT the
    "Expires" header field.

Roach Standards Track [Page 22] RFC 6665 SIP-Specific Event Notification July 2012

    The period of time for a subscription can be shortened to zero by
    the notifier.  In other words, it is perfectly valid for a
    SUBSCRIBE request with a non-zero expires to be answered with a
    NOTIFY request that contains "Subscription-Status:
    terminated;reason=expired".  This merely means that the notifier
    has shortened the subscription timeout to zero, and the
    subscription has expired instantaneously.  The body may contain
    valid state, or it may contain a neutral state (see
    Section 5.4.7).
 If the value of the "Subscription-State" header field 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 4.1.3.

4.2.3. PSTN/Internet Interworking (PINT) Compatibility

 The "Event" header field is considered mandatory for the purposes of
 this document.  However, to maintain compatibility with PINT (see
 [RFC2848]), notifiers MAY interpret a SUBSCRIBE request with no
 "Event" header field as requesting a subscription to PINT events.  If
 a notifier does not support PINT, it SHOULD return 489 (Bad Event) to
 any SUBSCRIBE requests without an "Event" header field.

4.3. Proxy Behavior

 Proxies need no additional behavior beyond that described in
 [RFC3261] to support SUBSCRIBE and NOTIFY transactions.  If a proxy
 wishes to see all of the SUBSCRIBE and NOTIFY requests for a given
 dialog, it MUST add a "Record-Route" header field to the initial
 SUBSCRIBE request and all NOTIFY requests.  It MAY choose to include
 "Record-Route" in subsequent SUBSCRIBE requests; however, these
 requests cannot cause the dialog's route set to be modified.
 Proxies that did not add a "Record-Route" header field to the initial
 SUBSCRIBE request MUST NOT add a "Record-Route" header field to any
 of the associated NOTIFY requests.
    Note that subscribers and notifiers may elect to use Secure/
    Multipurpose Internet Mail Extensions (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 [RFC3261].

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4.4. Common Behavior

4.4.1. Dialog Creation and Termination

 Dialogs usages are created upon completion of a NOTIFY transaction
 for a new subscription, unless the NOTIFY request contains a
 "Subscription-State" of "terminated."
 Because the dialog usage is established by the NOTIFY request, the
 route set at the subscriber is taken from the NOTIFY request itself,
 as opposed to the route set present in the 200-class response to the
 SUBSCRIBE request.
 NOTIFY requests are matched to such SUBSCRIBE requests if they
 contain the same "Call-ID", a "To" header field "tag" parameter that
 matches the "From" header field "tag" parameter of the SUBSCRIBE
 request, and the same "Event" header field.  Rules for comparisons of
 the "Event" header fields are described in Section 8.2.1.
 A subscription is destroyed after a notifier sends a NOTIFY request
 with a "Subscription-State" of "terminated", or in certain error
 situations described elsewhere in this document.  The subscriber will
 generally answer such final requests with a 200 (OK) response (unless
 a condition warranting an alternate response has arisen).  Except
 when the mechanism described in Section 4.5.2 is used, the
 destruction of a subscription results in the termination of its
 associated dialog.
    A subscriber may send a SUBSCRIBE request with an "Expires" header
    field 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 transaction with a "Subscription-State" of "terminated"
    completes.

4.4.2. Notifier Migration

 It is often useful to allow migration of subscriptions between
 notifiers.  Such migration may be effected by sending a NOTIFY
 request with a "Subscription-State" header field 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 request, 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 reuse
 the route set from the previous subscription dialog.

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 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.

4.4.3. 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 request 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 requests triggered by SUBSCRIBE requests with
 "Expires" header fields 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.
    Due to the requirement on notifiers to send a NOTIFY request
    immediately upon receipt of a SUBSCRIBE request, the state
    provided by polling is limited to the information that the
    notifier has immediate local access to when it receives the
    SUBSCRIBE request.  If, for example, the notifier generally needs
    to retrieve state from another network server, then that state
    will be absent from the NOTIFY request that results from polling.

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4.4.4. "Allow-Events" Header Field Usage

 The "Allow-Events" header field, if present, MUST include a
 comprehensive and inclusive list of tokens that indicates the event
 packages for which the user agent can act as a notifier.  In other
 words, a user agent sending an "Allow-Events" header field is
 advertising that it can process SUBSCRIBE requests and generate
 NOTIFY requests for all of the event packages listed in that header
 field.
 Any user agent that can act as a notifier for one or more event
 packages SHOULD include an appropriate "Allow-Events" header field
 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.
    On the other hand, it doesn't necessarily make much sense to
    indicate supported events inside a dialog established by a NOTIFY
    request if the only event package supported is the one associated
    with that subscription.
 Note that "Allow-Events" header fields MUST NOT be inserted by
 proxies.
 The "Allow-Events" header field does not include a list of the event
 template-packages supported by an implementation.  If a subscriber
 wishes to determine which event template-packages are supported by a
 notifier, it can probe for such support by attempting to subscribe to
 the event template-packages it wishes to use.
    For example: to check for support for the templatized package
    "presence.winfo", a client may attempt to subscribe to that event
    package for a known resource, using an "Expires" header value of
    0.  If the response is a 489 error code, then the client can
    deduce that "presence.winfo" is unsupported.

