[Docs] [txt|pdf] [draft-ietf-sipcor...] [Diff1] [Diff2]

PROPOSED STANDARD

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.









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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



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       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



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   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











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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------->|



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   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.






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   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.





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   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.








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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.





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   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.














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                          +-------------+
                          |    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.







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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).





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   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.

















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                         +-------------+
                         |    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



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      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.





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      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



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   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.




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      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.





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   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



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   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.





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      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".



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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.







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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.





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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



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   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.







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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.





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   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





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   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).







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   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.



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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:





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   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



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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].






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      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.





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   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.



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   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.




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   [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.





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   [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>.

























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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.







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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



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   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.




Roach                        Standards Track                   [Page 52]

RFC 6665             SIP-Specific Event Notification           July 2012


   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









Roach                        Standards Track                   [Page 53]


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