4.5. Targeting Subscriptions at Devices

 [RFC3265] defined a mechanism by which subscriptions could share
 dialogs with invite usages and with other subscriptions.  The purpose
 of this behavior was to allow subscribers to ensure that a
 subscription arrived at the same device as an established dialog.
 Unfortunately, the reuse of dialogs has proven to be exceedingly
 confusing.  [RFC5057] attempted to clarify proper behavior in a

Roach Standards Track [Page 26] RFC 6665 SIP-Specific Event Notification July 2012

 variety of circumstances; however, the ensuing rules remain confusing
 and prone to implementation error.  At the same time, the mechanism
 described in [RFC5627] now provides a far more elegant and
 unambiguous means to achieve the same goal.
 Consequently, the dialog reuse technique described in RFC 3265 is now
 deprecated.
 This dialog-sharing technique has also historically been used as a
 means for targeting an event package at a dialog.  This usage can be
 seen, for example, in certain applications of the REFER method
 [RFC3515].  With the removal of dialog reuse, an alternate (and more
 explicit) means of targeting dialogs needs to be used for this type
 of correlation.  The appropriate means of such targeting is left up
 to the actual event packages.  Candidates include the "Target-Dialog"
 header field [RFC4538], the "Join" header field [RFC3911], and the
 "Replaces" header field [RFC3891], depending on the semantics
 desired.  Alternately, if the semantics of those header fields do not
 match the event package's purpose for correlation, event packages can
 devise their own means of identifying dialogs.  For an example of
 this approach, see the Dialog Event Package [RFC4235].

4.5.1. Using GRUUs to Route to Devices

 Notifiers MUST implement the Globally Routable User Agent URI (GRUU)
 extension defined in [RFC5627], and MUST use a GRUU as their local
 target.  This allows subscribers to explicitly target desired
 devices.
 If a subscriber wishes to subscribe to a resource on the same device
 as an established dialog, it should check whether the remote contact
 in that dialog is a GRUU (i.e., whether it contains a "gr" URI
 parameter).  If so, the subscriber creates a new dialog, using the
 GRUU as the Request URI for the new SUBSCRIBE request.
    Because GRUUs are guaranteed to route to a specific device, this
    ensures that the subscription will be routed to the same place as
    the established dialog.

4.5.2. Sharing Dialogs

 For compatibility with older clients, subscriber and notifier
 implementations may choose to allow dialog sharing.  The behavior of
 multiple usages within a dialog are described in [RFC5057].
 Subscribers MUST NOT attempt to reuse dialogs whose remote target is
 a GRUU.

Roach Standards Track [Page 27] RFC 6665 SIP-Specific Event Notification July 2012

    Note that the techniques described in this section are included
    for backwards-compatibility purposes only.  Because subscribers
    cannot reuse dialogs with a GRUU for their remote target, and
    because notifiers must use GRUUs as their local target, any two
    implementations that conform to this specification will
    automatically use the mechanism described in Section 4.5.1.
    Further note that the prohibition on reusing dialogs does not
    exempt implicit subscriptions created by the REFER method.  This
    means that implementations complying with this specification are
    required to use the "Target-Dialog" mechanism described in
    [RFC4538] when the remote target is a GRUU.
 If a subscriber wishes to subscribe to a resource on the same device
 as an established dialog and the remote contact is not a GRUU, it MAY
 revert to dialog-sharing behavior.  Alternately, it MAY choose to
 treat the remote party as incapable of servicing the subscription
 (i.e., the same way it would behave if the remote party did not
 support SIP events at all).
 If a notifier receives a SUBSCRIBE request for a new subscription on
 an existing dialog, it MAY choose to implement dialog sharing
 behavior.  Alternately, it may choose to fail the SUBSCRIBE request
 with a 403 (Forbidden) response.  The error text of such 403
 responses SHOULD indicate that dialog sharing is not supported.
 To implement dialog sharing, subscribers and notifiers perform the
 following additional processing:
 o  When subscriptions exist in dialogs associated with INVITE-created
    application state and/or other subscriptions, these sets of
    application state do not interact beyond the behavior described
    for a dialog (e.g., route set handling).  In particular, multiple
    subscriptions within a dialog expire independently and require
    independent subscription refreshes.
 o  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.  This means that, when dialogs are
    reused, 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.

Roach Standards Track [Page 28] RFC 6665 SIP-Specific Event Notification July 2012

 o  Subscribers MAY include an "id" parameter in a SUBSCRIBE request's
    "Event" header field to allow differentiation between multiple
    subscriptions in the same dialog.  This "id" parameter, if
    present, contains an opaque token that identifies the specific
    subscription within a dialog.  An "id" parameter is only valid
    within the scope of a single dialog.
 o  If an "id" parameter is present in the SUBSCRIBE request used to
    establish a subscription, that "id" parameter MUST also be present
    in all corresponding NOTIFY requests.
 o  When a subscriber refreshes the subscription timer, the SUBSCRIBE
    request MUST contain the same "Event" header field "id" parameter
    as was present in the SUBSCRIBE request that created the
    subscription.  (Otherwise, the notifier will interpret the
    SUBSCRIBE request as a request for a new subscription in the same
    dialog.)
 o  When a subscription is created in the notifier, it stores any
    "Event" header field "id" parameter as part of the subscription
    information (along with the event package name).
 o  If an initial SUBSCRIBE request is sent on a pre-existing dialog,
    a matching NOTIFY request merely creates a new subscription
    associated with that dialog.

4.6. CANCEL Requests for SUBSCRIBE and NOTIFY Transactions

 Neither SUBSCRIBE nor NOTIFY requests can be canceled.  If a User
 Agent Server (UAS) receives a CANCEL request that matches a known
 SUBSCRIBE or NOTIFY transaction, it MUST respond to the CANCEL
 request, but otherwise ignore it.  In particular, the CANCEL request
 MUST NOT affect processing of the SUBSCRIBE or NOTIFY request in any
 way.
 UACs SHOULD NOT send CANCEL requests for SUBSCRIBE or NOTIFY
 transactions.

5. Event Packages

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

5.1. 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

Roach Standards Track [Page 29] RFC 6665 SIP-Specific Event Notification July 2012

 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" [RFC4485] 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 signaling volume;
 sending a notification every time a user's GPS position changes by
 one hundredth 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 that 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 that 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 the "Event" header field would be
 "presence.winfo".
    This scheme may be arbitrarily extended.  For example, application
    of the "winfo" package to the "presence.winfo" state of a resource
    would be represented by the name "presence.winfo.winfo".  It
    naturally follows from this syntax that the order in which
    templates are specified is significant.

Roach Standards Track [Page 30] RFC 6665 SIP-Specific Event Notification July 2012

    For example: consider a theoretical event template-package called
    "list".  The event "presence.winfo.list" would be the application
    of the "list" template to "presence.winfo", which would presumably
    be a list of winfo state associated with presence.  On the other
    hand, the event "presence.list.winfo" would represent the
    application of winfo to "presence.list", which would be represent
    the winfo state of a list of presence information.
 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 that 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.
 This problem has two possible solutions that event packages may
 choose to implement.

5.3.1. Complete State Information

 In general, event packages need to be able to convey a well-defined
 and complete state, rather than just a stream of events.  If it is
 not possible to describe complete system state for transmission in
 NOTIFY requests, then the problem set is not a good candidate for an
 event package.
 For packages that 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
 whenever 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".

Roach Standards Track [Page 31] RFC 6665 SIP-Specific Event Notification July 2012

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 requests
 contain state deltas instead of complete state.
 Such a scheme would work as follows: any NOTIFY request sent in
 immediate response to a SUBSCRIBE request contains full state
 information.  NOTIFY requests 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 field.
 If a NOTIFY request 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 request containing the state
 delta (except for the version number, which it retains to detect
 message loss), and re-sends a SUBSCRIBE request to force a NOTIFY
 request containing a complete state snapshot.

5.4. 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" requirements 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.
 For clarity: well-formed event package definitions contain sections
 addressing each of these issues, ideally in the same order and with
 the same titles as these subsections.

Roach Standards Track [Page 32] RFC 6665 SIP-Specific Event Notification July 2012

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
 that 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 field to modify
 the behavior of the event package, the syntax and semantics of such
 header fields MUST be clearly defined.
 Any "Event" header field parameters defined by an event package MUST
 be registered in the "Header Field Parameters and Parameter Values"
 registry defined by [RFC3968].  An "Event" header field parameter,
 once registered in conjunction with an event package, MUST NOT be
 reused with any other event package.  Non-event-package
 specifications MAY define "Event" header field parameters that apply
 across all event packages (with emphasis on "all", as opposed to
 "several"), such as the "id" parameter defined in this document.  The
 restriction of a parameter to use with a single event package only
 applies to parameters that are defined in conjunction with an event
 package.

5.4.3. SUBSCRIBE Request Bodies

 It is expected that most, but not all, event packages will define
 syntax and semantics for SUBSCRIBE request 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 reuse existing media types for message bodies
 where practical.  See [RFC4288] for information on media type
 specification and registration.
 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.

Roach Standards Track [Page 33] RFC 6665 SIP-Specific Event Notification July 2012

5.4.5. NOTIFY Request Bodies

 The NOTIFY request 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 bodies.
 Event packages also MUST define which media type is to be assumed if
 none are specified in the "Accept" header field 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.

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 request 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 what the deltas are 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
 requests that correspond to the SUBSCRIBE request (i.e., have

Roach Standards Track [Page 34] RFC 6665 SIP-Specific Event Notification July 2012

 matching "To", "From", "Call-ID", and "Event" header fields, as well
 as "From" header field "tag" parameter and "Event" header field "id"
 parameter) but that do not match the dialog would be rejected with a
 481 response.  Note that the 200-class response to the SUBSCRIBE
 request can arrive after a matching NOTIFY request has been received;
 such responses might not correlate to the same dialog established by
 the NOTIFY request.  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 request is allowed, the subscriber establishes a new dialog
 towards each notifier by returning a 200-class response to each
 NOTIFY request.  Each dialog is then handled as its own entity and is
 refreshed independently 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
 that 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) that 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 that allows
 subscribers to further limit the rate of notification.

5.4.11. State Aggregation

 Many event packages inherently work by collecting information about a
 resource from a number of other sources -- either through the use of
 PUBLISH [RFC3903], by subscribing to state information, or through
 other state-gathering mechanisms.
 Event packages that involve retrieval of state information for a
 single resource from more than one source need to consider how
 notifiers aggregate information into a single, coherent state.  Such
 packages MUST specify how notifiers aggregate information and how
 they provide authentication and authorization.

Roach Standards Track [Page 35] RFC 6665 SIP-Specific Event Notification July 2012

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 document
 for exact protocol details.

5.4.13. Use of URIs to Retrieve State

 Some types of event packages may define state information that 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.
 [RFC4483] defines a mechanism that can be used by event packages to
 convey information in such a fashion.

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 private.  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 are outside the scope of this document.

6.2. Notifier Privacy Mechanism

 The mere act of returning certain 400- and 600-class 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 200 (OK) response.  While the
 subsequent NOTIFY request 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.

Roach Standards Track [Page 36] RFC 6665 SIP-Specific Event Notification July 2012

 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 [RFC2779] 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 [CERT1998a].
 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 [RFC3261].

6.4. Replay Attacks

 Replaying of either SUBSCRIBE or NOTIFY requests can have detrimental
 effects.
 In the case of SUBSCRIBE requests, an attacker may be able to install
 any arbitrary subscription that it witnessed being installed at some
 point in the past.  Replaying of NOTIFY requests may be used to spoof
 old state information (although a good versioning mechanism in the
 body of the NOTIFY requests may help mitigate such an attack).  Note
 that the prohibition on sending NOTIFY requests to nodes that 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 [RFC3261].

6.5. Man-in-the-Middle Attacks

 Even with authentication, man-in-the-middle attacks using SUBSCRIBE
 requests 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"
 header fields (at a minimum) of SUBSCRIBE requests.  If SUBSCRIBE

Roach Standards Track [Page 37] RFC 6665 SIP-Specific Event Notification July 2012

 request 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 requests 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" header fields and the bodies of NOTIFY requests.
 Integrity protection of message header fields and bodies is discussed
 in [RFC3261].

6.6. Confidentiality

 The state information contained in a NOTIFY request 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 request 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 requests.
 The mechanisms for providing confidentiality are detailed in
 [RFC3261].

7. IANA Considerations

 With the exception of Section 7.2, the subsections here are for
 current reference, carried over from the original specification (RFC
 3265).  IANA has updated all registry references that pointed to RFC
 3265 to instead indicate this document and created the new "reason
 code" registry described in Section 7.2.

7.1. Event Packages

 This document defines an event-type namespace that requires a central
 coordinating body.  The body chosen for this coordination is the
 Internet Assigned Numbers Authority (IANA).

Roach Standards Track [Page 38] RFC 6665 SIP-Specific Event Notification July 2012

 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 is forbidden from sharing a
 name with a package.
 Policies for registration of SIP event packages and SIP event package
 templates are defined in Section 4.1 of [RFC5727].
 Registrations with the IANA are required to 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 that
 describes the event package.  Event template-package token
 registrations are also required to include a pointer to the published
 RFC that defines the event template-package.
 Registered tokens to designate packages and template-packages are
 disallowed from containing the character ".", which is used to
 separate template-packages from packages.

7.1.1. Registration Information

 This document specifies no package or template-package names.  All
 entries in this table are added by other documents.  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 four possible
 permutations of package type, contact, and reference.
 The table below lists the event packages and template-packages
 defined for use with the "SIP-Specific Event Notification" mechanism
 [RFC 6665].  Each name is designated as a package or a template-
 package under "Type".
 Package Name      Type         Contact      Reference
 ------------      ----         -------      ---------
 example1          package      [Doe]        [RFCnnnn]
 example2          package                   [RFCnnnn]
 example3          template     [Doe]        [RFCnnnn]
 example4          template                  [RFCnnnn]
 PEOPLE
 ------
 [Doe] John Doe <john.doe@example.com>
 REFERENCES
 ----------
 [RFCnnnn] Doe, J., "Sample Document", RFC nnnn, Month YYYY.

Roach Standards Track [Page 39] RFC 6665 SIP-Specific Event Notification July 2012

7.1.2. 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 8.2.1.)
 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:
    (self-explanatory)

7.2. Reason Codes

 This document further defines "reason" codes for use in the
 "Subscription-State" header field (see Section 4.1.3).
 Following the policies outlined in "Guidelines for Writing an IANA
 Considerations Section in RFCs" [RFC5226], new reason codes require a
 Standards Action.
 Registrations with the IANA include the reason code being registered
 and a reference to a published document that describes the event
 package.  Insertion of such values takes place as part of the RFC
 publication process or as the result of liaison activity between
 standards development organizations (SDOs), the result of which will
 be publication of an associated RFC.  New reason codes must conform
 to the syntax of the ABNF "token" element defined in [RFC3261].
 [RFC4660] defined a new reason code prior to the establishment of an
 IANA registry.  We include its reason code ("badfilter") in the
 initial list of reason codes to ensure a complete registry.
 The IANA registry for reason codes has been initialized with the
 following values:

Roach Standards Track [Page 40] RFC 6665 SIP-Specific Event Notification July 2012

 Reason Code            Reference
 -----------            ---------
 deactivated            [RFC6665]
 probation              [RFC6665]
 rejected               [RFC6665]
 timeout                [RFC6665]
 giveup                 [RFC6665]
 noresource             [RFC6665]
 invariant              [RFC6665]
 badfilter              [RFC4660]
 REFERENCES
 ----------
 [RFC6665]  A.B. Roach, "SIP-Specific Event Notification", RFC 6665,
            July 2012.
 [RFC4660]  Khartabil, H., Leppanen, E., Lonnfors, M., and
            J. Costa-Requena, "Functional Description of Event
            Notification Filtering", September 2006.

7.3. Header Field Names

 This document registers three new header field names, described
 elsewhere in this document.  These header fields are defined by the
 following information, which is to be added to the header field sub-
 registry under http://www.iana.org/assignments/sip-parameters.
 Header Name:   Allow-Events
 Compact Form:  u
 Header Name:   Subscription-State
 Compact Form:  (none)
 Header Name:   Event
 Compact Form:  o

7.4. 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

Roach Standards Track [Page 41] RFC 6665 SIP-Specific Event Notification July 2012

8. 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 [RFC3261] and contain references
 to elements defined therein.

8.1. New Methods

 This document describes two new SIP methods: SUBSCRIBE and NOTIFY.

8.1.1. 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.

8.1.2. 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 that has
 been requested by an earlier SUBSCRIBE method has occurred.  It may
 also provide further details about the event.

8.2. New Header Fields

8.2.1. "Event" Header Field

 Event is added to the definition of the element "message-header
 field" in the SIP message grammar.
 For the purposes of matching NOTIFY requests with SUBSCRIBE requests,
 the event-type portion of the "Event" header field is compared byte
 by byte, and the "id" parameter token (if present) is compared byte
 by byte.  An "Event" header field containing an "id" parameter never
 matches an "Event" header field without an "id" parameter.  No other
 parameters are considered when performing a comparison.  SUBSCRIBE
 responses are matched per the transaction handling rules in
 [RFC3261].

Roach Standards Track [Page 42] RFC 6665 SIP-Specific Event Notification July 2012

    Note that the foregoing 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 field.
 Multiple events per message are disallowed.
 The "Event" header field is defined only for use in SUBSCRIBE and
 NOTIFY requests and other requests whose definition explicitly calls
 for its use.  It MUST NOT appear in any other SIP requests and MUST
 NOT appear in responses.

8.2.2. "Allow-Events" Header Field

 "Allow-Events" is added to the definition of the element "general-
 header field" in the SIP message grammar.  Its usage is described in
 Section 4.4.4.
 User agents MAY include the "Allow-Events" header field in any
 request or response, as long as its contents comply with the behavior
 described in Section 4.4.4.

8.2.3. "Subscription-State" Header Field

 "Subscription-State" is added to the definition of the element
 "request-header" field in the SIP message grammar.  Its usage is
 described in Section 4.1.3.  "Subscription-State" header fields are
 defined for use in NOTIFY requests only.  They MUST NOT appear in
 other SIP requests or responses.

8.3. New Response Codes

8.3.1. 202 (Accepted) Response Code

 For historical purposes, the 202 (Accepted) response code is added to
 the "Success" header field definition.
 This document does not specify the use of the 202 response code in
 conjunction with the SUBSCRIBE or NOTIFY methods.  Previous versions
 of the SIP Events Framework assigned specific meaning to the 202
 response code.

Roach Standards Track [Page 43] RFC 6665 SIP-Specific Event Notification July 2012

 Due to response handling in forking cases, any 202 response to a
 SUBSCRIBE request may be absorbed by a proxy, and thus it can never
 be guaranteed to be received by the UAC.  Furthermore, there is no
 actual processing difference for a 202 as compared to a 200; a NOTIFY
 request is sent after the subscription is processed, and it conveys
 the correct state.  SIP interoperability tests found that
 implementations were handling 202 differently from 200, leading to
 incompatibilities.  Therefore, the 202 response is being deprecated
 to make it clear there is no such difference and 202 should not be
 handled differently than 200.
 Implementations conformant with the current specification MUST treat
 an incoming 202 response as identical to a 200 response and MUST NOT
 generate 202 response codes to SUBSCRIBE or NOTIFY requests.
 This document also updates [RFC4660], which reiterates the 202-based
 behavior in several places.  Implementations compliant with the
 present document MUST NOT send a 202 response to a SUBSCRIBE request
 and will send an alternate success response (such as 200) in its
 stead.

8.3.2. 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.

8.4. Augmented BNF Definitions

 The Augmented BNF [RFC5234] definitions for the various new and
 modified syntax elements follows.  The notation is as used in
 [RFC3261], 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 / "-"  / "!" / "%" / "*"
                          / "_" / "+" / "`" / "'" / "~" )
 ; The use of the "id" parameter is deprecated; it is included
 ; for backwards-compatibility purposes only.

Roach Standards Track [Page 44] RFC 6665 SIP-Specific Event Notification July 2012

 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"
                        / "invariant"
                        / event-reason-extension
 event-reason-extension = token

9. References

9.1. Normative References

 [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2848]    Petrack, S. and L. Conroy, "The PINT Service Protocol:
              Extensions to SIP and SDP for IP Access to Telephone
              Call Services", RFC 2848, June 2000.
 [RFC3261]    Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.
 [RFC3265]    Roach, A., "Session Initiation Protocol (SIP)-Specific
              Event Notification", RFC 3265, June 2002.
 [RFC3968]    Camarillo, G., "The Internet Assigned Number Authority
              (IANA) Header Field Parameter Registry for the Session
              Initiation Protocol (SIP)", BCP 98, RFC 3968,
              December 2004.

Roach Standards Track [Page 45] RFC 6665 SIP-Specific Event Notification July 2012

 [RFC4483]    Burger, E., "A Mechanism for Content Indirection in
              Session Initiation Protocol (SIP) Messages", RFC 4483,
              May 2006.
 [RFC5226]    Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.
 [RFC5234]    Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [RFC5627]    Rosenberg, J., "Obtaining and Using Globally Routable
              User Agent URIs (GRUUs) in the Session Initiation
              Protocol (SIP)", RFC 5627, October 2009.
 [RFC5727]    Peterson, J., Jennings, C., and R. Sparks, "Change
              Process for the Session Initiation Protocol (SIP) and
              the Real-time Applications and Infrastructure Area",
              BCP 67, RFC 5727, March 2010.

9.2. Informative References

 [RFC2779]    Day, M., Aggarwal, S., Mohr, G., and J. Vincent,
              "Instant Messaging / Presence Protocol Requirements",
              RFC 2779, February 2000.
 [RFC3515]    Sparks, R., "The Session Initiation Protocol (SIP) Refer
              Method", RFC 3515, April 2003.
 [RFC3840]    Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840, August 2004.
 [RFC3891]    Mahy, R., Biggs, B., and R. Dean, "The Session
              Initiation Protocol (SIP) "Replaces" Header", RFC 3891,
              September 2004.
 [RFC3903]    Niemi, A., "Session Initiation Protocol (SIP) Extension
              for Event State Publication", RFC 3903, October 2004.
 [RFC3911]    Mahy, R. and D. Petrie, "The Session Initiation Protocol
              (SIP) "Join" Header", RFC 3911, October 2004.
 [RFC4235]    Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
              Initiated Dialog Event Package for the Session
              Initiation Protocol (SIP)", RFC 4235, November 2005.

Roach Standards Track [Page 46] RFC 6665 SIP-Specific Event Notification July 2012

 [RFC4288]    Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", BCP 13, RFC 4288,
              December 2005.
 [RFC4485]    Rosenberg, J. and H. Schulzrinne, "Guidelines for
              Authors of Extensions to the Session Initiation Protocol
              (SIP)", RFC 4485, May 2006.
 [RFC4538]    Rosenberg, J., "Request Authorization through Dialog
              Identification in the Session Initiation Protocol
              (SIP)", RFC 4538, June 2006.
 [RFC4660]    Khartabil, H., Leppanen, E., Lonnfors, M., and J. Costa-
              Requena, "Functional Description of Event Notification
              Filtering", RFC 4660, September 2006.
 [RFC5057]    Sparks, R., "Multiple Dialog Usages in the Session
              Initiation Protocol", RFC 5057, November 2007.
 [RFC5839]    Niemi, A. and D. Willis, "An Extension to Session
              Initiation Protocol (SIP) Events for Conditional Event
              Notification", RFC 5839, May 2010.
 [CERT1998a]  CERT, "CERT Advisory CA-1998-01: Smurf IP Denial-of-
              Service Attacks", 1998,
              <http://www.cert.org/advisories/CA-1998-01.html>.

Roach Standards Track [Page 47] RFC 6665 SIP-Specific Event Notification July 2012

Appendix A. 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 first draft version, and the authors of the "SIP Extensions
 for Presence" document for their input to SUBSCRIBE and NOTIFY
 request semantics.
 I also owe a debt of gratitude to all the implementors who have
 provided feedback on areas of confusion or difficulty in the original
 specification.  In particular, Robert Sparks' Herculean efforts
 organizing, running, and collecting data from the SIPit events have
 proven invaluable in shaking out specification bugs.  Robert Sparks
 is also responsible for untangling the dialog usage mess, in the form
 of RFC 5057 [RFC5057].

Appendix B. Changes from RFC 3265

 This document represents several changes from the mechanism
 originally described in RFC 3265.  This section summarizes those
 changes.  Bug numbers refer to the identifiers for the bug reports
 kept on file at http://bugs.sipit.net/.

B.1. Bug 666: Clarify use of "expires=xxx" with "terminated"

 Strengthened language in Section 4.1.3 to clarify that "expires"
 should not be sent with "terminated", and must be ignored if
 received.

B.2. Bug 667: Reason code for unsub/poll not clearly spelled out

 Clarified description of "timeout" in Section 4.1.3. (n.b., the text
 in Section 4.4.3 is actually pretty clear about this).

B.3. Bug 669: Clarify: SUBSCRIBE for a duration might be answered with

    a NOTIFY/expires=0
 Added clarifying text to Section 4.2.2 explaining that shortening a
 subscription to zero seconds is valid.  Also added sentence to
 Section 3.1.1 explicitly allowing shortening to zero.

Roach Standards Track [Page 48] RFC 6665 SIP-Specific Event Notification July 2012

B.4. Bug 670: Dialog State Machine needs clarification

 The issues associated with the bug deal exclusively with the handling
 of multiple usages with a dialog.  This behavior has been deprecated
 and moved to Section 4.5.2.  This section, in turn, cites [RFC5057],
 which addresses all of the issues in Bug 670.

B.5. Bug 671: Clarify timeout-based removal of subscriptions

 Changed Section 4.2.2 to specifically cite Timer F (so as to avoid
 ambiguity between transaction timeouts and retransmission timeouts).

B.6. Bug 672: Mandate "expires" in NOTIFY

 Changed strength of including of "expires" in a NOTIFY from "SHOULD"
 to "MUST" in Section 4.2.2.

B.7. Bug 673: INVITE 481 response effect clarification

 This bug was addressed in [RFC5057].

B.8. Bug 677: SUBSCRIBE response matching text in error

 Fixed Section 8.2.1 to remove incorrect "...responses and..." --
 explicitly pointed to SIP for transaction response handling.

B.9. Bug 695: Document is not explicit about response to NOTIFY at

    subscription termination
 Added text to Section 4.4.1 indicating that the typical response to a
 terminal NOTIFY is a 200 (OK).

B.10. Bug 696: Subscription state machine needs clarification

 Added state machine diagram to Section 4.1.2 with explicit handling
 of what to do when a SUBSCRIBE never shows up.  Added definition of
 and handling for new Timer N to Section 4.1.2.4.  Added state machine
 to Section 4.2.2 to reinforce text.

B.11. Bug 697: Unsubscription behavior could be clarified

 Added text to Section 4.2.1.4 encouraging (but not requiring) full
 state in final NOTIFY request.  Also added text to Section 4.1.2.3
 warning subscribers that full state may or may not be present in the
 final NOTIFY.

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B.12. Bug 699: NOTIFY and SUBSCRIBE are target refresh requests

 Added text to both Sections 3.1 and 3.2 explicitly indicating that
 SUBSCRIBE and NOTIFY are target refresh methods.

B.13. Bug 722: Inconsistent 423 reason phrase text

 Changed reason phrase to "Interval Too Brief" in Sections 4.2.1.1 and
 4.2.1.4, to match 423 reason phrase in SIP [RFC3261].

B.14. Bug 741: Guidance needed on when to not include "Allow-Events"

 Added non-normative clarification to Section 4.4.4 regarding
 inclusion of "Allow-Events" in a NOTIFY for the one-and-only package
 supported by the notifier.

B.15. Bug 744: 5xx to NOTIFY terminates a subscription, but should not

 Issue of subscription (usage) termination versus dialog termination
 is handled in [RFC5057].  The text in Section 4.2.2 has been updated
 to summarize the behavior described by RFC 5057, and cites it for
 additional detail and rationale.

B.16. Bug 752: Detection of forked requests is incorrect

 Removed erroneous "CSeq" from list of matching criteria in
 Section 5.4.9.

B.17. Bug 773: Reason code needs IANA registry

 Added Section 7.2 to create and populate IANA registry.

B.18. Bug 774: Need new reason for terminating subscriptions to

     resources that never change
 Added new "invariant" reason code to Section 4.1.3 and to ABNF syntax
 in Section 8.4.

B.19. Clarify Handling of "Route"/"Record-Route" in NOTIFY

 Changed text in Section 4.3 in order to mandate "Record-Route" in
 initial SUBSCRIBE and all NOTIFY requests, and add "MAY"-level
 statements for subsequent SUBSCRIBE requests.

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B.20. Eliminate Implicit Subscriptions

 Added text to Section 4.2.1 explaining some of the problems
 associated with implicit subscriptions, and added normative language
 prohibiting them.  Removed language from Section 3.2 describing "non-
 SUBSCRIBE" mechanisms for creating subscriptions.  Simplified
 language in Section 4.2.2, now that the soft-state/non-soft-state
 distinction is unnecessary.

B.21. Deprecate Dialog Reuse

 Moved handling of dialog reuse and "id" handling to Section 4.5.2.
 It is documented only for backwards-compatibility purposes.

B.22. Rationalize Dialog Creation

 Section 4.4.1 has been updated to specify that dialogs should be
 created when the NOTIFY arrives.  Previously, the dialog was
 established by the SUBSCRIBE 200 or by the NOTIFY transaction.  This
 was unnecessarily complicated; the newer rules are easier to
 implement (and result in effectively the same behavior on the wire).

B.23. Refactor Behavior Sections

 Reorganized Section 4 to consolidate behavior along role lines
 (subscriber/notifier/proxy) instead of method lines.

B.24. Clarify Sections That Need to Be Present in Event Packages

 Added sentence to Section 5 clarifying that event packages are
 expected to include explicit sections covering the issues discussed
 in this section.

B.25. Make CANCEL Handling More Explicit

 Text in Section 4.6 now clearly calls out behavior upon receipt of a
 CANCEL.  We also echo the "...SHOULD NOT send..." requirement from
 [RFC3261].

B.26. Remove "State Agent" Terminology

 As originally planned, we anticipated a fairly large number of event
 packages that would move back and forth between end-user devices and
 servers in the network.  In practice, this has ended up not being the
 case.  Certain events, like dialog state, are inherently hosted at
 end-user devices; others, like presence, are almost always hosted in
 the network (due to issues like composition, and the ability to
 deliver information when user devices are offline).  Further, the

Roach Standards Track [Page 51] RFC 6665 SIP-Specific Event Notification July 2012

 concept of State Agents is the most misunderstood by event package
 authors.  In my expert review of event packages, I have yet to find
 one that got the concept of State Agents completely correct -- and
 most of them start out with the concept being 100% backwards from the
 way RFC 3265 described it.
 Rather than remove the ability to perform the actions previously
 attributed to the widely misunderstood term "State Agent", we have
 simply eliminated this term.  Instead, we talk about the behaviors
 required to create state agents (state aggregation, subscription
 notification) without defining a formal term to describe the servers
 that exhibit these behaviors.  In effect, this is an editorial change
 to make life easier for event package authors; the actual protocol
 does not change as a result.
 The definition of "State Agent" has been removed from Section 2.
 Section 4.4.2 has been retooled to discuss migration of subscription
 in general, without calling out the specific example of state agents.
 Section 5.4.11 has been focused on state aggregation in particular,
 instead of state aggregation as an aspect of state agents.

B.27. Miscellaneous Changes

 The following changes are relatively minor revisions to the document
 that resulted primarily from review of this document in the working
 group and IESG, rather than implementation reports.
 o  Clarified scope of "Event" header field parameters.  In RFC 3265,
    the scope is ambiguous, which causes problems with the registry in
    RFC 3968.  The new text ensures that "Event" header field
    parameters are unique across all event packages.
 o  Removed obsoleted language around IANA registration policies for
    event packages.  Instead, we now cite RFC 5727, which updates RFC
    3265, and is authoritative on event package registration policy.
 o  Several editorial updates after input from working group,
    including proper designation of "dialog usage" rather than
    "dialog" where needed.
 o  Clarified two normative statements about subscription termination
    by changing from plain English prose to RFC2119 language.
 o  Removed "Table 2" expansions, per WG consensus on how SIP Table 2
    is to be handled.
 o  Removed 202 response code.

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 o  Clarified that "Allow-Events" does not list event template-
    packages.
 o  Added clarification about proper response when the SUBSCRIBE
    indicates an unknown media type in its "Accept" header field.
 o  Minor clarifications to "Route" and "Record-Route" behavior.
 o  Added non-normative warning about the limitations of state
    polling.
 o  Added information about targeting subscriptions at specific
    dialogs.
 o  Added RFC 3261 to list of documents updated by this one (rather
    than the "2543" indicated by RFC 3265).
 o  Clarified text in Section 3.1.1 explaining the meaning of
    "Expires: 0".
 o  Changed text in definition of "probation" reason code to indicate
    that subscribers don't need to re-subscribe if the associated
    state is no longer of use to them.
 o  Specified that the termination of a subscription due to a NOTIFY
    transaction failure does not require sending another NOTIFY
    message.
 o  Clarified how order of template application affects the meaning of
    an "Event" header field value (e.g., "foo.bar.baz" is different
    than "foo.baz.bar").

Author's Address

 Adam Roach
 Tekelec
 17210 Campbell Rd.
 Suite 250
 Dallas, TX  75252
 US
 EMail: adam@nostrum.com

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